Dr. Michael Paulus

TU Dortmund University


  • Defect Creation in Surface-Mounted Metal-Organic Framework Thin Films
    Wang, Z. and Henke, S. and Paulus, M. and Welle, A. and Fan, Z. and Rodewald, K. and Rieger, B. and Fischer, R.A.
    ACS Applied Materials and Interfaces 12 (2020)
    Defect engineering is a strategy for tailoring the properties of metal-organic frameworks (MOFs). Plenty of efforts have been devoted to study the defect chemistry and structures of bulk MOFs; however, the reported example of a defect-engineered surface-mounted MOF (SURMOF) thin film is rare. In this work, defects were incorporated in SURMOF thin films by using defect-generating linkers and taking advantage of the liquid-phase stepwise epitaxial layer-by-layer growth (LBL). Two methods based on the LBL, named mixing method and alternating method, are proposed for incorporating defects in the prototypical SURMOF HKUST-1 by partially substituting the parent H3btc (benzene-1,3,5-tricarboxylic acid) linker with a set of defect-generating linkers H2ip (isophthalic acid), H2OH-ip (5-hydroxyisophthalic acid), and H2pydc (3,5-pyridinedicarboxylic acid). The crystallinity and phase purity of the obtained "defected" SURMOFs were confirmed by X-ray diffraction, infrared reflection absorption spectroscopy, and Raman spectroscopy. The incorporation of the defect-generating linkers and the types of induced defects were characterized by ultraviolet-visible spectroscopy, time-of-flight secondary ion mass spectrometry, methanol adsorption, scanning electron microscopy, and 1H nuclear magnetic resonance spectroscopy (after digestion of the samples). These two methods provide avenues for controlling the defect formation in MOF thin films. © 2019 American Chemical Society.
    view abstract10.1021/acsami.9b18672
  • Microstructural and Tribological Characteristics of Sn-Sb-Cu-Based Composite Coatings Deposited by Cold Spraying
    Tillmann, W. and Hagen, L. and Kensy, M.D. and Abdulgader, M. and Paulus, M.
    Journal of Thermal Spray Technology 29 (2020)
    Sn-based Babbitt coatings are widely used for sliding in hydrodynamic bearings. The Babbitting of bearing surfaces is among others accomplished by casting; however, this implies some disadvantages such as segregations, or susceptibility to shrinkage defects. Thermal spraying represents a promising method to overcome these challenges. To date, no studies on Babbitt coatings deposited by means of low-pressure cold spraying (LPCS) are available in the literature. In this study, a first attempt is made to produce a Sn-Sb-Cu-based composite coating reinforced with alumina particles by means of LPCS which enables the coating of internal diameters (IDs) of cylindrical components. A tailor-made feedstock was utilized which consists of a powder mixture of Sn, Sb, Cu and alumina. The composite coating is investigated with regard to its microstructural and tribological characteristics using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD), as well as dry sliding experiments. Metallographic investigations demonstrate the feasibility of depositing an alumina-reinforced Sn-Sb-Cu-based composite coating with a dense microstructure and low porosity. The composite coating mainly consists of Sn, SbSn, Cu and hexagonal CuSn. Despite a small fraction of alumina particles, the microhardness of the composite coating is primarily determined by the formation of SbSn intermetallic phases dispersed in the soft Sn-Sb-rich matrix. The composite coating possesses a coefficient of friction of 0.43 ± 0.01 and wear coefficient k of 17.27 ± 7.77 × 10−5 mm3 N m−1 sliding against a 100Cr6 counterbody. © 2020, ASM International.
    view abstract10.1007/s11666-020-01054-1
  • Microstructural characteristics in babbitt coatings deposited by LPCS
    Tillmann, W. and Hagen, L. and Abdulgader, M. and Kensy, M.D. and Paulus, M.
    Coatings 10 (2020)
    Studies have already established that themechanical properties of Babbitt coatings significantly depend on the microstructural characteristics, such as the amount and distribution of intermetallic compoundsdispersedin a soft solidsolutionmatrix. For Sn-Sb-Cu-based Babbitt coatings, the formation of SbSn-and CuSn-based precipitates has a substantial influence on the resulting microhardness and thus determines the maximum load carrying capacity. Thermal spraying of Sn-based Babbitt coatings results in a relatively more refined structure of these precipitates than in common manufacturing processes, such as casting, due to the thermal processing conditions. This study aims to evaluate the effect of the temperature of the propellant gas and substrate temperature on the microstructural characteristics of Sn-Sb-Cu-based Babbitt coatings deposited by low pressure cold spraying (LPCS). The deposits were examined for their phase composition, microhardness and mesoscopic structure. It was found that the coatings were mainly composed of Sb2Sn23, Sb0.49Sn0.51 and Sorosite (CuSn or CuSb0.115Sn0.835), regardless of the substrate temperature or temperature of the propellant gas to be investigated. For a gas temperature above 300 °C, an increased microhardness was observed, which correlates with the appearance of a more homogenous distribution of Sb0.49Sn0.51 dispersed in a soft Sn-rich solid solution matrix. © 2020 by the authors.
    view abstract10.3390/coatings10070689
  • Pressure stability of the first hydration shell of yttrium in aqueous YCl3 solution
    Elbers, M. and Sternemann, C. and Julius, K. and Paulus, M. and Surmeier, G. and König, N. and Nase, J. and Bolle, J. and Wagner, R. and Irifune, T. and Tolan, M.
    High Pressure Research (2020)
    The pressure stability of the first hydration shell of Y3+ ions in aqueous solution has been investigated by means of extended x-ray absorption fine-structure spectroscopy at the yttrium K-edge for hydrostatic pressures up to 4.5 kbar and concentrations between 1 M and 2.5 M. We find approximately 8.4 water molecules surrounding the yttrium cation at a mean distance of 2.370 Å at ambient conditions, independent on the concentration. The yttrium hydration shell has a low compressibility in the order of (-3.8 ± 0.7)×10-4 Å kbar-1 emphasizing its pressure stability in the kbar range. At the studied conditions, no indication for chloride complexation was observed. © 2020, © 2020 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/08957959.2020.1724998
  • The synergistic effect of heterostructured dissimilar metal-organic framework thin films on adsorption properties
    Wang, Z. and Wannapaiboon, S. and Henke, S. and Paulus, M. and Rodewald, K. and Rieger, B. and Fischer, R.A.
    Journal of Materials Chemistry A 8 (2020)
    A metal-organic framework (MOF) heterostructured thin film of 3D Cu3btc2 on 2D SURMOF-2 was developed for VOC adsorption. This heterostructured thin film shows higher VOC storage capacity than the two components and a counter-intuitive uptrend of adsorption ability (dimensionless normalized storage capacity) with increasing the size of VOCs. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c9ta10264g
  • Analyzing protein-ligand and protein-interface interactions using high pressure
    Levin, A. and Cinar, S. and Paulus, M. and Nase, J. and Winter, R. and Czeslik, C.
    Biophysical Chemistry 252 (2019)
    All protein function is based on interactions with the environment. Proteins can bind molecules for their transport, their catalytic conversion, or for signal transduction. They can bind to each other, and they adsorb at interfaces, such as lipid membranes or material surfaces. An experimental characterization is needed to understand the underlying mechanisms, but also to make use of proteins in biotechnology or biomedicine. When protein interactions are studied under high pressure, volume changes are revealed that directly describe spatial contributions to these interactions. Moreover, the strength of protein interactions with ligands or interfaces can be tuned in a smooth way by pressure modulation, which can be utilized in the design of drugs and bio-responsive interfaces. In this short review, selected studies of protein-ligand and protein-interface interactions are presented that were carried out under high pressure. Furthermore, a perspective on bio-responsive interfaces is given where protein-ligand binding is applied to create functional interfacial structures. © 2019 Elsevier B.V.
    view abstract10.1016/j.bpc.2019.106194
  • Cholesterol modulates the pressure response of DMPC membranes
    Surmeier, G. and Paulus, M. and Salmen, P. and Dogan, S. and Sternemann, C. and Nase, J.
    Biophysical Chemistry 252 (2019)
    In this work, the effect of cholesterol on the pressure response of solid-supported phospholipid multilayers is analyzed. It is shown that DMPC multilayers become highly pressure-responsive by the incorporation of low amounts of cholesterol, resulting in a strong pressure-induced expansion of the bilayer spacing. This is accompanied by a high tendency of the multilayer system to detach from the substrate. Increasing the cholesterol concentration reduces the pressure-induced expansion and the membrane structure remains largely unchanged upon pressurization, consequently the stability of the multilayers improves. For a determination of the influence of the substrate, the pressure-dependent behavior of multilayers is compared to that of solid-supported bilayers and multi-lamellar vesicles in bulk solution. While single-supported bilayers remain largely unaffected by external pressure independent of their cholesterol content, multi-lamellar vesicles and multilayers behave similarly. © 2019 Elsevier B.V.
    view abstract10.1016/j.bpc.2019.106210
  • Control of structural flexibility of layered-pillared metal-organic frameworks anchored at surfaces
    Wannapaiboon, S. and Schneemann, A. and Hante, I. and Tu, M. and Epp, K. and Semrau, A.L. and Sternemann, C. and Paulus, M. and Baxter, S.J. and Kieslich, G. and Fischer, R.A.
    Nature Communications 10 (2019)
    Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors. © 2019, The Author(s).
    view abstract10.1038/s41467-018-08285-5
  • Density variations of TMAO solutions in the kilobar range: Experiments, PC-SAFT predictions, and molecular dynamics simulations
    Knierbein, M. and Held, C. and Hölzl, C. and Horinek, D. and Paulus, M. and Sadowski, G. and Sternemann, C. and Nase, J.
    Biophysical Chemistry 253 (2019)
    We present measurements, molecular dynamics (MD) simulations, and predictions using Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) of the density of aqueous solutions in a pressure range from 1 bar to 5000 bar, a pressure regime that is highly relevant for both biochemical applications and the fundamental understanding of solvation. The accurate determination of density data of pressurized solutions remains challenging. We determined relative density changes from the variations in X-ray absorption through the sample and developed a new water parameter set for PC-SAFT modeling that is appropriate for high pressure conditions in the kilobar regime. As a showcase, we studied trimethylamine N-oxide (TMAO) solutions and demonstrated that their compressibility decreases with the TMAO content. This result is linked to the stabilizing effect of TMAO on the local H-bond network of water. Experiments and calculations, which represent two independent methods, are in very good agreement and are in accordance with results of force field molecular dynamics simulations of the same systems. © 2019 Elsevier B.V.
    view abstract10.1016/j.bpc.2019.106222
  • Impact of Macromolecular Crowding and Compression on Protein-Protein Interactions and Liquid-Liquid Phase Separation Phenomena
    Julius, K. and Weine, J. and Gao, M. and Latarius, J. and Elbers, M. and Paulus, M. and Tolan, M. and Winter, R.
    Macromolecules 52 (2019)
    We determined the intermolecular interaction potential, V(r), of dense lysozyme solutions, which governs the spatial distribution of the protein molecules and the location of its liquid-liquid phase separation (LLPS) region, in various crowding environments applying small-angle X-ray scattering in combination with liquid-state theory. We explored the effect of polyethylene glycol (PEG) on V(r) and the protein's phase behavior over a wide range of temperatures and pressures, crossing from the dilute to the semidilute polymer regime, thereby mimicking all crowding scenarios encountered in the heterogeneous biological cell. V(r) and hence the protein-protein distances and the phase boundary of the LLPS region strongly depend on the polymer-to-protein size ratio and the polymer concentration. The strongest effect is observed for small-sized PEG molecules, leading to a marked decrease of the mean intermolecular spacing of the protein molecules with increasing crowder concentration. The effect levels off at intermolecular distances where the proteins' second hydration shells start to penetrate each other. Strong repulsive forces like hydration-shell repulsion and/or soft enthalpic protein-PEG interactions must be operative at short distances which stabilize the protein against depletion-induced aggregation, also at pressures as high as encountered in the deep sea, where pressures up to the kbar-level are encountered. © 2019 American Chemical Society.
    view abstract10.1021/acs.macromol.8b02476
  • Influence of etching-pretreatment on nano-grained WC-Co surfaces and properties of PVD/HVOF duplex coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Krabiell, M. and Schröder, P. and Tiller, J. and Krumm, C. and Sternemann, C. and Paulus, M. and Elbers, M.
    Surface and Coatings Technology 374 (2019)
    The deposition of coatings by means of Physical Vapor Deposition (PVD)is an established process to enhance the lifetime and performance of carbide bulk tools. Although the effect of surface pretreatments on conventional WC-Co surfaces is well known, this investigation examines for the first time, how different surface pretreatments affect the surface integrity of thermally sprayed WC-Co substrates prior a subsequent PVD layer deposition and its resulting properties. Therefore, a WC-12Co feedstock with an average WC particle size of 100 nm was thermally sprayed on AISI M3 steel substrates using High Velocity Oxy-Fuel (HVOF)technique. Hereinafter, the HVOF sprayed WC-Co coatings were grounded and polished, thus serving as substrates for further surface pretreatments and the deposition of a CrAlN PVD hard coating by means of magnetron sputtering. To evaluate the influence of various surface pretreatments on the HVOF sprayed WC-Co coatings, several sequences such as heating, inert gas ion etching, metal ion etching, and High Power Impulse Magnetron Sputtering (HIPIMS)-etching were carried out. With respect to the subsequent PVD layer deposition, the results show that the pretreatment does neither affect the hardness nor Young's modulus of the CrAlN top layer. Yet, different effects on the WC-Co surface and PVD coating adhesion are observed. Inert gas ion etching leads to a faster removal of the carbides than of the Co-binder matrix. In contrast, metal ion etching provides a “micro-blasting” effect and removes the binder matrix as verified by Atomic Force Microscope (AFM)measurements. As a result, a decrease of the compressive residual stress state and an increase of the surface free energy are observed. With respect to HIPIMS-etching, a Cr-nanolayer was applied onto the WC-Co surface, which enhances the adhesion of the CrAlN top layer. Nevertheless, HRC Rockwell adhesion and scratch tests reveal a superior adhesion for samples pretreated with the metal ion etching. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.05.054
  • Microstructural characteristics of high-feed milled HVOF sprayed WC-Co coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Paulus, M. and Tolan, M. and Sakrowski, R. and Biermann, D. and Freiburg, D.
    Surface and Coatings Technology 374 (2019)
    Over the last decade, great efforts have been undertaken in science and industry to provide WC-Co feedstock with nano-sized WC particles that significantly improves tribo-mechanical coating properties. For tribologically stressed surfaces, superior surface characteristics can be achieved by applying tailored surfaces, using novel technologies in the field of production engineering such as High-Feed Milling (HFM). For the first time, textured surface patterns were produced onto HVOF sprayed WC-Co coatings with nano-sized WC particles by implementing a HFM post process. In dependence to two different textures resulting from the HFM, the microstructural characteristics of the produced surfaces are analyzed. Confocal microscopy revealed the machinability of textured patterns onto a HVOF-sprayed WC-12Co hard coating, which comprise of nano-sized WC particles, by means of HFM technology. X-ray diffraction analyses confirmed the insertion of macro- and micro-scale residual stresses. The experiments showed a significant insertion of compressive residual stresses transverse to the feed direction, whereas the insertion of compresses residual stresses with its feed direction being less pronounced. It was found that the HFM post process leads to a refinement of the WC crystallite size and a distinct increase of its internal strain, which both can be attributed to plastic deformations during HFM. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.06.012
  • New Approach to Structure–Property Correlations of Different Films of Sorbitan Esters and Their Self-Assembly into Viscoelastic Monolayers
    Demand, S. and Egger, S. and Degen, P. and Salmen, P. and Paulus, M. and Tolan, M. and Rehage, H.
    Journal of Surfactants and Detergents 22 (2019)
    This publication is focused on the structural origin of viscoelasticity in Langmuir monolayers. To improve the understanding of the structural origin of viscoelasticity of surfactant films, we systematically studied interfacial films of different sorbitan esters with saturated (Span 60 and 65) and unsaturated (Span 80 and 85) paraffin chains by means of surface rheology, Langmuir isotherms, X-ray reflectometry (XRR), and Brewster angle microscopy (BAM). The results of two-dimensional shear rheological measurements revealed the existence of temporarily cross-linked networks. In dynamic BAM experiments, we observed a swinging motion of the monolayers as a result of a sudden externally initiated mechanical perturbation. The viscoelastic film response, which relaxed with time as the external force vanished, could be traced back to the presence of foam-like supramolecular structures that interlinked solid-condensed domains. The temperature dependence of the elastic response implied that the solid domains decomposed at temperatures close to the bulk melting point of Span 60 and Span 65. We concluded that insoluble surfactants formed solid domains at the interface, which were linked with each other by nonsolid areas, giving viscoelastic films. These newly discovered insights into coherent film formations could provide new opportunities for designing mechanically stable surfactant interfaces. © 2019 AOCS
    view abstract10.1002/jsde.12261
  • Structure and mechanical properties of hafnium nitride films deposited by direct current, mid-frequency, and high-power impulse magnetron sputtering
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Tolan, M. and Paulus, M.
    Thin Solid Films 669 (2019)
    The structural properties of hafnium nitride films are mainly influenced by the deposition conditions, which are affected by the sputtering technique. A suitable use of the different sputtering modes allows to control the structural development of the films and thus to adjust the profile of the properties. NaCl-type hafnium nitride films were deposited using direct current magnetron sputtering (dcMS), mid-frequency magnetron sputtering (mfMS), and high-power impulse magnetron sputtering (HiPIMS). dcMS produces films with a columnar microstructure, whereas a fully-dense morphology is achieved by mfMS and HiPIMS. X-ray diffraction patterns show that films sputtered in dcMS mode have a (200) orientation, whereas mfMS and HiPIMS favor an orientation with the (111) plane parallel to the samples’ surface. mfMS leads to films with the largest crystal sizes and lowest stresses, which is ascribed to recrystallization mechanisms during the film growth. Hafnium films with an overstoichiometric composition show the highest hardness values. In this context, the dcMS-Hf49.8N50.2, mfMS-Hf50.4N49.6, and HiPIMS-Hf49.0N51.0 have a hardness of 28.2 ± 2.1, 32.4 ± 3.4, and 30.4 ± 3.1 GPa, respectively. In summary, the sputtering technique has a crucial role on the properties of the film and can be suitable used to adjust the structure and hardness of HfN films. © 2018 Elsevier B.V.
    view abstract10.1016/j.tsf.2018.10.035
  • 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 abstract10.1016/j.bpc.2019.106260
  • Surfactant-mediated formation of alginate layers at the water-air interface
    Degen, P. and Paulus, M. and Zwar, E. and Jakobi, V. and Dogan, S. and Tolan, M. and Rehage, H.
    Surface and Interface Analysis 51 (2019)
    The self-organization process of polysaccharide alginate with different cationic surfactants at the water-air interface was investigated over a wide concentration regime. The changes of surface properties determined by surface tension measurements, surface rheology, and X-ray reflectivity are correlated with changes of bulk properties measured by turbidity, light scattering, and zeta potential measurements. We demonstrate that the interactions between the alginate and cationic surfactants result in significant changes of bulk and interfacial properties. The results of surface shear experiments point to the existence of highly viscoelastic interfacial films. In combination with X-ray reflectivity, we demonstrate that these rheological features are related to polymer-surfactant associations at the interface. In the regime of high surfactant concentrations, we observed the existence of multilayer structures. © 2019 The Authors. Surface and Interface Analysis published by John Wiley & Sons Ltd
    view abstract10.1002/sia.6691
  • Temperature-induced formation of lubricous oxides in vanadium containing iron-based arc sprayed coatings
    Tillmann, W. and Hagen, L. and Kokalj, D. and Paulus, M. and Tolan, M.
    Coatings 9 (2019)
    In the field of surface engineering, the use of self-lubricous coatings with the incorporation of vanadium represent a promising approach to reduce friction, thus contributing to the wear behavior. For vanadium containing hard coatings produced by means of thin film technology, the reduction in friction at elevated temperatures was repeatedly attributed to temperature-induced and tribo-oxidatively formed oxides which act as solid lubricant. Only very few studies focused on the tribological characteristics of vanadium containing arc sprayed coatings. In this study, the tribological characteristics of a vanadium containing iron-based arc sprayed deposit were investigated in dry sliding experiments under ambient conditions and different temperatures. Types of wear at the worn surfaces and counterparts were examined by means of electron microscopy and energy dispersive X-ray (EDX) spectroscopy. The speciation of vanadium in the superficial layer was determined using X-ray absorption near edge structure (XANES) spectroscopy. It was found that the vanadium-containing coating exhibited a distinctly reduction of the coefficient of friction above 450 °C which further decreased with increasing temperature. XANES spectroscopy indicated an increased oxidation state for the V component on the coating surface, suggesting the prevalence of specific vanadium oxides which promote a self-lubricating ability of the coating. © 2018 by the authors.
    view abstract10.3390/coatings9010018
  • Adsorption Behavior of Lysozyme at Titanium Oxide-Water Interfaces
    Forov, Y. and Paulus, M. and Dogan, S. and Salmen, P. and Weis, C. and Gahlmann, T. and Behrendt, A. and Albers, C. and Elbers, M. and Schnettger, W. and Egger, S. and Zwar, E. and Rehage, H. and Kiesel, I. and Riedl, T. and Tolan, M.
    Langmuir 34 (2018)
    We present an in situ X-ray reflectivity study of the adsorption behavior of the protein lysozyme on titanium oxide layers under variation of different thermodynamic parameters, such as temperature, hydrostatic pressure, and pH value. Moreover, by varying the layer thickness of the titanium oxide layer on a silicon wafer, changes in the adsorption behavior of lysozyme were studied. In total, we determined less adsorption on titanium oxide compared with silicon dioxide, while increasing the titanium oxide layer thickness causes stronger adsorption. Furthermore, the variation of temperature from 20 to 80 °C yields an increase in the amount of adsorbed lysozyme at the interface. Additional measurements with variation of the pH value of the system in a region between pH 2 and 12 show that the surface charge of both protein and titanium oxide has a crucial role in the adsorption process. Further pressure-dependent experiments between 50 and 5000 bar show a reduction of the amount of adsorbed lysozyme with increasing pressure. © 2018 American Chemical Society.
    view abstract10.1021/acs.langmuir.8b00280
  • Different Breathing Mechanisms in Flexible Pillared-Layered Metal-Organic Frameworks: Impact of the Metal Center
    Schneemann, A. and Vervoorts, P. and Hante, I. and Tu, M. and Wannapaiboon, S. and Sternemann, C. and Paulus, M. and Wieland, D.C.F. and Henke, S. and Fischer, R.A.
    Chemistry of Materials 30 (2018)
    The pillared-layered metal-organic framework compounds M2(BME-bdc)2(dabco) (M2+ = Zn2+, Co2+, Ni2+, Cu2+; BME-bdc2- = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M2+. This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO2 pressure reveal different mechanisms for the np to lp phase transition during CO2 adsorption. © 2018 American Chemical Society.
    view abstract10.1021/acs.chemmater.7b05052
  • Human Apolipoprotein A1 at Solid/Liquid and Liquid/Gas Interfaces
    Dogan, S. and Paulus, M. and Forov, Y. and Weis, C. and Kampmann, M. and Cewe, C. and Kiesel, I. and Degen, P. and Salmen, P. and Rehage, H. and Tolan, M.
    Journal of Physical Chemistry B 122 (2018)
    An X-ray reflectivity study on the adsorption behavior of human apolipoprotein A1 (apoA1) at hydrophilic and hydrophobic interfaces is presented. It is shown that the protein interacts via electrostatic and hydrophobic interactions with the interfaces, resulting in the absorption of the protein. pH dependent measurements at the solid/liquid interface between silicon dioxide and aqueous protein solution show that in a small pH range between pH 4 and 6, adsorption is increased due to electrostatic attraction. Here, the native shape of the protein seems to be conserved. In contrast, the adsorption at the liquid/gas interface is mainly driven by hydrophobic effects, presumably by extending the hydrophobic regions of the amphipathic helices, and results in a conformational change of the protein during adsorption. However, the addition of differently charged membrane-forming lipids at the liquid/gas interface illustrates the ability of apoA1 to include lipids, resulting in a depletion of the lipids from the interface. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcb.7b12481
  • Investigation on the oxidation behavior of AlCrVxN thin films by means of synchrotron radiation and influence on the high temperature friction
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Applied Surface Science 427 (2018)
    Friction minimization is an important topic which is pursued in research and industry. In addition to the use of lubricants, friction-reducing oxide phases can be utilized which occur during. These oxides are called Magnéli phases and especially vanadium oxides exhibit good friction reducing properties. Thereby, the lubrication effect can be traced back to oxygen deficiencies. AlCrN thin films are being used as coatings for tools which have to withstand high temperatures. A further improvement of AlCrN thin films concerning their friction properties is possible by incorporation of vanadium. This study analyzes the temperature dependent oxidation behavior of magnetron sputtered AlCrVN thin films with different vanadium contents up to 13.5 at.-% by means of X-ray diffraction and X-ray absorption near-edge spectroscopy. Up to 400 °C the coatings show no oxidation. A higher temperature of 700 °C leads to an oxidation and formation of Magnéli phases of the coatings with vanadium contents above 10.7 at.-%. Friction coefficients, measured by ball-on-disk test are correlated with the oxide formation in order to figure out the effect of vanadium oxides. At 700 °C a decrease of the friction coefficient with increasing vanadium content can be observed, due to the formation of VO2, V2O3 and the Magnéli phase V4O7. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.09.029
  • The effects of osmolytes and crowding on the pressure-induced dissociation and inactivation of dimeric LADH
    Julius, K. and Al-Ayoubi, S.R. and Paulus, M. and Tolan, M. and Winter, R.
    Physical Chemistry Chemical Physics 20 (2018)
    Investigating the correlation between structure and activity of oligomeric enzymes at high pressure is essential for understanding intermolecular interactions and reactivity of proteins in cellulo of organisms thriving at extreme environmental conditions as well as for biotechnological applications, such as high-pressure enzymology. In a combined experimental effort employing small-angle X-ray scattering, FT-IR and fluorescence spectroscopy as well as stopped-flow enzyme kinetics in concert with high-pressure techniques, we reveal the pressure-induced conformational changes of the dimeric enzyme horse liver alcohol dehydrogenase (LADH) on the quaternary, secondary and tertiary structural level. Moreover, the effects of cosolutes and crowding agents, mimicking intracellular conditions, have been addressed. Our results show that beyond an increase of enzymatic activity at low pressures, loss of enzyme activity occurs around 600-800 bar, i.e. in a pressure regime where small conformational changes take place in the coenzyme's binding pocket, only. Whereas higher-order oligomers dissociate at low pressures, subunit dissociation of dimeric LADH takes place, depending on the solution conditions, between 2000 and 4000 bar, only. Oligomerization and subunit dissociation are modulated by cosolvents such as urea or trimethylamine-N-oxide as well as by the crowding agent polyethylene glycol, based on their tendency to bind to the protein's interface or act via their excluded volume effect, respectively. © the Owner Societies 2018.
    view abstract10.1039/c7cp08242h
  • Water-Mediated Protein-Protein Interactions at High Pressures are Controlled by a Deep-Sea Osmolyte
    Julius, K. and Weine, J. and Berghaus, M. and König, N. and Gao, M. and Latarius, J. and Paulus, M. and Schroer, M.A. and Tolan, M. and Winter, R.
    Physical Review Letters 121 (2018)
    The influence of natural cosolvent mixtures on the pressure-dependent structure and protein-protein interaction potential of dense protein solutions is studied and analyzed using small-angle X-ray scattering in combination with a liquid-state theoretical approach. The deep-sea osmolyte trimethylamine-N-oxide is shown to play a crucial and singular role in its ability to not only guarantee sustainability of the native protein's folded state under harsh environmental conditions, but it also controls water-mediated intermolecular interactions at high pressure, thereby preventing contact formation and hence aggregation of proteins. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.038101
  • A Study on the Tribological Behavior of Vanadium-Doped Arc Sprayed Coatings
    Tillmann, W. and Hagen, L. and Kokalj, D. and Paulus, M. and Tolan, M.
    Journal of Thermal Spray Technology 26 (2017)
    The formation of thin reactive films in sliding contacts under elevated temperature provides enhanced tribological properties since the formation of Magnéli phases leads to the ability of self-lubricating behavior. This phenomenon was studied for vanadium-doped coating systems which were produced using CVD and PVD technology. Vanadium-containing arc sprayed coatings were not widely examined so far. The aim of this study was to characterize Fe-V coatings deposited by the Twin Wire Arc Spraying process with respect to their oxidation behavior at elevated temperatures and to correlate the formation of oxides to the tribological properties. Dry sliding experiments were performed in the temperature range between 25 and 750 °C. The Fe-V coating possesses a reduced coefficient of friction and wear coefficient (k) at 650 and 750 °C, which were significant lower when compared to conventional Fe-based coatings. The evolution of oxide phases was identified in situ by x-ray diffraction for the investigated temperature range. Further oxidation of (pre-oxidized) arc sprayed Fe-V coatings, as verified by differential thermal analysis and thermo-gravimetric analysis, starts at about 500 °C. © 2017 ASM International
    view abstract10.1007/s11666-017-0524-y
  • Antibodies under pressure: A Small-Angle X-ray Scattering study of Immunoglobulin G under high hydrostatic pressure
    König, N. and Paulus, M. and Julius, K. and Schulze, J. and Voetz, M. and Tolan, M.
    Biophysical Chemistry 231 (2017)
    In the present work two subclasses of the human antibody Immunoglobulin G (IgG) have been investigated by Small-Angle X-ray Scattering under high hydrostatic pressures up to 5kbar. It is shown that IgG adopts a symmetric T-shape in solution which differs significantly from available crystal structures. Moreover, high-pressure experiments verify the high stability of the IgG molecule. It is not unfolded by hydrostatic pressures of up to 5kbar but a slight increase of the radius of gyration was observed at elevated pressures. © 2017 Elsevier B.V.
    view abstract10.1016/j.bpc.2017.05.016
  • Compositional fingerprint of soy sauces via hydrophobic surface interaction
    Jakobi, V. and Salmen, P. and Paulus, M. and Tolan, M. and Rosenhahn, A.
    Food Chemistry 218 (2017)
    In this work, the interaction of soy sauces with hydrophobic surfaces has been analyzed. Hydrophobic self-assembled monolayers on gold or silicon dioxide were used to harvest conditioning layers from soy sauce products with varying amounts of additives. The data was compared to adsorption of soy protein and glutamic acid as common ingredients. Spectral ellipsometry revealed that all tested sauces led to the formation of thin overlayers on hydrophobic surfaces. Products with less additives yielded adlayers in the same thickness range as pure soy protein. In contrast, sauces with more ingredients create distinctly thicker films. Using water contact angle goniometry, it is shown that all adlayers render the substrate more hydrophilic. Infrared spectroscopy provided a deeper insight into the adlayer chemistry and revealed that the adlayer composition is dominated by protein rich components. X-ray reflectivity on selected films provided further insight into the density profiles within the adlayers on the molecular scale. © 2016 Elsevier Ltd
    view abstract10.1016/j.foodchem.2016.09.045
  • Influence of bidisperse self-assembled monolayer structure on the slip boundary condition of thin polymer films
    McGraw, J.D. and Klos, M. and Bridet, A. and Hähl, H. and Paulus, M. and Castillo, J.M. and Horsch, M. and Jacobs, K.
    Journal of Chemical Physics 146 (2017)
    Alkylsilane self-assembled monolayers (SAMs) are often used as model substrates for their ease of preparation and hydrophobic properties. We have observed that these atomically smooth monolayers also provide a slip boundary condition for dewetting films composed of unentangled polymers. This slip length, an indirect measure of the friction between a given liquid and different solids, is switchable and can be increased [R. Fetzer et al., Phys. Rev. Lett. 95, 127801 (2005); O. Bäumchen et al., J. Phys.: Condens. Matter 24, 325102 (2012)] if the alkyl chain length is changed from 18 to 12 backbone carbons, for example. Typically, this change in boundary condition is affected in a quantized way, using one or the other alkyl chain length, thus obtaining one or the other slip length. Here, we present results in which this SAM structure is changed in a continuous way. We prepare bidisperse mixed SAMs of alkyl silanes, with the composition as a control parameter. We find that all the mixed SAMs investigated show an enhanced slip boundary condition as compared to the single-component SAMs. The slip boundary condition is accessed using optical and atomic force microscopy, and we describe these observations in the context of X-ray reflectivity measurements. The slip length, varying over nearly two orders of magnitude, of identical polymer melts on chemically similar SAMs is found to correlate with the density of exposed alkyl chains. Our results demonstrate the importance of a well characterized solid/liquid pair, down to the angstrom level, when discussing the friction between a liquid and a solid. © 2017 Author(s).
    view abstract10.1063/1.4978676
  • Investigation of the influence of the vanadium content on the high temperature tribo-mechanical properties of DC magnetron sputtered AlCrVN thin films
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Surface and Coatings Technology 328 (2017)
    The forming of high-strength steels or new aluminum alloys leads to a steady increase of the load of tools and coatings. One approach is to positively influence the manufacturing process by using thin solid films with self-lubricating features, provided by oxides at high temperatures with low decohesion energies. For the purpose of this study, AlCrN provides the matrix, while vanadium oxides are used to enhance the frictional and wear properties. However, it is not yet clear which minimum amount of vanadium has to be incorporated in DC magnetron sputtered AlCrN coatings to improve the tribological behavior. Therefore, in this study, AlCrVN coatings are synthesized with an increasing vanadium content by means of reactive DC magnetron sputtering. Additionally, a vanadium-free AlCrN coating is used as reference for the tribo-mechanical investigations. The coatings were synthesized up to a vanadium content of 13.5 at.-% and no phase change could be detected by means of x-ray diffraction. Moreover, no hexagonal AlN phase, which reduces the mechanical properties and the oxidation resistance, was formed. In contrast to the vanadium-free coating, the hardness of the coatings containing vanadium is slightly reduced. The coating with the smallest vanadium content shows the highest hardness of all analyzed coatings. A heat treatment at 400 °C does not lead to any significant changes with respect to mechanical properties, but at 700 °C hardness, modulus of elasticity and critical load decreased for all coatings, indicating a significant change in mechanical properties. The ball-on-disc test at room-temperature, 400 °C, and 700 °C shows the highest wear coefficient for the coating with the lowest vanadium content, due to the poor adhesion of the coating, although this coating shows the highest H/E-ratio. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.08.046
  • The Hydrophobic Gap at High Hydrostatic Pressures
    Wirkert, F.J. and Hölzl, C. and Paulus, M. and Salmen, P. and Tolan, M. and Horinek, D. and Nase, J.
    Angewandte Chemie - International Edition 56 (2017)
    We have gained new insight into the so-called hydrophobic gap, a molecularly thin region of decreased electron density at the interface between water and a solid hydrophobic surface, by X-ray reflectivity experiments and molecular dynamics simulations at different hydrostatic pressures. Pressure variations show that the hydrophobic gap persists up to a pressure of 5 kbar. The electron depletion in the interfacial region strongly decreases with an increase in pressure, indicating that the interfacial region is compressed more strongly than bulk water. The decrease is most significant up to 2 kbar; beyond that, the pressure response of the depletion is less pronounced. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201706662
  • Near-Surface and Bulk Behavior of Bicontinuous Microemulsions under High-Pressure Conditions
    Berghaus, M. and Paulus, M. and Salmen, P. and Al-Ayoubi, S. and Tolan, M. and Winter, R.
    Journal of Physical Chemistry B 120 (2016)
    The effect of hydrostatic pressure on the structure of a bicontinuous microemulsion in the presence of a solid interface has been studied by X-ray reflectometry and compared to the bulk behavior determined by small-angle X-ray scattering. Surface-induced lamellar ordering is observed close to the hydrophilic interface, which persists upon compression. The lamellar domains are compressed, but the correlation length of lamellar order does not change with pressure. SAXS measurements on the bulk microemulsion revealed an increased order upon pressurization. Although pressure can cause the formation of highly ordered lamellar phases from ordered bicontinuous cubic phases, such a scenario is not observed for the disordered analogue studied here. High pressure increases the stiffness of the interfacial surfactant layer, but this is not sufficient to overcome the loss in conformational entropy that would result from a transition to an ordered lamellar phase. Possible technological and biological implications of our results are briefly discussed. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcb.6b05639
  • Phase behavior of lysozyme solutions in the liquid-liquid phase coexistence region at high hydrostatic pressures
    Schulze, J. and Möller, J. and Weine, J. and Julius, K. and König, N. and Nase, J. and Paulus, M. and Tolan, M. and Winter, R.
    Physical Chemistry Chemical Physics 18 (2016)
    We present results from small-angle X-ray scattering and turbidity measurements on the effect of high hydrostatic pressure on the phase behavior of dense lysozyme solutions in the liquid-liquid phase separation region, and characterize the underlying intermolecular protein-protein interactions as a function of temperature and pressure under charge-screening conditions (0.5 M NaCl). A reentrant liquid-liquid phase separation region is observed at elevated pressures, which may originate in the pressure dependence of the solvent-mediated protein-protein interaction. A temperature-pressure-concentration phase diagram was constructed for highly concentrated lysozyme solutions over a wide range of temperatures, pressures and protein concentrations including the critical region of the liquid-liquid miscibility gap. © the Owner Societies 2016.
    view abstract10.1039/c6cp01791f
  • Polaron-induced lattice distortion of (In,Ga)As/GaAs quantum dots by optically excited carriers
    Tiemeyer, S. and Bombeck, M. and Göhring, H. and Paulus, M. and Sternemann, C. and Nase, J. and Wirkert, F.J. and Möller, J. and Büning, T. and Seeck, O.H. and Reuter, D. and Wieck, A.D. and Bayer, M. and Tolan, M.
    Nanotechnology 27 (2016)
    We report on a high resolution x-ray diffraction study unveiling the effect of carriers optically injected into (In,Ga)As quantum dots on the surrounding GaAs crystal matrix. We find a tetragonal lattice expansion with enhanced elongation along the [001] crystal axis that is superimposed on an isotropic lattice extension. The isotropic contribution arises from excitation induced lattice heating as confirmed by temperature dependent reference studies. The tetragonal expansion on the femtometer scale is tentatively attributed to polaron formation by carriers trapped in the quantum dots. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/42/425702
  • Solid-Supported Lipid Multilayers under High Hydrostatic Pressure
    Nowak, B. and Paulus, M. and Nase, J. and Salmen, P. and Degen, P. and Wirkert, F.J. and Honkimäki, V. and Tolan, M.
    Langmuir 32 (2016)
    In this work, the structure of solid-supported lipid multilayers exposed to increased hydrostatic pressure was studied in situ by X-ray reflectometry at the solid-liquid interface between silicon and an aqueous buffer solution. The layers' vertical structure was analyzed up to a maximum pressure of 4500 bar. The multilayers showed phase transitions from the fluid into different gel phases. With increasing pressure, a gradual filling of the sublayers between the hydrophilic head groups with water was observed. This process was inverted when the pressure was decreased, yielding finally smaller water layers than those in the initial state. As is commonly known, water has an abrasive effect on lipid multilayers by the formation of vesicles. We show that increasing pressure can reverse this process so that a controlled switching between multi- and bilayers is possible. © 2016 American Chemical Society.
    view abstract10.1021/acs.langmuir.5b04554
  • The detachment behavior of polycarbonate on thin films above the glass transition temperature
    Tillmann, W. and Hagen, L. and Hoffmann, F. and Dildrop, M. and Wibbeke, A. and Schöppner, V. and Resonnek, V. and Pohl, M. and Krumm, C. and Tiller, J.C. and Paulus, M. and Sternemann, C.
    Polymer Engineering and Science 56 (2016)
    When producing mono-axially stretched films made of amorphous polycarbonate, a self-reinforcement is generated due to the stretching process. This leads to an increase of the strength and stiffness. The mono-axial stretching process is conducted at temperatures above the glass transition temperature, whereas better mechanical properties are obtained at higher stretching temperatures. However, the film tends to adhere to the rolls, especially at temperatures from 10°C above the glass transition temperature. The rolls of the mono-axial stretching unit are made of an induction hardened and polished quenched and tempered steel 1.7225 – 42CrMo4. This work reports on the investigation of the detachment behavior of polycarbonate on different coatings as a function of the temperature and contact time. The main intention is to find a suitable coating on which the polycarbonate film adheres only slightly at temperatures clearly exceeding the glass transition temperature. POLYM. ENG. SCI., 56:786–797, 2016. © 2016 Society of Plastics Engineers. © 2016 Society of Plastics Engineers
    view abstract10.1002/pen.24307
  • The interaction between stearic acid monolayers and butane under elevated pressures
    Giebel, F. and Paulus, M. and Nase, J. and Kiesel, I. and Bieder, S. and Tolan, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 504 (2016)
    A study on the interaction between butane and lipid membranes is presented. Monomolecular films of stearic acid were prepared on a water surface and were exposed to dense gas phases of n-butane and isobutane. From X-ray reflectivity measurements and grazing incidence X-ray diffraction, the accumulation of gas molecules at the liquid-gas interface was analyzed. We show that the gas molecules penetrate into the lipid membrane and accumulate between the head group and tail group of the lipids. This process goes in hand with an increase of the long-range lateral crystallographic order within the lipid film. This and the formation of multilayer islands points to an expulsion of lipids by the gas molecules. At higher pressures close to the condensation pressure of the studied gases, the vertical and lateral order is lost, indicating an adsorbed liquid film in which the lipids are dissolved. © 2016 Elsevier B.V.
    view abstract10.1016/j.colsurfa.2016.05.067
  • X-ray diffraction (XRD)-studies on the temperature dependent interface reactions on hafnium, zirconium, and nickel coated monocrystalline diamonds used in grinding segments for stone and concrete machining
    Tillmann, W. and Tolan, M. and Pinho Ferreira, M. and Paulus, M. and Becke, M. and Stangier, D.
    Materialwissenschaft und Werkstofftechnik 47 (2016)
    Diamond impregnated metal matrix composites are the state of the art solution for the machining of mineral materials. The type of interface reactions between the metal matrix and diamond surface has an essential influence on the tool performance and durability. To improve the diamond retention, the diamonds can be coated by physical vapour deposition with metallic materials, which enforce interface reactions. Hence, this paper focuses on the investigation of the interfacial area on metal-coated monocrystalline diamonds. Hafnium and zirconium, both known as carbide forming elements, are used as coating materials. The third coating, which is used to determine its catalytic influences when applied as a physical vapour deposition (PVD)-layer, is nickel. Additionally, the coated diamond samples were heat-treated to investigate the starting point of the formation of new phases. X-ray diffraction-analyses revealed the assumed carbide formation on hafnium and zirconium coated samples. The formation temperature was identified between 800 °C and 1000 °C for hafnium and zirconium coatings. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/mawe.201600713
  • Salt induced reduction of lysozyme adsorption at charged interfaces
    Göhring, H. and Paulus, M. and Salmen, P. and Wirkert, F. and Kruse, T. and Degen, P. and Stuhr, S. and Rehage, H. and Tolan, M.
    Journal of Physics Condensed Matter 27 (2015)
    A study of lysozyme adsorption below a behenic acid membrane and at the solid-liquid interface between aqueous lysozyme solution and a silicon wafer in the presence of sodium chloride is presented. The salt concentration was varied between 1 mmol L-1 and 1000 mmol L-1. X-ray reflectivity data show a clear dependence of the protein adsorption on the salt concentration. Increasing salt concentrations result in a decreased protein adsorption at the interface until a complete suppression at high concentrations is reached. This effect can be attributed to a reduced attractive electrostatic interaction between the positively charged proteins and negatively charged surfaces by charge screening. The measurements at the solid-liquid interfaces show a transition from unoriented order of lysozyme in the adsorbed film to an oriented order with the short protein axis perpendicular to the solid-liquid interface with rising salt concentration. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/27/23/235103
  • Self-assembled silane monolayers: An efficient step-by-step recipe for high-quality, low energy surfaces
    Lessel, M. and Bäumchen, O. and Klos, M. and Hähl, H. and Fetzer, R. and Paulus, M. and Seemann, R. and Jacobs, K.
    Surface and Interface Analysis 47 (2015)
    Organosilane self-assembled monolayers (SAMs) are commonly used for modifying a wide range of substrates. Depending on the end group, highly hydrophobic or hydrophilic surfaces can be achieved. Silanization bases on the adsorption, self-assembly and covalent binding of silane molecules onto surfaces and results in a densely packed, SAM. Following wet chemical routines, the quality of the monolayer is often variable and, therefore, unsatisfactory. The process of self-assembly is not only affected by the chemicals involved and their purity but is also extremely sensitive to ambient parameters such as humidity or temperature and to contaminants. Here, a reliable and efficient wet-chemical recipe is presented for the preparation of ultra-smooth, highly ordered alkyl-terminated silane SAMs on Si wafers. The resulting surfaces are characterized by means of atomic force microscopy, X-ray reflectometry and contact angle measurements. Copyright © 2015-John Wiley & Sons, Ltd.
    view abstract10.1002/sia.5729
  • About the role of surfactants on the magnetic control over liquid interfaces
    Degen, P. and Zwar, E. and Paulus, M. and Tolan, M. and Rehage, H.
    Langmuir 30 (2014)
    The behavior of magnetically responsive aqueous Fe(III) surfactant solutions at liquid interfaces is analyzed. Such surfactants attracted much attention, because of the ability to manipulate interfaces by magnetic fields without any use of magnetic nanoparticles. A detailed analysis of the surface properties proves that the mixing of paramagnetic electrolyte solution with anionic, cationic and nonionic surfactants yields the similar magnetic response and no effect of the surfactant charge can be observed. We conclude that the observed magnetic shiftability of interfaces is caused by a combination of the paramagnetic behavior of the bulk liquid and a reduction of the surface tension. Thus, this work gives an alternative interpretation of the properties of 'magnetic surfactants' compared to the ones claimed in the literature. © 2014 American Chemical Society.
    view abstract10.1021/la503142q
  • PH controlled condensation of polysiloxane networks at the water-air interface
    Wieland, D.C.F. and Degen, P. and Paulus, M. and Schroer, M.A. and Rehage, H. and Tolan, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 455 (2014)
    Structural and mechanical properties of molecularly thick polysiloxane membranes were studied on different liquid subphases to investigate the impact of the subphase's pH value on the cross-linking process. The lateral structure of these films was studied in-situ by grazing incidence diffraction while torsions pendulum experiments reveal the response of the system to mechanical stress. The results show a hindered cross-linking on acidic subphases. At alkaline and neutral pH conditions the cross-linking process was not effected. The data revealed that the degree of polymerization can be tuned by regulating the subphase's pH value, which opens the opportunity to build complex polysiloxane membranes in a controlled manner. © 2014 Elsevier B.V.
    view abstract10.1016/j.colsurfa.2014.03.099
  • Reentrant liquid-liquid phase separation in protein solutions at elevated hydrostatic pressures
    Möller, J. and Grobelny, S. and Schulze, J. and Bieder, S. and Steffen, A. and Erlkamp, M. and Paulus, M. and Tolan, M. and Winter, R.
    Physical Review Letters 112 (2014)
    We present results from small-angle x-ray scattering data on the effect of high pressure on the phase behavior of dense lysozyme solutions in the liquid-liquid phase separation region, and characterize the underlying intermolecular protein-protein interactions as a function of temperature and pressure in this region of phase space. A reentrant liquid-liquid phase separation region has been discovered at elevated pressures, which originates in the pressure dependence of the solvent-mediated protein-protein interactions. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.112.028101
  • Specific anion effects on the pressure dependence of the protein-protein interaction potential
    Möller, J. and Grobelny, S. and Schulze, J. and Steffen, A. and Bieder, S. and Paulus, M. and Tolan, M. and Winter, R.
    Physical Chemistry Chemical Physics 16 (2014)
    We present a study on ion specific effects on the intermolecular interaction potential V(r) of dense protein solutions under high hydrostatic pressure conditions. Small-angle X-ray scattering in combination with a liquid-state theoretical approach was used to determine the effect of structure breaking/making salt anions (Cl-, SO4 2-, PO4 3-) on the intermolecular interaction of lysozyme molecules. It was found that besides the Debye-Hückel charge screening effect, reducing the repulsiveness of the interaction potential V(r) at low salt concentrations, a specific ion effect is observed at high salt concentrations for the multivalent kosmotropic anions, which modulates also the pressure dependence of the protein-protein interaction potential. Whereas sulfate and phosphate strongly influence the pressure dependence of V(r), chloride anions do not. The strong structure-making effect of the multivalent anions, dominating for the triply charged PO4 3-, renders the solution structure less bulk-water-like at high salt concentrations, which leads to an altered behavior of the pressure dependence of V(r). Hence, the particular structural properties of the salt solutions are able to influence the spatial organization and the intermolecular interactions of the proteins, in particular upon compression. These results are of interest for exploring the combined effects of ionic strength, temperature and pressure on the phase behavior of protein solutions, but may also be of relevance for understanding pressure effects on the hydration behavior of biological matter under extreme environmental conditions. This journal is © the Partner Organisations 2014.
    view abstract10.1039/c3cp55278k
  • Temperature-driven adsorption and desorption of proteins at solid-liquid interfaces
    Kiesel, I. and Paulus, M. and Nase, J. and Tiemeyer, S. and Sternemann, C. and Rüster, K. and Wirkert, F.J. and Mende, K. and Büning, T. and Tolan, M.
    Langmuir 30 (2014)
    The heat-induced desorption and adsorption of the proteins lysozyme, ribonuclease A, bovine serum albumin, and fibronectin at protein layers was investigated in two different environments: pure buffer and protein solution. Using two different environments allows us to distinguish between thermodynamic and kinetic mechanisms in the adsorption process. We observed a desorption in buffer and an adsorption in protein solution, depending upon protein properties, such as size, stability, and charge. We conclude that the desorption in buffer is mainly influenced by the mobility of the proteins at the interface, while the adsorption in protein solution is driven by conformational changes and, thereby, a gain in entropy. These results are relevant for controlling biofilm formation at solid-liquid interfaces. © 2014 American Chemical Society.
    view abstract10.1021/la404884a
  • The adsorption behavior of octafluoropropane at the water/gas interface
    Giebel, F. and Paulus, M. and Nase, J. and Bieder, S. and Kiesel, I. and Tolan, M.
    Journal of Applied Physics 116 (2014)
    We studied the adsorption behavior of the gas octafluoropropane at the water/gas interface as a function of different pressures. In a custom-made measurement cell, the gas pressure was varied in a range between 1 bar and close to the condensation pressure of octafluoropropane. The electron density profiles of the adsorption layers show that the layer thickness increases with pressure. The evolution of the layer electron density indicates that the bulk electron density is reached if a layer consisting of more than one monolayer of octafluoropropane is adsorbed on the water surface. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4902961
  • X-ray reflectivity measurements of liquid/solid interfaces under high hydrostatic pressure conditions
    Wirkert, F.J. and Paulus, M. and Nase, J. and Möller, J. and Kujawski, S. and Sternemann, C. and Tolan, M.
    Journal of Synchrotron Radiation 21 (2014)
    A high-pressure cell for in situ X-ray reflectivity measurements of liquid/solid interfaces at hydrostatic pressures up to 500 MPa (5 kbar), a pressure regime that is particularly important for the study of protein unfolding, is presented. The original set-up of this hydrostatic high-pressure cell is discussed and its unique properties are demonstrated by the investigation of pressure-induced adsorption of the protein lysozyme onto hydrophobic silicon wafers. The presented results emphasize the enormous potential of X-ray reflectivity studies under high hydrostatic pressure conditions for the in situ investigation of adsorption phenomena in biological systems.© 2014 International Union of Crystallography.
    view abstract10.1107/S1600577513021516
  • Carbon reactivity of binder metals in diamond-metal composites - Characterization by scanning electron microscopy and X-ray diffraction
    Tillmann, W. and Ferreira, M. and Steffen, A. and Rüster, K. and Möller, J. and Bieder, S. and Paulus, M. and Tolan, M.
    Diamond and Related Materials 38 (2013)
    Diamond tooling is a successfully used technique in machining of very hard materials such as minerals and concrete. The type and strength of bonding between the diamond grains, that are mainly responsible for the machining process (e.g. cutting or grinding), and the metallic binder phase is directly linked to the tools quality. Therefore it is of interest to investigate the carbon reactivity of commonly used binder materials. This paper reports about the investigation of the interfacial area between diamonds and one-component metallic binder matrices. As matrix material pure chromium, cobalt, copper, iron, and nickel was used. After the sintering process the diamonds were extracted from the metallic matrix and analyzed by scanning electron microscopy and X-ray diffraction. The morphology of the diamond surface was investigated and a phase analysis was done. These experimental studies support the hypothesis that the carbon reactivity of transition metals is linked to their d-orbital electron configuration. © 2013 Elsevier B.V.
    view abstract10.1016/j.diamond.2013.07.002
  • CsNaC2, CsKC2, CsRbC2 -syntheses and crystal structures of three new acetylides
    Liebig, S. and Paulus, M. and Sternemann, C. and Ruschewitz, U.
    Zeitschrift fur Anorganische und Allgemeine Chemie 639 (2013)
    Three new alkali metal acetylides CsNaC2, CsKC2, and CsRbC2 have been synthesized and characterized by means of synchrotron powder diffraction studies. As a new synthetic approach, the binary alkali metal acetylides were reacted at relatively low temperatures (200 °C). DSC measurements were performed to prove the general usability of this reaction. CsKC2 and CsRbC2 crystallize in a variant of the anti-PbCl2-type structure (Pnma, Z = 4), while for CsNaC2 a new structure type (Pbcm, Z = 4) is found. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/zaac.201300147
  • Formation of iron containing aggregates at the liquid-air interface
    Wieland, D.C.F. and Degen, P. and Paulus, M. and Schroer, M.A. and Bieder, S. and Sahle, C.J. and Möller, J. and Leick, S. and Chen, Z. and Struth, B. and Rehage, H. and Tolan, M.
    Colloids and Surfaces B: Biointerfaces 109 (2013)
    The early stages of the formation of inorganic aggregates, composed of iron compounds at the solution-air interface, were investigated in situ. The properties of the solution-air interface were changed by using different Langmuir layers. In order to get insight into the evolution of the sample system in situ, the processes were studied by X-ray scattering and spectroscopy techniques. The formation of aggregates was detected under cationic as well as under anionic Langmuir layers. The observed compounds lack long range order which indicates the formation of amorphous structures. This is supported by extended X-ray absorption fine structure measurements showing only minor order in the formed aggregates. © 2013 Elsevier B.V.
    view abstract10.1016/j.colsurfb.2013.03.006
  • Iron oxide film growth under ultrathin polysiloxane networks
    Degen, P. and Wieland, D.C.F. and Paulus, M. and Schroer, M.A. and Tolan, M. and Rehage, H.
    Colloid and Polymer Science 291 (2013)
    This study focuses on the preparation and characterization of magnetic switchable thin iron oxide-polymer films. In a series of experiments, the formation and growth of iron oxide under ultrathin polysiloxane layers was controlled by changing the concentration of iron ions in the aqueous subphase or by varying the residence time of ammonia in the gas phase above the liquid sample. The growth of the combined film structures is studied in situ by interfacial rheology, optical microscopy, and x-ray scattering experiments and ex situ by scanning electron microscopy. Different stages of iron oxide aggregation, from a very thin layer of amorphous iron oxide with thickness of a few nanometers up to micrometer thick coatings of crystalline maghemite (γ-Fe2O3) were investigated. The specific interactions between the inorganic iron oxide and the polymer membranes cause the creation of new composite materials which are sensitive to magnetic forces. © 2012 Springer-Verlag.
    view abstract10.1007/s00396-012-2767-y
  • Study of time and pressure dependent phenomena at the hard X-ray beamline BL9 of DELTA
    Wirkert, F.J. and Paulus, M. and Sternemann, C. and Nase, J. and Schroer, M.A. and Wieland, D.C.F. and Bieder, S. and Degen, P. and Rehage, H. and Tolan, M.
    Journal of Physics: Conference Series 425 (2013)
    The beamline BL9 of DELTA (Dortmund ELecTron Accelerator) is a multi-purpose beamline operating in an energy range between 4 and 27 keV. A short overview of the beamline and the experimental endstation is given. Exemplarily three typical applications, namely X-ray diffraction from interfaces, small angle X-ray scattering under high hydrostatic pressure and fast X-ray reflectivity measurements, are discussed in some detail in order to demonstrate the capabilities of the beamline.
    view abstract10.1088/1742-6596/425/20/202006
  • Adsorption of nanoparticles at the solid-liquid interface
    Brenner, T. and Paulus, M. and Schroer, M.A. and Tiemeyer, S. and Sternemann, C. and Möller, J. and Tolan, M. and Degen, P. and Rehage, H.
    Journal of Colloid and Interface Science 374 (2012)
    The adsorption of differently charged nanoparticles at liquid-solid interfaces was investigated by in situ X-ray reflectivity measurements. The layer formation of positively charged maghemite (γ-Fe 2O 3) nanoparticles at the aqueous solution-SiO 2 interface was observed while negatively charged gold nanoparticles show no adsorption at this interface. Thus, the electrostatic interaction between the particles and the charged surface was determined as the driving force for the adsorption process. The data analysis shows that a logarithmic particle size distribution describes the density profile of the thin adsorbed maghemite layer. The size distribution in the nanoparticle solution determined by small angle X-ray scattering shows an average particle size which is similar to that found for the adsorbed film. The formed magehemite film exhibits a rather high stability. © 2012 Elsevier Inc.
    view abstract10.1016/j.jcis.2012.02.010
  • On the spontaneous formation of clathrate hydrates at water-guest interfaces
    Boewer, L. and Nase, J. and Paulus, M. and Lehmkühler, F. and Tiemeyer, S. and Holz, S. and Pontoni, D. and Tolan, M.
    Journal of Physical Chemistry C 116 (2012)
    The formation of hydrates, cage-like water-gas structures, is of tremendous importance both in industries and research. Although of major significance, the formation process is not completely understood so far. We present a comprehensive study of hydrate formation at liquid-liquid interfaces between water and isobutane, propane, carbon dioxide, and at the liquid-gas interface between water and xenon. We investigated the structure of these interfaces under quiescent conditions in situ by means of X-ray reflectivity measurements both inside and outside the zone of hydrate stability. At the interfaces between water and liquid alkanes, no evidence for a structural change was found. In contrast, the accumulation of guest molecules inside nanothick interfacial layers was observed at the water-xenon and liquid-liquid water-CO 2 interfaces. We show that only those systems initially exhibiting such guest-enriched interfacial layers developed into macroscopic gas hydrates within our observation times (∼12 h). Therefore, these layers act as triggers for the spontaneous formation of macroscopic hydrates. © 2012 American Chemical Society.
    view abstract10.1021/jp211784w
  • Performance of fluorene and terthiophene copolymer in bilayer photovoltaic devices: The role of the polymer conformations
    Marchiori, C.F.N. and Yamamoto, N.A.D. and Grova, I.R. and MacEdo, A.G. and Paulus, M. and Sternemann, C. and Huotari, S. and Akcelrud, L. and Roman, L.S. and Koehler, M.
    Organic Electronics: physics, materials, applications 13 (2012)
    We report experiments using fluorene and terthiophene copolymer as the active layer in bilayer devices with C 60. The highest short circuit current, open circuit voltage and power conversion efficiency upon AM1.5 illumination were 6.8 mA/cm 2, 0.68 V and 2.33%, respectively. Density functional theory analysis was used to identify the most stable configurations of the terthiophene moieties in the polymer: the most stable form has the thiophene rings in the alternate configuration (anti) and the second conformation has the thiophene rings pointing to the same direction (syn). Comparing theoretical results with measurements of absorbance, X-ray diffraction, and X-ray reflectometry experiments, we conclude that the annealing treatment produces conformational anti to syn transition along the backbone of poly[9,9′-n-dihexyl-2,7-fluorene-alt-2,5-terthiophene] (LaPPS45). The syn segments of the chain condensed then in a lamellar ordered structure which increases the degree of crystallinity of the annealed samples and improve the light harvest at long wavelengths. From absorption measurements of films submitted to different annealing temperatures and with the help of theoretical calculations we propose a "wave-like" aggregation pattern to the syn segments in those lamellas. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.orgel.2012.08.002
  • Self-assembled iron oxide nanoparticle multilayer: X-ray and polarized neutron reflectivity
    Mishra, D. and Benitez, M.J. and Petracic, O. and Badini Confalonieri, G.A. and Szary, P. and Brüssing, F. and Theis-Bröhl, K. and Devishvili, A. and Vorobiev, A. and Konovalov, O. and Paulus, M. and Sternemann, C. and Toperverg, B.P. and Zabel, H.
    Nanotechnology 23 (2012)
    We have investigated the structure and magnetism of self-assembled, 20nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipoledipole interaction is rather strong, dominating the collective magnetic properties at room temperature. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/23/5/055707
  • Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity
    Hähl, H. and Evers, F. and Grandthyll, S. and Paulus, M. and Sternemann, C. and Loskill, P. and Lessel, M. and Hüsecken, A.K. and Brenner, T. and Tolan, M. and Jacobs, K.
    Langmuir 28 (2012)
    The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading. © 2012 American Chemical Society.
    view abstract10.1021/la300850g
  • The effect of ionic strength, temperature, and pressure on the interaction potential of dense protein solutions: From nonlinear pressure response to protein crystallization
    Möller, J. and Schroer, M.A. and Erlkamp, M. and Grobelny, S. and Paulus, M. and Tiemeyer, S. and Wirkert, F.J. and Tolan, M. and Winter, R.
    Biophysical Journal 102 (2012)
    Understanding the intermolecular interaction potential, V(r), of proteins under the influence of temperature, pressure, and salt concentration is essential for understanding protein aggregation, crystallization, and protein phase behavior in general. Here, we report small-angle x-ray scattering studies on dense lysozyme solutions of high ionic strength as a function of temperature and pressure. We show that the interaction potential changes in a nonlinear fashion over a wide range of temperatures, salt, and protein concentrations. Neither temperature nor protein and salt concentration lead to marked changes in the pressure dependence of V(r), indicating that changes of the water structure dominate the pressure dependence of the intermolecular forces. Furthermore, by analysis of the temperature, pressure, and ionic strength dependence of the normalized second virial coefficient, b2, we show that the interaction can be fine-tuned by pressure, which can be used to optimize b 2 values for controlled protein crystallization. © 2012 Biophysical Society.
    view abstract10.1016/j.bpj.2012.04.043
  • Concentration-induced wetting transition in water-tetrahydrofuran-isobutane systems
    Boewer, L. and Paulus, M. and Lehmkühler, F. and Tolan, M.
    Journal of Physical Chemistry C 115 (2011)
    The pressure-dependent wetting of isobutane at the aqueous tetrahydrofuran (THF) solution-isobutane interface was studied by means of X-ray reflectivity measurements. Using pure water and mixtures at low THF concentrations, a completely wetting isobutane layer is adsorbed onto the substrate. The pressure-dependent layer thickness can be described by a simple adsorption isotherm. In contrast, the formation of thick layers with low electron density is observed at high THF concentrations. The film growth shows an unpredictable behavior. This finding can be explained by the formation of partially wetting isobutane droplets on the water/THF substrate caused by a decrease of the liquids surface tension with increasing THF concentration. © 2011 American Chemical Society.
    view abstract10.1021/jp204663w
  • Dissolution of iron oxide nanoparticles inside polymer nanocapsules
    Möller, J. and Cebi, M. and Schroer, M.A. and Paulus, M. and Degen, P. and Sahle, C.J. and Wieland, D.C.F. and Leick, S. and Nyrow, A. and Rehage, H. and Tolan, M.
    Physical Chemistry Chemical Physics 13 (2011)
    The structure of poly(organosiloxane) nanocapsules partially filled with iron oxide cores of different sizes was revealed by small angle X-ray scattering and X-ray diffraction. The nanocapsules are synthesized by the formation of a poly(organosiloxane) shell around iron oxide nanoparticles and the simultaneous partial dissolution of these cores. Due to the high scattering contrast of the iron oxide cores compared to the polymer shell, the particle size distribution of the cores inside the capsules can be measured by small angle X-ray scattering. Additional information can be revealed by X-ray diffraction, which gives insights into the formation of the polymer network and the structure of the iron oxide cores. The study shows how the crystallinity and size of the nanoparticles as well as the shape and width of the size distribution can be altered by the synthesis parameters. © 2011 the Owner Societies.
    view abstract10.1039/c1cp22161b
  • Effect of magnetic nanoparticles on the surface rheology of surfactant films at the water surface
    Degen, P. and Wieland, D.C.F. and Leick, S. and Paulus, M. and Rehage, H. and Tolan, M.
    Soft Matter 7 (2011)
    The stability of fluid interfaces is important in many technical fields, e.g. suspensions, emulsions and foams. In this publication we investigated the influence of maghemite nanoparticles (γ-Fe<inf>2</inf>O<inf>3</inf>) on the surface stability of different surfactant films (SDS, CTAB, Brij 35). We investigated the interactions between nanoparticles and surfactant films by means of surface dilatation and surface shear rheological experiments. For further characterizations we used X-ray reflectivity (XRR) measurements, dynamic light scattering (DLS) and zeta (ζ)-potential measurements. For CTAB and more obvious for SDS it was found that at low to moderate surfactant concentrations, the viscoelasticity of the interface was increased drastically in the presence of the iron oxide nanoparticles. For films of Brij 35, however, the nanoparticles did not have any influence on the surface rheology. © The Royal Society of Chemistry 2011.
    view abstract10.1039/c1sm05248a
  • Exploring the piezophilic behavior of natural cosolvent mixtures
    Schroer, M.A. and Zhai, Y. and Wieland, D.C.F. and Sahle, C.J. and Nase, J. and Paulus, M. and Tolan, M. and Winter, R.
    Angewandte Chemie - International Edition 50 (2011)
    Marine organisms have evolved a surprising mechanism to counteract the deleterious effects of urea by trimethylammonium N-oxide (TMAO). The effect of pressure on the structure and intermolecular interactions of lysozyme in urea and TMAO solutions was studied (see picture). These findings help to understand the compensatory effect of urea-TMAO mixtures in deep-sea organisms. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201104380
  • Intercalation in layered metal-organic frameworks: Reversible inclusion of an extended π-system
    Arslan, H.K. and Shekhah, O. and Wieland, D.C.F. and Paulus, M. and Sternemann, C. and Schroer, M.A. and Tiemeyer, S. and Tolan, M. and Fischer, R.A. and Wöll, C.
    Journal of the American Chemical Society 133 (2011)
    We report the synthesis of layered [Zn 2(bdc) 2(H 2O) 2] and [Cu 2(bdc) 2(H 2O) 2] (bdc = benzdicarboxylate) metal-organic frameworks (MOF) carried out using the liquid-phase epitaxy approach employing self-assembled monolayer (SAM) modified Au-substrates. We obtain Cu and Zn MOF-2 structures, which have not yet been obtained using conventional, solvothermal synthesis methods. The 2D Cu 2+ dimer paddle wheel planes characteristic for the MOF are found to be strictly planar, with the planes oriented perpendicular to the substrate. Intercalation of an organic dye, DXP, leads to a reversible tilting of the planes, demonstrating the huge potential of these surface-anchored MOFs for the intercalation of large, planar molecules. © 2011 American Chemical Society.
    view abstract10.1021/ja2037996
  • Manipulating thin polymer films by changing the pH value
    Wieland, D.C.F. and Degen, P. and Paulus, M. and Schroer, M.A. and Rehage, H. and Tolan, M.
    Journal of Applied Physics 110 (2011)
    The structural change of Langmuir layers composed of alkyltrichlorosilanes under the influence of ammonia (NH3) was investigated. X-ray reflectivity and grazing incidence diffraction measurements along with surface pressure and surface potential measurements were performed in order to characterize the network structure. The data show an increase of the scattered intensity after addition of ammonia while the domain and unit cell size of the film did not change. These results show a higher surface coverage, which is not caused by a simple compression of the lipid tails. The effect can be attributed to a closing of voids in the polymer film caused by temporary breaking and annealing of the chemical bonds in the network by an increase of pH. © 2011 American Institute of Physics.
    view abstract10.1063/1.3661992
  • Multiple phase-transitions upon selective CO 2 adsorption in an alkyl ether functionalized metal-organic framework - An in situ X-ray diffraction study
    Henke, S. and Florian Wieland, D.C. and Meilikhov, M. and Paulus, M. and Sternemann, C. and Yusenko, K. and Fischer, R.A.
    CrystEngComm 13 (2011)
    The flexible alkyl ether functionalized metal-organic framework [Zn 2(BME-bdc) 2(dabco)] n (BME-bdc = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2. 2]octane) shows remarkable structural changes upon selective adsorption of CO 2 as determined by in situ X-ray diffraction at 195 K. Upon accommodation of carbon dioxide [Zn 2(BME-bdc) 2(dabco)] n transfers from a narrow pore form to an open pore form, which exhibits a much higher unit cell volume. Due to the slow adsorption kinetics an unexpected metastable intermediate form could be identified. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c1ce05446e
  • Nonlinear pressure dependence of the interaction potential of dense protein solutions
    Schroer, M.A. and Markgraf, J. and Wieland, D.C.F. and Sahle, C.J. and Möller, J. and Paulus, M. and Tolan, M. and Winter, R.
    Physical Review Letters 106 (2011)
    The influence of pressure on the structure and protein-protein interaction potential of dense protein solutions was studied and analyzed using small-angle x-ray scattering in combination with a liquid state theoretical approach. The structural as well as the interaction parameters of dense lysozyme solutions are affected by pressure in a nonlinear way. The structural properties of water lead to a modification of the protein-protein interactions below 4 kbar, which might have significant consequences for the stability of proteins in extreme natural environments. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.106.178102
  • Temperature-induced structural changes of tetrahydrofuran clathrate and of the liquid water/tetrahydrofuran mixture
    Lehmkühler, F. and Sakko, A. and Steinke, I. and Sternemann, C. and Hakala, M. and Sahle, C.J. and Buslaps, T. and Simonelli, L. and Galambosi, S. and Paulus, M. and Pylkkänen, T. and Tolan, M. and Hämäläinen, K.
    Journal of Physical Chemistry C 115 (2011)
    We present two complementary inelastic X-ray scattering studies on the structure of tetrahydrofuran (THF) clathrate hydrate and the supercooled stoichiometric liquid mixture of water and THF. Compton scattering experiments of the liquid mixture show that formation of hydrate precursors is unlikely. By comparing experimental spectra of THF hydrate and water/THF mixtures at temperatures above 250 K with density functional theory calculations, structural changes that manifest in OH bond length changes are observed. X-ray Raman scattering measurements of the oxygen K-edge in the same temperature range corroborate these results. The experimental results of THF hydrate at temperatures between 20 and 244 K can be modeled best by assuming thermal expansion only. Therefore, dependency on the system's temperature different structural behavior of THF hydrate is reported. © 2011 American Chemical Society.
    view abstract10.1021/jp207027p
  • Analysis of the ion distribution at a charged solid-liquid interface using X-ray standing waves
    Brücher, M. and Jacob, P. and Von Bohlen, A. and Franzke, J. and Sternemann, C. and Paulus, M. and Hergenröder, R.
    Langmuir 26 (2010)
    Functionalized solid-liquid interfaces were analyzed by X-ray standing waves (XSW) combined with, streaming current measurements to study surface charges, interfacial potential, and ion distributions. Thin films of aqueous solution containing Br- anions and Fe3+ cations at a concentration of 10 mg/L were prepared on functionalized silicon wafers. Functionalization of Si surfaces was accomplished by aminosilane groups shifting the interfacial potential, toward, positive values. The ion distribution was measured with nanometer resolution, which allows distinguishing between absorbed and mobile ions at the surface and in the diffusive layer, respectively. For Br-, different degrees of ion attraction were measured for the pH values 5.7 and 2.8. The ion Debye length values of the diffuse layer were 4 and 2 nm, respectively. © 2009 American Chemical Society.
    view abstract10.1021/la902385d
  • Effect of surface charge distribution on the adsorption orientation of proteins to lipid monolayers
    Tiemeyer, S. and Paulus, M. and Tolan, M.
    Langmuir 26 (2010)
    The adsorption orientation of the proteins lysozyme and ribonuclease A (RNase A) to a neutral 1,2-dipalmitoyl-snglycero- 3-phosphocholine (DPPC) and a negatively charged stearic acid lipid film was investigated by means of X-ray reflectivity. Both proteins adsorbed to the negatively charged lipid monolayer, whereas at the neutral monolayer, no adsorption was observed. For acquiring comprehensive information on the proteins' adsorption, X-ray reflectivity data were combined with electron densities obtained from crystallographic data. With this method, it is possible to determine the orientation of adsorbed proteins in solution underneath lipid monolayers. While RNase A specifically coupled with its positively charged active site to the negatively charged lipid monolayer, lysozyme prefers an orientation with its long axis parallel to the Langmuir film. In comparison to the electrostatic maps of the proteins, our results can be explained by the discriminative surface charge distribution of lysozyme and RNase A. © 2010 American Chemical Society.
    view abstract10.1021/la102616h
  • EuxSr1-xC2 (0 ≤ x ≤ 1): A dicarbide solid solution [1] with perfect vegard behavior
    Link, P. and Wandner, D. and Schellenberg, I. and Pöttgen, R. and Paulus, M. and Sahle, C.J. and Sternemann, C. and Ruschewitz, U.
    Zeitschrift fur Anorganische und Allgemeine Chemie 636 (2010)
    A solid solution EuxSr1-xC2 (0 ≤ x ≤ 1) was synthesized by direct reaction of the elements at 1123 K. The crystal structures of these compounds, investigated by synchrotron powder diffraction, depend upon x. For x &gt; 0.5 the monoclinic ThC2 type structure (C2/c, Z = 4) is observed and for x = 0.5 the ThC2 type structure coexists with the tetragonal CaC2 type structure (I4/mmm, Z = 2). The unit cell volumes per formula unit of all EuxSr1-xC2 compounds show perfect Vegard behavior, which is due to the almost identical ionic radii of Eu2+ and Sr2+. Mössbauer spectroscopic investigations indeed reveal that europium is in the divalent state over the whole composition range. EuxSr 1-xC2 exhibits several temperature dependent phase transitions that were studied by synchrotron powder diffraction and differential thermal analysis. The transition to a cubic high-temperature modification (Fm3̄ m, Z = 4) is of special interest, as it contains information about strain effects appearing inside the modifications with ordered C2 dumbbells (ThC2 and CaC2 type structures). The linear temperature dependence of the obtained transition temperatures TPh shows that no observable strain exists in EuxSr1-xC2, which is again due to the almost identical radii of Eu2+ and Sr 2+. EuxSr1-xC2 may therefore be described as a strain free dicarbide solid solution with perfect Vegard behavior. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/zaac.201000206
  • High-pressure SAXS study of folded and unfolded ensembles of proteins
    Schroer, M.A. and Paulus, M. and Jeworrek, C. and Krywka, C. and Schmacke, S. and Zhai, Y. and Wieland, D.C.F. and Sahle, C.J. and Chimenti, M. and Royer, C.A. and Garcia-Moreno, B. and Tolan, M. and Winter, R.
    Biophysical Journal 99 (2010)
    A structural interpretation of the thermodynamic stability of proteins requires an understanding of the structural properties of the unfolded state. High-pressure small-angle x-ray scattering was used to measure the effects of temperature, pressure, denaturants, and stabilizing osmolytes on the radii of gyration of folded and unfolded state ensembles of staphylococcal nuclease. A set of variants with the internal Val-66 replaced with Ala, Tyr, or Arg was used to examine how changes in the volume and polarity of an internal microcavity affect the dimensions of the native state and the pressure sensitivity of the ensemble. The unfolded state ensembles achieved for these proteins with high pressure were more compact than those achieved at high temperature, and were all very sensitive to the presence of urea and glycerol. Substitutions at the hydrophobic core detectably altered the conformation of the protein, even in the folded state. The introduction of a charged residue, such as Arg, inside the hydrophobic interior of a protein could dramatically alter the structural properties, even those of the unfolded state. The data suggest that a charge at an internal position can interfere with the formation of transient hydrophobic clusters in the unfolded state, and ensure that the pressure-unfolded form of a protein occupies the maximum volume possible. Only at high temperatures does the radius of gyration of the unfolded state ensemble approach the value for a statistical random coil. © 2010 by the Biophysical Society.
    view abstract10.1016/j.bpj.2010.09.046
  • Sticking polydisperse hydrophobic magnetite nanoparticles to lipid membranes
    Paulus, M. and Degen, P. and Brenner, T. and Tiemeyer, S. and Struth, B. and Tolan, M. and Rehage, H.
    Langmuir 26 (2010)
    The formation of a layer of hydrophobic magnetite (Fe3O 4) nanoparticles stabilized by lauric acid is analyzed by in situ X-ray reflectivity measurements. The data analysis shows that the nanoparticles partially disperse their hydrophobic coating. Consequently, a Langmuir layer was formed by lauric acid molecules that can be compressed into an untilted condensed phase. A majority of the nanoparticles are attached to the Langmuir film integrating lauric acid residue on their surface into the Langmuir film. Hence, the particles at the liquid-gas interface can be identified as so-called Janus beads, which are amphiphilic solids having two sides with different functionality. © 2010 American Chemical Society.
    view abstract10.1021/la102882j
  • The new x-ray lithography beamline BL1 at DELTA
    Lietz, D. and Paulus, M. and Sternemann, C. and Berges, U. and Hippert, B. and Tolan, M.
    AIP Conference Proceedings 1234 (2010)
    Lithography using synchrotron radiation in the x-ray regime provides a powerful method to produce mechanical components of sub-millimeter size with a very good quality for microtechnological applications. In recent years the demand for x-ray lithography beamtime for industrial production of microparts increased rapidly resulting in the development of new experimental endstations at synchrotron radiation sources dedicated for the production of micromechanical devices. We present in this work the layout of the new x-ray lithography beamline BL1 at the synchrotron radiation source DELTA in Dortmund and discuss first results of exposure tests. © 2010 American Institute of Physics.
    view abstract10.1063/1.3463213
  • γ-Fe2O3 nanoparticle adsorption at an OTS Langmuir monolayer
    Degen, P. and Paulus, M. and Leick, S. and Tolan, M. and Rehage, H.
    Colloid and Polymer Science 288 (2010)
    The assembling of magnetic nanoparticles in ordered structures as well as the preparation of very thin magnetic switchable polymer membranes is an important aim in many technical fields. We studied the influence of γ-Fe2O3 nanoparticles on the polymerization process and on the properties of the poly(organosiloxane)/nanoparticle-composite layer by surface rheological measurements, surface pressure/area (π/A) isotherm measurements, and Brewster angle microscopy. The adsorption process dynamics were studied by X-ray reflectivity and surface potential measurements. The results confirm the presence of attractive electrostatic interactions between the partial negatively charged monolayer and the positively charged nanoparticles. For further investigations, we prepared Langmuir-Blodgett layers of these polymer-nanoparticle composite and investigated them by atomic force microscopy and UV-Vis spectroscopy. We found that the concentration of nanoparticles was very low and the particles were mainly arranged below the polymer layer. © 2010 Springer-Verlag.
    view abstract10.1007/s00396-010-2191-0
  • membranes

  • surfaces

  • x-ray diffraction

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