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Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2021 • 167 Combining crystalline and polymeric excipients in API solid dispersions – Opportunity or risk?
    Veith, H. and Wiechert, F. and Luebbert, C. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 158 323-335 (2021)
    Amorphous solid dispersions (ASDs) are often metastable against crystallization of the active pharmaceutical ingredient (API) and thus might undergo unwanted changes during storage. The crystallization tendency of ASDs is influenced by the API crystallization driving force (CDF) and the mobility of the molecules in the ASD. Low molecular weight-excipients are known to stabilize amorphous APIs in so-called co-amorphous formulations. Due to their success in stabilizing co-amorphous APIs, low-molecular weight excipients might also enhance the stability of polymeric ASDs. In this work, we investigated the potential of combined low-molecular weight excipient/polymer formulations with in-silico tools and validated the predictions with long-term stability tests of the most promising excipient/polymer combinations. The considered critical quality attributes for the ASDs were the occurrence of amorphous phase separation, API CDF, and molecular mobility in the ASD. As an example, carbamazepine/polyvinylpyrrolidone ASDs were investigated combined with the excipients fructose, lactose, sucrose, trehalose, saccharin, tryptophan, and urea. Although all excipients had a negative impact on the ASD stability, saccharin still turned out to be the most promising one. Long-term stability studies with ASDs containing either saccharin or tryptophan verified -in agreement to the predictions- that API crystallization occurred faster than in the reference ASDs without additional excipient. This work showed that the addition of crystalline excipients to polymeric ASDs might not only offer opportunities but might also bear risks for the long-term stability of the ASD, even though the crystalline excipient stabilizes the polymer-free API. Consequently, excipients should be evaluated based on the thermodynamic phase behavior of the individual mixture of API/polymer/excipient, rather than based on pure-component properties of the excipient only. In-silico predictions proposed in this work remarkably decrease the number of screening tests for identifying suitable formulation excipients. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2020.11.025
  • 2021 • 166 Phase behavior of ASDs based on hydroxypropyl cellulose
    Luebbert, C. and Stoyanov, E. and Sadowski, G.
    International Journal of Pharmaceutics: X 3 (2021)
    Novel polymeric carriers for amorphous solid dispersions (ASDs) are highly demanded in pharmaceutical industry to improve the bioavailability of poorly-soluble drug candidates. Besides established polymer candidates, hydroxypropyl celluloses (HPC) comes more and more into the focus of ASD production since they have the availability to stabilize drug molecules in aqueous media against crystallization. The thermodynamic long-term stability of HPC ASDs with itraconazole and fenofibrate was predicted in this work with PC-SAFT and compared to three-months enduring long-term stability studies. The glass-transition temperature is a crucial attribute of a polymer, but in case of HPC hardly detectable by differential scanning calorimetry. By investigating the glass transition of HPC blends with a miscible polymer, we were for the first time able to estimate the HPC glass transition. Although both, fenofibrate and itraconazole reveal a very low crystalline solubility in HPC regardless of the HPC molecular weight, we observed that low-molecular weight HPC grades such as HPC-UL prevent fenofibrate crystallization for a longer period than the higher molecular weight HPC grades. As predicted, the ASDs with higher drug load underwent amorphous phase separation according to the differential scanning calorimetry thermograms. This work thus showed that it is possible to predict critical drug loads above which amorphous phase separation and/or crystallization occurs in HPC ASDs. © 2020 The Authors
    view abstractdoi: 10.1016/j.ijpx.2020.100070
  • 2021 • 165 Predicting process design spaces for spray drying amorphous solid dispersions
    Dohrn, S. and Rawal, P. and Luebbert, C. and Lehmkemper, K. and Kyeremateng, S.O. and Degenhardt, M. and Sadowski, G.
    International Journal of Pharmaceutics: X 3 (2021)
    Amorphous solid dispersions (ASDs) are commonly manufactured using spray-drying processes. The product quality can be decisively influenced by the choice of process parameters. Following the quality-by-design approach, the identification of the spray-drying process design space is thus an integral task in drug product development. Aiming a solvent-free and homogeneous ASD, API crystallization and amorphous phase separation needs to be avoided during drying. This publication provides a predictive approach for determining spray-drying process conditions via considering thermodynamic driving forces for solvent drying as well as ASD-specific API/polymer/solvent interactions and glass transitions. The ternary API/polymer/solvent phase behavior was calculated using the Perturbed-Chain Statistical Associating Theory (PC-SAFT) and combined with mass and energy balances to find appropriate spray-drying conditions. A process design space was identified for the ASDs of ritonavir and naproxen with either poly(vinylpyrrolidone) or poly(vinylpyrrolidone-co-vinylacetate) spray dried from the solvents acetone, dichloromethane, or ethanol. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.ijpx.2021.100072
  • 2021 • 164 Stability of pharmaceutical co-crystals at humid conditions can be predicted
    Veith, H. and Zaeh, M. and Luebbert, C. and Rodríguez-Hornedo, N. and Sadowski, G.
    Pharmaceutics 13 (2021)
    Knowledge of the stability of pharmaceutical formulations against relative humidity (RH) is essential if they are to become pharmaceutical products. The increasing interest in formulating active pharmaceutical ingredients as stable co-crystals (CCs) triggers the need for fast and reliable in-silico predictions of CC stability as a function of RH. CC storage at elevated RH can lead to deliquescence, which leads to CC dissolution and possible transformation to less soluble solid-state forms. In this work, the deliquescence RHs of the CCs succinic acid/nicotinamide, carbamazepine/nicotinamide, theophylline/citric acid, and urea/glutaric acid were predicted using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). These deliquescence RH values together with predicted phase diagrams of CCs in water were used to determine critical storage conditions, that could lead to CC instability, that is, CC dissolution and precipitation of its components. The importance of CC phase purity on RH conditions for CC stability is demonstrated, where trace levels of a separate phase of active pharmaceutical ingredient or of coformer can significantly decrease the deliquescence RH. The use of additional excipients such as fructose or xylitol was predicted to decrease the deliquescence RH even further. All predictions were successfully validated by stability measurements at 58%, 76%, 86%, 93%, and 98% RH and 25 °C. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/pharmaceutics13030433
  • 2020 • 163 A combined experimental and modelling approach for the Weimberg pathway optimisation
    Shen, L. and Kohlhaas, M. and Enoki, J. and Meier, R. and Schönenberger, B. and Wohlgemuth, R. and Kourist, R. and Niemeyer, F. and van Niekerk, D. and Bräsen, C. and Niemeyer, J. and Snoep, J. and Siebers, B.
    Nature Communications 11 (2020)
    The oxidative Weimberg pathway for the five-step pentose degradation to α-ketoglutarate is a key route for sustainable bioconversion of lignocellulosic biomass to added-value products and biofuels. The oxidative pathway from Caulobacter crescentus has been employed in in-vivo metabolic engineering with intact cells and in in-vitro enzyme cascades. The performance of such engineering approaches is often hampered by systems complexity, caused by non-linear kinetics and allosteric regulatory mechanisms. Here we report an iterative approach to construct and validate a quantitative model for the Weimberg pathway. Two sensitive points in pathway performance have been identified as follows: (1) product inhibition of the dehydrogenases (particularly in the absence of an efficient NAD+ recycling mechanism) and (2) balancing the activities of the dehydratases. The resulting model is utilized to design enzyme cascades for optimized conversion and to analyse pathway performance in C. cresensus cell-free extracts. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-020-14830-y
  • 2020 • 162 A new variational approach for the thermodynamic topology optimization of hyperelastic structures
    Junker, P. and Balzani, D.
    Computational Mechanics (2020)
    We present a novel approach to topology optimization based on thermodynamic extremal principles. This approach comprises three advantages: (1) it is valid for arbitrary hyperelastic material formulations while avoiding artificial procedures that were necessary in our previous approaches for topology optimization based on thermodynamic principles; (2) the important constraints of bounded relative density and total structure volume are fulfilled analytically which simplifies the numerical implementation significantly; (3) it possesses a mathematical structure that allows for a variety of numerical procedures to solve the problem of topology optimization without distinct optimization routines. We present a detailed model derivation including the chosen numerical discretization and show the validity of the approach by simulating two boundary value problems with large deformations. © 2020, The Author(s).
    view abstractdoi: 10.1007/s00466-020-01949-4
  • 2020 • 161 Constraints in thermodynamic extremal principles for non-local dissipative processes
    Hackl, K. and Fischer, F.D. and Svoboda, J.
    Continuum Mechanics and Thermodynamics 32 1337-1345 (2020)
    Phenomena treated by non-equilibrium thermodynamics can be very effectively described by thermodynamic variational principles. The remarkable advantage of such an approach consists in possibility to account for an arbitrary number of constraints among state or kinetic variables stemming, e.g., from conservation laws or balance equations. As shown in the current paper, the variational principles can provide original evolution equations for the state variables implicitly respecting the constraints. Moreover, the variational approach allows formulating the problem directly in discrete state variables and deriving their evolution equations without the necessity to solve partial differential equations. The variational approach makes it also possible to use different kinetic variables in formulation of dissipation and dissipation function. © 2019, The Author(s).
    view abstractdoi: 10.1007/s00161-019-00846-3
  • 2020 • 160 Influence of cytosolic conditions on the reaction equilibrium and the reaction enthalpy of the enolase reaction accessed by calorimetry and van ‘t HOFF
    Vogel, K. and Greinert, T. and Harms, H. and Sadowski, G. and Held, C. and Maskow, T.
    Biochimica et Biophysica Acta - General Subjects 1864 (2020)
    Background: Thermodynamic methods are finding more and more applications in systems biology, which attempts to understand cell functions mechanistically. Unfortunately, the state variables used (reaction enthalpy and Gibbs energy) do not take sufficient account of the conditions inside of cells, especially the crowding with macromolecules. Methods: For this reason, the influence of crowding agents and various other parameters such as salt concentrations, pH and temperature on equilibrium position and reaction enthalpy of the glycolytic example reaction 9 (2-Phospoglycerate - > Phosphoenolpyruvate + H2O) was investigated. The conditions were chosen to be as close as possible to the cytosolic conditions. Poly(ethylene glycol) MW = 20,000 g mol−1 (PEG 20,000) was used to analyze the influence of crowding with macromolecules. The equation of state electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) was applied to consider the influence of crowding agents on the reaction equilibria. Results and conclusions: For the reaction enthalpies and for the equilibria, it was found that the influence of salts and temperature is not pronounced while that of pH and PEG 20,000 concentration is considerable. Furthermore, it could be shown that under identical measurement conditions there are no differences between the van ‘t Hoff and the calorimetrically determined reaction enthalpy. General significance: The results show how important it is to consider the special cytosolic conditions when applying thermodynamic data in systems biology. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.bbagen.2020.129675
  • 2020 • 159 Phase behavior of pharmaceutically relevant polymer/solvent mixtures
    Dohrn, S. and Luebbert, C. and Lehmkemper, K. and Kyeremateng, S.O. and Degenhardt, M. and Sadowski, G.
    International Journal of Pharmaceutics 577 (2020)
    In the pharmaceutical industry, polymers are used as excipients for formulating poorly water-soluble active pharmaceutical ingredients (APIs) in so-called “amorphous solid dispersions” (ASDs). ASDs can be produced via solvent-based processes, where API and polymer are both dissolved in a solvent, followed by a solvent evaporation step (e.g. spray drying). Aiming at a homogeneous API/polymer formulation, phase separation of the components (API, polymer, solvent) during solvent evaporation must be avoided. The latter is often determined by the phase behavior of polymer/solvent mixtures used for ASD processing. Therefore, this work investigates the polymer-solvent interactions in these mixtures. Suitable polymer/solvent combinations investigated in this work comprise the pharmaceutically relevant polymers poly(vinylpyrrolidone) (PVP), poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA64), and hydroxyppropyl methylcellulose acetate succinate 126G (HPMCAS) as well as the solvents acetone, dichloromethane (DCM), ethanol, ethyl acetate, methanol, and water. Based on vapor-sorption experiments demixing of solvents and polymers were predicted using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). These were found to be correct for all investigated solvent/polymer mixtures. Acetone, DCM, ethanol, methanol, and water were found to be completely miscible with PVPVA64. DCM, ethanol, methanol, and water were found to be completely miscible with PVP K90, while none of the investigated solvents was appropriate for avoiding immiscibility with HPMCAS. In addition, the impact of temperature, polymer molecular weight, and solvent-mixture composition on miscibility was successfully predicted using PC-SAFT. Thus, the proposed methodology allows identifying suitable solvents or solvent mixtures relevant for solvent-based preparations of pharmaceutical ASD formulations with low experimental effort. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.ijpharm.2020.119065
  • 2020 • 158 Pressure-dependent electronic structure calculations using integral equation-based solvation models
    Pongratz, T. and Kibies, P. and Eberlein, L. and Tielker, N. and Hölzl, C. and Imoto, S. and Beck Erlach, M. and Kurrmann, S. and Schummel, P.H. and Hofmann, M. and Reiser, O. and Winter, R. and Kremer, W. and Kalbitzer, H.R. and...
    Biophysical Chemistry 257 (2020)
    Recent methodological progress in quantum-chemical calculations using the “embedded cluster reference interaction site model” (EC-RISM) integral equation theory is reviewed in the context of applying it as a solvation model for calculating pressure-dependent thermodynamic and spectroscopic properties of molecules immersed in water. The methodology is based on self-consistent calculations of electronic and solvation structure around dissolved molecules where pressure enters the equations via an appropriately chosen solvent response function and the pure solvent density. Besides specification of a dispersion-repulsion force field for solute-solvent interactions, the EC-RISM approach derives the electrostatic interaction contributions directly from the wave function. We further develop and apply the method to a variety of benchmark cases for which computational or experimental reference data are either available in the literature or are generated specifically for this purpose in this work. Starting with an enhancement to predict hydration free energies at non-ambient pressures, which is the basis for pressure-dependent molecular population estimation, we demonstrate the performance on the calculation of the autoionization constant of water. Spectroscopic problems are addressed by studying the biologically relevant small osmolyte TMAO (trimethylamine N-oxide). Pressure-dependent NMR shifts are predicted and compared to experiments taking into account proper computational referencing methods that extend earlier work. The experimentally observed IR blue-shifts of certain vibrational bands of TMAO as well as of the cyanide anion are reproduced by novel methodology that allows for weighing equilibrium and non-equilibrium solvent relaxation effects. Taken together, the model systems investigated allow for an assessment of the reliability of the EC-RISM approach for studying pressure-dependent biophysical processes. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.bpc.2019.106258
  • 2020 • 157 Standard Gibbs energy of metabolic reactions: IV. Triosephosphate isomerase reaction
    Greinert, T. and Baumhove, K. and Sadowski, G. and Held, C.
    Biophysical Chemistry 258 (2020)
    The glycolytic pathway is present in most organisms and represents a central part of the energy production mechanism in a cell. For a general understanding of glycolysis, the investigation from a thermodynamic point of view is essential and allows realising thermodynamic feasibility analyses under in vivo conditions. However, available literature standard Gibbs energies of reaction, ΔRg′0, are calculated using equilibrium-molality ratios Km′, which might lead to a misinterpretation of the glycolytic pathway. It was the aim of this work to thermodynamically investigate the triosephosphate isomerase (TPI) reaction to provide new activity-based reaction data. In vitro equilibrium experiments were performed, and activity coefficients were predicted with the equation of state electrolyte PC-SAFT (ePC-SAFT). The combination of experimental concentrations and predicted activity coefficients yielded the thermodynamic equilibrium constant Ka and a new value for ΔRg′0(298.15 K, pH 7) = 7.1 ± 0.3 kJ mol‑1. The availability of the new ΔRg′0 value allowed predicting influences of the reaction medium on the reaction equilibrium of the TPI reaction. In this work, influences of the initial substrate concentration, pH and Mg2+ concentration on the reaction equilibrium were investigated and a method is presented to predict these influences. The higher the substrate concentration and the higher the temperature, the stronger the reaction equilibrium is shifted on the product side. While the pH did not have a significant influence on the reaction equilibrium, Mg2+ yielded a shift of the reaction equilibrium to the substrate side. All these effects were predicted correctly with ePC-SAFT. Based on the ePC-SAFT predictions we concluded that a charge-reduction of the product by complexation of the product with Mg2+ was responsible for the strong influence of Mg2+ on the reaction equilibrium. Finally, the standard enthalpy of reaction of ΔRh′0(pH 7) = 18 ± 7 kJ mol‑1 was determined with the equilibrium constants Ka at 298.15 K, 304.15 K and 310.15 K using the van ‘t Hoff equation. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.bpc.2020.106330
  • 2020 • 156 Standard Gibbs energy of metabolic reactions: V. Enolase reaction
    Greinert, T. and Vogel, K. and Seifert, A.I. and Siewert, R. and Andreeva, I.V. and Verevkin, S.P. and Maskow, T. and Sadowski, G. and Held, C.
    Biochimica et Biophysica Acta - Proteins and Proteomics 1868 (2020)
    The glycolytic pathway is one of the most important pathways for living organisms, due to its role in energy production and as supplier of precursors for biosynthesis in living cells. This work focuses on determination of the standard Gibbs energy of reaction ΔRg′0 of the enolase reaction, the ninth reaction in the glycolysis pathway. Exact ΔRg′0 values are required to predict the thermodynamic feasibility of single metabolic reactions or even of metabolic reaction sequences under cytosolic conditions. So-called “apparent” standard data from literature are only valid at specific conditions. Nevertheless, such data are often used in pathway analyses, which might lead to misinterpretation of the results. In this work, equilibrium measurements were combined with activity coefficients in order to obtain new standard values ΔRg′0 for the enolase reaction that are independent of the cytosolic conditions. Reaction equilibria were measured at different initial substrate concentrations and temperatures of 298.15 K, 305.15 K and 310.15 K at pH 7. The activity coefficients were predicted using the equation of state electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT). The ePC-SAFT parameters were taken from literature or fitted to new experimentally determined osmotic coefficients and densities. At 298.15 K and pH 7, a ΔRg′0(298.15 K, pH 7) value of −2.8 ± 0.2 kJ mol− 1 was obtained. This value differs by up to 5 kJ mol− 1 from literature data. Reasons are the poorly defined “standard” conditions and partly undefined reaction conditions of literature works. Finally, using temperature-dependent equilibrium constants and the van ‘t Hoff equation, the standard enthalpy of reaction of ΔRh′0(298.15 K, pH 7) = 27 ± 10 kJ mol− 1 was determined, and a similar value was found by quantum-chemistry calculations. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.bbapap.2020.140365
  • 2020 • 155 Thermodynamics and kinetics of glycolytic reactions. Part i: Kinetic modeling based on irreversible thermodynamics and validation by calorimetry
    Vogel, K. and Greinert, T. and Reichard, M. and Held, C. and Harms, H. and Maskow, T.
    International Journal of Molecular Sciences 21 1-20 (2020)
    In systems biology, material balances, kinetic models, and thermodynamic boundary conditions are increasingly used for metabolic network analysis. It is remarkable that the reversibility of enzyme‐catalyzed reactions and the influence of cytosolic conditions are often neglected in kinetic models. In fact, enzyme‐catalyzed reactions in numerous metabolic pathways such as in glycolysis are often reversible, i.e., they only proceed until an equilibrium state is reached and not until the substrate is completely consumed. Here, we propose the use of irreversible thermodynamics to describe the kinetic approximation to the equilibrium state in a consistent way with very few adjustable parameters. Using a flux‐force approach allowed describing the influence of cytosolic conditions on the kinetics by only one single parameter. The approach was applied to reaction steps 2 and 9 of glycolysis (i.e., the phosphoglucose isomerase reaction from glucose 6‐ phosphate to fructose 6‐phosphate and the enolase‐catalyzed reaction from 2‐phosphoglycerate to phosphoenolpyruvate and water). The temperature dependence of the kinetic parameter fulfills the Arrhenius relation and the derived activation energies are plausible. All the data obtained in this work were measured efficiently and accurately by means of isothermal titration calorimetry (ITC). The combination of calorimetric monitoring with simple flux‐force relations has the potential for adequate consideration of cytosolic conditions in a simple manner. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms21218341
  • 2020 • 154 Thermodynamics and kinetics of glycolytic reactions. Part II: Influence of cytosolic conditions on thermodynamic state variables and kinetic parameters
    Vogel, K. and Greinert, T. and Reichard, M. and Held, C. and Harms, H. and Maskow, T.
    International Journal of Molecular Sciences 21 1-20 (2020)
    For systems biology, it is important to describe the kinetic and thermodynamic properties of enzyme-catalyzed reactions and reaction cascades quantitatively under conditions prevailing in the cytoplasm. While in part I kinetic models based on irreversible thermodynamics were tested, here in part II, the influence of the presumably most important cytosolic factors was investigated using two glycolytic reactions (i.e., the phosphoglucose isomerase reaction (PGI) with a uni-uni-mechanism and the enolase reaction with an uni-bi-mechanism) as examples. Crowding by macromolecules was simulated using polyethylene glycol (PEG) and bovine serum albumin (BSA). The reactions were monitored calorimetrically and the equilibrium concentrations were evaluated using the equation of state ePC-SAFT. The pH and the crowding agents had the greatest influence on the reaction enthalpy change. Two kinetic models based on irreversible thermodynamics (i.e., single parameter flux-force and two-parameter Noor model) were applied to investigate the influence of cytosolic conditions. The flux-force model describes the influence of cytosolic conditions on reaction kinetics best. Concentrations of magnesium ions and crowding agents had the greatest influence, while temperature and pH-value had a medium influence on the kinetic parameters. With this contribution, we show that the interplay of thermodynamic modeling and calorimetric process monitoring allows a fast and reliable quantification of the influence of cytosolic conditions on kinetic and thermodynamic parameters. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/ijms21217921
  • 2020 • 153 Thin interface limit of the double-sided phase-field model with convection
    Subhedar, A. and Galenko, P.K. and Varnik, F.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 378 (2020)
    The thin interface limit of the phase-field model is extended to include transport via melt convection. A double-sided model (equal diffusivity in liquid and solid phases) is considered for the present analysis. For the coupling between phase-field and Navier-Stokes equations, two commonly used schemes are investigated using a matched asymptotic analysis: (i) variable viscosity and (ii) drag force model. While for the variable viscosity model, the existence of a thin interface limit can be shown up to the second order in the expansion parameter, difficulties arise in satisfying no-slip boundary condition at this order for the drag force model. Nevertheless, detailed numerical simulations in two dimensions show practically no difference in dendritic growth profiles in the presence of forced melt flow obtained for the two coupling schemes. This suggests that both approaches can be used for the purpose of numerical simulations. Simulation results are also compared to analytic theory, showing excellent agreement for weak flow. Deviations at higher fluid velocities are discussed in terms of the underlying theoretical assumptions. © 2020 The Author(s) Published by the Royal Society. All rights reserved.
    view abstractdoi: 10.1098/rsta.2019.0540
  • 2020 • 152 Viscosity of ASDs at humid conditions
    Wolbert, F. and Stecker, J. and Luebbert, C. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 154 387-396 (2020)
    Many amorphous solid dispersions (ASDs) are thermodynamically unstable. Thus, the active pharmaceutical ingredient (API) might crystallize over time. The crystallization kinetics and therewith the long-term stability of ASDs depends on the storage conditions temperature and relative humidity (RH) as they determine the molecular mobility of the API in the polymer. To quantify the molecular mobility, the rheological behavior of two different ASDs with ibuprofen and either poly(vinyl acetate) or poly(vinylpyrrolidone-co-vinyl acetate) was analyzed as function of temperature and relative humidity by means of an oscillatory rheometer. The plasticizing effect of ibuprofen and absorbed water on the zero-shear viscosity of the polymer could be fully explained by the reduction of the glass-transition temperature of the mixture compared to the one of the pure polymer. Moreover, this work proposes an approach to predict the zero-shear viscosity of an ASD based on only the temperature dependence of the zero-shear viscosity of the pure polymer as well as the predicted water content in the ASD at certain RH using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2020.07.024
  • 2019 • 151 Combined co-solvent and pressure effect on kinetics of a peptide hydrolysis: An activity-based approach
    Knierbein, M. and Wangler, A. and Luong, T.Q. and Winter, R. and Held, C. and Sadowski, G.
    Physical Chemistry Chemical Physics 21 22224-22229 (2019)
    The application of co-solvents and high pressure has been reported to be an efficient means to tune the kinetics of enzyme-catalyzed reactions. Co-solvents and pressure can lead to increased reaction rates without sacrificing enzyme stability, while temperature and pH operation windows are generally very narrow. Quantitative prediction of co-solvent and pressure effects on enzymatic reactions has not been successfully addressed in the literature. Herein, we are introducing a thermodynamic approach that is based on molecular interactions in the form of activity coefficients of substrate and of enzyme in the multi-component solution. This allowed us to quantitatively predict the combined effect of co-solvent and pressure on the kinetic constants, i.e.The Michaelis constant KM and the catalytic constant kcat, of an α-CT-catalyzed peptide hydrolysis reaction. The reaction was studied in the presence of different types of co-solvents and at pressures up to 2 kbar, and quantitative predictions could be obtained for KM, kcat, and finally even primary Michaelis-Menten plots using activity coefficients provided by the thermodynamic model PC-SAFT. This journal is © the Owner Societies.
    view abstractdoi: 10.1039/c9cp03868j
  • 2019 • 150 Development of Multilayer Sinter Cladding of Cold Work Tool Steel on Hadfield Steel Plates for Wear-Resistant Applications
    Farayibi, P.K. and Blüm, M. and Theisen, W. and Weber, S.
    Journal of Materials Engineering and Performance 28 1833-1847 (2019)
    Machinery components used for mining and mineral processing activities are often subjected to high impact loads and wear which have placed demands for the development of materials with high resistance to dynamic loads and aggressive wear conditions. In this study, a multilayered cladding of high alloyed cold work tool steel (X245VCrMo9-4), interlayered with Hadfield steel (X120Mn12) plates, which was also used as substrate using super-solidus liquid-phase sintering technique was investigated. A stack of the cold work tool steel powder was prepared with interlayered X120Mn12 steel plates in an alumina crucible at tap density with the substrate placed on it and was sintered in a vacuum furnace at 1250 °C at a heating rate of 10 K/min, held for 30 min under a nitrogen atmosphere at 0.08 MPa and furnace-cooled. Sample from the as-sintered cladding was subjected to austenization at 1000 °C, quenched in oil and tempered at 150 °C for 2 h. Samples were subjected to microstructural examination using optical and scanning electron microscopy. The microstructural investigations were supplemented by hardness and impact wear tests. Computational thermodynamics was used to support experimental findings. The results revealed that a near-net densification of the sintered X245 was achieved with 99.93 ± 0.01% density. The sintered X245 was characterized by a dispersion of vanadium carbonitride precipitates, especially at the grain boundaries. The heat-treated X245 sample had the highest hardness of 680 ± 7 HV30 due to the matrix of tempered martensitic microstructure when compared to as-sintered with hardness of 554 ± 2 HV30. The X245/X120 interface was characterized by diffusion of Cr, Mo, Mn and C, which resulted in metallurgical bonding between the cladded materials. The impact wear resistance of the sintered X245 was eight times that of the X120; hence, a tough and wear-resistant tool is anticipated when the X120 work hardened in service. © 2019, ASM International.
    view abstractdoi: 10.1007/s11665-019-03942-2
  • 2019 • 149 Dissociation of the Signaling Protein K-Ras4B from Lipid Membranes Induced by a Molecular Tweezer
    Li, L. and Erwin, N. and Möbitz, S. and Niemeyer, F. and Schrader, T. and Winter, R.H.A.
    Chemistry - A European Journal 25 9827-9833 (2019)
    Oncogenic Ras mutations occur in more than 30 % of human cancers. K-Ras4B is the most frequently mutated isoform of Ras proteins. Development of effective K-Ras4B inhibitors has been challenging, hence new approaches to inhibit this oncogenic protein are urgently required. The polybasic domain of K-Ras4B with its stretch of lysine residues is essential for its plasma membrane targeting and localization. Employing CD and fluorescence spectroscopy, confocal fluorescence, and atomic force microscopy we show that the molecular tweezer CLR01 is able to efficiently bind to the lysine stretch in the polybasic domain of K-Ras4B, resulting in dissociation of the K-Ras4B protein from the lipid membrane and disintegration of K-Ras4B nanoclusters in the lipid bilayer. These results suggest that targeting of the polybasic domain of K-Ras4B by properly designed tweezers might represent an effective strategy for inactivation of K-Ras4B signaling. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201901861
  • 2019 • 148 High-surface-area corundum by mechanochemically induced phase transformation of boehmite
    Amrute, A.P. and Łodziana, Z. and Schreyer, H. and Weidenthaler, C. and Schüth, F.
    Science 366 485-489 (2019)
    In its nanoparticulate form, corundum (a-Al2O3) could lead to several applications. However, its production into nanoparticles (NPs) is greatly hampered by the high activation energy barrier for its formation from cubic close-packed oxides and the sporadic nature of its nucleation. We report a simple synthesis of nanometer-sized a-Al2O3 (particle diameter ~13 nm, surface areas ~140 m2 g-1) by the mechanochemical dehydration of boehmite (g-AlOOH) at room temperature. This transformation is accompanied by severe microstructural rearrangements and might involve the formation of rare mineral phases, diaspore and tohdite, as intermediates. Thermodynamic calculations indicate that this transformation is driven by the shift in stability from boehmite to a-Al2O3 caused by milling impacts on the surface energy. Structural water in boehmite plays a crucial role in generating and stabilizing a-Al2O3 NPs. © 2019 American Association for the Advancement of Science. All rights reserved.
    view abstractdoi: 10.1126/science.aaw9377
  • 2019 • 147 Solubility of pharmaceutical ingredients in triglycerides
    Brinkmann, J. and Huxoll, F. and Luebbert, C. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 145 113-120 (2019)
    Lipid-based drug delivery systems (LBDDS) are highly relevant as pharmaceutical formulations significantly enhancing the bioavailability of active pharmaceutical ingredients (APIs). These formulations often are complex mixtures of APIs, various lipids, and other excipients (e.g. surfactants). In their simplest form, LBDDS contain one API being dissolved in a pure lipid, which often is a triglyceride (TG). In this work, solubilities of the APIs indomethacin, ibuprofen, and fenofibrate in pure TGs of different chain lengths (C chain 8–18) and degree of saturation were investigated. Solubilities of APIs in TGs were measured via differential scanning calorimetry, hot-stage microscopy, high-performance liquid chromatography, and Raman spectroscopy. The influence of fatty-acid chain length and degree of saturation on the API solubility in the TGs was investigated. APIs showed a higher solubility in saturated (wIBU = 10.5 wt% at 25 °C in tricaprylin) TGs compared to unsaturated ones (wIBU = 4.0 wt% at 25 °C in triolein). The fatty-acid chain length of TGs only slightly affects the solubility of ibuprofen and fenofibrate, but strongly influences the eutectic temperature of the API/TG mixtures. API solubilities in TGs and TG mixtures (mixtures of tricaprylin and tricaprin) were successfully modeled using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) accounting for the intermolecular API/TG interactions providing a deep understanding of the energetic and structural impact of the TGs on API solubility. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2019.10.012
  • 2019 • 146 Structure and thermodynamics of aqueous urea solutions from ambient to kilobar pressures: From thermodynamic modeling, experiments, and first principles simulations to an accurate force field description
    Hölzl, C. and Kibies, P. and Imoto, S. and Noetzel, J. and Knierbein, M. and Salmen, P. and Paulus, M. and Nase, J. and Held, C. and Sadowski, G. and Marx, D. and Kast, S.M. and Horinek, D.
    Biophysical Chemistry 254 (2019)
    Molecular simulations based on classical force fields are a powerful method for shedding light on the complex behavior of biomolecules in solution. When cosolutes are present in addition to water and biomolecules, subtle balances of weak intermolecular forces have to be accounted for. This imposes high demands on the quality of the underlying force fields, and therefore force field development for small cosolutes is still an active field. Here, we present the development of a new urea force field from studies of urea solutions at ambient and elevated hydrostatic pressures based on a combination of experimental and theoretical approaches. Experimental densities and solvation shell properties from ab initio molecular dynamics simulations at ambient conditions served as the target properties for the force field optimization. Since urea is present in many marine life forms, elevated hydrostatic pressure was rigorously addressed: densities at high pressure were measured by vibrating tube densitometry up to 500 bar and by X-ray absorption up to 5 kbar. Densities were determined by the perturbed-chain statistical associating fluid theory equation of state. Solvation properties were determined by embedded cluster integral equation theory and ab initio molecular dynamics. Our new force field is able to capture the properties of urea solutions at high pressures without further high-pressure adaption, unlike trimethylamine-N-oxide, for which a high-pressure adaption is necessary. © 2019
    view abstractdoi: 10.1016/j.bpc.2019.106260
  • 2019 • 145 The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells
    Stolterfoht, M. and Caprioglio, P. and Wolff, C.M. and Márquez, J.A. and Nordmann, J. and Zhang, S. and Rothhardt, D. and Hörmann, U. and Amir, Y. and Redinger, A. and Kegelmann, L. and Zu, F. and Albrecht, S. and Koch, N. and K...
    Energy and Environmental Science 12 2778-2788 (2019)
    Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (VOC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the VOC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the VOC of the device. Importantly, the VOC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the VOC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the VOC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ee02020a
  • 2019 • 144 Thermodynamic Activity-Based Solvent Design for Bioreactions
    Wangler, A. and Held, C. and Sadowski, G.
    Trends in Biotechnology 37 1038-1041 (2019)
    To improve the kinetics of enzyme-catalyzed reactions, cosolvents are commonly added to reaction mixtures. The search for a good cosolvent is still empirical and experimentally based. We discuss a thermodynamic activity-based approach that improves biocatalytic processes by predicting cosolvent influences on Michaelis constants, ultimately reducing time and cost. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.tibtech.2019.04.015
  • 2018 • 143 Carbide types in an advanced microalloyed bainitic/ferritic Cr–Mo Steel – TEM observations and thermodynamic calculations [Karbide in einem mikrolegierten bainitisch-ferritischen Cr–Mo-Stahl – TEM Charakterisierung und thermodynamische Berechnungen]
    Wang, H. and Somsen, C. and Eggeler, G. and Detemple, E.
    Materialwissenschaft und Werkstofftechnik 49 726-740 (2018)
    The strength and toughness of low alloyed ferritic/bainitic steels depend on their microstructure, which evolves during thermo-mechanical treatments along the processing chain. Chromium-molybdenum steel microstructures are complex. Therefore, only a limited number of attempts have been made to fully characterize carbide populations in such steels. In the present work, analytical transmission electron microscopy is employed to study the microstructure of a low alloyed chromium-molybdenum steel, which features ferritic (F, mainly α-iron and niobium-carbides) and bainitic (B, α-phase, dislocation, grain/subgrain boundaries, various MxCy carbides) regions. The crystal structure and chemical nature of more than 200 carbides are determined and their distributions in the two microstructural regions are analyzed. The present work shows how particles can be identified in an effective manner and how the microstructural findings can be interpreted on the basis of thermodynamic calculations. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201700186
  • 2018 • 142 Co-solvent effects on reaction rate and reaction equilibrium of an enzymatic peptide hydrolysis
    Wangler, A. and Canales, R. and Held, C. and Luong, T.Q. and Winter, R. and Zaitsau, D.H. and Verevkin, S.P. and Sadowski, G.
    Physical Chemistry Chemical Physics 20 11317-11326 (2018)
    This work presents an approach that expresses the Michaelis constant KaM and the equilibrium constant Kth of an enzymatic peptide hydrolysis based on thermodynamic activities instead of concentrations. This provides KaM and Kth values that are independent of any co-solvent. To this end, the hydrolysis reaction of N-succinyl-l-phenylalanine-p-nitroanilide catalysed by the enzyme α-chymotrypsin was studied in pure buffer and in the presence of the co-solvents dimethyl sulfoxide, trimethylamine-N-oxide, urea, and two salts. A strong influence of the co-solvents on the measured Michaelis constant (KM) and equilibrium constant (Kx) was observed, which was found to be caused by molecular interactions expressed as activity coefficients. Substrate and product activity coefficients were used to calculate the activity-based values KaM and Kth for the co-solvent free reaction. Based on these constants, the co-solvent effect on KM and Kx was predicted in almost quantitative agreement with the experimental data. The approach presented here does not only reveal the importance of understanding the thermodynamic non-ideality of reactions taking place in biological solutions and in many technological applications, it also provides a framework for interpreting and quantifying the multifaceted co-solvent effects on enzyme-catalysed reactions that are known and have been observed experimentally for a long time. © the Owner Societies.
    view abstractdoi: 10.1039/c7cp07346a
  • 2018 • 141 Experimental and Numerical Investigations on Interdiffusion Profiles in Compounds Produced by Sinter-Cladding
    Blüm, M. and Theisen, W. and Weber, S.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 4991-5000 (2018)
    Tools used for mineral processing applications are affected by strong abrasive wear and high dynamic loads. This results in opposing demands on the mechanical properties of these tools. Therefore, modern concepts for the manufacturing of mineral processing tools include a composite tool concept consisting of a low-alloyed substrate and a high-alloyed, wear-resistant cladding material. These coatings can be applied using different production processes such as composite casting, deposit welding, and HIP cladding. During the deposition of the cladding, interdiffusion between the substrate and cladding material occurs. This interdiffusion may have a negative impact on the compound, since characteristics such as wear resistance, mechanical properties, and the local microstructure are influenced. This article is focused on the investigation and simulation of interdiffusion processes in supersolidus-sintered compounds using computational thermodynamics, diffusion calculations, optical emission spectrometry, hardness profiles, and microstructural investigations. It is shown that the interdiffusion processes between the solid substrate and the semisolid cladding can be simulated using a dispersed phase model to give results with a close concordance to optical emission spectrometry measurements. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-018-4750-9
  • 2018 • 140 Phase nucleation through confined spinodal fluctuations at crystal defects evidenced in Fe-Mn alloys
    Kwiatkowski Da Silva, A. and Ponge, D. and Peng, Z. and Inden, G. and Lu, Y. and Breen, A. and Gault, B. and Raabe, D.
    Nature Communications 9 (2018)
    Analysis and design of materials and fluids requires understanding of the fundamental relationships between structure, composition, and properties. Dislocations and grain boundaries influence microstructure evolution through the enhancement of diffusion and by facilitating heterogeneous nucleation, where atoms must overcome a potential barrier to enable the early stage of formation of a phase. Adsorption and spinodal decomposition are known precursor states to nucleation and phase transition; however, nucleation remains the less well-understood step in the complete thermodynamic sequence that shapes a microstructure. Here, we report near-atomic-scale observations of a phase transition mechanism that consists in solute adsorption to crystalline defects followed by linear and planar spinodal fluctuations in an Fe-Mn model alloy. These fluctuations provide a pathway for austenite nucleation due to the higher driving force for phase transition in the solute-rich regions. Our observations are supported by thermodynamic calculations, which predict the possibility of spinodal decomposition due to magnetic ordering. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03591-4
  • 2018 • 139 Physical stability of API/polymer-blend amorphous solid dispersions
    Lehmkemper, K. and Kyeremateng, S.O. and Bartels, M. and Degenhardt, M. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 124 147-157 (2018)
    The preparation of amorphous solid dispersions (ASDs) is a well-established strategy for formulating active pharmaceutical ingredients by embedding them in excipients, usually amorphous polymers. Different polymers can be combined for designing ASDs with desired properties like an optimized dissolution behavior. One important criterion for the development of ASD compositions is the physical stability. In this work, the physical stability of API/polymer-blend ASDs was investigated by thermodynamic modeling and stability studies. Amorphous naproxen (NAP) and acetaminophen (APAP) were embedded in blends of hydroxypropyl methylcellulose acetate succinate (HPMCAS) and either poly(vinylpyrrolidone) (PVP) or poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA64). Parameters for modeling the API solubility in the blends and the glass-transition temperature curves of the water-free systems with Perturbed-Chain Statistical Associating Fluid Theory and Kwei equation, respectively, were correlated to experimental data. The phase behavior for standardized storage conditions (0%, 60% and 75% relative humidity (RH)) was predicted and compared to six months-long stability studies. According to modeling and experimental results, the physical stability was reduced with increasing HPMCAS content and increasing RH. This trend was observed for all investigated systems, with both APIs (NAP and APAP) and both polymer blends (PVP/HPMCAS and PVPVA64/HPMCAS). PC-SAFT and the Kwei equation turned out to be suitable tools for modeling and predicting the physical stability of the investigated API/polymer-blends ASDs. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2017.12.002
  • 2018 • 138 Thermodynamic prediction of the solvent effect on a transesterification reaction
    Lemberg, M. and Schomäcker, R. and Sadowski, G.
    Chemical Engineering Science 176 264-269 (2018)
    This work focuses on the thermodynamic prediction of solvent effects on the transesterification of butyl acetate with ethanol to butanol and ethyl acetate in the solvent heptane at 293.15 K and 303.15 K. Both, the reaction equilibrium and the reaction kinetics have been investigated experimentally by Schmidt et al. (1999). They found that the solvent heptane does not affect the reaction equilibrium but significantly influences the reaction kinetics. They described the solvent effect on the reaction kinetics by empirically correlating the experimentally-observed apparent rate constants with the dielectric constants of the different reaction mixtures. In this work we re-evaluated the experimental data and now present a thermodynamic approach to consistently predict the solvent effect on both, the reaction equilibrium and the reaction kinetics. Accounting for the activity coefficients of the reactants/products obtained from the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) allowed for considering the interactions of the reactants/products among themselves and also with the solvent heptane. Accounting for those, it is shown that the solvent effect on the reaction equilibrium as well as on the reaction rate can even be predicted in very good agreement with the experimental data. © 2017
    view abstractdoi: 10.1016/j.ces.2017.10.033
  • 2017 • 137 A variational approach to the modelling of grooving in a three-dimensional setting
    Hackl, K. and Fischer, F.D. and Svoboda, J.
    Acta Materialia 129 331-342 (2017)
    We present a theory of thermal grooving, i.e. surface motion due to surface diffusion, based solely on geometrical and energetic arguments and a variational approach involving a thermodynamic extremal principle. The theory is derived for a fully three-dimensional setting. All interface and contact conditions at junction lines and points of the material aggregate are derived rigorously and without ambiguity. A finite element implementation of the model is employed. Numerical examples are presented and compared with experimental results from the literature. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.01.064
  • 2017 • 136 Amorphous-amorphous phase separation in API/polymer formulations
    Luebbert, C. and Huxoll, F. and Sadowski, G. and Van Den Mooter, G. and Grohganz, H.
    Molecules 22 (2017)
    The long-term stability of pharmaceutical formulations of poorly-soluble drugs in polymers determines their bioavailability and therapeutic applicability. However, these formulations do not only often tend to crystallize during storage, but also tend to undergo unwanted amorphous-amorphous phase separations (APS). Whereas the crystallization behavior of APIs in polymers has been measured and modeled during the last years, the APS phenomenon is still poorly understood. In this study, the crystallization behavior, APS, and glass-transition temperatures formulations of ibuprofen and felodipine in polymeric PLGA excipients exhibiting different ratios of lactic acid and glycolic acid monomers in the PLGA chain were investigated by means of hot-stage microscopy and DSC. APS and recrystallization was observed in ibuprofen/PLGA formulations, while only recrystallization occurred in felodipine/PLGA formulations. Based on a successful modeling of the crystallization behavior using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), the occurrence of APS was predicted in agreement with experimental findings. © 2017 by the authors; licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/molecules22020296
  • 2017 • 135 Electro-chemical aspects of IPMCs within the framework of the theory of porous media
    Leichsenring, P. and Serdas, S. and Wallmersperger, T. and Bluhm, J. and Schröder, J.
    Smart Materials and Structures 26 (2017)
    Ionomeric polymer-metal composites (IPMCs) consist of an ionomer with bound anionic groups and mobile counterions. They are plated with noble impermeable metal cover layers. By application of an electric voltage, a transport of the mobile ions towards the respective electrode occurs. Due to local electrostatic and ionic forces, a local deformation of the IPMC can be observed. Therefore IPMCs are promising candidates for electrochemical transducers. In the present research, the chemo-electro-mechanical behavior of IPMCs is described within the framework of the theory of porous media. First, the field equations are derived with respect to the second law of thermodynamics. Second, a reduced set of equations for the chemo-electric behavior is formulated and discretized by applying the finite element method. In the numerical investigations a parametric study of the time and space dependent behavior is carried out in order to quantify the influence of different material compositions. Based on this study, the characteristic response of IPMC to the application of an electric voltage can be predicted. Concluding, the obtained computational framework is an excellent tool for the design of electrochemical transducers. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-665X/aa590e
  • 2017 • 134 Investigation of local heat transfer in random particle packings by a fully resolved LBM-approach
    Kravets, B. and Kruggel-Emden, H.
    Powder Technology 318 293-305 (2017)
    Static particle/fluid systems occur in a wide range of technical processes in chemical industry and in energy technology. Direct numerical simulations (DNS) can be applied to model the fluid flow in particle packings and thus to examine heat and momentum transfer in those systems. Besides traditional CFD simulations, the Lattice-Boltzmann method (LBM) has been established as an alternative and efficient approach. Thermal LBM (TLBM) relies on a set of two distribution functions which represent the fluid flow and its internal energy and cover convection-diffusion problems. In the present study a D3Q19 model with a multiple-relaxation-time (MRT) collision model for density distributions and a Bhatnagar–Gross–Krook (BGK) collision model for energy distributions is used. The fluid-solid boundaries are represented by interpolated bounce-back methods. Numerical investigations are performed for single sphere and random particle packings. Particle averaged and local heat transfer coefficients are considered. Obtained results are compared against available state of the art correlations and recent numerical results from the literature. The results demonstrate the accuracy of the derived LBM approach. Particle averaged and local heat transfer coefficients in sphere packings are presented in a range of Rep = 20 − 100 and ε = 0.6 − 1. A correlation of local heat transfer coefficients in random sphere packings is derived. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2017.05.039
  • 2017 • 133 Measurement and PC-SAFT modeling of solid-liquid equilibrium of deep eutectic solvents of quaternary ammonium chlorides and carboxylic acids
    Pontes, P.V.A. and Crespo, E.A. and Martins, M.A.R. and Silva, L.P. and Neves, C.M.S.S. and Máximo, G.J. and Hubinger, M.D. and Batista, E.A.C. and Pinho, S.P. and Coutinho, J.A.P. and Sadowski, G. and Held, C.
    Fluid Phase Equilibria (2017)
    In this study the solid-liquid equilibria (SLE) of 15 binary mixtures composed of one of three different symmetrical quaternary ammonium chlorides and one of five different fatty acids were measured. The experimental data obtained showed extreme negative deviations to ideality causing large melting-temperature depressions (up to 300 K) that are characteristic for deep eutectic systems. The experimental data revealed that cross-interactions between quaternary ammonium salt and fatty acid increase with increasing alkyl chain length of the quaternary ammonium chloride and with increasing chain length of the carboxylic acid. The pronounced decrease of melting temperatures in these deep eutectic systems is mainly caused by strong hydrogen-bonding interactions, and thermodynamic modeling required an approach that takes hydrogen bonding into account. Thus, the measured phase diagrams were modeled with perturbed-chain statistical associating fluid theory based on the classical molecular homonuclear approach. The model showed very good agreement with the experimental data using a semi-predictive modeling approach, in which binary interaction parameters between quaternary ammonium chloride and carboxylic acid correlated with chain length of the components. This supports the experimental findings on the phase behavior and interactions present in these systems and it allows estimating eutectic points of such highly non-ideal mixtures. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2017.04.007
  • 2017 • 132 Measuring and Predicting Thermodynamic Limitation of an Alcohol Dehydrogenase Reaction
    Voges, M. and Fischer, F. and Neuhaus, M. and Sadowski, G. and Held, C.
    Industrial and Engineering Chemistry Research 56 5535-5546 (2017)
    The knowledge of thermodynamic limitations on enzymatic reactions and of influencing factors thereon is essential for process optimization to increase space-time yields and to reduce the amount of solvent or energy consumption. In this work, the alcohol dehydrogenase (ADH) catalyzed reaction from acetophenone and 2-propanol to 1-phenylethanol and acetone in aqueous solution was investigated in a temperature range of 293.15-303.15 K at pH 7. It serves as a model reaction to demonstrate the use of biothermodynamics in order to investigate and predict limitations of enzymatic reactions. Experimental molalities of the reacting agents at equilibrium were measured yielding the position of reaction equilibrium (Km) at different reaction conditions (temperature, initial reactant molalities). The maximum initial acetophenone molality under investigation was 0.02 mol·kg-1 due to solubility limitations with a 1- to 50-fold excess of 2-propanol. It was shown that Km strongly depends on the initial reactant molalities as well as on reaction temperature. Experimental Km values were in the range of 0.20 to 0.49. Thermodynamic key properties (thermodynamic equilibrium constant, standard Gibbs energy and standard enthalpy of reaction) were determined by measured Km values and activity coefficients of the reacting agents predicted with the thermodynamic model ePC-SAFT. In addition, ePC-SAFT was used to predict Km at different initial molalities. Experimental and predicted results were in quantitative agreement (root-mean-square error of experimental versus predicted Km was 0.053), showing that ePC-SAFT is a promising tool to identify process conditions that might increase/decrease Km values and, thus, shift the position of reactions for industrial applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.7b01228
  • 2017 • 131 Moisture-induced phase separation and recrystallization in amorphous solid dispersions
    Luebbert, C. and Sadowski, G.
    International Journal of Pharmaceutics 532 635-646 (2017)
    Active Pharmaceutical Ingredients (APIs) are often dissolved in polymeric matrices to control the gastrointestinal dissolution and to stabilize the amorphous state of the API. During the pharmaceutical development of new formulations, stability studies via storage at certain temperature and relative humidity (RH) have to be carried out to verify the long-term thermodynamic stability of these formulations against unwanted recrystallization and moisture-induced amorphous–amorphous phase separation (MIAPS). This study focuses on predicting the MIAPS of API/polymer formulations at elevated RH. In a first step, the phase behavior of water-free formulations of ibuprofen (IBU) and felodipine (FEL) combined with the polymers poly(vinyl pyrrolidone) (PVP), poly(vinyl acetate) (PVAC) and poly (vinyl pyrrolidone-co-vinyl acetate) (PVPVA64) was determined experimentally by differential scanning calorimetry (DSC). The phase behavior of these water-free formulations was modeled using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). Based on this, the API solubility and MIAPS in the above-mentioned formulations at humid conditions was predicted in perfect agreement with the results of two-year lasting stability studies at 25 °C/0% RH and 40 °C/75% RH. MIAPS was predicted and also experimentally found for the FEL/PVP, FEL/PVPVA64 and IBU/PVP formulations, whereas MIAPS was neither predicted nor measured for the IBU/PVPVA64 system and PVAC-containing formulations. It was thus shown that the results of time-consuming long-term stability tests can be correctly predicted via thermodynamic modeling with PC-SAFT. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.ijpharm.2017.08.121
  • 2017 • 130 Molecular dynamics study of stability and disintegration of long rod-like micelles: Dodecyltrimethylammonium chloride in solutions of hydroxybenzoates
    Gujt, J. and Bešter-Rogač, M. and Spohr, E.
    Journal of Molecular Liquids 228 150-159 (2017)
    Recently it was found out that different positions of the hydroxylic group on hydroxybenzoate anions (HB) crucially affect the thermodynamics of the self-organization of the cationic surfactant dodecyltrimethylammonium chloride (DTAC) and also the structure of resulting aggregates. In our previous work, the properties of stable long cylindrical DTAC micelles in the presence of NaHB at 1:1 DTAC/NaHB molar ratio in aqueous solutions were investigated by atomistic molecular dynamics simulations. In the present work, we first study the decay of cylindrical DTAC micelles in water without added salt and then extend our research to systems with low NaHB concentrations (DTAC/NaHB molar ratios of 4:1 and 2:1) in order to approach the real experimental conditions more closely. The geometry and structural properties of DTAC micelles in water are investigated, and also the decomposition of long cylindrical micelles and the solvent accessible surface area of micelles is discussed. We observe that the initial DTAC micelle without NaHB quickly disintegrates into smaller stable spherical micelles. At the 2:1 DTAC/NaHB molar ratio we find all initial DTAC micelles to remain stable; however, their geometry deviates significantly from initial cylindrical one. Furthermore, it is observed that o-HB induces a more ordered internal structure of the micelle, and is more strongly oriented than the other two isomers, which agrees well with the experiments and observations reported in our previous work. When the NaHB concentration is decreased to 4:1 DTAC/NaHB molar ratio, an initial DTAC micelle disintegrates forming smaller aggregates of spherical or elongated shapes regardless of the nature of the HB isomer present. The microscopic structure of the resultant micelles is very similar to the structure observed at higher NaHB concentration, however, the effect of HB ions is smaller. It was also observed that the micelle remains stable longer in the presence of o-HB than in the presence of the other two isomers. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.molliq.2016.09.067
  • 2017 • 129 Reaction Equilibrium of the ω-Transamination of (S)-Phenylethylamine: Experiments and ePC-SAFT Modeling
    Voges, M. and Abu, R. and Gundersen, M.T. and Held, C. and Woodley, J.M. and Sadowski, G.
    Organic Process Research and Development 21 976-986 (2017)
    This work focuses on the thermodynamic equilibrium of the ω-transaminase-catalyzed reaction of (S)-phenylethylamine with cyclohexanone to acetophenone and cyclohexylamine in aqueous solution. For this purpose, the equilibrium concentrations of the reaction were experimentally investigated under varying reaction conditions. It was observed that the temperature (30 and 37 °C), the pH (between pH 7 and pH 9), as well as the initial reactant concentrations (between 5 and 50 mmol·kg-1) influenced the equilibrium position of the reaction. The position of the reaction equilibrium was moderately shifted toward the product side by either decreasing temperature or decreasing pH. In contrast, the initial ratio of the reactants showed only a marginal influence on the equilibrium position. Further experiments showed that increasing the initial reactant concentrations significantly shifted the equilibrium position to the reactant side. In order to explain these effects, the activity coefficients of the reacting agents were calculated and the activity-based thermodynamic equilibrium constant Kth of the reaction was determined. For this purpose, the activity coefficients of the reacting agents were modeled at their respective experimental equilibrium concentrations using the equation of state electrolyte PC-SAFT (ePC-SAFT). The combination of the concentrations of the reacting agents at equilibrium and their respective activity coefficients provided the thermodynamically consistent equilibrium constant Kth. Unexpectedly, the experimental Km values deviated by a factor of up to four from the thermodynamic equilibrium constant Kth. The observed concentration dependency of the experimental Km values could be explained by the influence of concentration on activity coefficients. Further, these activity coefficients were found to be strongly temperature dependent, which is important for the determination of standard enthalpy of reactions, which in this work was found to be +7.7 ± 2.8 kJ·mol-1. Using the so-determined Kth and activity coefficients of the reacting agents (ePC-SAFT), the equilibrium concentrations of the reaction were predicted for varying initial reactant concentrations, which were found to be in good agreement with the experimental behavior. These results showed a non-negligible influence of the activity coefficients of the reacting agents on the equilibrium position and, thus, on the product yield. Experiments and ePC-SAFT predictions showed that the equilibrium position can only be described accurately by taking activity coefficients into account. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.oprd.7b00078
  • 2017 • 128 Size- and density-controlled deposition of Ag nanoparticle films by a novel low-temperature spray chemical vapour deposition method—research into mechanism, particle growth and optical simulation
    Liu, Y. and Plate, P. and Hinrichs, V. and Köhler, T. and Song, M. and Manley, P. and Schmid, M. and Bartsch, P. and Fiechter, S. and Lux-Steiner, M.C. and Fischer, C.-H.
    Journal of Nanoparticle Research 19 (2017)
    Ag nanoparticles have attracted interest for plasmonic absorption enhancement of solar cells. For this purpose, well-defined particle sizes and densities as well as very low deposition temperatures are required. Thus, we report here a new spray chemical vapour deposition method for producing Ag NP films with independent size and density control at substrate temperatures even below 100 °C, which is much lower than for many other techniques. This method can be used on different substrates to deposit Ag NP films. It is a reproducible, low-cost process which uses trimethylphosphine (hexafluoroacetylacetonato) silver as a precursor in alcoholic solution. By systematic variation of deposition parameters and classic experiments, mechanisms of particle growth and of deposition processes as well as the low decomposition temperature of the precursor could be explained. Using the 3D finite element method, absorption spectra of selected samples were simulated, which fitted well with the measured results. Hence, further applications of such Ag NP films for generating plasmonic near field can be predicted by the simulation. © 2017, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-017-3834-6
  • 2017 • 127 Standard Gibbs energy of metabolic reactions: II. Glucose-6-phosphatase reaction and ATP hydrolysis
    Meurer, F. and Do, H. T. and Sadowski, G. and Held, C.
    Biophysical Chemistry 223 30--38 (2017)
    ATP (adenosine triphosphate) is a key reaction for metabolism. Tools from systems biology require standard reaction data in order to predict metabolic pathways accurately. However, literature values for standard Gibbs energy of ATP hydrolysis are highly uncertain and differ strongly from each other. Further, such data usually neglect the activity coefficients of reacting agents, and published data like this is apparent (condition-dependent) data instead of activity-based standard data. In this work a consistent value for the standard Gibbs energy of ATP hydrolysis was determined. The activity coefficients of reacting agents were modeled with electrolyte Perturbed Chain Statistical Associating Fluid Theory (ePC-SAFT). The Gibbs energy of ATP hydrolysis was calculated by combining the standard Gibbs energies of hexokinase reaction and of glucose-6-phosphate hydrolysis. While the standard Gibbs energy of hexokinase reaction was taken from previous work, standard Gibbs energy of glucose-6-phosphate hydrolysis reaction was determined in this work. For this purpose, reaction equilibrium molalities of reacting agents were measured at pH 7 and pH 8 at 298.15 K at varying initial reacting agent molalities. The corresponding activity coefficients at experimental equilibrium molalities were predicted with ePC-SAFT yielding the Gibbs energy of glucose-6-phosphate hydrolysis of -13.72 +/- 0.75 kJ. mol(-1). Combined with the value for hexokinase, the standard Gibbs energy of ATP hydrolysis was finally found to be - 31.55 +/- 127 kJ. mol(-1). For both, ATP hydrolysis and glucose-6-phosphate hydrolysis, a good agreement with own and literature values were obtained when influences of pH, temperature, and activity coefficients were explicitly taken into account in order to calculate standard Gibbs energy at pH 7, 298.15 K and standard state. (C) 2017 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.bpc.2017.02.005
  • 2017 • 126 The hpCADD NDDO Hamiltonian: Parametrization
    Thomas, H.B. and Hennemann, M. and Kibies, P. and Hoffgaard, F. and Güssregen, S. and Hessler, G. and Kast, S.M. and Clark, T.
    Journal of Chemical Information and Modeling 57 1907-1922 (2017)
    A neglect of diatomic differential overlap (NDDO) Hamiltonian has been parametrized as an electronic component of a polarizable force field. Coulomb and exchange potentials derived directly from the NDDO Hamiltonian in principle can be used with classical potentials, thus forming the basis for a new generation of efficiently applicable multipolar polarizable force fields. The new hpCADD Hamiltonian uses force-field-like atom types and reproduces the electrostatic properties (dipole moment, molecular electrostatic potential) and Koopmans' theorem ionization potentials closely, as demonstrated for a large training set and an independent test set of small molecules. The Hamiltonian is not intended to reproduce geometries or total energies well, as these will be controlled by the classical force-field potentials. In order to establish the hpCADD Hamiltonian as an electronic component in force-field-based calculations, we tested its performance in combination with the 3D reference interaction site model (3D RISM) for aqueous solutions. Comparison of the resulting solvation free energies for the training and test sets to atomic charges derived from standard procedures, exact solute-solvent electrostatics based on high-level quantum-chemical reference data, and established semiempirical Hamiltonians demonstrates the advantages of the hpCADD parametrization. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jcim.7b00080
  • 2017 • 125 Thermodynamics of enzyme-catalyzed esterifications: II. Levulinic acid esterification with short-chain alcohols
    Altuntepe, E. and Emel’yanenko, V.N. and Forster-Rotgers, M. and Sadowski, G. and Verevkin, S.P. and Held, C.
    Applied Microbiology and Biotechnology 101 7509-7521 (2017)
    Levulinic acid was esterified with methanol, ethanol, and 1-butanol with the final goal to predict the maximum yield of these equilibrium-limited reactions as function of medium composition. In a first step, standard reaction data (standard Gibbs energy of reaction ΔRg0) were determined from experimental formation properties. Unexpectedly, these ΔRg0 values strongly deviated from data obtained with classical group contribution methods that are typically used if experimental standard data is not available. In a second step, reaction equilibrium concentrations obtained from esterification catalyzed by Novozym 435 at 323.15 K were measured, and the corresponding activity coefficients of the reacting agents were predicted with perturbed-chain statistical associating fluid theory (PC-SAFT). The so-obtained thermodynamic activities were used to determine ΔRg0 at 323.15 K. These results could be used to cross-validate ΔRg0 from experimental formation data. In a third step, reaction-equilibrium experiments showed that equilibrium position of the reactions under consideration depends strongly on the concentration of water and on the ratio of levulinic acid: alcohol in the initial reaction mixtures. The maximum yield of the esters was calculated using ΔRg0 data from this work and activity coefficients of the reacting agents predicted with PC-SAFT for varying feed composition of the reaction mixtures. The use of the new ΔRg0 data combined with PC-SAFT allowed good agreement to the measured yields, while predictions based on ΔRg0 values obtained with group contribution methods showed high deviations to experimental yields. © 2017, Springer-Verlag GmbH Germany.
    view abstractdoi: 10.1007/s00253-017-8481-4
  • 2017 • 124 Thermodynamics of enzyme-catalyzed esterifications: I. Succinic acid esterification with ethanol
    Altuntepe, E. and Greinert, T. and Hartmann, F. and Reinhardt, A. and Sadowski, G. and Held, C.
    Applied Microbiology and Biotechnology 101 5973-5984 (2017)
    Succinic acid (SA) was esterified with ethanol using Candida antarctica lipase B immobilized on acrylic resin at 40 and 50 °C. Enzyme activity in the reaction medium was assured prior to reaction experiments. Reaction-equilibrium experiments were performed for varying initial molalities of SA and water in the reaction mixtures. This allowed calculating the molality-based apparent equilibrium constant Km as function of concentration and temperature. Km was shown to depend strongly on the molality of water and SA as well as on temperature. It could be concluded that increasing the molality of SA shifted the reaction equilibrium towards the products. Water had a strong effect on the activity of the enzyme and on Km. The concentration dependence of Km values was explained by the activity coefficients of the reacting agents. These were predicted with the thermodynamic models Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), NRTL, and Universal Quasichemical Functional Group Activity Coefficients (UNIFAC), yielding the ratio of activity coefficients of products and reactants Kγ. All model parameters were taken from literature. The models yielded Kγ values between 25 and 115. Thus, activity coefficients have a huge impact on the consistent determination of the thermodynamic equilibrium constants Kth. Combining Km and PC-SAFT-predicted Kγ allowed determining Kth and the standard Gibbs energy of reaction as function of temperature. This value was shown to be in very good agreement with results obtained from group contribution methods for Gibbs energy of formation. In contrast, inconsistencies were observed for Kth using Kγ values from the classical gE-models UNIFAC and NRTL. The importance of activity coefficients opens the door for an optimized reaction setup for enzymatic esterifications. © 2017, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00253-017-8287-4
  • 2017 • 123 Trace Adsorption of Ethane, Propane, and n-Butane on Microporous Activated Carbon and Zeolite 13X at Low Temperatures
    Birkmann, F. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 62 1973-1982 (2017)
    Removing of trace-leveled light hydrocarbons from exhaust air or gas streams becomes an increasingly important issue in the field of process and environmental technology, e.g., storage and transport of liquefied natural gas. Adsorption processes at temperatures below 0°C have great potential to meet process specifications or environmental regulatory limits. Designing of such adsorption processes requires a profound insight into the thermodynamics of adsorption at low temperatures, which is not available yet. Therefore, this work provides adsorption isotherms of ethane, propane, and n-butane on microporous activated carbon and zeolite 13X in a temperature range from -40 to +60°C and at partial pressures from 5 to 1000 Pa. The influence of temperature on the adsorbed amount on activated carbon and zeolite 13X is discussed for each adsorptive considering isosteric heats of adsorption and specific interactions between the adsorptive and the adsorbent surface. (Graph Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.6b01068
  • 2016 • 122 Benzoic Acid and Chlorobenzoic Acids: Thermodynamic Study of the Pure Compounds and Binary Mixtures with Water
    Reschke, T. and Zherikova, K.V. and Verevkin, S.P. and Held, C.
    Journal of Pharmaceutical Sciences 105 1050-1058 (2016)
    Benzoic acid is a model compound for drug substances in pharmaceutical research. Process design requires information about thermodynamic phase behavior of benzoic acid and its mixtures with water and organic solvents. This work addresses phase equilibria that determine stability and solubility. In this work, Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) was used to model the phase behavior of aqueous and organic solutions containing benzoic acid and chlorobenzoic acids. Absolute vapor pressures of benzoic acid and 2-, 3-, and 4-chlorobenzoic acid from literature and from our own measurements were used to determine pure-component PC-SAFT parameters. Two binary interaction parameters between water and/or benzoic acid were used to model vapor-liquid and liquid-liquid equilibria of water and/or benzoic acid between 280 and 413 K. The PC-SAFT parameters and 1 binary interaction parameter were used to model aqueous solubility of the chlorobenzoic acids. Additionally, solubility of benzoic acid in organic solvents was predicted without using binary parameters. All results showed that pure-component parameters for benzoic acid and for the chlorobenzoic acids allowed for satisfying modeling phase equilibria. The modeling approach established in this work is a further step to screen solubility and to predict the whole phase region of mixtures containing pharmaceuticals. © 2016 American Pharmacists Association®.
    view abstractdoi: 10.1016/j.xphs.2015.12.020
  • 2016 • 121 Complex and liquid hydrides for energy storage
    Callini, E. and Atakli, Z.Ö.K. and Hauback, B.C. and Orimo, S.-I. and Jensen, C. and Dornheim, M. and Grant, D. and Cho, Y.W. and Chen, P. and Hjörvarsson, B. and de Jongh, P. and Weidenthaler, C. and Baricco, M. and Paskevicius...
    Applied Physics A: Materials Science and Processing 122 (2016)
    The research on complex hydrides for hydrogen storage was initiated by the discovery of Ti as a hydrogen sorption catalyst in NaAlH4 by Boris Bogdanovic in 1996. A large number of new complex hydride materials in various forms and combinations have been synthesized and characterized, and the knowledge regarding the properties of complex hydrides and the synthesis methods has grown enormously since then. A significant portion of the research groups active in the field of complex hydrides is collaborators in the International Energy Agreement Task 32. This paper reports about the important issues in the field of complex hydride research, i.e. the synthesis of borohydrides, the thermodynamics of complex hydrides, the effects of size and confinement, the hydrogen sorption mechanism and the complex hydride composites as well as the properties of liquid complex hydrides. This paper is the result of the collaboration of several groups and is an excellent summary of the recent achievements. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-016-9881-5
  • 2016 • 120 Exploring the mineral-water interface: Reduction and reaction kinetics of single hematite (α-Fe2O3) nanoparticles
    Shimizu, K. and Tschulik, K. and Compton, R.G.
    Chemical Science 7 1408-1414 (2016)
    In spite of their natural and technological importance, the intrinsic electrochemical properties of hematite (α-Fe2O3) nanoparticles are not well understood. In particular, particle agglomeration, the presence of surface impurities, and/or inadequate proton concentrations are major obstacles to uncover the fundamental redox activities of minerals in solution. These are particularly problematic when samples are characterized in common electrochemical analyses such as cyclic voltammetry in which nanoparticles are immobilized on a stationary electrode. In this work, the intrinsic reaction kinetics and thermodynamics of individual hematite nanoparticles are investigated by particle impact chronoamperometry. The particle radius derived from the integrated area of spikes recorded in a chronoamperogram is in excellent agreement with electron microscopy results, indicating that the method provides a quantitative analysis of the reduction of the nanoparticles to the ferrous ion. A key finding is that the suspended individual nanoparticles undergo electrochemical reduction at potentials much more positive than those immobilized on a stationary electrode. The critical importance of the solid/water interface on nanoparticle activity is further illustrated by a kinetic model. It is found that the first electron transfer process is the rate determining step of the reductive dissolution of hematite nanoparticles, while the overall process is strongly affected by the interfacial proton concentration. This article highlights the effects of the interfacial proton and ferrous ion concentrations on the reductive dissolution of hematite nanoparticles and provides a highly effective method that can be readily applied to study a wide range of other mineral nanoparticles. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5sc03678j
  • 2016 • 119 Finite element simulation of coating-induced heat transfer: application to thermal spraying processes
    Berthelsen, R. and Tomath, D. and Denzer, R. and Menzel, A.
    Meccanica 51 291-307 (2016)
    Thermal spraying is a widely applied coating technique. The optimisation of the thermal spraying process with respect to temperature or temperature induced residual stress states requires a numerical framework for the simulation of the coating itself as well as of the quenching procedure after the application of additional material. This work presents a finite element framework for the simulation of mass deposition due to coating by means of thermal spraying combined with the simulation of nonlinear heat transfer of a rigid heat conductor. The approach of handling the dynamic problem size is highlighted with focus on the thermodynamical consistency of the derived model. With the framework implemented, numerical examples are employed and material parameters are fitted to experimental data of steel as well as of tungsten-carbide–cobalt-coating. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11012-015-0236-7
  • 2016 • 118 Framework for non-coherent interface models at finite displacement jumps and finite strains
    Ottosen, N.S. and Ristinmaa, M. and Mosler, J.
    Journal of the Mechanics and Physics of Solids 90 124-141 (2016)
    This paper deals with a novel constitutive framework suitable for non-coherent interfaces, such as cracks, undergoing large deformations in a geometrically exact setting. For this type of interface, the displacement field shows a jump across the interface. Within the engineering community, so-called cohesive zone models are frequently applied in order to describe non-coherent interfaces. However, for existing models to comply with the restrictions imposed by (a) thermodynamical consistency (e.g., the second law of thermodynamics), (b) balance equations (in particular, balance of angular momentum) and (c) material frame indifference, these models are essentially fiber models, i.e. models where the traction vector is collinear with the displacement jump. This constraints the ability to model shear and, in addition, anisotropic effects are excluded. A novel, extended constitutive framework which is consistent with the above mentioned fundamental physical principles is elaborated in this paper. In addition to the classical tractions associated with a cohesive zone model, the main idea is to consider additional tractions related to membrane-like forces and out-of-plane shear forces acting within the interface. For zero displacement jump, i.e. coherent interfaces, this framework degenerates to existing formulations presented in the literature. For hyperelasticity, the Helmholtz energy of the proposed novel framework depends on the displacement jump as well as on the tangent vectors of the interface with respect to the current configuration - or equivalently - the Helmholtz energy depends on the displacement jump and the surface deformation gradient. It turns out that by defining the Helmholtz energy in terms of the invariants of these variables, all above-mentioned fundamental physical principles are automatically fulfilled. Extensions of the novel framework necessary for material degradation (damage) and plasticity are also covered. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2016.02.034
  • 2016 • 117 Improved thermodynamic treatment of vacancy-mediated diffusion and creep
    Fischer, F.D. and Hackl, K. and Svoboda, J.
    Acta Materialia 108 347-354 (2016)
    Approximately a decade ago a new concept to describe the kinetics of one-phase solid state systems evolving by diffusion and activity of vacancies has been published by the authors. The concept is based on the Onsager-Ziegler Thermodynamic Extremal Principle (TEP). In course of the last decade several improvements and corrections have been performed, which justify an overworking of the concept. A short introduction of the TEP is followed by a detail investigation of the Gibbs energy and its rate as well as of dissipation and dissipation function due to multicomponent diffusion process coupled with vacancy activity provoking swelling/shrinkage and creep and thus internal stress state development. The application of TEP allows an exact derivation of driving forces for the coupled processes. The Manning theory of diffusion is applied and the derivation of evolution equations for all system parameters (site fractions, swelling/shrinkage and creep strain tensor) is provided. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.01.017
  • 2016 • 116 Metal hydrides for concentrating solar thermal power energy storage
    Sheppard, D.A. and Paskevicius, M. and Humphries, T.D. and Felderhoff, M. and Capurso, G. and Bellosta von Colbe, J. and Dornheim, M. and Klassen, T. and Ward, P.A. and Teprovich, J.A., Jr. and Corgnale, C. and Zidan, R. and Grant...
    Applied Physics A: Materials Science and Processing 122 (2016)
    The development of alternative methods for thermal energy storage is important for improving the efficiency and decreasing the cost of concentrating solar thermal power. We focus on the underlying technology that allows metal hydrides to function as thermal energy storage (TES) systems and highlight the current state-of-the-art materials that can operate at temperatures as low as room temperature and as high as 1100 °C. The potential of metal hydrides for thermal storage is explored, while current knowledge gaps about hydride properties, such as hydride thermodynamics, intrinsic kinetics and cyclic stability, are identified. The engineering challenges associated with utilising metal hydrides for high-temperature TES are also addressed. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-016-9825-0
  • 2016 • 115 Novel Displacement Agents for Aqueous 2-Phase Extraction Can Be Estimated Based on Hybrid Shortcut Calculations
    Kress, C. and Sadowski, G. and Brandenbusch, C.
    Journal of Pharmaceutical Sciences 105 3030-3038 (2016)
    The purification of therapeutic proteins is a challenging task with immediate need for optimization. Besides other techniques, aqueous 2-phase extraction (ATPE) of proteins has been shown to be a promising alternative to cost-intensive state-of-the-art chromatographic protein purification. Most likely, to enable a selective extraction, protein partitioning has to be influenced using a displacement agent to isolate the target protein from the impurities. In this work, a new displacement agent (lithium bromide [LiBr]) allowing for the selective separation of the target protein IgG from human serum albumin (represents the impurity) within a citrate–polyethylene glycol (PEG) ATPS is presented. In order to characterize the displacement suitability of LiBr on IgG, the mutual influence of LiBr and the phase formers on the aqueous 2-phase system (ATPS) and partitioning is investigated. Using osmotic virial coefficients (B22 and B23) accessible by composition gradient multiangle light-scattering measurements, the precipitating effect of LiBr on both proteins and an estimation of both protein partition coefficients is estimated. The stabilizing effect of LiBr on both proteins was estimated based on B22 and experimentally validated within the citrate–PEG ATPS. Our approach contributes to an efficient implementation of ATPE within the downstream processing development of therapeutic proteins. © 2016 American Pharmacists Association®
    view abstractdoi: 10.1016/j.xphs.2016.06.006
  • 2016 • 114 On Local Phase Equilibria and the Appearance of Nanoparticles in the Microstructure of Single-Crystal Ni-Base Superalloys
    Yardley, V. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Parsa, A.B. and Kostka, A. and Somsen, C. and Dlouhy, A. and Neuking, K. and George, E.P. and Eggeler, G.
    Advanced Engineering Materials 18 1556-1567 (2016)
    High-resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni-base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′-volume fractions. Using thermodynamic calculations one can explain, why γ-nanoparticles are observed in the central regions of large cuboidal γ′-particles and why tertiary γ′-nanoparticles form in the γ-channels. The chemical compositions of the γ-channels and of the newly formed γ-particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′-particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate after homogenization is discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201600237
  • 2016 • 113 Partitioning of Cr and Si between cementite and ferrite derived from first-principles thermodynamics
    Sawada, H. and Kawakami, K. and Körmann, F. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Acta Materialia 102 241-250 (2016)
    Partitioning of Cr and Si between cementite and ferrite was investigated by first-principles thermodynamics taking into account vibrational, electronic, and magnetic Gibbs energy contributions. At finite temperatures, these contributions lower the partitioning Gibbs energy and compete with the configurational entropy, which favors impurity segregation to ferrite due to its larger volume fraction compared to cementite. Due to the large positive partitioning enthalpy contribution of Si at T = 0 K, partitioning of Si to cementite is virtually absent in agreement with experiment. The situation is drastically different for Cr impurities. Incorporation of finite-temperature effects resolves the discrepancy between experimental observations and previous T = 0 K first-principles calculations. Cr strongly segregates to cementite due to the enhanced magnetic entropy of cementite above 400 K, i.e., near the Curie temperature of cementite. The increasing magnetic fluctuations in ferrite cause a strong reduction of the partitioning coefficient in the temperature range from 773 to 973 K in qualitative agreement with experiment. Quantitative agreement with calphad data and experimental data for equilibrium Cr concentrations in a wide range of alloy compositions is achieved by renormalizing the theoretical magnetic partitioning Gibbs energy by a constant scaling factor. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.09.010
  • 2016 • 112 Predicting the solubility of pharmaceutical cocrystals in solvent/anti-solvent mixtures
    Lange, L. and Heisel, S. and Sadowski, G.
    Molecules 21 (2016)
    In this work, the solubilities of pharmaceutical cocrystals in solvent/anti-solvent systems were predicted using PC-SAFT in order to increase the efficiency of cocrystal formation processes. Modeling results and experimental data were compared for the cocrystal system nicotinamide/succinic acid (2:1) in the solvent/anti-solvent mixtures ethanol/water, ethanol/acetonitrile and ethanol/ethyl acetate at 298.15 K and in the ethanol/ethyl acetate mixture also at 310.15 K. The solubility of the investigated cocrystal slightly increased when adding small amounts of anti-solvent to the solvent, but drastically decreased for high anti-solvent amounts. Furthermore, the solubilities of nicotinamide, succinic acid and the cocrystal in the considered solvent/anti-solvent mixtures showed strong deviations from ideal-solution behavior. However, by accounting for the thermodynamic non-ideality of the components, PC-SAFT is able to predict the solubilities in all above-mentioned solvent/anti-solvent systems in good agreement with the experimental data. © 2016 by the authors; licensee MDPI.
    view abstractdoi: 10.3390/molecules21050593
  • 2016 • 111 Standard Gibbs Energy of Metabolic Reactions: I. Hexokinase Reaction
    Meurer, F. and Bobrownik, M. and Sadowski, G. and Held, C.
    Biochemistry 55 5665-5674 (2016)
    The standard Gibbs energy of reaction enables calculation of the driving force of a (bio)chemical reaction. Gibbs energies of reaction are required in thermodynamic approaches to determine fluxes as well as single reaction conversions of metabolic bioreactions. The hexokinase reaction (phosphorylation of glucose) is the entrance step of glycolysis, and thus its standard Gibbs energy of reaction (ΔRg°) is of great impact. ΔRg° is accessible from equilibrium measurements, and the very small concentrations of the reacting agents cause usually high error bars in data reduction steps. Even worse, works from literature do not account for the nonideal behavior of the reacting agents (activity coefficients were assumed to be unity); thus published ΔRg° values are not standard data. Consistent treatment of activity coefficients of reacting agents is crucial for the accurate determination of standard Gibbs energy from equilibrium measurements. In this work, equilibrium molalities of hexokinase reaction were measured with an enzyme kit. These results were combined with reacting agents' activity coefficients obtained with the thermodynamic model ePC-SAFT. Pure-component parameters for adenosine triphosphate (ATP) and adenosine diphosphate (ADP) were fitted to experimental osmotic coefficients (water + Na2ATP, water + NaADP). ΔRg° of the hexokinase reaction at 298.15 K and pH 7 was found to be -17.83 ± 0.52 kJ·mol-1. This value was compared with experimental literature data; very good agreement between the different ΔRg° values was obtained by accounting for pH, pMg, and the activity coefficients of the reacting agents. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.biochem.6b00471
  • 2016 • 110 Structure and Stability of Long Rod-like Dodecyltrimethylammonium Chloride Micelles in Solutions of Hydroxybenzoates: A Molecular Dynamics Simulation Study
    Gujt, J. and Bešter-Rogač, M. and Spohr, E.
    Langmuir 32 8275-8286 (2016)
    The relative position of the hydroxylic and carboxylic groups in the isomeric hydroxybenzoate (HB) anions is experimentally known to have a large impact on the thermodynamics of micellization of cationic surfactants, such as dodecyltrimethylammonium chloride (DTAC), and on the structure of the resulting micelles. To understand the effect of the different isomers on the molecular level, we employed atomistic molecular dynamics simulations to study systems containing infinitely long cylindrical DTAC micelles in aqueous solutions of the sodium salts of all three isomers of HB at a temperature and a pressure of 298.15 K and 1 atm. In all studied systems, the number of DTAC unimers is identical to the number of HB anions. At this concentration, the initially cylindrical micelles remain stable, irrespective of the nature of the isomer, whereas micelles rapidly disintegrated in the absence of HB anions. The HB isomers decrease the line density of unimers along the micellar axis and its concomitant thickness in the order o-HB > m-HB > p-HB. It is further observed that o-HB anions penetrate more deeply into the micellar core, induce a more ordered internal structure of the micelle, and are oriented more strongly than the other two isomers. In addition, the ortho isomer shows two different preferential orientations with respect to the radial direction of the cylindrical micelle; it can either be incorporated almost completely into the micelle or it can be attached through hydrogen bonding to one of those o-HB anions that are already incorporated into the micelle, and thus stick out of the micellar surface. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.6b02076
  • 2016 • 109 Structure and thermodynamics of nondipolar molecular liquids and solutions from integral equation theory
    Frach, R. and Heil, J. and Kast, S.M.
    Molecular Physics 114 2461-2476 (2016)
    Solvent-induced solute polarisation of nondipolar solvents originates mainly from specific directional interactions and higher electrostatic multipole moments. Popular continuum solvation models such as the polarisable continuum models ignore such interactions and, therefore, cannot adequately model solvation effects on electronic structure in these environments. Important examples of nondipolar solvents that are indistinguishable by continuum methods are benzene and hexafluorobenzene. Both substances have very similar macroscopic properties, while solutes dissolved in either benzene or hexafluorobenzene behave differently due to their inverted electrostatic quadrupole moments and slightly different size. As a first step towards a proper and computationally feasible description of nondipolar molecular solvents, we present here integral equation theory results based on various forms of the reference interaction site model coupled to quantum-chemical calculations for benzene and hexafluorobenzene solutions of small molecules. We analyse solvation structures, also in comparison with molecular dynamics simulations, and show that predictions of transfer Gibbs energies, which define partition constants, benefit substantially from considering the exact, wave function-derived electrostatic field distribution beyond a simple point charge solute model in comparison with experimental data. Moreover, by constructing artificial uncharged and charge-inverted toy models of the solvents, it is possible to dissect the relative importance of dispersion and quadrupolar electrostatic effects on the partitioning equilibria. Such insight can help to design specifically optimised solvents to control solubility and selectivity for a wide range of applications. © 2016 Informa UK Limited, trading as Taylor & Francis Group
    view abstractdoi: 10.1080/00268976.2016.1167266
  • 2016 • 108 Temperature Dependent Adsorption of Sulfur Components, Water, and Carbon Dioxide on a Silica-Alumina Gel Used in Natural Gas Processing
    Chowanietz, V. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 61 3208-3216 (2016)
    Adsorption is one of the key technologies for the removal of sulfur compounds in trace levels from natural gas prior to a technical utilization. To improve the design of these coupled adsorption-desorption processes a profound insight into the thermodynamics of adsorption is necessary. Therefore, this article provides adsorption isotherms of ethyl mercaptan, methyl mercaptan, hydrogen sulfide, water, and carbon dioxide on a commercial silica-alumina gel used in natural gas purification. The experimental data spans a temperature range between 25 and 300 °C at concentrations between 0 and 2000 mol-ppm at total pressure of 1.3 bar. Equilibrium capacities and isosteric heats of adsorption are compared and discussed based on an analysis of specific interactions between the adsorptives and the adsorbent's chemical surface functionality. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.6b00301
  • 2016 • 107 The OpenCalphad thermodynamic software interface
    Sundman, B. and Kattner, U.R. and Sigli, C. and Stratmann, M. and Le Tellier, R. and Palumbo, M. and Fries, S.G.
    Computational Materials Science 125 188-196 (2016)
    Thermodynamic data are needed for all kinds of simulations of materials processes. Thermodynamics determines the set of stable phases and also provides chemical potentials, compositions and driving forces for nucleation of new phases and phase transformations. Software to simulate materials properties needs accurate and consistent thermodynamic data to predict metastable states that occur during phase transformations. Due to long calculation times thermodynamic data are frequently pre-calculated into “lookup tables” to speed up calculations. This creates additional uncertainties as data must be interpolated or extrapolated and conditions may differ from those assumed for creating the lookup table. Speed and accuracy requires that thermodynamic software is fully parallelized and the OpenCalphad (OC) software is the first thermodynamic software supporting this feature. This paper gives a brief introduction to computational thermodynamics and introduces the basic features of the OC software and presents four different application examples to demonstrate its versatility. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2016.08.045
  • 2016 • 106 Theoretical framework of modeling of ionic EAPs within the Theory of Porous Media
    Bluhm, J. and Serdas, S. and Schröder, J.
    Archive of Applied Mechanics 86 3-19 (2016)
    A thermo-electromechanical formulation for the description of ionic electroactive polymers is derived within the framework of the Theory of Porous Media. The model consists of an electrically charged porous solid saturated with an ionic solution. The saturated porous medium is assumed to be incompressible. Different constituents following different kinematic paths are considered such as solid, fluid, anions, cations and free charges. The electromechanical and the electrodynamic field equations are discussed. Based on the second law of thermodynamics, a consistent model is developed. With respect to the closure problem of the model, the needed constitutive relations and evolution equations are presented. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-015-1110-8
  • 2016 • 105 Thermodynamics of Bioreactions
    Held, C. and Sadowski, G.
    Annual Review of Chemical and Biomolecular Engineering 7 395-414 (2016)
    Thermodynamic principles have been applied to enzyme-catalyzed reactions since the beginning of the 1930s in an attempt to understand metabolic pathways. Currently, thermodynamics is also applied to the design and analysis of biotechnological processes. The key thermodynamic quantity is the Gibbs energy of reaction, which must be negative for a reaction to occur spontaneously. However, the application of thermodynamic feasibility studies sometimes yields positive Gibbs energies of reaction even for reactions that are known to occur spontaneously, such as glycolysis. This article reviews the application of thermodynamics in enzyme-catalyzed reactions. It summarizes the basic thermodynamic relationships used for describing the Gibbs energy of reaction and also refers to the nonuniform application of these relationships in the literature. The review summarizes state-of-the-art approaches that describe the influence of temperature, pH, electrolytes, solvents, and concentrations of reacting agents on the Gibbs energy of reaction and, therefore, on the feasibility and yield of biological reactions. Copyright © 2016 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev-chembioeng-080615-034704
  • 2016 • 104 Thermodynamics of a model biological reaction: A comprehensive combined experimental and theoretical study
    Emel'yanenko, V.N. and Yermalayeu, A.V. and Voges, M. and Held, C. and Sadowski, G. and Verevkin, S.P.
    Fluid Phase Equilibria 422 99-110 (2016)
    In this work we applied experimental and theoretical thermodynamics to methyl ferulate hydrolysis, a model biological reaction, in order to calculate the equilibrium constant and reaction enthalpy. In the first step, reaction data was collected. Temperature-dependent equilibrium concentrations of methyl ferulate hydrolysis have been measured. These were combined with activity coefficients predicted with electrolyte PC-SAFT in order to derive thermodynamic equilibriums constants K a as a function of temperature.In a second step, thermochemical properties of the highly pure reaction participants methyl ferulate and ferulic acid were measured by complementary thermochemical methods including combustion and differential scanning calorimetry. Vapor pressures and sublimation enthalpies of these compounds were measured by transpiration and TGA methods over a broad temperature range. Thermodynamic data on methyl ferulate and ferulic acid available in the literature were evaluated and combined with our own experimental results. Further, the standard molar enthalpy of methyl ferulate hydrolysis reaction calculated according to the Hess's Law applied to the reaction participants was found to be in agreement with the experimental reaction enthalpy from the equilibrium study.In a final step, the gas-phase equilibrium constant of methyl ferulate hydrolysis at 298.15 K was calculated with the G3MP2 method. This value was adjusted to the liquid phase using the experimental vapor pressures of the reaction participants. As a result, the liquid phase K a value calculated by quantum chemistry with additional data on the pure reaction participants was in good agreement with the experimental K a reported in the literature for the aqueous phase. The thermodynamic procedure based on the quantum-chemical calculations is found to be a valuable option for assessment of thermodynamic properties of biologically relevant chemical reactions. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2016.01.035
  • 2016 • 103 Thermodynamics of the alanine aminotransferase reaction
    Voges, M. and Schmidt, F. and Wolff, D. and Sadowski, G. and Held, C.
    Fluid Phase Equilibria 422 87-98 (2016)
    The thermodynamic equilibrium of the aminotransferase reaction from l-alanine and 2-oxoglutarate to l-glutamate and pyruvate in aqueous solution was investigated in a temperature range between 25 and 37 °C and pH between at 5 and 9.Prior to considering the reaction equilibria, measurements were carried out to ensure the enzyme activity in the aqueous reaction media. After that, equilibrium concentrations of reacting agents were measured by HPLC-analysis. At constant temperature and pH, reaction equilibrium was shown to depend on the absolute molalities (0.005-0.130 mol kg-1) as well as on the ratio of initial molalities of the reactants. It could be concluded that reaction equilibrium was shifted towards the product site upon increasing reactant molalities, increasing temperature, and increasing pH. Further, yields of pyruvate were increased upon excess initial molality of l-alanine compared to 2-oxoglutarate.The thermodynamic equilibrium constant Ka* was determined by extrapolating the ratio of product equilibrium molalites and reactant equilibrium molalites to infinite dilution of all reacting agents. The activity-coefficient ratio of products and reactants in the reaction media was predicted with ePC-SAFT. Combining Ka* and the activity-coefficient ratio allowed quantitatively predicting the influence of temperature, pH, and reacting-agent molalities on the reaction equilibrium. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2016.01.023
  • 2015 • 102 A general approach to study the thermodynamics of ligand adsorption to colloidal surfaces demonstrated by means of catechols binding to zinc oxide quantum dots
    Lin, W. and Walter, J. and Burger, A. and Maid, H. and Hirsch, A. and Peukert, W. and Segets, D.
    Chemistry of Materials 27 358-369 (2015)
    A general strategy to study the thermodynamics of ligand adsorption to colloidal surfaces was established. The versatility of our approach is demonstrated by means of catechols binding to ZnO quantum dots (QDs). First, isothermal titration calorimetry (ITC) was used to extract all relevant thermodynamic parameters, namely association constant, enthalpy, entropy, and free energy of ligand binding. Noteworthy, the determined δG of -20.3 ± 0.4 kJ mol-1 indicates a strong, reproducible, and exothermic interaction between the catechol anchor group and the oxide particle surface. To confirm the characterization of ligand binding by measuring the heat of adsorption, the free energy was cross-validated by mass-based adsorption isotherms. A combination of inductively coupled plasma optical emission spectroscopy (ICP-OES) and UV/vis spectroscopy was developed to quantitatively determine the mass of bound catechols with respect to the available particle surface. The association constant K was determined by a Langmuir fit to be 2618 M-1 which leads to δG = -19.50 kJ mol-1 according to δG = -RTln K. To close the mass balance, analytical ultracentrifugation (AUC) was applied to detect the amount of the free, unbound catechol in solution. Finally, Raman spectroscopy and nuclear magnetic resonance spectroscopy (NMR) were performed to quantify the amount of remaining acetate from particle synthesis and to distinguish bound (chemisorbed) and unbound (physisorbed) catechol. Our results reveal that approximately 65 wt % of acetate is replaced, and physisorbed catechol will not affect the amount of remaining acetate on the ZnO surface. Moreover, no pronounced chemical shift peak as it would be expected for free catechol is observed by NMR at all. This indicates a highly dynamic adsorption-desorption equilibrium between the free and the physisorbed state of catechol on the particle surface. Our concept of combined analytics is seen to be a generally applicable strategy for particle-ligand interfacial studies. It gives detailed insight into thermodynamics, binding states, and ligand composition and is thus considered as an important step toward tailored colloidal surface properties. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm504080d
  • 2015 • 101 A novel approach for analyzing the dissolution mechanism of solid dispersions
    Ji, Y. and Paus, R. and Prudic, A. and Lübbert, C. and Sadowski, G.
    Pharmaceutical Research 32 2559-2578 (2015)
    Purpose To analyze the dissolution mechanism of solid dispersions of poorly water-soluble active pharmaceutical ingredients (APIs), to predict the dissolution profiles of the APIs and to find appropriate ways to improve their dissolution rate. Methods The dissolution profiles of indomethacin and naproxen from solid dispersions in PVP K25 were measured in vitro using a rotating-disk system (USP II). A chemical-potential-gradient model combined with the thermodynamic model PC-SAFT was developed to investigate the dissolution mechanism of indomethacin and naproxen from their solid dispersions at different conditions and to predict the dissolution profiles of these APIs. Results The results show that the dissolution of the investigated solid dispersions is controlled by dissolution of both, API and PVP K25 as they codissolve according to the initial API loading. Moreover, the dissolution of indomethacin and naproxen was improved by decreasing the API loading in polymer (leading to amorphous solid dispersions) and increasing stirring speed, temperature and pH of the dissolution medium. The dissolution of indomethacin and naproxen from their amorphous solid dispersions is mainly controlled by the surface reaction, which implies that indomethacin and naproxen dissolution can be effectively improved by formulation design and by improving their solvation performance. Conclusions The chemical-potential-gradient model combined with PC-SAFTcan be used to analyze the dissolution mechanism of solid dispersions and to describe and predict the dissolution profiles of API as function of stirring speed, temperature and pH value of the medium. This work helps to find appropriate ways to improve the dissolution rate of poorly-soluble APIs. © Springer Science+Business Media New York 2015.
    view abstractdoi: 10.1007/s11095-015-1644-z
  • 2015 • 100 Ab initio thermodynamics of the CoCrFeMnNi high entropy alloy: Importance of entropy contributions beyond the configurational one
    Ma, D. and Grabowski, B. and Körmann, F. and Neugebauer, J. and Raabe, D.
    Acta Materialia 100 90-97 (2015)
    We investigate the thermodynamic properties of the prototype equi-atomic high entropy alloy (HEA) CoCrFeMnNi by using finite-temperature ab initio methods. All relevant free energy contributions are considered for the hcp, fcc, and bcc structures, including electronic, vibrational, and magnetic excitations. We predict the paramagnetic fcc phase to be most stable above room temperature in agreement with experiment. The corresponding thermal expansion and bulk modulus agree likewise well with experimental measurements. A careful analysis of the underlying entropy contributions allows us to identify that the originally postulated dominance of the configurational entropy is questionable. We show that vibrational, electronic, and magnetic entropy contributions must be considered on an equal footing to reliably predict phase stabilities in HEA systems. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.08.050
  • 2015 • 99 Ab initio-based bulk and surface thermodynamics of InGaN alloys: Investigating the effects of strain and surface polarity
    Duff, A.I. and Lymperakis, L. and Neugebauer, J.
    Physica Status Solidi (B) Basic Research 252 855-865 (2015)
    The growth of high In content InGaN with sufficiently high crystal quality is challenging due to the differences in the GaN and InN thermodynamics. The surprisingly different thermodynamics is due to a complex competition between strain and chemistry and mediated by the different indium and gallium atomic radii as well as their different bonding enthalpies with nitrogen. In the present work, we investigate bulk and surface thermodynamics of molecular beam epitaxial (MBE) growth of In<inf>x</inf>Ga<inf>1-x</inf>N for the technologically relevant (0001) and (0001-) growth planes by means of density functional theory calculations. Our calculations confirm that coherent growth fully suppresses phase separation through spinodal decomposition. However, the biaxial strain is found to have a marginal effect on the critical temperatures for In<inf>x</inf>Ga<inf>1-x</inf>N decomposition. Furthermore, the thermal stability of excess indium is found to be remarkably higher on N-polar surfaces than on the Ga-polar surfaces. Phase diagram of In covered (a) Ga-polar and (b) N-polar surfaces as function of In pressure. InGaN alloys are key materials for solid state lighting applications in the green region of the optical spectrum. This Feature Article provides an overview of recent advances in ab initio-based InGaN bulk and surface thermodynamics calculations and elucidates the effects of strain and surface polarity on indium incorporation. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201451687
  • 2015 • 98 Benchmark Thermochemistry for Biologically Relevant Adenine and Cytosine. A Combined Experimental and Theoretical Study
    Emel'yanenko, V.N. and Zaitsau, D.H. and Shoifet, E. and Meurer, F. and Verevkin, S.P. and Schick, C. and Held, C.
    Journal of Physical Chemistry A 119 9680-9691 (2015)
    The thermochemical properties available in the literature for adenine and cytosine are in disarray. A new condensed phase standard (p° = 0.1 MPa) molar enthalpy of formation at T = 298.15 K was measured by using combustion calorimetry. New molar enthalpies of sublimation were derived from the temperature dependence of vapor pressure measured by transpiration and by the quarz-crystal microbalance technique. The heat capacities of crystalline adenine and cytosine were measured by temperature-modulated DSC. Thermodynamic data on adenine and cytosine available in the literature were collected, evaluated, and combined with our experimental results. Thus, the evaluated collection of data together with the new experimental results reported here has helped to resolve contradictions in the available enthalpies of formation. A set of reliable thermochemical data is recommended for adenine and cytosine for further thermochemical calculations. Quantum-chemical calculations of the gas phase molar enthalpies of formation of adenine and cytosine have been performed by using the G4 method and results were in excellent agreement with the recommended experimental data. The standard molar entropies of formation and the standard molar Gibbs functions of formation in crystal and gas state have been calculated. Experimental vapor-pressure data measured in this work were used to estimate pure-component PC-SAFT parameters. This allowed modeling solubility of adenine and cytosine in water over the temperature interval 278-310 K. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.5b04753
  • 2015 • 97 Biopolymer foaming with supercritical CO2 - Thermodynamics, foaming behaviour and mechanical characteristics
    Frerich, S.C.
    Journal of Supercritical Fluids 96 349-358 (2015)
    Polymer foams, especially those based on biodegradable polymers, are in high demand for energy saving applications, for example as thermal insulations or packaging materials. To understand and predict the quality and material properties of polymer foams, concise knowledge of the factors influencing the foaming behaviour, especially pressure and temperature, is required. Therefore, three biodegradable polyesters, namely poly (lactide) (PLA), poly (butylene succinate) (PBS) and a blend of poly (lactide) and poly (hydroxy butyrate) (PLA-PHB), have been subjected to a direct foaming procedure using compressed carbon dioxide as blowing agent, studying the influence of saturation temperature (ranging from 95 °C to 175°C) and applied pressure (ranging from atmospheric pressure to 30 MPa) on the resulting foam material. As these results are strongly depending on the melting behaviour of the polymers, all three polymers were subjected to calorimetric analysis in a scanning transitiometer that allows for applying pressure levels of up to 45 MPa. The created porous materials were characterized by determining their density, porosity and morphology, using SEM analysis. Their mechanical behaviour was investigated by using compressive strength tests. It is shown that the quality of the produced foam structures and its properties is strongly depending on the foaming conditions. In order to obtain foams with a high quality, the saturation temperature and pressure have to be adapted to the phase transition liquid-solid of the polymer-gas system. The results obtained via scanning transitiometer represent the SLG-line of the binary systems polymer and CO2, and their influence on the foaming behaviour enabled the identification of ideal foaming conditions for the three polymers regarded in this study. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.supflu.2014.09.043
  • 2015 • 96 Boron-alloyed Fe-Cr-C-B tool steels - Thermodynamic calculations and experimental validation
    Röttger, A. and Lentz, J. and Theisen, W.
    Materials and Design 88 420-429 (2015)
    This study focuses on the development of boron-alloyed tool steels. The influence of Cr additions from 0 to 10mass% on microstructural changes were investigated for a constant metalloid content (C+B=2.4mass%). In the first step, thermodynamic calculations were performed to map the quaternary Fe-Cr-C-B system. In the second step, thermodynamic calculations were validated with laboratory melts that were investigated with respect to the microstructure and phase composition. The results of thermodynamic calculations correspond to real material behavior of Fe-Cr-C-B alloys. Furthermore, the influence of chromium on hard phase formation was investigated by means of phase analysis methods, X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). Nanoindentation was used to determine hard phase properties (hardness, Young's modulus). It was shown that chromium promotes the formation of M2B-type borides. An increase in the Cr content within the M2B phase led to a transformation from the tetragonal structure into an orthorhombic structure. This transformation is accompanied by an increase in hardness and in the Young's modulus. In contrast, Cr also promotes the formation of Cr-rich carboborides of type M23(C,B)6. However, an increased Cr content within the M23(C,B)6 phase is not associated with an increase in hardness or elastic modulus. © 2015 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2015.08.157
  • 2015 • 95 Combining X-ray crystallography and molecular modeling toward the optimization of pyrazolo[3,4-d ]pyrimidines as potent c-Src inhibitors active in vivo against neuroblastoma
    Tintori, C. and Fallacara, A.L. and Radi, M. and Zamperini, C. and Dreassi, E. and Crespan, E. and Maga, G. and Schenone, S. and Musumeci, F. and Brullo, C. and Richters, A. and Gasparrini, F. and Angelucci, A. and Festuccia, C. a...
    Journal of Medicinal Chemistry 58 347-361 (2015)
    c-Src is a tyrosine kinase belonging to the Src-family kinases. It is overexpressed and/or hyperactivated in a variety of cancer cells, thus its inhibition has been predicted to have therapeutic effects in solid tumors. Recently, the pyrazolo[3,4-d]pyrimidine 3 was reported as a dual c-Src/Abl inhibitor. Herein we describe a multidisciplinary drug discovery approach for the optimization of the lead 3 against c-Src. Starting from the X-ray crystal structure of c-Src in complex with 3, Monte Carlo free energy perturbation calculations were applied to guide the design of c-Src inhibitors with improved activities. As a result, the introduction of a meta hydroxyl group on the C4 anilino ring was computed to be particularly favorable. The potency of the synthesized inhibitors was increased with respect to the starting lead 3. The best identified compounds were also found active in the inhibition of neuroblastoma cell proliferation. Furthermore, compound 29 also showed in vivo activity in xenograft model using SH-SY5Y cells. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jm5013159
  • 2015 • 94 Coordinate-invariant phase field modeling of ferro-electrics, part I: Model formulation and single-crystal simulations
    Schrade, D. and Keip, M.-A. and Thai, H. and Schröder, J. and Svendsen, B. and Müller, R. and Gross, D.
    GAMM Mitteilungen 38 102-114 (2015)
    An electro-mechanically coupled phase field model for ferroelectric domain evolution is introduced. Based on Gurtin's concept of a microforce balance, a generalized Ginzburg-Landau evolution equation is derived from the second law of thermodynamics. The thermodynamic potential is formulated for transversely isotropic material behavior by adopting a coordinateinvariant formulation. The model is reduced to 2D and implemented into a finite element framework. The numerical simulations concern the microstructure evolution in mechanically clamped BaTiO3 single-crystals. In the second part of this contribution Keip et al. [1], the poling behavior of ferroelectric composites and polycrystals is investigated with regard to size effects and the influence of a discontinuous order parameter field across grain boundaries. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510005
  • 2015 • 93 Dissolution of crystalline pharmaceuticals: Experimental investigation and thermodynamic modeling
    Paus, R. and Ji, Y. and Braak, F. and Sadowski, G.
    Industrial and Engineering Chemistry Research 54 731-742 (2015)
    In this work, a two-step chemical-potential-gradient model based on nonequilibrium thermodynamic principles was developed to investigate the dissolution mechanism of crystalline active pharmaceutical ingredients (APIs). The perturbed-chain statistical associating fluid theory was used to calculate the required solubilities and chemical potentials of the investigated APIs. The statistical rate theory was used to describe the mass-transfer rate of the APIs at the solid-liquid interface during the dissolution process. Dissolution profiles of indomethacin, naproxen, and glibenclamide in water and in buffered solutions at pH 5.0, 6.5, and 7.2 were measured using a rotating-disk system (USP II). The specific dissolution mechanisms of the APIs, such as surface reaction and diffusion, were analyzed by applying the proposed model to identify the rate-controlling step. The results show that the dissolution mechanisms of indomethacin, naproxen, and glibenclamide change with varying pH values of the solution medium. On the basis of the calculated rate constants, the dissolution profiles were modeled with a high degree of accuracy when compared with the experimental data. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ie503939w
  • 2015 • 92 Dynamic strain-induced transformation: An atomic scale investigation
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Springer, H. and Raabe, D.
    Scripta Materialia 109 23-27 (2015)
    Phase transformations provide the most versatile access to the design of complex nanostructured alloys in terms of grain size, morphology, local chemical constitution etc. Here we study a special case of deformation induced phase transformation. More specifically, we investigate the atomistic mechanisms associated with dynamic strain-induced transformation (DSIT) in a dual-phased multicomponent iron-based alloy at high temperatures. DSIT phenomena and the associated secondary phase nucleation were observed at atomic scale using atom probe tomography. The obtained local chemical composition was used for simulating the nucleation process which revealed that DSIT, occurring during load exertion, proceeds by a diffusion-controlled nucleation process. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2015.07.010
  • 2015 • 91 Effect of ruthenium on the precipitation of topologically close packed phases in Ni-based superalloys of 3rd and 4th generation
    Matuszewski, K. and Rettig, R. and Matysiak, H. and Peng, Z. and Povstugar, I. and Choi, P. and Müller, J. and Raabe, D. and Spiecker, E. and Kurzydłowski, K.J. and Singer, R.F.
    Acta Materialia 95 274-283 (2015)
    The precipitation of topologically close packed (TCP) phases is detrimental for the high temperature strength of high refractory Ni-based superalloys. The beneficial influence of Ru with respect to this so called instability is nowadays well accepted. In the present paper the precipitation of topologically close packed (TCP) phases is studied quantitatively in two experimental alloys (one Ru-free and one with addition of Ru) to clarify the mechanism of the Ru effect. It is confirmed that the TCP phase precipitates undergo sequential phase transformation with the tetragonal σ-phase precipitating first. Ru retards the phase transformation and leads to decreased equilibrium volume fraction of TCP phases. The results clearly indicate that Ru decreases the driving force for TCP phase precipitation. Investigations of crystallography and chemistry of the TCP/matrix interface point to an additional effect by increase of misfit strain energy. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.05.033
  • 2015 • 90 Effect of the specific surface area on thermodynamic and kinetic properties of nanoparticle anatase TiO2 in lithium-ion batteries
    Madej, E. and Klink, S. and Schuhmann, W. and Ventosa, E. and La Mantia, F.
    Journal of Power Sources 297 140-148 (2015)
    Anatase TiO<inf>2</inf> nanoparticles with a specific surface area of 100 m2 g-1 and 300 m2 g-1 have been investigated as negative insertion electrode material for lithium-ion batteries. Galvanostatic intermittent titration (GITT) and electrochemical impedance spectroscopy (EIS) were used to investigate the effect of the specific surface area on the performance of the material. GITT was performed at C/10 rate, followed by an EIS measurement after each relaxation step. Separation of kinetic and thermodynamic contributions to the overpotential of the phase transformation on Li+ (de-)insertion allowed revealing a dependency of both terms on the specific surface area. The material with higher surface area undergoes intrinsic transformation during the initial cycles affecting the thermodynamics of (de-)insertion while the sample with lower surface area shows large and asymmetric kinetic hindrances. For the material with 15 nm particles, Li+ de-insertion appears to have a higher resistance than lithium insertion. © 2015, Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2015.07.079
  • 2015 • 89 Element-resolved thermodynamics of magnetocaloric lafe13-xsix
    Gruner, M.E. and Keune, W. and Roldan Cuenya, B. and Weis, C. and Landers, J. and Makarov, S.I. and Klar, D. and Hu, M.Y. and Alp, E.E. and Zhao, J. and Krautz, M. and Gutfleisch, O. and Wende, H.
    Physical Review Letters 114 (2015)
    By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe13-xSix. These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.057202
  • 2015 • 88 Incorporating the CALPHAD sublattice approach of ordering into the phase-field model with finite interface dissipation
    Zhang, L. and Stratmann, M. and Du, Y. and Sundman, B. and Steinbach, I.
    Acta Materialia 88 156-169 (2015)
    A new approach to incorporate the sublattice models in the CALPHAD (CALculation of PHAse Diagram) formalism directly into the phase-field formalism is developed. In binary alloys, the sublattice models can be classified into two types (i.e., "Type I" and "Type II"), depending on whether a direct one-to-one relation between the element site fraction in the CALPHAD database and the phase concentration in the phase-field model exists (Type I), or not (Type II). For "Type II" sublattice models, the specific site fractions, corresponding to a given mole fraction, have to be established via internal relaxation between different sublattices. Internal minimization of sublattice occupancy and solute evolution during microstructure transformation leads, in general, to a solution superior to the separate solution of the individual problems. The present coupling technique is validated for Fe-C and Ni-Al alloys. Finally, the model is extended into multicomponent alloys and applied to simulate the nucleation process of VC monocarbide from austenite matrix in a steel containing vanadium. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.037
  • 2015 • 87 Influence of humidity on the phase behavior of API/polymer formulations
    Prudic, A. and Ji, Y. and Luebbert, C. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 94 352-362 (2015)
    Amorphous formulations of APIs in polymers tend to absorb water from the atmosphere. This absorption of water can induce API recrystallization, leading to reduced long-term stability during storage. In this work, the phase behavior of different formulations was investigated as a function of relative humidity. Indomethacin and naproxen were chosen as model APIs and poly(vinyl pyrrolidone) (PVP) and poly(vinyl pyrrolidone-co-vinyl acetate) (PVPVA64) as excipients. The formulations were prepared by spray drying. The water sorption in pure polymers and in formulations was measured at 25 °C and at different values of relative humidity (RH = 25%, 50% and 75%). Most water was absorbed in PVP-containing systems, and water sorption was decreasing with increasing API content. These trends could also be predicted in good agreement with the experimental data using the thermodynamic model PC-SAFT. Furthermore, the effect of absorbed water on API solubility in the polymer and on the glass-transition temperature of the formulations was predicted with PC-SAFT and the Gordon-Taylor equation, respectively. The absorbed water was found to significantly decrease the API solubility in the polymer as well as the glass-transition temperature of the formulation. Based on a quantitative modeling of the API/polymer phase diagrams as a function of relative humidity, appropriate API/polymer compositions can now be selected to ensure long-term stable amorphous formulations at given storage conditions. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2015.06.009
  • 2015 • 86 On the thermomechanical coupling in dissipative materials: A variational approach for generalized standard materials
    Bartels, A. and Bartel, T. and Canadija, M. and Mosler, J.
    Journal of the Mechanics and Physics of Solids 82 218-234 (2015)
    This paper deals with the thermomechanical coupling in dissipative materials. The focus lies on finite strain plasticity theory and the temperature increase resulting from plastic deformation. For this type of problem, two fundamentally different modeling approaches can be found in the literature: (a) models based on thermodynamical considerations and (b) models based on the so-called Taylor-Quinney factor. While a naive straightforward implementation of thermodynamically consistent approaches usually leads to an over-prediction of the temperature increase due to plastic deformation, models relying on the Taylor-Quinney factor often violate fundamental physical principles such as the first and the second law of thermodynamics. In this paper, a thermodynamically consistent framework is elaborated which indeed allows the realistic prediction of the temperature evolution. In contrast to previously proposed frameworks, it is based on a fully three-dimensional, finite strain setting and it naturally covers coupled isotropic and kinematic hardening - also based on non-associative evolution equations. Considering a variationally consistent description based on incremental energy minimization, it is shown that the aforementioned problem (thermodynamical consistency and a realistic temperature prediction) is essentially equivalent to correctly defining the decomposition of the total energy into stored and dissipative parts. Interestingly, this decomposition shows strong analogies to the Taylor-Quinney factor. In this respect, the Taylor-Quinney factor can be well motivated from a physical point of view. Furthermore, certain intervals for this factor can be derived in order to guarantee that fundamental physically principles are fulfilled a priori. Representative examples demonstrate the predictive capabilities of the final constitutive modeling framework. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2015.04.011
  • 2015 • 85 Phase stability of non-equiatomic CoCrFeMnNi high entropy alloys
    Ma, D. and Yao, M. and Pradeep, K.G. and Tasan, C.C. and Springer, H. and Raabe, D.
    Acta Materialia 98 288-296 (2015)
    Abstract The objective of this study is to experimentally and theoretically investigate the phase stability of non-equiatomic Fe<inf>x</inf>Mn<inf>62-x</inf>Ni<inf>30</inf>Co<inf>6</inf>Cr<inf>2</inf> based high entropy alloys, where x ranges from 22 to 42 at.%. Another aim is to systematically and critically assess the predictive capability of the CALPHAD approach for such high entropy alloy systems. We find that the CALPHAD simulations provide a very consistent assessment of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases after non-equilibrium solidification processes, unfavorable segregation profiles inherited from solidification together with the associated nucleation and growth of low temperature phases, and undesired martensitic transformation effects. Encouraged by these consistent theoretical and experimental results, we extend our simulations to other alloy systems with equiatomic compositions reported in the literature. Using these other equiatomic model systems we demonstrate how systematic CALPHAD simulations can improve and accelerate the design of multicomponent alloy systems. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.07.030
  • 2015 • 84 Predicting the Solubility Advantage of Amorphous Pharmaceuticals: A Novel Thermodynamic Approach
    Paus, R. and Ji, Y. and Vahle, L. and Sadowski, G.
    Molecular Pharmaceutics 12 2823-2833 (2015)
    For the solubility and bioavailability of poorly soluble active pharmaceutical ingredients (APIs) to be improved, the transformation of crystalline APIs to the amorphous state has often been shown to be advantageous. As it is often difficult to measure the solubility of amorphous APIs, the application of thermodynamic models is the method of choice for determining the solubility advantage. In this work, the temperature-dependent solubility advantage of an amorphous API versus its crystalline form was predicted for five poorly soluble APIs in water (glibenclamide, griseofulvin, hydrochlorothiazide, indomethacin, and itraconazole) based on modeling the API/solvent phase diagrams using the perturbed-chain statistical associating fluid theory (PC-SAFT). Evaluation of the performance of this approach was performed by comparing the predicted solubility advantage to experimental data and to the solubility advantage calculated by the commonly applied Gibbs-energy-difference method. For all of the systems considered, PC-SAFT predictions of the solubility advantage are significantly more accurate than the results obtained from the Gibbs-energy-difference method. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/mp500824d
  • 2015 • 83 Reverse engineering of fluid selection for thermodynamic cycles with cubic equations of state, using a compression heat pump as example
    Roskosch, D. and Atakan, B.
    Energy 81 202--212 (2015)
    Fluid selection for thermodynamic cycles like refrigeration cycles, heat pumps or organic Rankine cycles remains an actual topic. Generally the search for a working fluid is based on experimental approaches or on a not very systematic trial and error approach, far from being elegant. An alternative method may be a theory based reverse engineering approach, proposed and investigated here: The design process should start with an optimal process and with (abstract) properties of the fluid needed to fit into this optimal process, best described by some general equation of state and the corresponding fluid-describing parameters. These should be analyzed and optimized with respect to the defined model process, which also has to be optimized simultaneously. From this information real fluids can be selected or even synthesized which have fluid defining properties in the optimum regime like critical temperature or ideal gas capacities of heat, allowing to find new worldng fluids, not considered so far. The number and kind of the fluid-defining parameters is mainly based on the choice of the used EOS (equation of state). The property model used in the present work is based on the cubic Peng-Robinson equation, chosen due to its moderate numerical expense, sufficient accuracy as well as a general availability of the fluid-defining parameters for many compounds. The considered model-process works between the temperature levels of 273.15 and 333.15 K and can be used as heat pump for supplying buildings with heat, typically. The objective functions are the COP (coefficient of performance) and the VHC (volumetric heating capacity) as a function of critical pressure, critical temperature, acentric factor and two coefficients for the temperature-dependent isobaric ideal gas heat capacity. Also, the steam quality at the compressor entrance has to be regarded as a problem variable. The results give clear hints regarding optimal fluid parameters of the analyzed process and deepen the thermodynamic understanding of the process. Finally, for the COP optimization a strategy for screening large databases is explained. Several fluids from different substance groups like hydrogen iodide (COP = 3.68), formaldehyde (3.61) or cyclopropane (3.42) were found to have higher COPs than the often used R134a (3.12). These fluids will also have to fulfill further criteria, prior to their usage, but the method appears to be a good base for fluid selection. (C) 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.energy.2014.12.025
  • 2015 • 82 Thermodynamic phase behaviour of indomethacin/PLGA formulations
    Prudic, A. and Lesniak, A.-K. and Ji, Y. and Sadowski, G.
    European Journal of Pharmaceutics and Biopharmaceutics 93 88-94 (2015)
    In the current study, the phase behaviour of indomethacin and poly(lactic-co-glycolic acid) (PLGA) formulations was investigated as a function of the molecular weight and the copolymer composition of PLGA. The formulations were prepared by ball milling, and the phase behaviour, comprised of the glass-transition temperature of the formulations and the solubility of indomethacin in PLGA, was measured using modulated differential scanning calorimetry (mDSC). The results determined that the solubility of indomethacin in PLGA at room temperature was very low and increased with a corresponding decrease in the molecular weight of PLGA. The copolymer composition of PLGA had a minor effect on the indomethacin solubility. The effect of PLGA's molecular weight and copolymer composition on the solubility of indomethacin could be modelled using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) with a high degree of accuracy when compared with the experimental data. The glass-transition temperatures had a negative deviation from the weighted mean of the glass-transition temperatures of the pure substances, which could be described by the Kwei-equation. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.ejpb.2015.01.029
  • 2015 • 81 Thermodynamic Modeling for Efficient Cocrystal Formation
    Lange, L. and Sadowski, G.
    Crystal Growth and Design 15 4406-4416 (2015)
    The purpose of this work is to increase the efficiency of the cocrystal formation process by thermodynamic modeling using perturbed-chain statistical associating fluid theory (PC-SAFT). By accounting for the thermodynamic nonideality of the components in the cocrystal system, PC-SAFT is able to model and predict the solubility behavior of pharmaceutical cocrystals based solely on the knowledge of a single cocrystal solubility point in any solvent and at any temperature. Furthermore, the cocrystal solubility in other solvents and for other temperatures can be predicted without the need for additional measurements. The (+)-mandelic acid/(-)-mandelic acid (1:1), caffeine/glutaric acid (1:1), and carbamazepine/nicotinamide (1:1) cocrystal systems were modeled, and the results were in excellent agreement with the experimental data. (Figure Presented). © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.5b00735
  • 2014 • 80 A novel homogenization method for phase field approaches based on partial rank-one relaxation
    Mosler, J. and Shchyglo, O. and Montazer Hojjat, H.
    Journal of the Mechanics and Physics of Solids 68 251-266 (2014)
    This paper deals with the analysis of homogenization assumptions within phase field theories in a finite strain setting. Such homogenization assumptions define the average bulks energy within the diffusive interface region where more than one phase co-exist. From a physical point of view, a correct computation of these energies is essential, since they define the driving force of material interfaces between different phases. The three homogenization assumptions considered in this paper are: (a) Voigt/Taylor model, (b) Reuss/Sachs model, and (c) Khachaturyan model. It is shown that these assumptions indeed share some similarities and sometimes lead to the same results. However, they are not equivalent. Only two of them allow the computation of the individual energies of the co-existing phases even within the aforementioned diffusive interface region: the Voigt/Taylor and the Reuss/Sachs model. Such a localization of the averaged energy is important in order to determine and to subsequently interpret the driving force at the interface. Since the Voigt/Taylor and the Reuss/Sachs model are known to be relatively restrictive in terms of kinematics (Voigt/Taylor) and linear momentum (Reuss/Sachs), a novel homogenization approach is advocated. Within a variational setting based on (incremental) energy minimization, the results predicted by the novel approach are bounded by those corresponding to the Voigt/Taylor and the Reuss/Sachs model. The new approach fulfills equilibrium at material interfaces (continuity of the stress vector) and it is kinematically compatible. In sharp contrast to existing approaches, it naturally defines the mismatch energy at incoherent material interfaces. From a mathematical point of view, it can be interpreted as a partial rank-one convexification. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmps.2014.04.002
  • 2014 • 79 A thermodynamic investigation of the glucose-6-phosphate isomerization
    Hoffmann, P. and Held, C. and Maskow, T. and Sadowski, G.
    Biophysical Chemistry 195 22-31 (2014)
    In this work, ΔRg+ values for the enzymatic G6P isomerization were determined as a function of the G6P equilibrium molality between 25 °C and 37 °C. The reaction mixtures were buffered at pH = 8.5. In contrast to standard literature work, ΔRg+ values were determined from activity-based equilibrium constants instead of molality-based apparent values. This yielded a ΔRg+ value of 2.55 ± 0.05 kJ mol- 1 at 37 °C, independent of the solution pH between 7.5 and 8.5. Furthermore, ΔRh + was measured at pH = 8.5 and 25 °C yielding 12.05 ± 0.2 kJ mol- 1. Accounting for activity coefficients turned out to influence ΔRg+ up to 30% upon increasing the G6P molality. This result was confirmed by predictions using the thermodynamic model ePC-SAFT. Finally, the influence of the buffer and of potassium glutamate as an additive on the reaction equilibrium was measured and predicted with ePC-SAFT in good agreement. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.bpc.2014.08.002
  • 2014 • 78 An invariant formulation for phase field models in ferroelectrics
    Schrade, D. and Müller, R. and Gross, D. and Keip, M.-A. and Thai, H. and Schröder, J.
    International Journal of Solids and Structures 51 2144-2156 (2014)
    This paper introduces an electro-mechanically coupled phase field model for ferroelectric domain evolution based on an invariant formulation for transversely isotropic piezoelectric material behavior. The thermodynamic framework rests upon Gurtin's notion of a micro-force system in conjunction with an associated micro-force balance. This leads to a formulation of the second law, from which a generalized Ginzburg-Landau evolution equation is derived. The invariant formulation of the thermodynamic potential provides a consistent way to obtain the order parameter dependent elastic stiffness, piezoelectric, and dielectric tensor. The model is reduced to 2d and implemented into a finite element framework. The material constants used in the simulations are adapted to meet the thermodynamic condition of a vanishing micro-force. It is found that the thermodynamic potential taken from the literature has to be extended in order to avoid a loss of positive definiteness of the stiffness and the dielectric tensor. The small-signal response is investigated in the presence and in the absence of the additional regularizing terms in the potential. The simulations show the pathological behavior of the model in case these terms are not taken into account. The paper closes with microstructure simulations concerning a ferroelectric nanodot subjected to an electric field, a cracked single crystal, and a ferroelectric bi-crystal. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2014.02.021
  • 2014 • 77 Beneficial effects of stoichiometry and nanostructure for a LiBH 4-MgH2 hydrogen storage system
    Hu, J. and Witter, R. and Shao, H. and Felderhoff, M. and Fichtner, M.
    Journal of Materials Chemistry A 2 66-72 (2014)
    The hydrogen storage system [MgH2-2LiBH4] shows attractive properties such as favorable thermodynamics, high hydrogen capacity and reversibility. However, there exists an incubation period that amounts up to 10 hours in the dehydrogenation steps, which restricts this system as a practical material. In this study, the influences of stoichiometry and the nanoscale MgH2 were investigated for the system. Considerably shortened incubation times were achieved with deficit amounts of LiBH 4 or by using nanoscale MgH2. In addition, the application of nanoscale MgH2 prevented or suppressed the formation of [B 12H12]2- in the dehydrogenation, which is otherwise an issue concerning the re-cyclability. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3ta13775a
  • 2014 • 76 Breakdown of the arrhenius law in describing vacancy formation energies: The importance of local anharmonicity revealed by Ab initio thermodynamics
    Glensk, A. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Physical Review X 4 (2014)
    We study the temperature dependence of the Gibbs energy of vacancy formation in Al and Cu from T = 0 K up to the melting temperature, fully taking into account anharmonic contributions. Our results show that the formation entropy of vacancies is not constant as often assumed but increases almost linearly with temperature. The resulting highly nonlinear temperature dependence in the Gibbs formation energy naturally explains the differences between positron annihilation spectroscopy and differential dilatometry data and shows that nonlinear thermal corrections are crucial to extrapolate high-temperature experimental data to T = 0 K. Employing these corrections-rather than the linear Arrhenius extrapolation that is commonly assumed in analyzing experimental data-revised formation enthalpies are obtained that differ up to 20% from the previously accepted ones. Using the revised experimental formation enthalpies, we show that a large part of the discrepancies between DFT-GGA and unrevised experimental vacancy formation energies disappears. The substantial shift between previously accepted and the newly revised T = 0 K formation enthalpies also has severe consequences in benchmarking ab initio methods against experiments, e.g., in deriving corrections that go beyond commonly used LDA and GGA exchangecorrelation functionals such as the AM05 functional.
    view abstractdoi: 10.1103/PhysRevX.4.011018
  • 2014 • 75 FT-IR and FT-Raman spectra of 5-fluoroorotic acid with solid state simulation by DFT methods
    Cuellar, A. and Alcolea Palafox, M. and Rastogi, V.K. and Kiefer, W. and Schlücker, S. and Rathor, S.K.
    Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy 132 430-445 (2014)
    FT-Raman and FT-IR studies of the biomolecule 5-fluoroorotic acid in the solid state were carried out. The unit cell found in the crystal was simulated as a tetramer form by density functional calculations. They were performed to clarify wavenumber assignments of the experimental observed bands in the spectra. Correlations with the molecule of uracil were made, and specific scale equations were employed to scale the wavenumbers of 5-fluoroorotic acid. Good reproduction of the experimental wavenumbers is obtained and the % error is very small in the majority of the bands. This fact confirms our simplified solid state model. The molecular structure was fully optimized using DFT and MP2 methods. The relative stability of both the syn and anti conformations was investigated, and the anti-form was found to be slightly more stable, by 7.49 kJ/mol at the MP2 level. The structures of all possible tautomeric forms were determined. The keto-form appeared as the most stable one. The NBO atomic charges and several thermodynamic parameters were also calculated. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.saa.2014.04.107
  • 2014 • 74 Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces
    Raabe, D. and Herbig, M. and Sandlöbes, S. and Li, Y. and Tytko, D. and Kuzmina, M. and Ponge, D. and Choi, P.-P.
    Current Opinion in Solid State and Materials Science 18 253-261 (2014)
    Grain boundaries influence mechanical, functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as 'segregation engineering' solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.cossms.2014.06.002
  • 2014 • 73 Influence of copolymer composition on the phase behavior of solid dispersions
    Prudic, A. and Kleetz, T. and Korf, M. and Ji, Y. and Sadowski, G.
    Molecular Pharmaceutics 11 4189-4198 (2014)
    The incorporation of poorly soluble active pharmaceutical ingredients (APIs) into excipients (e.g., polymers) to formulate an amorphous solid dispersion is a promising strategy to improve the oral bioavailability of the API. The application of copolymer excipients allows access to combinations of different monomers and thus to the design of excipients to improve solid-dispersion properties. In this work, the thermodynamic phase behavior of solid dispersions was investigated as a function of the API, type of monomer, and copolymer composition. The glass-transition temperatures and API solubilities in the solid dispersions of naproxen and indomethacin in polyvinylpyrrolidone, polyvinyl acetate, and copolymers with different weight fractions of vinylpyrrolidone and vinyl actetate were investigated. It is shown that the thermodynamic phase behavior of API/copolymer solid dispersions is a function of monomer type and copolymer composition. This effect was also predicted by using the perturbed-chain statistical associating fluid theory (PC-SAFT). The glass-transition temperature of the solid dispersions was calculated with the Gordon-Taylor equation. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/mp500412d
  • 2014 • 72 Influence of molecular hydrogen on acetylene pyrolysis: Experiment and modeling
    Aghsaee, M. and Dürrstein, S.H. and Herzler, J. and Böhm, H. and Fikri, M. and Schulz, C.
    Combustion and Flame 161 2263-2269 (2014)
    The effect of molecular hydrogen on the formation of molecular carbonaceous species important for soot formation is studied through a combination of shock-tube experiments with high-repetition-rate time-of-flight mass spectrometry and detailed chemistry modeling. The experiment allows to simultaneously measure the concentration-time profiles for various species with a time resolution of 10μs. Concentration histories of reactants and polyacetylene intermediates (C2xH2, x=1-4) are measured during the pyrolysis of acetylene with and without H2 added to the gas mixture for a wide range of conditions. In the 1760-2565K temperature range, reasonable agreement between the experiment and the model predictions for C2H2, C4H2, C6H2, and C8H2 is achieved. H2 addition leads to the depletion of important building blocks for particle formation, namely of polyacetylenes due to an enhanced consumption of important radicals by H2, which are required for the fast build-up of carbonaceous material. © 2014 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2014.03.012
  • 2014 • 71 Influence of solution nitriding of supersolidus-sintered cold work tool steels on their hardenability
    Blüm, M. and Conrads, J. and Weber, S. and Theisen, W.
    HTM - Journal of Heat Treatment and Materials 69 273-281 (2014)
    Powder metallurgical steel grades offer higher quality and performance compared to cast and forged steel grades due to their finer microstructure without segregations or textures. Because high-alloyed, cold work tool steels cannot be compacted by solid-state sintering, hot isostatic pressing is state of the art. This process, however, is comparatively expensive and there is thus a high demand for alternative densification processes. Supersolidus liquid-phase sintering represents an alternative to hot isostatic pressing for densification of these steels to theoretical density. During sintering, the steel powder interacts with the sintering atmosphere, which can be a vacuum, hydrogen, hydrogen plus nitrogen, or nitrogen. In a nitrogen atmosphere, there may be nitrogen uptake by the sintered material, which changes the chemical composition of the steel and thus results in a decrease in the sintering temperature. The aim of this work is to investigate the influence of nitrogen uptake on the hardenability of a high-alloyed and supersolidus-sintered cold work tool steel. Computational thermodynamics using the Calphad method were applied to calculate the optimal parameters for direct quenching from the sintering temperature. In addition, the tempering response was investigated as a function of the heat-treatment parameters. It was found that nitriding exerts a significant influence on the hardenability, which is also dependent on the cooling rate from the sintering temperature to the quenching temperature. Hardenability was predicted qualitatively on the basis of equilibrium carbon concentrations of the austenitic matrix calculated with the Calphad method. © 2014 Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/105.110234
  • 2014 • 70 Local visualization of catalytic activity at gas evolving electrodes using frequency-dependent scanning electrochemical microscopy
    Chen, X. and Maljusch, A. and Rincón, R.A. and Battistel, A. and Bandarenka, A.S. and Schuhmann, W.
    Chemical Communications 50 13250-13253 (2014)
    A new concept for the localized characterization of gas evolving electrodes based on scanning electrochemical microscopy (SECM) is suggested. It offers information about the spatial distribution of the predominant locations, which represent the most active catalytic sites, and dynamic characteristics of gas-bubble departure. The knowledge about gas-bubble departure is critical for the assessment and development of new electrode materials for energy applications. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4cc06100d
  • 2014 • 69 Ostwald-Freundlich diffusion-limited dissolution kinetics of nanoparticles
    Ely, D.R. and Edwin García, R. and Thommes, M.
    Powder Technology 257 120-123 (2014)
    For many years, nanoparticles have garnered increasing interest in pharmaceutical investigations. It is well known that the solubility of nanoparticles increases with decreasing size due to the Gibbs-Thomson effect. However, there are currently no analytical models to describe the kinetics of nanoparticle dissolution. The purpose of this article is to provide a Thermodynamics-based description of the kinetics of nanoparticle dissolution. In particular, the Ostwald-Freundlich relation is used to correct dissolution times for small particles, which have higher solubilities than larger particles. The developed model is an extension of the Hixson-Crowell cube root law in which the total normalized dissolution time is corrected by a "solubility size factor" that approaches unity for increasing initial particle size. This model enables rapid estimation of the total dissolution time of spherical nanoparticles in a gently agitated, zero solute concentration reservoir. The total dissolution time predicted differs from Hixson-Crowell by nearly 10% for initial particle sizes fifty times larger than the characteristic particle size, and increases to more than a factor of six at the characteristic particle size. This work provides a physics-based description of the nanoparticle dissolution kinetics and details the reaches and limitations of the developed model. The theoretical framework provided herein is valid for a wide range of dissolution processes and size scales affording it a high level of practicality. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2014.01.095
  • 2014 • 68 Spin noise spectroscopy beyond thermal equilibrium and linear response
    Glasenapp, P. and Sinitsyn, N.A. and Yang, L. and Rickel, D.G. and Roy, D. and Greilich, A. and Bayer, M. and Crooker, S.A.
    Physical Review Letters 113 (2014)
    Per the fluctuation-dissipation theorem, the information obtained from spin fluctuation studies in thermal equilibrium is necessarily constrained by the system's linear response functions. However, by including weak radio frequency magnetic fields, we demonstrate that intrinsic and random spin fluctuations even in strictly unpolarized ensembles can reveal underlying patterns of correlation and coupling beyond linear response, and can be used to study nonequilibrium and even multiphoton coherent spin phenomena. We demonstrate this capability in a classical vapor of K41 alkali atoms, where spin fluctuations alone directly reveal Rabi splittings, the formation of Mollow triplets and Autler-Townes doublets, ac Zeeman shifts, and even nonlinear multiphoton coherences. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.113.156601
  • 2014 • 67 Synthesis of 7-pentafluorophenyl-1 H -indole: An anion receptor for anion-π interactions
    Sun, Z.-H. and Albrecht, M. and Giese, M. and Pan, F. and Rissanen, K.
    Synlett 25 2075-2077 (2014)
    7-Pentafluorophenyl-1H-indole has the potential to be a key compound for the investigation of anion-π interactions in solution. Unfortunately, it was not possible to obtain it by aryl-aryl coupling reaction. Finally, it has been prepared by Bartoli indole synthesis. The key compound as well as analogues were submitted to preliminary studies of anion binding. Single crystals of two key receptors were obtained. © Georg Thieme Verlag Stuttgart New York.
    view abstractdoi: 10.1055/s-0034-1378449
  • 2014 • 66 Thermodynamic phase behavior of API/polymer solid dispersions
    Prudic, A. and Ji, Y. and Sadowski, G.
    Molecular Pharmaceutics 11 2294-2304 (2014)
    To improve the bioavailability of poorly soluble active pharmaceutical ingredients (APIs), these materials are often integrated into a polymer matrix that acts as a carrier. The resulting mixture is called a solid dispersion. In this work, the phase behaviors of solid dispersions were investigated as a function of the API as well as of the type and molecular weight of the carrier polymer. Specifically, the solubility of artemisinin and indomethacin was measured in different poly(ethylene glycol)s (PEG 400, PEG 6000, and PEG 35000). The measured solubility data and the solubility of sulfonamides in poly(vinylpyrrolidone) (PVP) K10 and PEG 35000 were modeled using the perturbed-chain statistical associating fluid theory (PC-SAFT). The results show that PC-SAFT predictions are in a good accordance with the experimental data, and PC-SAFT can be used to predict the whole phase diagram of an API/polymer solid dispersion as a function of the kind of API and polymer and of the polymers molecular weight. This remarkably simplifies the screening process for suitable API/polymer combinations. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/mp400729x
  • 2013 • 65 Ab initio study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Zhu, L.-F. and Friák, M. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 20 296-304 (2013)
    We employ ab initio calculations and investigate the single-crystalline elastic properties of (Ca,Mg)CO3 crystals covering the whole range of concentrations from pure calcite CaCO3 to pure magnesite MgCO3. Studying different distributions of Ca and Mg atoms within 30-atom supercells, our theoretical results show that the energetically most favorable configurations are characterized by elastic constants that nearly monotonously increase with the Mg content. Based on the first principles-derived single-crystalline elastic anisotropy, the integral elastic response of (Ca,Mg)CO3 polycrystals is determined employing a mean-field self-consistent homogenization method. As in case of single-crystalline elastic properties, the computed polycrystalline elastic parameters sensitively depend on the chemical composition and show a significant stiffening impact of Mg atoms on calcite crystals in agreement with the experimental findings. Our analysis also shows that it is not advantageous to use a higher-scale two-phase mix of stoichiometric calcite and magnesite instead of substituting Ca atoms by Mg ones on the atomic scale. Such two-phase composites are not significantly thermodynamically favorable and do not provide any strong additional stiffening effect. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.01.030
  • 2013 • 64 Adsorption of amino acids on the magnetite-(111)-surface: A force field study
    Bürger, A. and Magdans, U. and Gies, H.
    Journal of Molecular Modeling 19 851-857 (2013)
    Magnetite (Fe3O4) is an important biomineral, e.g., used by magnetotactic bacteria. The connection between the inorganic magnetite-(111)-surface and the organic parts of the bacteria is the magnetosome membrane. The membrane is built by different magnetosome membrane proteins (MMPs), which are dominated by the four amino acids glycine (Gly), aspartic acid (Asp), leucine (Leu) and glutamic acid (Glu). Force field simulations of the interaction of the magnetite-(111)-surface and the main amino acid compounds offer the possibility to investigate if and how the membrane proteins could interact with the mineral surface thus providing an atomistic view on the respective binding sites. In a force field simulation the four amino acids were docked on the Fe-terminated magnetite-(111)-surface. The results show that it is energetically favorable for the amino acids to adsorb on the surface with Fe-O-distances between 2.6 Å and 4.1 Å. The involved O-atoms belong to the carboxyl-group (Asp and Glu) or to the carboxylate-group (Gly, Leu and Glu). Electrostatic interactions dominate the physisorption of the amino acids. During the simulations, according to the frequency of the best results, the global minimum for the docking interaction could be attained for all amino acids analyzed. © 2012 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00894-012-1606-x
  • 2013 • 63 Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography
    Pradeep, K.G. and Wanderka, N. and Choi, P. and Banhart, J. and Murty, B.S. and Raabe, D.
    Acta Materialia 61 4696-4706 (2013)
    We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc-bcc solid solution but instead a composite of bcc structured Ni-Al-, Cr-Fe- and Fe-Cr-based regions and of fcc Cu-Zn-based regions. The Cu-Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr-Fe phase that was inherited from the hot compacted state. The Ni-Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20-30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.04.059
  • 2013 • 62 Comparison of analytic and numerical bond-order potentials for W and Mo
    Čák, M. and Hammerschmidt, T. and Drautz, R.
    Journal of Physics Condensed Matter 25 (2013)
    Bond-order potentials (BOPs) are derived from the tight-binding approximation and provide a linearly-scaling computation of the energy and forces for a system of interacting atoms. While the numerical BOPs involve the numerical integration of the response (Green's) function, the expressions for the energy and interatomic forces are analytical within the formalism of the analytic BOPs. In this paper we present a detailed comparison of numerical and analytic BOPs. We use established parametrizations for the bcc refractory metals W and Mo and test structural energy differences; tetragonal, trigonal, hexagonal and orthorhombic deformation paths; formation energies of point defects as well as phonon dispersion relations. We find that the numerical and analytic BOPs generally are in very good agreement for the calculation of energies. Different from the numerical BOPs, the forces in the analytic BOPs correspond exactly to the negative gradients of the energy. This makes it possible to use the analytic BOPs in dynamical simulations and leads to improved predictions of defect energies and phonons as compared to the numerical BOPs. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/26/265002
  • 2013 • 61 DFT+ U study of arsenate adsorption on FeOOH surfaces: Evidence for competing binding mechanisms
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Journal of Physical Chemistry C 117 15571-15582 (2013)
    On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites - directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1-As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2-As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1-As = 3.26-3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2-As = 3.31-3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with d Fe1-As = 3.55-3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2-As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2-As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe-As distance. Our results show that the Fe-As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe-As distance cannot be used as a unique criterion to discriminate between adsorption modes. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp400649m
  • 2013 • 60 Direct gas-phase synthesis of single-phase β-FeSi2 nanoparticles
    Bywalez, R. and Orthner, H. and Mehmedovic, E. and Imlau, R. and Kovacs, A. and Luysberg, M. and Wiggers, H.
    Journal of Nanoparticle Research 15 (2013)
    For the first time, phase-pure β-FeSi2 nanoparticles were successfully produced by gas-phase synthesis. We present a method to fabricate larger quantities of semiconducting β-FeSi2 nanoparticles, with crystallite sizes between 10 and 30 nm, for solar and thermoelectric applications utilizing a hot-wall reactor. A general outline for the production of those particles by thermal decomposition of silane and iron pentacarbonyl is provided based on kinetic data. The synthesized particles are investigated by X-ray diffraction and transmission electron microscopy, providing evidence that the as-prepared materials are indeed β-FeSi2, while revealing morphological characteristics inherent to the nanoparticles created. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1878-9
  • 2013 • 59 High-temperature deformation and recrystallization: A variational analysis and its application to olivine aggregates
    Hackl, K. and Renner, J.
    Journal of Geophysical Research: Solid Earth 118 943-967 (2013)
    We develop a framework for a variational analysis of microstructural evolution during inelastic high-temperature deformation accommodated by dislocation mechanisms and diffusive mass transport. A polycrystalline aggregate is represented by a distribution function characterizing the state of individual grains by three variables, dislocation density, grain size, and elastic strain. The aggregate's free energy comprises elastic energy and energies of lattice distortions due to dislocations and grain boundaries. The work performed by the external loading is consumed by changes in the number of defects and their migration leading to inelastic deformation. The variational approach minimizes the rate of change of free energy with the evolution of the state variables under constraints on the aggregate volume, on a relation between changes in plastic strain and dislocation density, and on the form of the dissipation functionals for defect processes. The constrained minimization results in four basic evolution equations, one each for the evolution in grain size and dislocation density and flow laws for dislocation and diffusion creep. Analytical steady state scaling relations between stress and dislocation density and grain size (piezometers) are derived for quasi-homogeneous materials characterized by a unique relation between grain size and dislocation density. Our model matches all currently available experimental observations regarding high-temperature deformation of olivine aggregates with plausible values for the involved micromechanical model parameters. The relation between strain rate and stress for olivine aggregates maintaining a steady state microstructure is distinctly nonlinear in stark contrast to the majority of geodynamical modeling relying on linear relations, i.e., Newtonian behavior. Key Points Analytical derivation of steady-state piezometers using variational analysis Matches observations for olivine rocks with plausible micromechanical parameters Provides insight into rheology of olivine aggregates, e.g., lifetime of grains ©2013. American Geophysical Union. All Rights Reserved.
    view abstractdoi: 10.1002/jgrb.50125
  • 2013 • 58 Hofmeister effect of sodium halides on the switching energetics of thermoresponsive polymer brushes
    Naini, C.A. and Thomas, M. and Franzka, S. and Frost, S. and Ulbricht, M. and Hartmann, N.
    Macromolecular Rapid Communications 34 417-422 (2013)
    A laser temperature-jump technique is used to probe the impact of sodium halides on the temperature-dependent switching kinetics and thermodynamics of poly(N-isopropylacrylamide) brushes. An analysis on the basis of a two-state model reveals van't Hoff enthalpy and entropy changes. Sodium halides increase the endothermicity and the entropic gain of the switching process below and above Tc following the Hofmeister series: NaCl &gt; NaBr &gt; NaI. In contrast, enthalpic and entropic changes at Tc remain virtually unaffected. This provides an unprecedented insight into the underlying switching energetics of this classic stimuli-responsive polymer. Because of its model character, these results represent an essential reference on the way to unpuzzle the molecular driving forces of the Hofmeister effect. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201200681
  • 2013 • 57 Interaction of antitumor flavonoids with dsDNA in the absence and presence of Cu(II)
    Temerk, Y.M. and Ibrahim, M.S. and Kotb, M. and Schuhmann, W.
    Analytical and Bioanalytical Chemistry 405 3839-3846 (2013)
    The binding of antitumor flavonoids, namely 3-hydroxyflavone (3HF) and hesperidin (Hesp) with dsDNA was investigated in the absence and presence of Cu(II) using cyclic voltammetry and square wave voltammetry at the hanging mercury drop electrode. The reduction currents of 3HF, 3HF-Cu complex, and the 3HF-β-cyclodextrin inclusion complex decreased after intercalation into dsDNA. The intercalation of Hesp into dsDNA is weak. dsDNA is reduced at a potential of -1.48 V overlaying the reduction of Hesp. In contrast, in the presence of Cu(II), the interaction of Hesp with dsDNA leads to a much stronger intercalation. The binding constants of the flavonoid-Cu complex with dsDNA were evaluated and calibration graphs for the determination of dsDNA were obtained from the decrease in the peak current in the cyclic voltammograms of 3HF in the presence of dsDNA. The proposed method exhibited good recovery and reproducibility for indirect determination of dsDNA. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00216-012-6675-2
  • 2013 • 56 Microscopic structure of water at elevated pressures and temperatures
    Sahle, C.J. and Sternemann, C. and Schmidt, C. and Lehtola, S. and Jahn, S. and Simonelli, L. and Huotari, S. and Hakala, M. and Pylkkänen, T. and Nyrow, A. and Mende, K. and Tolan, M. and Hämäläinen, K. and Wilke, M.
    Proceedings of the National Academy of Sciences of the United States of America 110 6301-6306 (2013)
    We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley's K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6at600 °C and p = 134 MPa.
    view abstractdoi: 10.1073/pnas.1220301110
  • 2013 • 55 Molecular tweezers with varying anions: A comparative study
    Dutt, S. and Wilch, C. and Gersthagen, T. and Talbiersky, P. and Bravo-Rodriguez, K. and Hanni, M. and Sánchez-García, E. and Ochsenfeld, C. and Klärner, F.-G. and Schrader, T.
    Journal of Organic Chemistry 78 6721-6734 (2013)
    Selective binding of the phosphate-substituted molecular tweezer 1a to protein lysine residues was suggested to explain the inhibition of certain enzymes and the aberrant aggregation of amyloid petide Aβ42 or α-synuclein, which are assumed to be responsible for Alzheimer's and Parkinson's disease, respectively. In this work we systematically investigated the binding of four water-soluble tweezers 1a-d (substituted by phosphate, methanephosphonate, sulfate, or O-methylenecarboxylate groups) to amino acids and peptides containing lysine or arginine residues by using fluorescence spectroscopy, NMR spectroscopy, and isothermal titration calorimetry (ITC). The comparison of the experimental results with theoretical data obtained by a combination of QM/MM and ab initio1H NMR shift calculations provides clear evidence that the tweezers 1a-c bind the amino acid or peptide guest molecules by threading the lysine or arginine side chain through the tweezers' cavity, whereas in the case of 1d the guest molecule is preferentially positioned outside the tweezer's cavity. Attractive ionic, CH-π, and hydrophobic interactions are here the major binding forces. The combination of experiment and theory provides deep insight into the host-guest binding modes, a prerequisite to understanding the exciting influence of these tweezers on the aggregation of proteins and the activity of enzymes. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jo4009673
  • 2013 • 54 Nanocrystalline Fe-C alloys produced by ball milling of iron and graphite
    Chen, Y.Z. and Herz, A. and Li, Y.J. and Borchers, C. and Choi, P. and Raabe, D. and Kirchheim, R.
    Acta Materialia 61 3172-3185 (2013)
    A series of nanocrystalline Fe-C alloys with different carbon concentrations (xtot) up to 19.4 at.% (4.90 wt.%) are prepared by ball milling. The microstructures of these alloys are characterized by transmission electron microscopy and X-ray diffraction, and partitioning of carbon between grain boundaries and grain interiors is determined by atom probe tomography. It is found that the segregation of carbon to grain boundaries of α-ferrite can significantly reduce its grain size to a few nanometers. When the grain boundaries of ferrite are saturated with carbon, a metastable thermodynamic equilibrium between the matrix and the grain boundaries is approached, inducing a decreasing grain size with increasing xtot. Eventually the size reaches a lower limit of about 6 nm in alloys with x tot &gt; 6.19 at.% (1.40 wt.%); a further increase in xtot leads to the precipitation of carbon as Fe3C. The observed presence of an amorphous structure in 19.4 at.% C (4.90 wt.%) alloy is ascribed to a deformation-driven amorphization of Fe3C by severe plastic deformation. By measuring the temperature dependence of the grain size for an alloy with 1.77 at.% C additional evidence is provided for a metastable equilibrium reached in the nanocrystalline alloy. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.02.006
  • 2013 • 53 On the macroscopic description of yield surface evolution by means of distortional hardening models: Application to magnesium
    Shi, B. and Mosler, J.
    International Journal of Plasticity 44 1-22 (2013)
    Texture evolution in polycrystals due to rotation of the atomic lattice in single grains results in a complex macroscopic mechanical behavior which cannot be reasonably captured only by classical isotropic or kinematic hardening in general. More precisely and focusing on standard rate-independent plasticity theory, the complex interplay at the microscale of a polycrystal leads to an evolving macroscopic anisotropy of the yield surface, also known as distortional or differential hardening. This effect is of utmost importance, if non-radial loading paths such as those associated with forming processes are to be numerically analyzed. In the present paper, different existing distortional hardening models are critically reviewed. For a better comparison, they are reformulated into the modern framework of hyperelastoplasticity, and the same objective time derivative is applied to all evolution equations. Furthermore, since the original models are based on a Hill-type yield function not accounting for the strength differential effect as observed in hcp metals such as magnesium, respective generalizations are also discussed. It is shown that only one of the resulting models can fulfill the second law of thermodynamics. That model predicts a high curvature of the yield function in loading direction, while the opposite region of the yield function is rather flat. Indeed, such a response can be observed for some materials such as aluminum alloys. In the case of magnesium, however, that does not seem to be true. Therefore, a novel constitutive model is presented. Its underlying structure is comparably simple and the model is thermodynamically consistent. Conceptually, distortional hardening is described by an Armstrong-Frederick-type evolution equation. The predictive capabilities of the final model are demonstrated by comparisons to experimentally measured data. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2012.11.007
  • 2013 • 52 Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys
    Otto, F. and Yang, Y. and Bei, H. and George, E.P.
    Acta Materialia 61 2628-2638 (2013)
    High configurational entropies have been hypothesized to stabilize solid solutions in equiatomic, multi-element alloys which have attracted much attention recently as "high-entropy" alloys with potentially interesting properties. To evaluate the usefulness of configurational entropy as a predictor of single-phase (solid solution) stability, we prepared five new equiatomic, quinary alloys by replacing individual elements one at a time in a CoCrFeMnNi alloy that was previously shown to be single-phase [1]. An implicit assumption here is that, if any one element is replaced by another, while keeping the total number of elements constant, the configurational entropy of the alloy is unchanged; therefore, the new alloys should also be single-phase. Additionally, the substitute elements that we chose, Ti for Co, Mo or V for Cr, V for Fe, and Cu for Ni, had the same room temperature crystal structure and comparable size/electronegativity as the elements being replaced to maximize solid solubility consistent with the Hume-Rothery rules. For comparison, the base CoCrFeMnNi alloy was also prepared. After three-day anneals at elevated temperatures, multiple phases were observed in all but the base CoCrFeMnNi alloy, suggesting that, by itself, configurational entropy is generally not able to override the competing driving forces that also govern phase stability. Thermodynamic analyses were carried out for each of the constituent binaries in the investigated alloys (Co-Cr, Fe-Ni, Mo-Mn, etc.). Our experimental results combined with the thermodynamic analyses suggest that, in general, enthalpy and non-configurational entropy have greater influences on phase stability in equiatomic, multi-component alloys. Only when the alloy microstructure is a single-phase, approximately ideal solid solution does the contribution of configurational entropy to the total Gibbs free energy become dominant. Thus, high configurational entropy provides a way to rationalize, after the fact, why a solid solution forms (if it forms), but it is not a useful a priori predictor of which of the so-called high-entropy alloys will form thermodynamically stable single-phase solid solutions.
    view abstractdoi: 10.1016/j.actamat.2013.01.042
  • 2013 • 51 The Haber-Bosch process revisited: On the real structure and stability of "ammonia iron" under working conditions
    Kandemir, T. and Schuster, M.E. and Senyshyn, A. and Behrens, M. and Schlögl, R.
    Angewandte Chemie - International Edition 52 12723-12726 (2013)
    In situ neutron diffraction was used to study the structural properties of an industrial ammonia synthesis catalyst under working conditions similar to those of the Haber-Bosch process. Despite favorable thermodynamics, no indications of reversible bulk nitridation of the iron catalyst was observed in a self-generated ammonia concentration of 12 vol % at 425 °C and 75 bar after 88 h on stream. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201305812
  • 2013 • 50 The role of activity coefficients in bioreaction equilibria: Thermodynamics of methyl ferulate hydrolysis
    Hoffmann, P. and Voges, M. and Held, C. and Sadowski, G.
    Biophysical Chemistry 173-174 21-30 (2013)
    The Gibbs energy of reaction (ΔRg) is the key quantity in the thermodynamic characterization of biological reactions. Its calculation requires precise standard Gibbs energy of reaction (ΔRg +) values. The value of ΔRg+ is usually determined by measuring the apparent (concentration-dependent) equilibrium constants K, e.g., the molality-based Km. However, the thermodynamically consistent determination of ΔRg+ requires the thermodynamic (activity-based) equilibrium constant Ka. These values (Km and Ka) are equal only if the ratio of the activity coefficients of the reactants to the activity coefficients of the products (Kγ) is equal to unity. In this work, the impact of Kγ on the estimation of Ka for biological reactions was investigated using methyl ferulate (MF) hydrolysis as a model reaction. The value of Kγ was experimentally determined from Km values that were measured at different reactant concentrations. Moreover, K γ was independently predicted using the thermodynamic model ePC-SAFT. Both the experimentally determined and the predicted K γ values indicate that this value cannot be assumed to be unity in the considered reaction. In fact, in the reaction conditions considered in this work, Kγ was shown to be in the range of 3 < K γ < 6 for different reactant molalities (2 < mmol MF kg- 1 < 10). The inclusion of Kγ and thus the use of the thermodynamically correct Ka value instead of Km lead to remarkable differences (almost 40%) in the determination of ΔRg+. Moreover, the new value for ΔRg+ increases the concentration window at which the reaction can thermodynamically occur. The influence of additives was also investigated both experimentally and theoretically. Both procedures consistently indicated that the addition of NaCl (0 to 1 mol kg- 1 water) moderately decreased the value of Kγ, which means that the values of Km increase and that a higher amount of products is obtained as a result of the addition of salt. Additionally, Km was found to strongly depend on pH. A ten-fold increase in the Km values was observed in the pH range of 6 to 7; this increase corresponds to a change of more than 100% in the value of ΔRg+. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.bpc.2012.12.006
  • 2013 • 49 Thermodynamics of carbon solubility in ferrite and vacancy formation in cementite in strained pearlite
    Nematollahi, G.A. and Von Pezold, J. and Neugebauer, J. and Raabe, D.
    Acta Materialia 61 1773-1784 (2013)
    In order to investigate the thermodynamic driving force for the experimentally observed accumulation of C in ferritic layers of severely plastically deformed pearlitic wires, the stabilities of C interstitials in ferrite and of C vacancies in cementite are investigated as a function of uniaxial stain, using density-functional theory. In the presence of an applied strain along [1 1 0] or [1 1 1], the C interstitial in ferrite is significantly stabilized, while the C vacancy in cementite is moderately destabilized by the corresponding strain states in cementite [1 0 0] and ([0 1 0]). The enhanced stabilization of the C interstitial gives rise to an increase in the C concentration within the ferritic layers by up to two orders of magnitude. Our results thus suggest that in addition to the generally assumed non-equilibrium, dislocation-based mechanism, there is also a strain-induced thermodynamic driving force for the experimentally observed accumulation of C in ferrite. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.12.001
  • 2013 • 48 Universal frozen spectra after time-dependent symmetry restoring phase transitions
    Queisser, F. and Navez, P. and Schützhold, R.
    Journal of Physics Condensed Matter 25 (2013)
    For a general O(N) model, we study the time-dependent phase transition from a state with broken symmetry to the symmetric phase . During this non-equilibrium process, the primordial quantum (or thermal) fluctuations of the initial Goldstone modes are frozen and result in a deviation from the final ground (or thermal) state. For very slow transitions, we find that these fluctuations display a universal scaling behaviour. Their spectra are universal functions of a single parameter, which combines the initial frequency of the Goldstone modes and the sweep rate. As a result, the final two-point function is not exponentially suppressed at large distances Δr = r - r′ (as it would be in the ground state) but decays polynomially in 1/|Δr|. Finally, we exemplify this universal behaviour for the transition from the super-fluid phase to the Mott state in the Bose-Hubbard model. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/40/404215
  • 2012 • 47 Ab initio-based prediction of phase diagrams: Application to magnetic shape memory alloys
    Hickel, T. and Uijttewaal, M. and Al-Zubi, A. and Dutta, B. and Grabowski, B. and Neugebauer, J.
    Advanced Engineering Materials 14 547-561 (2012)
    An ultimate goal of material scientists is the prediction of the thermodynamics of tailored materials solely based on first principles methods. The present work reviews recent methodological developments and advancements providing thereby an up-to-date basis for such an approach. Key ideas and the performance of these methods are discussed with respect to the Heusler alloy Ni-Mn-Ga - a prototype magnetic shape-memory alloy of great technological interest for various applications. Ni-Mn-Ga shows an interesting and complex sequence of phase transitions, rendering it a significant theoretical challenge for any first principles approach. The primary goal of this investigation is to determine the composition dependence of the martensitic transition temperature in these alloys. Quasiharmonic phonons and the magnetic exchange interactions as well as the delicate interplay of vibrational and magnetic excitations are taken into account employing density functional theory. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200092
  • 2012 • 46 Acoustics of two-component porous materials with anisotropic tortuosity
    Albers, B. and Wilmanski, K.
    Continuum Mechanics and Thermodynamics 24 403-416 (2012)
    The paper is devoted to the analysis of monochromatic waves in two-component poroelastic materials described by a Biot-likemodelwhose stress-strain relations are isotropic but the permeability is anisotropic. This anisotropy is induced by the anisotropy of the tortuosity which is given by a second order symmetric tensor. This is a new feature of the model while in earlier papers only isotropic permeabilities were considered. We show that this new model describes four modes of propagation. For our special choice of orientation of the direction of propagation these are two pseudo longitudinal modes P1 and P2, one pseudo transversal mode S2 and one transversal mode S1. The latter becomes also pseudo transversal in the general case of anisotropy. We analyze the speeds of propagation and the attenuation of these waves as well as the polarization properties in dependence on the orientation of the principal directions of the tortuosity.We indicate the practical importance of different shear (transversal)modes of propagation in a possible new nondestructive test of geophysical materials. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00161-011-0218-5
  • 2012 • 45 Adsorption of anionic-azo dye from aqueous solution by lignocellulose- biomass jute fiber: Equilibrium, kinetics, and thermodynamics study
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Adhikari, B. and Majumder, S.B.
    Industrial and Engineering Chemistry Research 51 12095-12106 (2012)
    The present investigation describes the evaluation of feasibility of lignocellulosic-biomass jute fiber (JF) toward adsorptive removal of anionic-azo dye from aqueous solution. Batch studies illustrated that dye uptake was highly dependent on different process variables, pH, initial dye concentration of solution, adsorbent dosage, and temperature. Further, an attempt has been taken to correlate these process variables with dye absorption and was optimized through a full-factorial central composite design (CCD) in response surface methodology (RSM). Maximum adsorption capacity (29.697 mg/g) under optimum conditions of variables (pH 3.91, adsorbent dose 2.04 g/L, adsorbate concentration 244.05 mg/L, and temperature 30 °C), as predicted by RSM, was found to be very close to the experimentally determined value (28.940 mg/g). Exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. Equilibrium adsorption data were highly consistent with Langmuir isotherm yielding R 2 = 0.999. Kinetic studies revealed that adsorption followed pseudo second-order model regarding the intraparticle diffusion. Activation parameters for the adsorption process were computed using Arrhenius and Eyring equations. Maximum desorption efficiency of spent adsorbent was achieved using sodium hydroxide solution (0.1 M). © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ie301708e
  • 2012 • 44 Advancing density functional theory to finite temperatures: Methods and applications in steel design
    Hickel, T. and Grabowski, B. and Körmann, F. and Neugebauer, J.
    Journal of Physics Condensed Matter 24 (2012)
    The performance of materials such as steels, their high strength and formability, is based on an impressive variety of competing mechanisms on the microscopic/atomic scale (e.g. dislocation gliding, solid solution hardening, mechanical twinning or structural phase transformations). Whereas many of the currently available concepts to describe these mechanisms are based on empirical and experimental data, it becomes more and more apparent that further improvement of materials needs to be based on a more fundamental level. Recent progress for methods based on density functional theory (DFT) now makes the exploration of chemical trends, the determination of parameters for phenomenological models and the identification of new routes for the optimization of steel properties feasible. A major challenge in applying these methods to a true materials design is, however, the inclusion of temperature-driven effects on the desired properties. Therefore, a large range of computational tools has been developed in order to improve the capability and accuracy of first-principles methods in determining free energies. These combine electronic, vibrational and magnetic effects as well as structural defects in an integrated approach. Based on these simulation tools, one is now able to successfully predict mechanical and thermodynamic properties of metals with a hitherto not achievable accuracy. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/24/5/053202
  • 2012 • 43 Efficient modeling of microstructure evolution in magnesium by energy minimization
    Homayonifar, M. and Mosler, J.
    International Journal of Plasticity 28 1-20 (2012)
    The description of the complex interplay between deformation-induced twinning and dislocation slip, typical for metals showing an hcp structure such as magnesium, is of utmost importance for understanding their deformation behavior. In the present paper, an incremental energy principle is presented for that purpose. Within this principle, dislocation slip is modeled by crystal plasticity theory, while the phase decomposition associated with twinning is considered by a mixture theory. This mixture theory naturally avoids the decomposition of the twinning process into so-called pseudo-dislocations followed by a reorientation of the total crystal. By way of contrast, the proposed model captures the transformation of the crystal lattice due to twinning in a continuous fashion by simultaneously taking dislocation slip within both, possibly co-existent, phases into account. The shear strain induced by twinning as well as the deformation history are consistently included within the twinned domain by an enhanced multiplicative decomposition of the deformation gradient. Kinematic compatibility between the different phases is enforced by a Hadamard-type compatibility condition, while compatibility with respect to the boundary conditions requires the introduction of a boundary layer. The evolution of all state variables such as the twinning volume and the plastic strains associated with dislocation slip follow jointly and conveniently from minimizing the stress power of the total crystal. This canonical variational principle is closely related to the postulate of maximum dissipation and guarantees thermodynamical consistency of the resulting model. Particularly, the second law of thermodynamics is fulfilled. In contrast to previous models suitable for the analysis of the deformation systems in magnesium, the Helmholtz energy of the twinning interfaces and that of the aforementioned boundary layer are considered. Analogously, the energy due to twinning nucleation and that related to twinning growth are accounted for by suitable dissipation functionals. By doing so, the number of twinning laminates becomes an additional unknown within the minimization principle and thus, the thickness of the lamellas can be computed. Interestingly, by interpreting this thickness as the mean free path of dislocations, a size effect of Hall-Petch-type can naturally be included within the novel model. The predictive capabilities of the resulting approach are finally demonstrated by analyzing the channel die test. For that purpose, a certain rank-two laminate structure is considered. However, it bears emphasis that the proposed framework is very general and consequently, it can also be applied to other materials. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijplas.2011.05.011
  • 2012 • 42 Epidermal growth factor receptor (EGFR) signaling and covalent EGFR inhibition in lung cancer
    Heuckmann, J.M. and Rauh, D. and Thomas, R.K.
    Journal of Clinical Oncology 30 3417-3420 (2012)
    doi: 10.1200/JCO.2012.43.1825
  • 2012 • 41 Exploring the thermodynamic derivatives of the structure factor of dense protein solutions
    Schroer, M.A. and Tolan, M. and Winter, R.
    Physical Chemistry Chemical Physics 14 9486-9491 (2012)
    Using small-angle X-ray scattering data of concentrated solutions of the protein lysozyme taken at different pressures and temperatures, the isothermal pressure derivative and the isobaric temperature derivative of the structure factor S(q) were determined. The pressure derivative of S(q) allows us to test various models for the triplet correlation function g 3. Significant differences were found in comparison to simple liquids reflecting the more complex interaction potential in dense protein solutions. © 2012 the Owner Societies.
    view abstractdoi: 10.1039/c2cp41041a
  • 2012 • 40 Influence of processing and heat treatment on corrosion resistance and properties of high alloyed steel coatings
    Hill, H. and Weber, S. and Raab, U. and Theisen, W. and Wagner, L.
    Journal of Thermal Spray Technology 21 987-994 (2012)
    Corrosion and abrasive wear are two important aspects to be considered in numerous engineering applications. Looking at steels, high-chromium high-carbon tool steels are proper and cost-efficient materials. They can either be put into service as bulk materials or used as comparatively thin coatings to protect lower alloyed construction or heat treatable steels from wear and corrosion. In this study, two different corrosion resistant tool steels were used for the production of coatings and bulk material. They were processed by thermal spraying and super solidus liquid phase sintering as both processes can generally be applied to produce coatings on low alloyed substrates. Thermally sprayed (high velocity oxygen fuel) coatings were investigated in the as-processed state, which is the most commonly used condition for technical applications, and after a quenching and tempering treatment. In comparison, sintered steels were analyzed in the quenched and tempered condition only. Significant influence of alloy chemistry, processing route, and heat treatment on tribological properties was found. Experimental investigations were supported by computational thermodynamics aiming at an improvement of tribological and corrosive resistance. © 2012 ASM International.
    view abstractdoi: 10.1007/s11666-012-9788-4
  • 2012 • 39 Pearlite revisited
    Steinbach, I. and Plapp, M.
    Continuum Mechanics and Thermodynamics 24 665-673 (2012)
    Zener's model of pearlite transformation in steels can be viewed as the prototype of many microstructure evolution models in materials science. It links principles of thermodynamics and kinetics to the scale of the microstructure. In addition it solves a very practical problem: How the hardness of steel is correlated to the conditions of processing. Although the model is well established since the 1950s, quantitative explanation of growth kinetics was missing until very recently. The present paper will shortly review the classical model of pearlite transformation. Zener's conjecture of maximum entropy production will be annotated by modern theoretical and experimental considerations of a band of stable (sometimes oscillating) states around the state of maximum entropy production. Finally, an explanation of the growth kinetics observed in experiments is proposed based on diffusion fluxes driven by stress gradients due to large transformation strain. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00161-011-0204-y
  • 2012 • 38 Phase-field model with finite interface dissipation: Extension to multi-component multi-phase alloys
    Zhang, L. and Steinbach, I.
    Acta Materialia 60 2702-2710 (2012)
    A previously developed phase-field model with finite interface dissipation for binary dual-phase alloys out of chemical equilibrium is generalized to a multi-component multi-phase model in the framework of the multi-phase-field formalism, allowing the description of multiple junctions with an arbitrary number of phases and components. In multiple junctions, each phase concentration is assigned to a dynamic equation to account for finite interface dissipation, and its formulation is proposed in two different models. The overall mass conservation between the phases of a multiple junction is used in model I, whereas the concentrations of each pair of phases have to be conserved during the transformations for model II. Both models demonstrate the decomposition of the nonlinear interactions between different phases into pairwise interaction of phases in multiple junctions. They converge to the same equilibrium state while in non-equilibrium states different predictions are given. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.02.032
  • 2012 • 37 SintClad: A new approach for the production of wear-resistant tools
    Blüm, M. and Hill, H. and Moll, H. and Weber, S. and Theisen, W.
    Journal of Materials Engineering and Performance 21 756-763 (2012)
    Tools used in the mineral processing industry are required to feature high wear resistance to facilitate an adequate cost efficiency. These kinds of tools are made of composite materials based on a low-alloyed substrate material and a high-alloyed coating. The coatings can be applied in different ways using production processes like HIP cladding, deposit welding, and composite casting. The article is concerned with the problem of a novel and cost-effective coating alternative: sinter cladding, using the principle of supersolidus liquid-phase sintering (SLPS). Usually SLPS represents a sintering technique, which is used for the compaction of high-alloyed metal powders. However, no recognizable efforts were made to use the SLPSprocess for applying a PM-coating on a bulk substrate material. Sinter cladding for the first time uses SLPS to combine the process of powder compaction with the application of a coating to a solid steel substrate into one single step. Another advantage of the process is the possibility to produce massive bulk coatings with thicknesses exceeding 20 mm. This article is original in the scope of question and investigation methods in terms of microstructure, hardness profiles, EDX measurements, diffusion calculations, and computational thermodynamics. © ASM International.
    view abstractdoi: 10.1007/s11665-012-0199-y
  • 2012 • 36 Thermodynamic properties of aqueous salt containing urea solutions
    Sadeghi, M. and Held, C. and Samieenasab, A. and Ghotbi, C. and Abdekhodaie, M.J. and Taghikhani, V. and Sadowski, G.
    Fluid Phase Equilibria 325 71-79 (2012)
    Urea and inorganic ions are present in some of the physiological systems, e.g. urine. Understanding the interactions in urea/salt/water is a preliminary step to shed light on more complicated behavior of multi-component physiological systems. State-of-the-art models as well as thermophysical properties can be applied to understand the interactions in these systems. In order to determine such interactions densities, mean ionic activity coefficients (MIACs), osmotic coefficients, and solubility were measured in aqueous solutions of urea and different salts. Densities were determined at temperatures 293.15, 303.15, and 313.15K for urea concentrations up to 3molal and up to 1molal for NaCl. Osmotic coefficients and MIACs were obtained at 310.15K by using the vapor-pressure osmometry and potentiometry methods, respectively. Ternary aqueous urea solutions containing NaCl, KCl, NaBr, KBr, LiBr, NaNO 3, and LiNO 3 at two different concentrations of urea (0.3 and 1molal) as well as at three salt concentrations (0.25, 0.5, and 0.75molal) were considered. Moreover, urea solubility was also measured at 310.15K in 3 and 5molal NaCl solutions in the present work. Experimental data obtained in this work showed that the salt primarily dictates the volumetric properties and the MIAC while the solute with higher concentration determines the behavior of osmotic coefficients in these solutions. The ePC-SAFT model (without any adjustable mixture parameter) was used to accurately predict the experimental densities, activity and osmotic coefficients, and solubilities of the studied mixtures. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2012.04.003
  • 2012 • 35 Thermo-fluid dynamic Time-of-Flight flow sensor system
    Ecin, O. and Zhao, R. and Hosticka, B.J. and Grabmaier, A.
    Proceedings of IEEE Sensors (2012)
    The authors report on a novel linear time-invariant (LTI) modeling of a flow sensor system based on thermal Time-of-Flight (TOF) principle by using pulsed hot wire anemometry. Thermal heat pulses are electrically generated at the hot wire centered in a pipe, carried along by the fluid in flow direction, and detected at several positions downstream. The flow sensor consisting of a hot wire and several temperature sensors is entirely regarded as an LTI system which is characterized by the fluid velocity, the fluid media and some heat transfer effects. Hence, a mathematical model with thermodynamic and fluid mechanical parameters is built. The thermo-fluid dynamic step and impulse responses of the hot wire are analyzed with respect to their signal parameters like rise and fall time and referred to the parameters of thermodynamics and fluid mechanics. The flow velocity and flow media are determined by signal analysis of the hot wire and by the thermo-fluid dynamic TOF measurement. © 2012 IEEE.
    view abstractdoi: 10.1109/ICSENS.2012.6411270
  • 2011 • 34 A density functional theory based estimation of the anharmonic contributions to the free energy of a polypeptide helix
    Ismer, L. and Ireta, J. and Neugebauer, J.
    Journal of Chemical Physics 135 (2011)
    We have employed density functional theory to determine the temperature dependence of the intrinsic stability of an infinite poly-L-alanine helix. The most relevant helix types, i.e., the - and the 310 - helix, and several unfolded conformations, which serve as reference for the stability analysis, have been included. For the calculation of the free energies for the various chain conformations we have explicitly included both, harmonic and anharmonic contributions. The latter have been calculated by means of a thermodynamic integration approach employing stochastic Langevin molecular dynamics, which is shown to provide a dramatic increase in the computational efficiency as compared to commonly employed deterministic molecular dynamics schemes. Employing this approach we demonstrate that the anharmonic part of the free energy amounts to the order of 0.1-0.4 kcal/mol per peptide unit for all analysed conformations. Although small, the anharmonic contribution stabilizes the helical conformations with respect to the fully extended structure. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3629451
  • 2011 • 33 A journey through 12 years of interacting molecules: From artificial amino acid receptors to the recognition of biomolecules and switchable nanomaterials
    Schmuck, C.
    Synlett 1798-1815 (2011)
    Research in supramolecular chemistry has been carried out in my laboratory for the past 12 years. Intrigued by the fascinating power of supramolecular chemistry, as seen in biomolecular recognition events in nature, we started out by trying to mimic the basic principles of such recognition events in small artificial model systems. Our first targets were amino acids and oligopeptides. We then moved on to proteins and nucleic acids, and we started to develop supramolecular systems that, besides simple binding, also featured functions, e.g. shutting down enzymes or allowing gene delivery into cells. In recent years and in a completely different yet related field, we have begun to develop self-assembling nanomaterials. The basic idea for this derived from an accidental discovery of the interesting self-assembling properties of a simple zwitterion, which was a synthetic intermediate on the route to our amino acid receptors. This personal account summarizes this journey and is intended not only to present the most-important findings from our laboratory so far, but also to shed light on how all these projects developed over the years and how our journey took us to where we are now. 1 Introduction 2 How It All Began 3 The Beginning: The Design of a New Oxoanion Binding Site 4 Tailor-Made Receptors for Small Peptides 5 Combinatorial Development of Peptide Receptors 6 From Small Peptides to Proteins 7 Nucleic Acids as Targets 8 Self-Assembling Zwitterions 9 pH-Switchable Nanostructures 10 Conclusion. © Georg Thieme Verlag Stuttgart . New York.
    view abstractdoi: 10.1055/s-0030-1260946
  • 2011 • 32 A thermodynamically and variationally consistent class of damage-type cohesive models
    Mosler, J. and Scheider, I.
    Journal of the Mechanics and Physics of Solids 59 1647-1668 (2011)
    A novel class of cohesive constitutive models suitable for the analysis of material separation such as that related to cracks, shear bands or delamination processes is presented. The proposed framework is based on a geometrically exact description (finite deformation) and it naturally accounts for material anisotropies. For that purpose, a Helmholtz energy depending on evolving structural tensors is introduced. In sharp contrast to previously published anisotropic cohesive models with finite strain kinematics based on a spatial description, all models belonging to the advocated class are thermodynamically consistent, i.e., they are rigorously derived by applying the Coleman and Noll procedure. Although this procedure seems nowadays to be standard for stressstrain-type constitutive laws, this is not the case for cohesive models at finite strains. An interesting new finding from the Coleman and Noll procedure is the striking analogy between cohesive models and boundary potential energies. This analogy gives rise to the introduction of additional stress tensors which can be interpreted as deformational surface shear. To the best knowledge of the authors, those stresses which are required for thermodynamical consistency at finite strains, have not been taken into account in existing models yet. Furthermore, the additional stress tensors can result in an effective traction-separation law showing a non-trivial stress-free configuration consistent with the underlying Helmholtz energy. This configuration is not predicted by previous models. Finally, the analogy between cohesive models and boundary potential energies leads to a unique definition of the controversially discussed fictitious intermediate configuration. More precisely, traction continuity requires that the interface geometry with respect to the deformed configuration has to be taken as the average of both sides. It will be shown that the novel class of interface models does not only fulfill the second law of thermodynamics, but also it shows an even stronger variational structure, i.e., the admissible states implied by the novel model can be interpreted as stable energy minimizers. This variational structure is used for deriving a variationally consistent numerical implementation. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2011.04.012
  • 2011 • 31 An experimental and kinetic modeling study of methyl formate low-pressure flames
    Dooley, S. and Dryer, F.L. and Yang, B. and Wang, J. and Cool, T.A. and Kasper, T. and Hansen, N.
    Combustion and Flame 158 732-741 (2011)
    The oxidation of methyl formate (CH3OCHO), the simplest methyl ester, is studied in a series of burner-stabilized laminar flames at pressures of 22-30Torr and equivalence ratios (Φ) from 1.0 to 1.8 for flame conditions of 25-35% fuel. Flame structures are determined by quantitative measurements of species mole fractions with flame-sampling molecular-beam synchrotron photoionization mass spectrometry (PIMS). Methyl formate is observed to be converted to methanol, formaldehyde and methane as major intermediate species of mechanistic relevance. Smaller amounts of ethylene and acetylene are also formed from methyl formate oxidation. Reactant, product and major intermediate species profiles are in good agreement with the computations of a recently developed kinetic model for methyl formate oxidation [S. Dooley, M.P. Burke, M. Chaos, Y. Stein, F.L. Dryer, V.P. Zhukov, O. Finch, J.M. Simmie, H.J. Curran, Int. J. Chem. Kinet. 42 (2010) 527-529] which shows that hydrogen abstraction reactions dominate fuel consumption under the tested flame conditions. Radical-radical reactions are shown to be significant in the formation of a number of small concentration intermediates, including the production of ethyl formate (C2H5OCHO), the subsequent decomposition of which is the major source of observed ethylene concentrations. The good agreement of model computations with this set of experimental data provides a further test of the predictive capabilities of the proposed mechanism of methyl formate oxidation. Other salient issues in the development of this model are discussed, including recent controversy regarding the methyl formate decomposition mechanism, and uncertainties in the experimental measurement and modeling of low-pressure flame-sampling experiments. Kinetic model computations show that worst-case disturbances to the measured temperature field, which may be caused by the insertion of the sampling cone into the flame, do not alter mechanistic conclusions provided by the kinetic model. However, such perturbations are shown to be responsible for disparities in species location between measurement and computation. © 2010 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2010.11.003
  • 2011 • 30 Corrosion properties of a plastic mould steel with special focus on the processing route
    Hill, H. and Huth, S. and Weber, S. and Theisen, W.
    Materials and Corrosion 62 436-443 (2011)
    Applications in plastics processing bear increased requirements for the used materials, especially with respect to their corrosion and wear resistance. For this reason, special powder metallurgical tools steels were developed that fulfil these demands. The common processing route for their production is hot isostatic pressing (HIP) of pre-alloyed powders which is followed by hot working if semi-finished parts are to be produced. As an alternative to HIP, super solidus liquid phase sintering (SLPS) permits the consolidation of pre-alloyed tool steel powders to near net-shape parts. It can be performed in different sintering atmospheres. In this work, the plastic mould steel X190CrVMo20-4 was processed by SLPS in vacuum as well as under nitrogen atmosphere. The resulting materials were analysed with respect to their microstructure, tempering behaviour and corrosion resistance in 0.5 molar sulphuric acid in dependence of the heat treatment. As a reference, the HIPed and the HIPed and worked state were also investigated. The results show that different heat treatments alter the ranking of the sintered and the HIPed state with respect to corrosion resistance. As expected, a high tempering for maximum secondary hardness causes a significant loss of corrosion resistance. The experimental findings were supported by thermodynamic calculations based on slight alterations in chemical composition that result from the different manufacturing processes. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.200905570
  • 2011 • 29 Critical investigation of high temperature gas nitriding of a PM tool steel
    Weber, S. and Theisen, W.
    International Journal of Materials Research 102 17-24 (2011)
    The nitrogen uptake of high-alloyed steels, especially tool steels, from gaseous atmospheres at elevated temperatures is a well known effect. Besides, for instance, the nitrogen-based case hardening, one application arises in the field of powder metallurgy. Here, introducing nitrogen containing atmospheres during sintering of high-alloyed powder metal-lurgically produced tool steels could lead to an optimization of the sintering process and an improvement of the materials properties. In this context, several works deal with the application of computational thermodynamics for considering the nitrogen uptake. The scope of this contribution is to investigate in detail the agreement of calculated and experimentally found nitrogen contents. Therefore, one typical gas-atomized powder of a ledeburitic cold work steel was sintered at a temperature of 1230 °C, varying the nitrogen partial pressure during sintering between 0.02 MPa and 0.2 MPa. The measured nitrogen contents are compared with values obtained from equilibrium calculations. Additionally, the distribution of nitrogen in the microstructure is investigated by time-resolved optical emission spectrometry and energy dispersive X-ray spectrometry measurements. The results exhibit good agreement between calculated and measured values for low partial pressures of less than 0.1 MPa and an increasing deviation for higher partial pressures. Furthermore, the transformation of vanadium-containing MC carbides to M(C, N) carbonitrides was verified. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110447
  • 2011 • 28 Development and characterization of novel corrosion-resistant TWIP steels
    Mujica, L. and Weber, S. and Hunold, G. and Theisen, W.
    Steel Research International 82 26-31 (2011)
    Austenitic steels exhibiting twinning induced plasticity (TWIP) are materials with exceptional mechanical properties. In this work, the development of new grades of TWIP steels exhibiting corrosion resistance is presented. The alloy development was supported by thermodynamic and diffusion calculations within the (Fe-Mn-Cr)-(C-N) alloy system. For the calculations ambient pressure and primary austenitic solidification were considered as necessary to avoid nitrogen degassing in all processing steps. Manganese is used as an austenite stabilizer, chromium to increase nitrogen solubility and provide corrosion resistance, while carbon and nitrogen provide mechanical strength. Diffusion calculations were used in order to predict the extent of micro segregations and additionally to evaluate the effect of diffusion annealing treatments. The material was cast in a laboratory scale with a nominal composition of Fe-20Mn-12Cr-0.25C-0.3N. Diffusion annealing was followed by hot rolling and solution annealing resulting in a fully austenitic microstructure. Tensile tests at room temperature were performed, exhibiting yield strengths of 430 MPa and elongation to fracture of 93%. In addition, not only the mechanical properties but also the weldability was studied, focussing on the characterization of the microstructure of bead on plate welds obtained by laser and TIG welding. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000219
  • 2011 • 27 Effects of counterpoise correction and basis set extrapolation on the MP2 geometries of hydrogen bonded dimers of ammonia, water, and hydrogen fluoride
    Boese, A.D. and Jansen, G. and Torheyden, M. and Höfener, S. and Klopper, W.
    Physical Chemistry Chemical Physics 13 1230-1238 (2011)
    We study the combined effects of counterpoise correction and basis set extrapolation on the second-order Møller-Plesset (MP2) geometries of three hydrogen bonded dimers, namely (NH3)2, (H 2O)2 and (HF)2. For (NH3) 2, we study three characteristic structures on its potential energy surface. In addition, we look at the basis set convergence when diffuse functions on the hydrogen atoms are left out, as well as the errors introduced by including core correlation with valence-only correlation-consistent basis sets. Overall, the counterpoise-corrected and extrapolated geometries appear to be very reliable and are in convincing agreement with the geometries from explicitly correlated MP2-F12 calculations. Obtaining geometries with errors of less than 0.001 Ångstrom and 0.5 degrees compared to the basis set limit is, however, even with these advanced methods a difficult task. © the Owner Societies 2011.
    view abstractdoi: 10.1039/c0cp01493a
  • 2011 • 26 From polariton condensates to highly photonic quantum degenerate states of bosonic matter
    Aßmann, M. and Tempel, J.-S. and Veit, F. and Bayer, M. and Rahimi-Iman, A. and Löffler, A. and Höfling, S. and Reitzenstein, S. and Worschech, L. and Forchel, A.
    Proceedings of the National Academy of Sciences of the United States of America 108 1804-1809 (2011)
    Bose-Einstein condensation (BEC) is a thermodynamic phase transition of an interacting Bose gas. Its key signatures are remarkable quantum effects like superfluidity and a phonon-like Bogoliubov excitation spectrum, which have been verified for atomic BECs. In the solid state, BEC of exciton-polaritons has been reported. Polaritons are strongly coupled light-matter quasiparticles in semiconductor microcavities and composite bosons. However, they are subject to dephasing and decay and need external pumping to reach a steady state. Accordingly the polariton BEC is a nonequilibrium process of a degenerate polariton gas in self-equilibrium, but out of equilibrium with the baths it is coupled to and therefore deviates from the thermodynamic phase transition seen in atomic BECs. Here we show that key signatures of BEC can even be observed without fulfilling the self-equilibrium condition in a highly photonic quantum degenerate nonequilibrium system.
    view abstractdoi: 10.1073/pnas.1009847108
  • 2011 • 25 Hydrogen storage properties of nanostructured MgH2/TiH 2 composite prepared by ball milling under high hydrogen pressure
    Shao, H. and Felderhoff, M. and Schüth, F.
    International Journal of Hydrogen Energy 36 10828-10833 (2011)
    Nanostructured MgH2/0.1TiH2 composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH2 from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van't Hoff equation. The values for the MgH2/0.1TiH2 nano-composite system are 77.4 kJ mol-1 H2 and 137.5 J K-1 mol-1 H2, respectively. These values are in agreement with those obtained for a commercial MgH2 system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.05.180
  • 2011 • 24 Mechanistic studies of Fc-PNA(·DNA) surface dynamics based on the kinetics of electron-transfer processes
    Hüsken, N. and Gȩbala, M. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    Chemistry - A European Journal 17 9678-9690 (2011)
    N-Terminally ferrocenylated and C-terminally gold-surface-grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(·DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron-transfer process, a nanoscopic view of the Fc-PNA(·DNA) surface dynamics was obtained. Loosely packed, surface-confined Fc-PNA single strands were found to render the charge-transfer process of the tethered Fc moiety diffusion-limited, whereas surfaces modified with Fc-PNA·DNA duplexes exhibited a charge-transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron-transfer rate constant, k 0. An in-depth understanding of the dynamics of surface-bound PNA and PNA·DNA strands is of utmost importance for the development of DNA biosensors using (Fc-)PNA recognition layers. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201003764
  • 2011 • 23 Molecular dynamics simulation study of microstructure evolution during cyclic martensitic transformations
    Kastner, O. and Eggeler, G. and Weiss, W. and Ackland, G.J.
    Journal of the Mechanics and Physics of Solids 59 1888-1908 (2011)
    Shape memory alloys (SMA) exhibit a number of features which are not easily explained by equilibrium thermodynamics, including hysteresis in the phase transformation and reverse shape memory in the high symmetry phase. Processing can change these features: repeated cycling can train the reverse shape memory effect, while changing the amount of hysteresis and other functional properties. These effects are likely to be due to formations of localised defects and these can be studied by atomistic methods. Here we present a molecular dynamics simulation study of such behaviour employing a two-dimensional, binary LennardJones model. Our atomistic model exhibits a symmetry breaking, displacive phase transition from a high temperature, entropically stabilised, austenite-like phase to a low temperature martensite-like phase. The simulations show transformations in this model material proceed by non-diffusive nucleation and growth processes and produce distinct microstructures. We observe the generation of persistent lattice defects during forward-and-reverse transformations which serve as nucleation centres in subsequent transformation processes. These defects interfere the temporal and spatial progression of transformations and thereby affect subsequent product morphologies. During cyclic transformations we observe accumulations of lattice defects so as to establish new microstructural elements which represent a memory of the previous morphologies. These new elements are self-organised and they provide a basis of the reversible shape memory effect in the model material. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmps.2011.05.009
  • 2011 • 22 Nucleation and precipitation kinetics of M23C6 and M2N in an Fe-Mn-Cr-C-N austenitic matrix and their relationship with the sensitization phenomenon
    Mújica Roncery, L. and Weber, S. and Theisen, W.
    Acta Materialia 59 6275-6286 (2011)
    This work addresses the austenite decomposition in Fe-20Mn-12Cr-0.24C-0.32N steel. Decomposition of austenite in C + N steels leads to the formation of M23C6 and M2N precipitates. Owing to the fact that decomposition is a temperature- and time-dependent phenomenon, thermodynamic and diffusion simulations of nucleation and growth of precipitates were performed based on the CALPHAD method. A well-known consequence of decomposition is sensitization of the areas surrounding the carbides and nitrides, which is normally associated with Cr depletion. In this study, the influence of element redistribution on the Gibbs free energy of the austenitic phase was analyzed. From the electrochemical equivalence of the difference in the Gibbs energy and the difference in the potential, it was found that the sensitized areas exhibit a less noble potential. A mechanism based on a sensitized anode and a matrix cathode is proposed and is correlated with experimental measures of intergranular corrosion. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.06.038
  • 2011 • 21 On the thermomechanical coupling in finite strain plasticity theory with non-linear kinematic hardening by means of incremental energy minimization
    Canadija, M. and Mosler, J.
    International Journal of Solids and Structures 48 1120-1129 (2011)
    The thermomechanical coupling in finite strain plasticity theory with non-linear kinematic hardening is analyzed within the present paper. This coupling is of utmost importance in many applications, e.g., in those showing low cycle fatigue (LCF) under large strain amplitudes. Since the by now classical thermomechanical coupling originally proposed by Taylor and Quinney cannot be used directly in case of kinematic hardening, the change in heat as a result of plastic deformation is computed by applying the first law of thermodynamics. Based on this balance law, together with a finite strain plasticity model, a novel variationally consistent method is elaborated. Within this method and following Stainier and Ortiz (2010), all unknown variables are jointly and conveniently computed by minimizing an incrementally defined potential. In sharp contrast to previously published works, the evolution equations are a priori enforced by employing a suitable parameterization of the flow rule and the evolution equations. The advantages of this parameterization are, at least, twofold. First, it leads eventually to an unconstrained stationarity problem which can be directly applied to any yield function being positively homogeneous of degree one, i.e., the approach shows a broad range of application. Secondly, the parameterization provides enough flexibility even for a broad range of non-associative models such as kinematic hardening of Armstrong-Frederick-type. Different to Stainier and Ortiz (2010), the continuous variational problem is approximated by a standard, fully-implicit time integration. The applicability of the resulting numerical implementation is finally demonstrated by analyzing the thermodynamically coupled response for a loading cycle. © 2011 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijsolstr.2010.12.018
  • 2011 • 20 Stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes
    Jakobi, J. and Menéndez-Manjón, A. and Chakravadhanula, V.S.K. and Kienle, L. and Wagener, P. and Barcikowski, S.
    Nanotechnology 22 (2011)
    Charged Pt-Ir alloy nanoparticles are generated through femtosecond laser ablation of a Pt9Ir target in acetone without using chemical precursors or stabilizing agents. Preservation of the target's stoichiometry in the colloidal nanoparticles is confirmed by transmission electron microscopy (TEM)-energy-dispersive x-ray spectroscopy (EDX), high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM)-EDX elemental maps, high resolution TEM and selected area electron diffraction (SAED) measurements. Results are discussed with reference to thermophysical properties and the phase diagram. The nanoparticles show a lognormal size distribution with a mean Feret particle size of 26nm. The zeta potential of - 45mV indicates high stability of the colloid with a hydrodynamic diameter of 63nm. The charge of the particles enables electrophoretic deposition of nanoparticles, creating nanoscale roughness on three-dimensional PtIr neural electrodes within a minute. In contrast to coating with Pt or Ir oxides, this method allows modification of the surface roughness without changing the chemical composition of PtIr. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/14/145601
  • 2011 • 19 The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer-Tropsch synthesis: A quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study
    Graf, B. and Muhler, M.
    Physical Chemistry Chemical Physics 13 3701-3710 (2011)
    The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H 2 at 303 K, whereas the presence of H 2 induced CO dissociation at higher temperatures leading to the formation of CH 4 and H 2O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO 2 formation according to C ads + 2O ads → CO 2(g). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol -1, whereas it increased to 110 kJ mol -1 on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol -1, which increased to 225 kJ mol -1 in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites. © the Owner Societies 2011.
    view abstractdoi: 10.1039/c0cp01875a
  • 2011 • 18 Thermodynamics and molecular dynamics investigation of a possible new critical size for surface and inner cohesive energy of Al nanoparticles
    Chamaani, A. and Marzbanrad, E. and Rahimipour, M. R. and Yaghmaee, M. S. and Aghaei, A. and Kamachali, R. D. and Behnamian, Y.
    Journal of Nanoparticle Research 13 6059--6067 (2011)
    In this study, the authors first review the previously developed, thermodynamics-based theory for size dependency of the cohesion energy of free-standing spherically shaped Al nanoparticles. Then, this model is extrapolated to the cubic and truncated octahedron Al nanoparticle shapes. A series of computations for Al nanoparticles with these two new shapes are presented for particles in the range of 1-100 nm. The thermodynamics computational results reveal that there is a second critical size around 1.62 and 1 nm for cubes and truncated octahedrons, respectively. Below this critical size, particles behave as if they consisted only of surface-energy-state atoms. A molecular dynamics simulation is used to verify this second critical size for Al nanoparticles in the range of 1-5 nm. MD simulation for cube and truncated octahedron shapes shows the second critical point to be around 1.63 and 1.14 nm, respectively. According to the modeling and simulation results, this second critical size seems to be a material property characteristic rather than a shape-dependent feature.
    view abstractdoi: 10.1007/s11051-011-0258-6
  • 2011 • 17 Thermodynamic modeling of solubility
    Rüther, F. and Sadowski, G.
    Chemie-Ingenieur-Technik 83 496-502 (2011)
    The knowledge of the solubility in pure solvents and solvent mixtures is crucial for designing the crystallization process. However, it is practically impossible to scan all possible solvent mixtures and mixture compositions experimentally. Therefore, a physically well-founded thermodynamic model which allows for solubility predictions in pure solvents and solvent mixtures based only on a small amount of experimental data is required. In this work, we demonstrated the applicability of the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state to model and to predict the solubility, especially of pharmaceuticals and complex (bio-) molecules, in pure solvents and solvent mixtures. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201000207
  • 2010 • 16 Ab initio study of thermodynamic, structural, and elastic properties of Mg-substituted crystalline calcite
    Elstnerová, P. and Friák, M. and Fabritius, H.O. and Lymperakis, L. and Hickel, T. and Petrov, M. and Nikolov, S. and Raabe, D. and Ziegler, A. and Hild, S. and Neugebauer, J.
    Acta Biomaterialia 6 4506-4512 (2010)
    Arthropoda, which represent nearly 80% of all known animal species, are protected by an exoskeleton formed by their cuticle. The cuticle represents a hierarchically structured multifunctional biocomposite based on chitin and proteins. Some groups, such as Crustacea, reinforce the load-bearing parts of their cuticle with calcite. As the calcite sometimes contains Mg it was speculated that Mg may have a stiffening impact on the mechanical properties of the cuticle (Becker et al., Dalton Trans. (2005) 1814). Motivated by these facts, we present a theoretical parameter-free quantum-mechanical study of the phase stability and structural and elastic properties of Mg-substituted calcite crystals. The Mg-substitutions were chosen as examples of states that occur in complex chemical environments typical for biological systems in which calcite crystals contain impurities, the role of which is still the topic of debate. Density functional theory calculations of bulk (Ca,Mg)CO3 were performed employing 30-atom supercells within the generalized gradient approximation as implemented in the Vienna Ab-initio Simulation Package. Based on the calculated thermodynamic results, low concentrations of Mg atoms are predicted to be stable in calcite crystals in agreement with experimental findings. Examining the structural characteristics, Mg additions nearly linearly reduce the volume of substituted crystals. The predicted elastic bulk modulus results reveal that the Mg substitution nearly linearly stiffens the calcite crystals. Due to the quite large size-mismatch of Mg and Ca atoms, Mg substitution results in local distortions such as off-planar tilting of the CO32- group. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actbio.2010.07.015
  • 2010 • 15 Compatible solutes: Thermodynamic properties and biological impact of ectoines and prolines
    Held, C. and Neuhaus, T. and Sadowski, G.
    Biophysical Chemistry 152 28-39 (2010)
    Compatible solutes like ectoine and its derivatives are deployed by halophile organisms as osmolytes to sustain the high salt concentration in the environment. This work investigates the relation of the thermodynamic properties of compatible solutes and their impact as osmolytes. The ectoines considered in this work are ectoine, hydroxyectoine, and homoectoine. Besides solution densities (15-45. °C) and solubilities in water (3-80. °C), component activity coefficients in the aqueous solutions were determined in the temperature range between 0 and 50. °C. The latter is important for adjusting a certain water activity and therewith a respective osmotic pressure within a cell. The characteristic effect of ectoines is compared to that of prolines, as well as to that of incompatible solutes as salts and urea. The experimental results show that the influence on the activity (coefficient) of water is quite different for compatible and incompatible solutes: whereas compatible solutes cause decreasing water activity coefficients, incompatible solutes lead to an increase in water activity coefficients. Based on this quantity, the paper discusses the impact of various osmolytes on biological systems and contributes to the explanation why some osmolytes are more often and at other temperatures used than others. Moreover, it was found that the anti-stress effect of an osmolyte is weakened in the presence of a salt.Finally, it is shown that the thermodynamic properties of compatible solutes can be modeled and even predicted using the thermodynamic model PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory). © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.bpc.2010.07.003
  • 2010 • 14 Comprehensive investigations of the supersolidus liquid-phase sintering of two plastic mold steels
    Hill, H. and Weber, S. and Siebert, S. and Huth, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 686-695 (2010)
    The processing of plastics, particularly reinforced composites, necessitates the use of corrosion- and wear-resistant materials for tools that come into contact with the polymer. For such applications, plastic mold steels were developed that offer not only a good wear resistance due to the presence of carbides in a martensitic matrix, but also good corrosion resistance provided primarily by a sufficient amount of dissolved chromium. The common processing route for these high-alloyed materials is the hot isostatic pressing (HIP) of gas-atomized powders (PM-HIP). In this context, sintering plays an insignificant role, except for the processing of metal-matrix composites (MMCs). The development of novel wear- and corrosion-resistant MMCs based on plastic mold steels requires knowledge of the sintering behavior of prealloyed powders of such tool steels. It is well known that alloyed powders can be processed by supersolidus liquid-phase sintering (SLPS), a method leading to almost full densification and to microstructures without significant coarsening effects. In this work, two different gas-atomized powders of plastic mold steels were investigated by computational thermodynamics, thermal analysis, sintering experiments, and microstructural characterization. The results show that both powders can be sintered to almost full density (1 to 3 pct porosity) by SLPS in a vacuum or a nitrogen atmosphere. Experimental findings on the densification behavior, nitrogen uptake, and carbide volume fractions are in good agreement with calculations performed by computational thermodynamics. © 2010 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-009-0148-z
  • 2010 • 13 Configurational forces and couples in fracture mechanics accounting for microstructures and dissipation
    Stumpf, H. and Makowski, J. and Hackl, K.
    International Journal of Solids and Structures 47 2380-2389 (2010)
    Configurational forces and couples acting on a dynamically evolving fracture process region as well as their balance are studied with special emphasis to microstructure and dissipation. To be able to investigate fracture process regions preceding cracks of mode I, II and III we choose as underlying continuum model the polar and micropolar, respectively, continuum with dislocation motion on the microlevel. As point of departure balance of macroforces, balance of couples and balance of microforces acting on dislocations are postulated. Taking into account results of the second law of thermodynamics the stress power principle for dissipative processes is derived. Applying this principle to a fracture process region evolving dynamically in the reference configuration with variable rotational and crystallographic structure, the configurational forces and couples are derived generalizing the well-known Eshelby tensor. It is shown that the balance law of configurational forces and couples reflects the structure of the postulated balance laws on macro- and microlevel: the balance law of configurational forces and configurational couples are coupled by field variable, while the balance laws of configurational macro- and microforces are coupled only by the form of the free energy. They can be decoupled by corresponding constitutive assumption. Finally, it is shown that the second law of thermodynamics leads to the result that the generalized Eshelby tensor for micropolar continua with dislocation motion consists of a non-dissipative part, derivable from free and kinetic energy, and a dissipative part, derivable from a dissipation pseudo-potential. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2010.04.032
  • 2010 • 12 Effects of Ti, Zr, and Hf on the phase stability of Mo_ss + Mo3Si + Mo5SiB2 alloys at 1600 °C
    Yang, Y. and Bei, H. and Chen, S. and George, E.P. and Tiley, J. and Chang, Y.A.
    Acta Materialia 58 541-548 (2010)
    Understanding the stability of the three-phase Mo_ss + Mo3Si + Mo5SiB2 region is important for alloy design of Mo-Si-B-based refractory metal intermetallic composites. In this work, thermodynamic modeling is coupled with guided experiments to study phase stability in this three-phase region of the Mo-Si-B-X (X = Ti, Zr, Hf) system. Both the calculated and experimental results show that additions of Zr and Hf limit significantly the stability of the three-phase region because of the formation of the ternary phases MoSiZr and MoSiHf, while Ti addition leads to a much larger region of stability for the three-phase equilibrium. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2009.09.032
  • 2010 • 11 Erratum: Chemistry and physical properties of the phosphide telluride Zr2PTe2 (Eur. J. Inorg. Chem. (2009) (3102-3110))
    Tschulik, K. and Ruck, M. and Binnewies, M. and Milke, E. and Hoffmann, S. and Schnelle, W. and Fokwa, B.P.T. and Gilleßen, M. and Schmidt, P.
    European Journal of Inorganic Chemistry 2201 (2010)
    doi: 10.1002/ejic.201000335
  • 2010 • 10 Excitation and relaxation of swift heavy ion irradiated dielectrics
    Osmani, O. and Medvedev, N. and Schleberger, M. and Rethfeld, B.
    e-Journal of Surface Science and Nanotechnology 8 278-282 (2010)
    The structural modifications of insulators after swift heavy ion irradiation are studied by applying a combination of the Monte-Carlo method (MC), used to describe the excitation and relaxation induced by the penetrating ion in the electronic subsystem, with the two temperature model (TTM) describing the temporal evolution of the lattice temperature heated by the hot electronic system. This MC-TTM combination demonstrates that secondary ionizations play a very important role for the track formation process and is capable to compute important material parameters which are often unaccessible by experiments. It is found that the secondary electron generation leads to an additional energy source in the heat diffusion equation related to energy storage in holes. © 2010 The Surface Science Society of Japan.
    view abstractdoi: 10.1380/ejssnt.2010.278
  • 2010 • 9 Formation, stability and crystal structure of the r phase in Mo-Re-Si alloys
    Bei, H. and Yang, Y. and Viswanathan, G.B. and Rawn, C.J. and George, E.P. and Tiley, J. and Chang, Y.A.
    Acta Materialia 58 6027-6034 (2010)
    The formation, stability and crystal structure of the σ phase in Mo-Re-Si alloys were investigated. Guided by thermodynamic calculations, six critically selected alloys were arc melted and annealed at 1600 °C for 150 h. Their as-cast and annealed microstructures, including phase fractions and distributions, the compositions of the constituent phases and the crystal structure of the r phase were analyzed by thermodynamic modeling coupled with experimental characterization by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. Two key findings resulted from this work. One is the large homogeneity range of the r phase region, extending from binary Mo-Re to ternary Mo-Re-Si. The other is the formation of a r phase in Mo-rich alloys either through the peritectic reaction of liquid + Moss → σ or primary solidification. These findings are important in understanding the effects of Re on the microstructure and providing guidance on the design of Mo-Re-Si alloys. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.020
  • 2010 • 8 Influence of Ni on martensitic phase transformations in NiTi shape memory alloys
    Frenzel, J. and George, E.P. and Dlouhy, A. and Somsen, C. and Wagner, M.F.-X. and Eggeler, G.
    Acta Materialia 58 3444-3458 (2010)
    High-precision data on phase transformation temperatures in NiTi, including numerical expressions for the effect of Ni on MS, MF, AS, AF and T0, are obtained, and the reasons for the large experimental scatter observed in previous studies are discussed. Clear experimental evidence is provided confirming the predictions of Tang et al. 1999 [19] regarding deviations from a linear relation between the thermodynamic equilibrium temperature and Ni concentration. In addition to affecting the phase transition temperatures, increasing Ni contents are found to decrease the width of thermal hysteresis and the heat of transformation. These findings are rationalized on the basis of the crystallographic data of Prokoshkin et al. 2004 [68] and the theory of Ball and James [25]. The results show that it is important to document carefully the details of the arc-melting procedure used to make shape memory alloys and that, if the effects of processing are properly accounted for, precise values for the Ni concentration of the NiTi matrix can be obtained. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.02.019
  • 2010 • 7 Multi-axial behavior of ferroelectrics with two types of micro-macro mechanical models
    Jayabal, K. and Arockiarajan, A. and Menzel, A. and Sivakumar, S.M.
    IUTAM Bookseries 19 95-102 (2010)
    Ferroelectric ceramics exhibit a significantly different nonlinear behavior with external electric and mechanical fields applied at angles to the initial poled direction. This angle dependent response of the ferroelectric polycrystals are predicted by two types of models based on irreversible thermodynamics and physics of domain switching. The first type is a uniaxial model dealing with simultaneous evolution of three variants at a given instant. The back stress and electric fields, assumed as linear functions of remnant strain and polarization developed by the domain switching process, are introduced in the model to assist or resist further switching process. The second type is a three dimensional model that considers all six variants of a tetragonal crystal in each grain and the dissipation associated with grain boundary constraints are brought into the model through switching criterion. The pressure dependent constraints imposed by the surrounding grains on the grain of interest at its boundary during domain switching process is correlated with the resistance experienced by a ferroelectric single crystal on its boundary during domain switching. Taking all the domain switching possibilities, the volume fractions of each of the variants are tracked and homogenized for macroscopic behavior. Numerical simulations were carried out for the behavior of ferroelectrics using both the models and the outcome was found to be qualitatively comparable with experimental observations given in literature. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/978-90-481-3771-8-10
  • 2010 • 6 Prediction of tautomer ratios by embedded-cluster integral equation theory
    Kast, S.M. and Heil, J. and Güssregen, S. and Schmidt, K.F.
    Journal of Computer-Aided Molecular Design 24 343-353 (2010)
    The "embedded cluster reference interaction site model" (EC-RISM) approach combines statistical-mechanical integral equation theory and quantum-chemical calculations for predicting thermodynamic data for chemical reactions in solution. The electronic structure of the solute is determined self-consistently with the structure of the solvent that is described by 3D RISM integral equation theory. The continuous solvent-site distribution is mapped onto a set of discrete background charges ("embedded cluster") that represent an additional contribution to the molecular Hamiltonian. The EC-RISM analysis of the SAMPL2 challenge set of tautomers proceeds in three stages. Firstly, the group of compounds for which quantitative experimental free energy data was provided was taken to determine appropriate levels of quantum-chemical theory for geometry optimization and free energy prediction. Secondly, the resulting workflow was applied to the full set, allowing for chemical interpretations of the results. Thirdly, disclosure of experimental data for parts of the compounds facilitated a detailed analysis of methodical issues and suggestions for future improvements of the model. Without specifically adjusting parameters, the EC-RISM model yields the smallest value of the root mean square error for the first set (0.6 kcal mol-1) as well as for the full set of quantitative reaction data (2.0 kcal mol-1) among the SAMPL2 participants. © 2010 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s10822-010-9340-x
  • 2010 • 5 The reweighted path ensemble
    Rogal, J. and Lechner, W. and Juraszek, J. and Ensing, B. and Bolhuis, P.G.
    Journal of Chemical Physics 133 (2010)
    We introduce a reweighting scheme for the path ensembles in the transition interface sampling framework. The reweighting allows for the analysis of free energy landscapes and committor projections in any collective variable space. We illustrate the reweighting scheme on a two dimensional potential with a nonlinear reaction coordinate and on a more realistic simulation of the Trp-cage folding process. We suggest that the reweighted path ensemble can be used to optimize possible nonlinear reaction coordinates. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3491817
  • 2010 • 4 Thermodynamic properties of cementite (Fe3 C)
    Hallstedt, B. and Djurovic, D. and von Appen, J. and Dronskowski, R. and Dick, A. and Körmann, F. and Hickel, T. and Neugebauer, J.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 34 129-133 (2010)
    Cementite (Fe3 C) is one of the most common phases in steel. In spite of its importance, thermodynamic investigations, either experimental or theoretical, of cementite are infrequent. In the present work, the thermodynamic properties of cementite are reevaluated and Gibbs energy functions valid from 0 K upwards presented. At high temperature (1000 K and above), the Gibbs energy is practically unchanged compared to previous evaluations. The energy of formation at 0 K was also calculated using density functional theory. This energy of formation (+8 kJ/mol at 0 K) is in reasonable agreement with the present thermodynamic evaluation (+23.5 kJ/mol at 0 K and +27.0 kJ/mol at 298.15 K) and with a solution calorimetric measurement of the enthalpy of formation (+18.8 kJ/mol at 298.15 K). In addition, the heat capacity was calculated theoretically using ab initio data combined with statistical concepts such as the quasiharmonic approximation. The theoretical calculation agrees equally well as the present evaluation with experimental data, but suggests a different weighting of the experimental data. In order to use it directly in the thermodynamic evaluation further modifications in the Fe-C system, primarily of the fcc phase, would be required in order to reproduce phase equilibrium data with sufficient accuracy. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.calphad.2010.01.004
  • 2010 • 3 Unique features of the folding landscape of a repeat protein revealed by pressure perturbation
    Rouget, J.-B. and Schroer, M.A. and Jeworrek, C. and Pühse, M. and Saldana, J.-L. and Bessin, Y. and Tolan, M. and Barrick, D. and Winter, R. and Royer, C.A.
    Biophysical Journal 98 2712-2721 (2010)
    The volumetric properties of proteins yield information about the changes in packing and hydration between various states along the folding reaction coordinate and are also intimately linked to the energetics and dynamics of these conformations. These volumetric characteristics can be accessed via pressure perturbation methods. In this work, we report high-pressure unfolding studies of the ankyrin domain of the Notch receptor (Nank1-7) using fluorescence, small-angle x-ray scattering, and Fourier transform infrared spectroscopy. Both equilibrium and pressure-jump kinetic fluorescence experiments were consistent with a simple two-state folding/unfolding transition under pressure, with a rather small volume change for unfolding compared to proteins of similar molecular weight. High-pressure fluorescence, Fourier transform infrared spectroscopy, and small-angle x-ray scattering measurements revealed that increasing urea over a very small range leads to a more expanded pressure unfolded state with a significant decrease in helical content. These observations underscore the conformational diversity of the unfolded-state basin. The temperature dependence of pressure-jump fluorescence relaxation measurements demonstrated that at low temperatures, the folding transition state ensemble (TSE) lies close in volume to the folded state, consistent with significant dehydration at the barrier. In contrast, the thermal expansivity of the TSE was found to be equivalent to that of the unfolded state, indicating that the interactions that constrain the folded-state thermal expansivity have not been established at the folding barrier. This behavior reveals a high degree of plasticity of the TSE of Nank1-7. © 2010 by the Biophysical Society.
    view abstractdoi: 10.1016/j.bpj.2010.02.044
  • 2006 • 2 Dependence of dissociation pressure upon doping level of Ti-doped sodium alanate - A possibility for "thermodynamic tailoring" of the system
    Streukens, G. and Bogdanović, B. and Felderhoff, M. and Schüth, F.
    Physical Chemistry Chemical Physics 8 2889-2892 (2006)
    Pressure-concentration isotherms have been recorded for NaAlH4 with different doping levels of titanium. It is well known that titanium accelerates the hydrogenation and dehydrogenation reactions in this system. Our studies have shown that the titanium doping also significantly alters the thermodynamics of the system, which is demonstrated by the change of the dissociation pressure with doping level. This can be explained by changes in the systems energy by dilution of the TiAl-alloy present after doping: Such an alloy forms as a result of the doping reaction. Aluminum generated during the dehydrogenation reaction dilutes this alloy, which gives an additional contribution to the free energy of the system. © the Owner Societies 2006.
    view abstractdoi: 10.1039/b603268k
  • 2005 • 1 One-step direct synthesis of a Ti-doped sodium alanate hydrogen storage material
    Von Colbe, J.M.B. and Felderhoff, M. and Bogdanović, B. and Schüth, F. and Weidenthaler, C.
    Chemical Communications 4732-4734 (2005)
    Titanium-doped NaAlH4 which has among the best reported hydrogen adsorption-desorption rates, has been synthesized in a one-step direct hydrogenation process under ball-milling in a special mill, allowing process parameters such as temperature and pressure to be recorded in situ. © The Royal Society of Chemistry 2005.
    view abstractdoi: 10.1039/b506502j