Prof. Dr. Roland Böhmer

Experimental Physics
TU Dortmund University

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  • First- And third-order shear nonlinearities across the structural relaxation peak of the deeply supercooled pharmaceutical liquid indomethacin
    Moch, K. and Bierwirth, S.P. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 155 (2021)
    Nonlinear rheological properties of viscous indomethacin are studied in the frequency range of its structural relaxation, that is, in a range so far inaccessible to standard techniques involving medium-amplitude oscillatory shear amplitudes. The first- and third-order nonlinearity parameters thus recorded using a sequence of small and large shear excitations in a time efficient manner are compared with predictions from rheological models. By properly phase cycling the shear amplitudes, build-up and decay transients are recorded. Analogous to electrical-field experiments, these transients yield direct access to the structural relaxation times under linear and nonlinear shearing conditions. To demonstrate the broader applicability of the present approach, transient analyses are also carried out for the glass formers glycerol, ortho-terphenyl, and acetaminophen. © 2021 Author(s).
    view abstract10.1063/5.0065572
  • Isomeric effects in structure formation and dielectric dynamics of different octanols
    Bolle, J. and Bierwirth, S.P. and Požar, M. and Perera, A. and Paulus, M. and Münzner, P. and Albers, C. and Dogan, S. and Elbers, M. and Sakrowski, R. and Surmeier, G. and Böhmer, R. and Tolan, M. and Sternemann, C.
    Physical Chemistry Chemical Physics 23 (2021)
    The understanding of the microstructure of associated liquids promoted by hydrogen-bonding and constrained by steric hindrance is highly relevant in chemistry, physics, biology and for many aspects of daily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and molecular dynamics simulations to reveal temperature induced changes in the microstructure of different octanol isomers,i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groups form the basis of chain-, cyclic- or loop-like bonded structures that are separated by outwardly directed alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-ray scattering and simulations. The charge ordering which pilots OH aggregation can be linked to the strength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used here converge to the same interpretation: as one moves from 1-octanol to the branched octanols, the cluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates. All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, which can be understood as a change in microstructure promoted by chain association with increased chain length possibly assisted by ring-opening effects. All these results tend to support the intuitive picture of the entropic constraint provided by branching through the alkyl tails and highlight its capital entropic role in supramolecular assembly. © the Owner Societies 2021.
    view abstract10.1039/d1cp02468j
  • Non-ideal mixing behavior in dibutyl phosphate-propylamine binary liquids: Dielectric and nuclear magnetic resonance investigations
    Calandra, P. and Turco Liveri, V. and Proietti, N. and Capitani, D. and Lombardo, D. and Gainaru, C. and Böhmer, R. and Kozak, M. and Dobies, M. and Fojud, Z. and Pochylski, M.
    Journal of Molecular Liquids 323 (2021)
    Owing to the amphiphilic nature of their constituent molecules, binary mixtures of pure liquid surfactants are usually characterized by enhanced nano-segregation and thus can exhibit interesting transport properties and complex macroscopic behavior. In this ambit it was recently shown by Turco Liveri et al. (J. Mol. Liq. 263 (2018) 274–281) at room temperature that mixtures of short aliphatic chains compounds, such as dibutyl phosphate (DBP) and n-propylamine (PA) liquids, due to their ability to allow for phosphate-to-amine proton transfer, display ionic liquid–like behavior with composition-dependent enhanced conductivity, viscosity, and magnetically-induced birefringence. To understand the molecular mechanisms at the basis of this behavior, in the present study a combination of nuclear magnetic resonance (NMR) and dielectric spectroscopy investigations has been carried out for the same materials for different amine molar ratios. It was found that at certain compositions all studied dynamical processes (conformational changes, local hopping of “free” protons among neighboring polar headgroups, long-range charge migration) exhibit significant deviations from ideal mixing behavior. The microscopic origin of these deviations is discussed. © 2020
    view abstract10.1016/j.molliq.2020.114963
  • Rheology based estimates of self- And collective diffusivities in viscous liquids
    Gainaru, C. and Ahlmann, S. and Röwekamp, L.S. and Moch, K. and Bierwirth, S.P. and Böhmer, R.
    Journal of Chemical Physics 155 (2021)
    The self-diffusion coefficient of viscous liquids is estimated on the basis of a simple analysis of their rheological shear spectra. To this end, the Almond-West approach, previously employed to access single-particle diffusivities in ionic conductors, is generalized for application to molecular dynamics in supercooled liquids. Rheology based estimates, presented for indomethacin, ortho-terphenyl, and trinaphthylbenzene, reveal relatively small, yet systematic differences when compared with diffusivity data directly measured for these highly viscous liquids. These deviations are discussed in terms of mechanical Haven ratios, introduced to quantify the magnitude of collective translational effects that have an impact on the viscous flow. © 2021 Author(s).
    view abstract10.1063/5.0055811
  • The relationship between charge and molecular dynamics in viscous acid hydrates
    Ahlmann, S. and Münzner, P. and Moch, K. and Sokolov, A.P. and Böhmer, R. and Gainaru, C.
    Journal of Chemical Physics 155 (2021)
    Oscillatory shear rheology has been employed to access the structural rearrangements of deeply supercooled sulfuric acid tetrahydrate (SA4H) and phosphoric acid monohydrate, the latter in protonated (PA1H) and deuterated (PA1D) forms. Their viscoelastic responses are analyzed in relation to their previously investigated electric conductivity. The comparison of the also presently reported dielectric response of deuterated sulfuric acid tetrahydrate (SA4D) and that of its protonated analog SA4H reveals an absence of isotope effects for the charge transport in this hydrate. This finding clearly contrasts with the situation known for PA1H and PA1D. Our analyses also demonstrate that the conductivity relaxation profiles of acid hydrides closely resemble those exhibited by classical ionic electrolytes, even though the charge transport in phosphoric acid hydrates is dominated by proton transfer processes. At variance with this dielectric simplicity, the viscoelastic responses of these materials depend on their structural compositions. While SA4H displays a “simple liquid”-like viscoelastic behavior, the mechanical responses of PA1H and PA1D are more complex, revealing relaxation modes, which are faster than their ubiquitous structural rearrangements. Interestingly, the characteristic rates of these fast mechanical relaxations agree well with the characteristic frequencies of the charge rearrangements probed in the dielectric investigations, suggesting appearance of a proton transfer in mechanical relaxation of phosphoric acid hydrates. These findings open the exciting perspective of exploiting shear rheology to access not only the dynamics of the matrix but also that of the charge carriers in highly viscous decoupled conductors. © 2021 Author(s).
    view abstract10.1063/5.0055179
  • The relaxation behavior of supercooled and glassy imidacloprid
    Mansuri, A. and Münzner, P. and Feuerbach, T. and Vermeer, A.W.P. and Hoheisel, W. and Böhmer, R. and Thommes, M. and Gainaru, C.
    Journal of Chemical Physics 155 (2021)
    Employing dielectric spectroscopy, oscillatory shear rheology, and calorimetry, the present work explores the molecular dynamics of the widely used insecticide imidacloprid above and below its glass transition temperature. In its supercooled liquid regime, the applied techniques yield good agreement regarding the characteristic structural (alpha) relaxation times of this material. In addition, the generalized Gemant-DiMarzio-Bishop model provides a good conversion between the frequency-dependent dielectric and shear mechanical responses in its viscous state, allowing for an assessment of imidacloprid's molecular hydrodynamic radius. In order to characterize the molecular dynamics in its glassy regime, we employ several approaches. These include the application of frequency-temperature superposition (FTS) to its isostructural dielectric and rheological responses as well as use of dielectric and calorimetric physical aging and the Adam-Gibbs-Vogel model. While the latter approach and dielectric FTS provide relaxation times that are close to each other, the other methods predict notably longer times that are closer to those reflecting a complete recovery of ergodicity. This seemingly conflicting dissimilarity demonstrates that the molecular dynamics of glassy imidacloprid strongly depends on its thermal history, with high relevance for the use of this insecticide as an active ingredient in technological applications. © 2021 Author(s).
    view abstract10.1063/5.0067404
  • Translational and reorientational dynamics in deep eutectic solvents
    Reuter, D. and Münzner, P. and Gainaru, C. and Lunkenheimer, P. and Loidl, A. and Böhmer, R.
    Journal of Chemical Physics 154 (2021)
    We performed rheological measurements of the typical deep eutectic solvents (DESs) glyceline, ethaline, and reline in a very broad temperature and dynamic range, extending from the low-viscosity to the high-viscosity supercooled-liquid regime. We find that the mechanical compliance spectra can be well described by the random free-energy barrier hopping model, while the dielectric spectra on the same materials involve significant contributions arising from reorientational dynamics. The temperature-dependent viscosity and structural relaxation time, revealing non-Arrhenius behavior typical for glassy freezing, are compared to the ionic dc conductivity and relaxation times determined by broadband dielectric spectroscopy. For glyceline and ethaline, we find essentially identical temperature dependences for all dynamic quantities. These findings point to a close coupling of the ionic and molecular translational and reorientational motions in these systems. However, for reline, the ionic charge transport appears decoupled from the structural and reorientational dynamics, following a fractional Walden rule. In particular, at low temperatures, the ionic conductivity in this DES is enhanced by about one decade compared to expectations based on the temperature dependence of the viscosity. The results for all three DESs can be understood without invoking a revolving-door mechanism previously considered as a possible charge-transport mechanism in DESs. © 2021 Author(s).
    view abstract10.1063/5.0045448
  • From Ultraslow to Extremely Fast Dynamics in Sodium Nitrate: an 17O NMR Study
    Beerwerth, J. and Siegel, R. and Hoffmann, L. and Plaga, L.S. and Storek, M. and Bojer, B. and Senker, J. and Hiller, W. and Böhmer, R.
    Applied Magnetic Resonance 51 (2020)
    Increasing dynamics in solids featuring nuclei subjected to second-order quadrupolar interactions lead to central-transition spectra that undergo two consecutive line-shaped transitions. Conventional motional narrowing occurs when the molecular exchange rate is on the order of the strength of the dominant interaction. In a second step, the resulting intermediately narrowed spectra change further when the motion becomes faster than the Larmor precession rate, leading to terminally narrowed spectra that can display a residual quadrupolar shift. We derive analytic expressions for this shift and analyze the quadrupolar central-transition spectra in terms of CN symmetrical cone models. Increasing the number of sites to N ≥ 3, the terminally narrowed spectra remain unaltered, while the intermediately narrowed spectra remain unaltered only for N ≥ 5. This finding relates to the different (cubic vs. icosahedral) symmetries that are required to average out the spatial second- and fourth-rank terms in the second-order quadrupolar interaction. Following recent work (Hung et al., Solid State Nucl Magn Reson 84:14–19, 2017), 17O NMR is applied to examine the three-site rotation of the nitrate group in NaNO3. Line shapes are measured and analyzed, and in addition to prior work, satellite-transition and stimulated-echo experiments are carried out. The final-state amplitudes extracted from the latter are reproduced using model calculations. It is shown how two-dimensional exchange spectra relating to N-site cone motions can be decomposed in terms of effective two-site-jump spectra. This latter approach is successfully tested for NaNO3. © 2020, The Author(s).
    view abstract10.1007/s00723-020-01201-5
  • Reorientational dynamics of trimethoxyboroxine: A molecular glass former studied by dielectric spectroscopy and 11B nuclear magnetic resonance
    Hoffmann, L. and Beerwerth, J. and Greim, D. and Senker, J. and Sternemann, C. and Hiller, W. and Böhmer, R.
    Journal of Chemical Physics 152 (2020)
    In this work, trimethoxyboroxine (TMB) is identified as a small-molecule glass former. In its viscous liquid as well as glassy states, static and dynamic properties of TMB are explored using various techniques. It is found that, on average, the structure of the condensed TMB molecules deviates from threefold symmetry so that TMB's electric dipole moment is nonzero, thus rendering broadband dielectric spectroscopy applicable. This method reveals the super-Arrhenius dynamics that characterizes TMB above its glass transition, which occurs at about 204 K. To extend the temperature range in which the molecular dynamics can be studied, 11B nuclear magnetic resonance experiments are additionally carried out on rotating and stationary samples: Exploiting dynamic second-order shifts, spin-relaxation times, line shape effects, as well as stimulated-echo and two-dimensional exchange spectroscopy, a coherent picture regarding the dynamics of this glass former is gained. © 2020 Author(s).
    view abstract10.1063/1.5129769
  • Suppression of Orientational Correlations in the Viscous-Liquid State of Hyperquenched Pressure-Densified Glycerol
    Gainaru, C. and Nelson, H. and Huebinger, J. and Grabenbauer, M. and Böhmer, R.
    Physical Review Letters 125 (2020)
    Glycerol pressurized to 2 kbar and hyperquenched from the bulk liquid at rates of about-10 000 K/s, has been frozen to an extreme out-of-equilibrium state. As compared to conventionally cooled melts, the resulting material exhibits lower orientational correlations, enabling the observation of a secondary relaxation peak in the ambient-pressure dielectric response. The hyperquenching rather than the pressurizing part of the preparation protocol induces the observed structural changes. These vanish entirely only well above the glass transition temperature of the equilibrium liquid and are evidence for strong similarities between hyperquenched and vapor-deposited glass formers. © 2020 American Physical Society.
    view abstract10.1103/PhysRevLett.125.065503
  • Systematic differences in the relaxation stretching of polar molecular liquids probed by dielectric vs magnetic resonance and photon correlation spectroscopy
    Körber, T. and Stäglich, R. and Gainaru, C. and Böhmer, R. and Rössler, E.A.
    Journal of Chemical Physics 153 (2020)
    Relaxation spectra of molecular glass formers devoid of secondary relaxation maxima, as measured by dielectric spectroscopy (DS), nuclear magnetic resonance (NMR) relaxometry, photon correlation spectroscopy (PCS), and Fabry-Perot interferometry, are quantitatively compared in terms of the Kohlrausch stretching parameter βK. For a reliable estimate of βK, the excess wing contribution has to be included in the spectral analysis. The relaxation stretching probed by PCS and NMR varies only weakly among the liquids (βK = 0.58 ± 0.06). It is similar to that found in DS, provided that the liquid is sufficiently nonpolar (relaxation strength Δϵ≲6). For larger strengths, larger βKDS (narrowed relaxation spectra) are found when compared to those reported from NMR and PCS. Frequencyerature superposition (FTS) holds for PCS and NMR. This is demonstrated by data scaling and, for the few glass formers for which results are available, by the equivalence of the susceptibilities χPCS″ωτ∝χNMR″τ∝χNMR″ω, i.e., measuring at a constant frequency is equivalent to measuring at a constant temperature or constant correlation time. In this context, a plot of the spin-lattice relaxation rate R1(T) as a function of the spin-spin relaxation rate R2(T) is suggested to reveal the stretching parameter without the need to perform frequency-dependent investigations. Dielectrically, we identify a trend of increasing deviations from FTS with increasing Δϵ. Depending on the technique and glass former, the relative relaxation strength of the excess wing varies, whereas its exponent appears to be method independent for a given substance. For polar liquids, we discuss possible reasons for the discrepancy between the results from PCS and NMR as compared to those from DS. © 2020 Author(s).
    view abstract10.1063/5.0022155
  • Time Scales of the Quasitetrahedral Motion in KMnO4Observed by 17O Central-Transition NMR Spectroscopy
    Beerwerth, J. and Bojer, B. and Senker, J. and Böhmer, R.
    Journal of Physical Chemistry C 124 (2020)
    Oxygen-17 spin relaxation and stimulated-echo experiments are carried out to study temperature-dependent reorientational time scales in potassium permanganate. From an analysis of the present data, an energy barrier of 0.35 eV is found. This activation energy is much larger than the one reported previously (Jakobsen et al. Quantitative Dynamics and Structure for Crystalline Cs2WO4 and KMnO4 Determined from High-Field 17O Variable-Temperature MAS NMR Experiments, J. Phys. Chem. C 2014, 118, 20639-20646), but it is on the order of magnitude expected from observations for the dynamics of other tetraoxoanions in ionic crystals. The various contributions to the measured spin relaxation times and details regarding the two-time correlation functions for KMnO4 are discussed. © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c04704
  • Tuning the dynamics of imidazolium-based ionic liquids via hydrogen bonding. I. The viscous regime
    Thomann, C.A. and Münzner, P. and Moch, K. and Jacquemin, J. and Goodrich, P. and Sokolov, A.P. and Böhmer, R. and Gainaru, C.
    Journal of Chemical Physics 153 (2020)
    Combining results from impedance spectroscopy and oscillatory shear rheology, the present work focuses on the relation between the mass and charge flows and on how these are affected by the H-bonding in viscous ionic liquids (ILs). In particular, we compare the relaxational behaviors of the paradigmatic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) and its OH-functionalized counterpart 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OHEMIM-TFSI). Our results and their analysis demonstrate that the presence of cationic OH-groups bears a strong impact on the overall dynamics of OHEMIM-TFSI, although no signatures of suprastructural relaxation modes could be identified in their dielectric and mechanical responses. To check whether at the origin of this strong variation is the H-bonding or merely the difference between the corresponding cation sizes (controlling both the hydrodynamic volume and the inter-charge distance), the present study includes 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIM-TFSI), mixtures of EMIM-TFSI and PMIM-TFSI with lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and mixtures of OHEMIM-TFSI with PMIM-TFSI. Their investigation clearly reveals that the dynamical changes induced by H-bonding are significantly larger than those that can be attributed to the change in the ion size. Moreover, in the mixtures of OHEMIM-TFSI with PMIM-TFSI, a dilution of the OH-groups leads to strong deviations from ideal mixing behavior, thus highlighting the common phenomenological ground of hydroxy-functionalized ILs and other H-bonded liquids. © 2020 Author(s).
    view abstract10.1063/5.0026144
  • Amorphous and crystalline ices studied by dielectric spectroscopy
    Plaga, L.J. and Raidt, A. and Fuentes Landete, V. and Amann-Winkel, K. and Massani, B. and Gasser, T.M. and Gainaru, C. and Loerting, T. and Böhmer, R.
    Journal of Chemical Physics 150 (2019)
    This work reports on frequency dependent ambient-pressure dielectric measurements of hyperquenched glassy water, ice IV, ice VI, as well as a CO2-filled clathrate hydrate, the latter featuring a chiral water network. The dipolar time scales and the spectral shapes of the loss spectra of these specimens are mapped out and compared with literature data on low-density and high-density amorphous ices as well as on amorphous solid water. There is a trend that the responses of the more highly dense amorphous ices are slightly more dynamically heterogeneous than those of the lower-density amorphous ices. Furthermore, practically all of the amorphous ices, for which broadband dielectric spectra are available, display a curved high-frequency wing. Conversely, the high-frequency flanks of the nominally pure ice crystals including ice V and ice XII can be characterized by an approximate power-law behavior. While the spectral shapes of the nominally pure ices thus yield some hints regarding their amorphicity or crystallinity, a comparison of their time scale appears less distinctive in this respect. In the accessible temperature range, the relaxation times of the crystalline ices are between those of low-density and high-density amorphous ice. Hence, with reference also to previous work, the application of suitable doping currently seems to be the best dielectric spectroscopy approach to distinguish amorphous from crystalline ices. © 2019 Author(s).
    view abstract10.1063/1.5100785
  • Deeply supercooled aqueous LiCl solution studied by frequency-resolved shear rheology
    Münzner, P. and Hoffmann, L. and Böhmer, R. and Gainaru, C.
    Journal of Chemical Physics 150 (2019)
    To characterize the structural relaxation of an aqueous solution of LiCl, frequency-dependent shear rheological experiments are carried out near its glass transition. Analyzed within the fluidity representation, the generic spectral shape that was previously found for a range of different kinds of glass formers is confirmed for the currently studied hydrogen-bonded fluid as well. Furthermore, the validity of the rheological equivalent of the Barton-Nakajima-Namikawa relation is demonstrated for the aqueous LiCl solution. Its mechanical response is compared with that obtained using dielectric spectroscopy, a technique which is sensitive to both the reorientational dynamics of the water molecules and the translational dynamics of the ionic species. The extent to which these electrical polarization processes are coupled to those governing the viscoelastic response is discussed, also in comparison with the behavior of other ion conducting liquids. © 2019 Author(s).
    view abstract10.1063/1.5100600
  • First-Order and Third-Order Nonlinearities from Medium-Amplitude Oscillatory Shearing of Hydrogen-Bonded Polymers and Other Viscoelastic Materials
    Bierwirth, S.P. and Honorio, G. and Gainaru, C. and Böhmer, R.
    Macromolecules (2019)
    Medium-amplitude oscillatory shear experiments are employed to study the supramolecular relaxation modes in hydroxyl-terminated polydimethylsiloxane. For these associating melts, this allowed us to identify the nonlinear rheological signature of hydrogen-bond-mediated structures occurring in addition to those of the covalent chains. First-order and third-order nonlinearity parameters are determined for the supramolecular and chainlike modes to compare them with the predictions of an array of rheological models. Fingerprints of the two supramolecular modes can be recognized in the nonlinear response. By collecting third-order nonlinearity parameters 3Q0 for about 35 materials including polymer melts, polymer solutions, and other viscoelastic fluids, it is demonstrated that the maximum value of 3Q0 correlates with the magnitude of the shear modulus at the frequency at which this maximum occurs. The implications of this experimental finding are discussed. © 2019 American Chemical Society.
    view abstract10.1021/acs.macromol.9b01650
  • Linear and nonlinear shear studies reveal supramolecular responses in supercooled monohydroxy alcohols with faint dielectric signatures
    Bierwirth, S.P. and Honorio, G. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 150 (2019)
    Monohydroxy alcohols (MAs) with methyl and hydroxyl side groups attached to the same carbon atom in the alkyl backbone can display very weak structural and supramolecular dielectric relaxation processes when probed in the regime of small electrical fields. This can render their separation and assignment difficult in the pure liquids. When mixing with bromoalkanes, a faint Debye-like process can be resolved dielectrically for 4-methyl-4-heptanol. To achieve a separation of structural and supramolecular processes in pure 4-methyl-4-heptanol and 3-methyl-3-heptanol, mechanical experiments are carried out in the linear-response regime as well as using medium-angle oscillatory shear amplitudes. It is demonstrated that first-order and third-order nonlinear mechanical effects allow for a clear identification of supramolecular viscoelastic modes even for alcohols in which they leave only a weak signature in the linear-response shear modulus. Additionally, the nonlinear rheological behavior of 2-ethyl-1-hexanol is studied, revealing that its linearly detected terminal mode does not coincide with that revealed beyond the linear-response regime. This finding contrasts with those for the other MAs studied in this work. © 2019 Author(s).
    view abstract10.1063/1.5086529
  • Local and global dynamics of the viscous ion conductors 2Ca(NO 3 ) 2 -3KNO 3 and 2Ca(NO 3 ) 2 -3RbNO 3 probed by 87 Rb nuclear magnetic resonance and shear rheology
    Beerwerth, J. and Bierwirth, S.P. and Adam, J. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 150 (2019)
    The microscopic and macroscopic dynamics of calcium alkali nitrate melts are studied in their supercooled regime by means of shear rheology and nuclear magnetic resonance (NMR). The structural relaxation is probed using shear rheology to access the viscoelastic flow as well as using physical aging experiments. By exploiting the strongly quadrupole-perturbed 87 Rb nucleus, the local dynamics is probed on the milliseconds to nanoseconds range using various NMR methods involving central-transition stimulated-echo techniques, line shape analyses, spin relaxations, and second-order dynamic shift effects. The time scales monitored via the local Rb probe are in harmony with the electrical conductivity relaxation times. The low-temperature NMR line shapes agree excellently with those predicted by the Czjzek model. The temperature dependent second-order dynamic frequency shift is described using the imaginary part of the spectral density. It is demonstrated how the latter quantity can be generalized to include effects of correlation time distributions. © 2019 Author(s).
    view abstract10.1063/1.5093973
  • Local and global dynamics of the viscous ion conductors 2Ca(NO3)2-3KNO3 and 2Ca(NO3)2-3RbNO3 probed by 87Rb nuclear magnetic resonance and shear rheology
    Beerwerth, J. and Bierwirth, S.P. and Adam, J. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 150 (2019)
    The microscopic and macroscopic dynamics of calcium alkali nitrate melts are studied in their supercooled regime by means of shear rheology and nuclear magnetic resonance (NMR). The structural relaxation is probed using shear rheology to access the viscoelastic flow as well as using physical aging experiments. By exploiting the strongly quadrupole-perturbed 87Rb nucleus, the local dynamics is probed on the milliseconds to nanoseconds range using various NMR methods involving central-transition stimulated-echo techniques, line shape analyses, spin relaxations, and second-order dynamic shift effects. The time scales monitored via the local Rb probe are in harmony with the electrical conductivity relaxation times. The low-temperature NMR line shapes agree excellently with those predicted by the Czjzek model. The temperature dependent second-order dynamic frequency shift is described using the imaginary part of the spectral density. It is demonstrated how the latter quantity can be generalized to include effects of correlation time distributions. © 2019 Author(s).
    view abstract10.1063/1.5093973
  • Nature of Water's Second Glass Transition Elucidated by Doping and Isotope Substitution Experiments
    Fuentes-Landete, V. and Plaga, L.J. and Keppler, M. and Böhmer, R. and Loerting, T.
    Physical Review X 9 (2019)
    Based on calorimetry and dielectric spectroscopy, the influence of dopants as well as H/D-isotope substitution on the dynamics and thermodynamics of expanded high-density amorphous ice (eHDA) is studied. We find that dopants do not significantly alter the phase behavior, the dielectric relaxation times, and the calorimetric glass transition of eHDA. These observations starkly contrast those made for crystalline ices such as ice Ih, ice V, ice VI, and ice XII, where suitable dopants enhance the dielectric dynamics by several orders of magnitude and can trigger hydrogen order-disorder transitions, then taking place below the orientational glass transition temperature of undoped samples. This conspicuous contrast to the behavior of crystalline ices strongly argues against point-defect dynamics in amorphous ices and against a previously suggested "crystallinelike" nature of the amorphous ices. Furthermore, H/D substitution also does not affect the calorimetric glass transition in eHDA much, whereas for crystalline ices, the heat capacity increase at the glass transition is roughly halved. In addition, the H/D-isotope shift of the glass transition onset is much larger for crystalline ices than it is for amorphous ices. This observation favors the notion of eHDA's glass transition as a glass-to-liquid transition and is evidence against a mere molecular-reorientation unfreezing at water's second glass transition. Comparing the isotope effect on activation energies for dielectric relaxation with ice V suggests that in amorphous ice water molecules move translationally above Tg. Thus, the present work strongly supports that above this glass transition, water does indeed exist in its contested high-density liquid state. © 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevX.9.011015
  • Nonlinear electrical and rheological spectroscopies identify structural and supramolecular relaxations in a model peptide
    Honorio, G. and Bierwirth, S.P. and Gainaru, C. and Böhmer, R.
    Soft Matter 15 (2019)
    Supercooled liquid secondary amides display an electrical absorption peak characterized by an almost Debye-like shape, indicative of a close-to-exponential polarization response. This response, believed to be supramolecular in nature, is so enormously intense that the amide's structural process, contributing only a few percent to the total relaxation strength, is hard to resolve reliably using standard dielectric spectroscopy. To overcome this issue, nonlinear dielectric spectroscopy involving field-induced structural recovery and temperature-induced physical aging, was applied near the calorimetric glass transition of a mixture of N-methylformamide and N-ethylacetamide. Without the need to rely on cumbersome deconvolution procedures, it is thus demonstrated that the supramolecular response is by a factor of 6 slower than the structural relaxation. Conversely, in linear rheological experiments only the structural relaxation could be resolved, but not the supramolecular one. However, medium-amplitude oscillatory shear experiments carried out at 160 K do reveal the supramolecular process. Hence, the combination of linear and nonlinear mechanical measurements corroborates the dielectrically uncovered spectral separation of the two processes. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9sm00434c
  • Structure and dynamics of short-chain polymerized ionic liquids
    Wieland, F. and Bocharova, V. and Münzner, P. and Hiller, W. and Sakrowski, R. and Sternemann, C. and Böhmer, R. and Sokolov, A.P. and Gainaru, C.
    Journal of Chemical Physics 151 (2019)
    Combining experimental results obtained with X-ray scattering and field-gradient nuclear magnetic resonance (NMR) and an assessment of new and previous dielectric and rheology data, our study focuses on the molecular weight (Mw) evolution of local structure and dynamics in a homologous series of covalently bonded ionic liquids. Performed on a family of electrolytes with a tailored degree of ionic decoupling, this study reveals the differences between monomeric and oligomeric melts with respect to their structural organization, mass and charge transport, and molecular diffusion. Our study demonstrates that for the monomeric compound, the broadband conductivity and mechanical spectra reflect the same underlying distribution of activation barriers and that the Random Barrier Model describes fairly well both the ionic and structural relaxation processes in these materials. Moreover, the oligomers with chains comprising ten segments only exhibit both structural and dynamical fingerprints of a genuine polymer. A comparison of conductivity levels estimated using the self-diffusion coefficients probed via NMR and those probed directly with dielectric spectroscopy reveals the emerging of ion correlations which are affecting the macroscopic charge transport in these materials in a chain-length dependent manner. © 2019 Author(s).
    view abstract10.1063/1.5109228
  • Chapter 7: NMR Studies of Ionic Dynamics in Solids
    Böhmer, R. and Storek, M. and Vogel, M.
    New Developments in NMR 2018-January (2018)
    We outline NMR methods for studies of ionic dynamics in solids, including, but not limited to 7Li NMR approaches to lithium ion conductors. The covered techniques enable detailed characterization of ionic motions in wide ranges of time and length scales. © The Royal Society of Chemistry 2018.
    view abstract10.1039/9781788010467-00193
  • Coexistence of two structural relaxation processes in monohydroxy alcohol-alkyl halogen mixtures: Dielectric and rheological studies
    Bierwirth, S.P. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 149 (2018)
    Evidence for the existence of two glass transitions is found in binary mixtures of monohydroxy alcohols with an aprotic alkyl halide by means of dielectric spectroscopy and, markedly, also shear rheology. In the mechanical data, an enormous separation of two components becomes obvious for suitable compositions. The observation of bimodal motional heterogeneity is possible despite the fact that the glass transition temperatures of these substances differ by only 40 K. Obviously, the hydrogen-bond driven formation of supramolecular structures in one of the mixture components facilitates the emergence of dynamic contrast which for other binary liquids was so far only observed in the presence of much larger glass transition temperature differences. © 2018 Author(s).
    view abstract10.1063/1.5037037
  • Communication: Correlation of terminal relaxation rate and viscosity enhancement in supramolecular small-molecule liquids
    Bierwirth, S.P. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 148 (2018)
    Monohydroxy alcohols with a large supramolecular Debye-type dielectric process often exhibit a significant decoupling between the Debye mode and the structural relaxation. Using shear rheology, a technique that is sensitive to both processes as well, the current work reveals a widely applicable correlation in terms of the dynamical onset and the viscosity enhancement of the supramolecular shear mode with respect to the structural relaxation. Rheological data from an array of about 50 oligomers, associating polymers, (polymerized) ionic liquids, and aqueous solutions corroborate this correlation which thus appears to be generic to many classes of complex fluids. © 2018 Author(s).
    view abstract10.1063/1.5037803
  • Dynamic signatures of the transition from stacking disordered to hexagonal ice: Dielectric and nuclear magnetic resonance studies
    Gainaru, C. and Vynokur, E. and Köster, K.W. and Fuentes-Landete, V. and Spettel, N. and Zollner, J. and Loerting, T. and Böhmer, R.
    Journal of Chemical Physics 148 (2018)
    Using various temperature-cycling protocols, the dynamics of ice I were studied via dielectric spectroscopy and nuclear magnetic resonance relaxometry on protonated and deuterated samples obtained by heating high-density amorphous ices as well as crystalline ice XII. Previous structural studies of ice I established that at temperatures of about 230 K, the stacking disorder of the cubic/hexagonal oxygen lattice vanishes. The present dielectric and nuclear magnetic resonance investigations of spectral changes disclose that the memory of the existence of a precursor phase is preserved in the hydrogen matrix up to 270 K. This finding of hydrogen mobility lower than that of the undoped hexagonal ice near the melting point highlights the importance of dynamical investigations of the transitions between various ice phases and sheds new light on the dynamics in ice I in general. © 2018 Author(s).
    view abstract10.1063/1.5023178
  • Erratum: Author Correction: Dynamics enhanced by HCl doping triggers 60% Pauling entropy release at the ice XII-XIV transition (Nature communications (2015) 6 (7349))
    Köster, K.W. and Fuentes-Landete, V. and Raidt, A. and Seidl, M. and Gainaru, C. and Loerting, T. and Böhmer, R.
    Nature communications 9 (2018)
    This corrects the article DOI: 10.1038/ncomms8349.
    view abstract10.1038/ncomms16189
  • Experiments indicating a second hydrogen ordered phase of ice VI
    Gasser, T.M. and Thoeny, A.V. and Plaga, L.J. and Köster, K.W. and Etter, M. and Böhmer, R. and Loerting, T.
    Chemical Science 9 (2018)
    In the last twelve years five new ice phases were experimentally prepared. Two of them are empty clathrate hydrates and three of them represent hydrogen ordered counterparts of previously known disordered ice phases. Here, we report on hydrogen ordering in ice VI samples produced by cooling at pressures up to 2.00 GPa. Based on results from calorimetry, dielectric relaxation spectroscopy, Raman spectroscopy, and powder X-ray diffraction the existence of a second hydrogen ordered polymorph related to ice VI is suggested. Powder X-ray data show the oxygen network to be the one of ice VI. For the 1.80 GPa sample the activation energy from dielectric spectroscopy is 45 kJ mol-1, which is much larger than for the known hydrogen ordered proxy of ice VI, ice XV. Raman spectroscopy indicates the 1.80 GPa sample to be more ordered than ice XV. It is further distinct from ice XV in that it experiences hydrogen disordering above ≈103 K which is 26 K below the ice XV to ice VI disordering transition. Consequently, below 103 K it is thermodynamically more stable than ice XV, adding a stability region to the phase diagram of water. For the time being we suggest to call this new phase ice β-XV and to relabel it ice XVIII once its crystal structure is known. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8sc00135a
  • Heterogeneous rotational and translational dynamics in glasses and other disordered materials studied by NMR
    Böhmer, R. and Storek, M. and Vogel, M.
    Modern Magnetic Resonance (2018)
    Disordered materials such as glass formers and amorphous solid electrolytes are characterized by ubiquitous nonexponential molecular and/or ionic dynamics. This chapter focuses mostly on their investigation via two-time stimulated-echobased correlation functions. Underlying concepts are briefly reviewed and recent experimental examples from 2H, 7Li, 17O, 23Na, and 31P NMR are presented which encompass nonselective and selective central-transition excitation and a variety of relevant spin quantum numbers. Several recent methodological developments render also four-time stimulated-echo techniques applicable to a large array of probe nuclei. The higher-order correlation functions thus accessible enable quantitative insights into the origins of the nonexponentiality of atomic, ionic, or molecular motions. Provided that heterogeneous dynamics prevails, these experiments elucidate the temporal evolution of fast and slow subensembles, in particular by monitoring exchange processes among them. Together with corresponding frequency-domain techniques that are also touched upon, NMR methods to unravel the nature of nonexponentiality in a host of materials have become available for almost any probe nuclei. © Springer International Publishing AG, part of Springer Nature 2018.
    view abstract10.1007/978-3-319-28388-3_136
  • Thermodynamic and kinetic isotope effects on the order-disorder transition of ice XIV to ice XII
    Fuentes-Landete, V. and Köster, K.W. and Böhmer, R. and Loerting, T.
    Physical Chemistry Chemical Physics 20 (2018)
    Isotope effects accompanying the order-disorder transition of ice XIV to ice XII are studied using calorimetry, X-ray diffraction, and dielectric spectroscopy. Particular emphasis is placed on the impact of the cooling rate applied during high-pressure production and during ambient-pressure recooling on the degree of hydrogen order in the low-Temperature ice XIV phase. For specimens from D2O, ordering is harder to achieve in the sense that despite smaller cooling rates, the degree of order is less than in crystals produced from H2O. The degree of ordering can be quantified in terms of the Pauling entropy using calorimetry and manifests itself in structural and dynamical features that were examined using X-ray diffraction and dielectric spectroscopy, respectively. In hydrogen chloride doped samples, H/D substitution was found to slow down the dipolar dynamics up to about 30-fold and shifts the order-disorder transition by 4-6 K. By contrast to earlier assumptions it is possible to reach a high degree of ordering also at ambient pressure, provided the cooling rate is small enough. That is, at ambient pressure, orthorhombic stress slows down the dipolar reorientation near the ordering transition by a factor of 300-2000 for H2O and 30-100 for D2O samples. Furthermore, by long-Term storage of our samples at 77 K we have reached surprisingly large increases in degree of order. For the D2O samples we observed an unprecedented high order, corresponding to more than 45% of the Pauling entropy. © the Owner Societies 2018.
    view abstract10.1039/c8cp03786h
  • Transient Nonlinear Response of Dynamically Decoupled Ionic Conductors
    Wieland, F. and Sokolov, A.P. and Böhmer, R. and Gainaru, C.
    Physical Review Letters 121 (2018)
    The present study demonstrates that large electric fields progressively enhance the conductivity of ionic systems up to timescales corresponding to those on which their structural rearrangements take place. Yet, in many ionic materials, some regarded as candidates for electrical energy storage applications, the structural relaxation process can be tremendously slower than (or highly decoupled from) the charge fluctuations. Consequently, nonlinear dielectric spectroscopy may be employed to access rheological information in dynamically decoupled ionic conductors, whereas the combination of large electric power density and good mechanical stability, both technologically highly desired, imposes specific experimental constraints to reliably determine the steady-state conductivity of such materials. © 2018 American Physical Society.
    view abstract10.1103/PhysRevLett.121.064503
  • Two-site jumps in dimethyl sulfone studied by one- and two-dimensional 17O NMR spectroscopy
    Beerwerth, J. and Storek, M. and Greim, D. and Lueg, J. and Siegel, R. and Cetinkaya, B. and Hiller, W. and Zimmermann, H. and Senker, J. and Böhmer, R.
    Journal of Magnetic Resonance 288 (2018)
    Polycrystalline dimethyl sulfone is studied using central-transition oxygen-17 exchange NMR. The quadrupolar and chemical shift tensors are determined by combining quantum chemical calculations with line shape analyses of rigid-lattice spectra measured for stationary and rotating samples at several external magnetic fields. Quantum chemical computations predict that the largest principal axes of the chemical shift anisotropy and electrical field gradient tensors enclose an angle of about 73°. This prediction is successfully tested by comparison with absorption spectra recorded at three different external magnetic fields. The experimental one-dimensional motionally narrowed spectra and the two-dimensional exchange spectrum are compatible with model calculations involving jumps of the molecules about their two-fold symmetry axis. This motion is additionally investigated by means of two-time stimulated-echo spectroscopy which allows for a determination of motional correlation functions over a wider temperature range than previously reported using carbon and deuteron NMR. On the basis of suitable second-order quadrupolar frequency distributions, sin-sin stimulated-echo amplitudes are calculated for a two-site model in the limit of vanishing evolution time and compared with experimental findings. The present study thus establishes oxygen-17 NMR as a powerful method that will be particularly useful for the study of solids and liquids devoid of nuclei governed by first-order anisotropies. © 2018 Elsevier Inc.
    view abstract10.1016/j.jmr.2018.01.016
  • Communication: Heterogeneous water dynamics on a clathrate hydrate lattice detected by multidimensional oxygen nuclear magnetic resonance
    Adjei-Acheamfour, M. and Storek, M. and Böhmer, R.
    Journal of Chemical Physics 146 (2017)
    Previous deuteron nuclear magnetic resonance studies revealed conflicting evidence regarding the possible motional heterogeneity of the water dynamics on the hydrate lattice of an ice-like crystal. Using oxygen-17 nuclei as a sensitive quadrupolar probe, the reorientational two-time correlation function displays a clear nonexponentiality. To check whether this dispersive behavior is a consequence of dynamic heterogeneity or rather of an intrinsic nonexponentiality, a multidimensional, four-time magnetic resonance experiment was devised that is generally applicable to strongly quadrupolarly perturbed half-integer nuclei such as oxygen-17. Measurements of an appropriate four-time function demonstrate that it is possible to select a subensemble of slow water molecules. Its mean time scale is compared to theoretical predictions and evidence for significant motional heterogeneity is found. © 2017 Author(s).
    view abstract10.1063/1.4983043
  • Communication: Nonadditive dielectric susceptibility spectra of associating liquids
    Bierwirth, S.P. and Münzner, P. and Knapp, T.A. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 146 (2017)
    Highly unusual linear-response spectra involving contributions from hydrogen-bonded supramolecular processes and from structural relaxations are found in 4-methyl-3-heptanol mixed with 2-ethyl-1-hexylbromide. Although the mean time scales of the underlying relaxations are separated by more than 3 decades, the overall spectra cannot be decomposed into a sum of these processes. This finding challenges the ubiquitous practice of disentangling susceptibility spectra of Debye liquids by adding suitable subspectra. The spectral shape of the studied viscous mixtures is excellently described using the Williams ansatz, here a necessary approach and not as previously considered merely an alternative to additive analyses. © 2017 Author(s).
    view abstract10.1063/1.4978228
  • Connecting structurally and dynamically detected signatures of supramolecular Debye liquids
    Büning, T. and Lueg, J. and Bolle, J. and Sternemann, C. and Gainaru, C. and Tolan, M. and Böhmer, R.
    Journal of Chemical Physics 147 (2017)
    The monohydroxy alcohol 2-ethyl-1-hexanol mixed with the halogen-substituted alkyl halides 2-ethyl-1-hexyl chloride and 2-ethyl-1-hexyl bromide was studied using synchrotron-based x-ray scattering. In the diffraction patterns, an oxygen-related prepeak appears. The concentration dependence of its intensity, shape, and position indicates that the formation of the hydrogen-bonded associates of monohydroxy alcohols is largely hindered by the halogen alkane admixture. Using dielectric spectroscopy and high-resolution rheology on the same liquid mixtures, it is shown that these structural features are correlated with the relaxation mechanisms giving rise to supramolecular low-frequency dynamics. © 2017 Author(s).
    view abstract10.1063/1.4986866
  • Four-time 7Li stimulated-echo spectroscopy for the study of dynamic heterogeneities: Application to lithium borate glass
    Storek, M. and Tilly, J.F. and Jeffrey, K.R. and Böhmer, R.
    Journal of Magnetic Resonance 282 (2017)
    To study the nature of the nonexponential ionic hopping in solids a pulse sequence was developed that yields four-time stimulated-echo functions of previously inaccessible spin-3/2-nuclei such as 7Li. It exploits combined Zeeman and octupolar order as longitudinal carrier state. Higher-order correlation functions were successfully generated for natural-abundance and isotopically-enriched lithium diborate glasses. Four-time 7Li measurements are presented and compared with two-time correlation functions. The results are discussed with reference to approaches devised to quantify the degree of nonexponentiality in glass forming systems and evidence for the occurrence of dynamic heterogeneities and dynamic exchange were found. Additional experiments using the 6Li species illustrate the challenge posed by subensemble selection when the dipolar interactions are not very much smaller than the quadrupolar ones. © 2017 Elsevier Inc.
    view abstract10.1016/j.jmr.2017.06.010
  • Generic Primary Mechanical Response of Viscous Liquids
    Bierwirth, S.P. and Böhmer, R. and Gainaru, C.
    Physical Review Letters 119 (2017)
    Four decades ago a seminal review by Jonscher [Nature (London) 267, 673 (1977)NATUAS0028-083610.1038/267673a0] revealed that the dielectric response of conducting materials is characterized by a "remarkable universality". Demonstrating that the same response pattern is exhibited also by shear rheological spectra of nonpolymeric viscous liquids, the present contribution connects two branches of condensed matter physics: Concepts developed for charge transport can be employed for the description of mass flow and vice versa. Based on the virtual equivalence of the two dynamics a connection is established between microscopic and macroscopic viscoelastic characteristics of liquids, resembling the Barton-Nakajima-Namikawa relation for conductivity. © 2017 American Physical Society.
    view abstract10.1103/PhysRevLett.119.248001
  • Interplay of defect doping and Bernal-Fowler rules: A simulation study of the dynamics on ice lattices
    Köster, K.W. and Klocke, T. and Wieland, F. and Böhmer, R.
    Physical Review B 96 (2017)
    Protonic defects on ice lattices induced by doping with acids such as HCl and HF or bases such as KOH can facilitate order-disorder transitions. In laboratory experiments KOH doping is efficient in promoting the ordering transition from hexagonal ice I to ice XI, but it is ineffective for other known ice phases, for which HCl can trigger hydrogen ordering. Aiming at understanding these differences, random-walk simulations of the defect diffusion are performed on two- and three-dimensional ice lattices under the constraints imposed by the Bernal-Fowler ice rules. Effective defect diffusion coefficients are calculated for a range of dopants, concentrations, and ice phases. The interaction of different defects, incorporated by different dopants, is investigated to clarify the particular motion-enhancing role played by complementary defect pairs. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.134301
  • Relaxation dynamics and transformation kinetics of deeply supercooled water: Temperature, pressure, doping, and proton/deuteron isotope effects
    Lemke, S. and Handle, P.H. and Plaga, L.J. and Stern, J.N. and Seidl, M. and Fuentes-Landete, V. and Amann-Winkel, K. and Köster, K.W. and Gainaru, C. and Loerting, T. and Böhmer, R.
    Journal of Chemical Physics 147 (2017)
    Above its glass transition, the equilibrated high-density amorphous ice (HDA) transforms to the low-density pendant (LDA). The temperature dependence of the transformation is monitored at ambient pressure using dielectric spectroscopy and at elevated pressures using dilatometry. It is found that near the glass transition temperature of deuterated samples, the transformation kinetics is 300 times slower than the structural relaxation, while for protonated samples, the time scale separation is at least 30 000 and insensitive to doping. The kinetics of the HDA to LDA transformation lacks a proton/deuteron isotope effect, revealing that this process is dominated by the restructuring of the oxygen network. The x-ray diffraction experiments performed on samples at intermediate transition stages reflect a linear combination of the LDA and HDA patterns implying a macroscopic phase separation, instead of a local intermixing of the two amorphous states. © 2017 Author(s).
    view abstract10.1063/1.4993790
  • Simple-liquid dynamics emerging in the mechanical shear spectra of poly(propylene glycol)
    Gainaru, C. and Hecksher, T. and Fan, F. and Xing, K. and Cetinkaya, B. and Olsen, N.B. and Dyre, J.C. and Sokolov, A.P. and Böhmer, R.
    Colloid and Polymer Science 295 (2017)
    The present dielectric investigations of methyl-terminated poly(propylene glycol) (PPG) oligomers reveal that near the glass transition the normal modes and segmental relaxation merge in a single-process susceptibility spectrum, similar to previous observations on OH-terminated species. Moreover, the present shear-mechanical measurements demonstrate that the vanishing of chain modes can be monitored without recourse to dielectric investigations, which are able to access chain dynamics only for the relatively small fraction of type A polymers. As the normal and segmental modes merge, the viscosity displays a crossover from a polymer-like regime governed by the chain dynamics, to a simple-liquid regime governed by the structural relaxation. © 2017 Springer-Verlag GmbH Germany
    view abstract10.1007/s00396-017-4206-6
  • Submillimeter coils for stimulated-echo spectroscopy of a solid sodium ion conductor by nonselective excitation of MHz broad 23Na quadrupolar satellite spectra
    Jessat, T. and Adjei-Acheamfour, M. and Storek, M. and Böhmer, R.
    Solid State Nuclear Magnetic Resonance 82-83 (2017)
    In solids the detection of ionic motion covering the time range of milliseconds and longer is often accomplished using stimulated-echo spectroscopy. For spectral line widths much below or much above 1 MHz nonselective or fully selective radio-frequency pulse excitation, respectively, is typically applied in such experiments. To enable the study of samples with quadrupolarly broadened satellite spectra featuring intermediate widths (in the lower MHz range) the present work exploits microcoils. Using such coils, stimulated-echo spectroscopy can be performed under conditions of nonselective excitation for instance with 23Na as a nuclear probe. Nutation experiments are carried out to assess the coil performance. The impact of second-order quadrupolar interactions is studied using explicit density-matrix calculations. The applicability of the present approach is successfully tested for a sodium orthophosphate based solid ion conductor. © 2016 Elsevier Inc.
    view abstract10.1016/j.ssnmr.2016.12.010
  • Colloquium: Water's controversial glass transitions
    Amann-Winkel, K. and Böhmer, R. and Fujara, F. and Gainaru, C. and Geil, B. and Loerting, T.
    Reviews of Modern Physics 88 (2016)
    Water is the most common and, judged from its numerous anomalous properties, the weirdest of all known liquids and the complexity of its pressure-temperature map is unsurpassed. A major obstacle on the way to a full understanding of water's structure and dynamics is the hard-to-explore territory within this map, colloquially named the no man's land. Many experiments suggest that just before stepping across its low-temperature border, amorphous ices undergo glass-to-liquid transitions while other interpretations emphasize the importance of underlying disordered (nano)crystalline states. Prospects for reconciling the conflicting views regarding the nature of water's glass transitions are discussed. © 2016 American Physical Society.
    view abstract10.1103/RevModPhys.88.011002
  • Doping-enhanced dipolar dynamics in ice v as a precursor of hydrogen ordering in ice XIII
    Köster, K.W. and Raidt, A. and Fuentes Landete, V. and Gainaru, C. and Loerting, T. and Böhmer, R.
    Physical Review B 94 (2016)
    Dielectric spectroscopy measurements are carried out in the temperature range from about 100 to 145 K on nominally pure ice V as well as on crystals doped with KOH and with HCl in order to investigate their reorientation dynamics at ambient pressure. The orientational glass transition temperature of pure ice V is detected at 123 K, in agreement with previous indications from calorimetry. KOH doped ice V displays an about 60-fold enhanced hydrogen dynamics and the dipolar relaxation induced by HCl doping is even by a factor of about 40 000 faster than that of the undoped material. The phase transition of HCl doped ice V to ice XIII is accompanied by a significant reorientational slowdown and a pronounced freeze-out of the electrical susceptibility. The results obtained near this transition are discussed in relation to other order/disorder ice pairs such as ice I/XI and ice XII/XIV. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.184306
  • Erratum: Corrigendum to “Quadrupolar transients, cosine correlation functions, and two-dimensional exchange spectra of non-selectively excited spin-3/2 nuclei: A 7Li NMR study of the superionic conductor lithium indium phosphate” (J. Magn. Reson. (2015) 260 (116–126))
    Storek, M. and Böhmer, R.
    Journal of Magnetic Resonance 272 (2016)
    The authors regret that Eqs. (13) and (14) of the article, M. Storek and R. Böhmer, J. Magn. Reson. 260 (2015) 116–126, contained typographical errors. These equations should have read as follows:(13)SE+=120[-5-3cos(2φ)]+120[-3+3cos(2φ)]〈cos(ωQtp)+cos(ωQta)〉+120[-9-3cos(2φ)]〈c2(tp,ta)〉+35cos(2φ)〈s2(tp,ta)〉 (14)SE-=-140[7cosφ+9cos(3φ)]-940[cosφ-cos(3φ)]〈cos(ωQtp)+cos(ωQta)〉-340[5cosφ+3cos(3φ)]〈c2(tp,ta)〉+35cosφ〈s2(tp,ta)〉. The computations in the article which refer to Eqs. (13) and (14), however, were carried out using the form of the equations reproduced here. Furthermore, in Appendix A of the article the first correction term in Eq. (A.2) is O(tp6) and Eq. (A.1) should read as follows: (A.1)〈cos[ωD(0)tp]cos[ωD(tm)tp]〉=1-735tp2+tp47325+17〈P2(cosθ1)P2(cosθ2)〉+81175〈P4(cosθ1)P4(cosθ2)〉+O(tp6) so that for the ℓ-dependent reorientation times, τℓ = [ℓ(ℓ + 1)]−1τc, one finds τℓ =2 = 10/3τℓ =4. The corrections given in this Corrigendum do not affect in any other way the contents and conclusions of the article, neither in the main text nor in its appendices or in the supplementary material. The authors would like to apologize for any inconvenience caused. © 2016
    view abstract10.1016/j.jmr.2016.08.017
  • Interchannel Hopping in Single Crystalline Lithium Triborate Probed by 7Li NMR: Spin Relaxation, Line Shape Analysis, Selective-Inversion Spin Alignment, and Two-Dimensional Exchange Spectra
    Storek, M. and Böhmer, R.
    Journal of Physical Chemistry C 120 (2016)
    The ion dynamics of single crystalline lithium triborate, LiB3O5, an important material in nonlinear optics, is studied using various 7Li and 11B nuclear magnetic resonance (NMR) techniques at temperatures from about 480 to 780 K in order to elucidate the apparent discrepancies underlying previous interpretations of NMR line shape analyses and results from dielectric spectroscopy. Rotating frame spin-lattice relaxation as well as line shape measurements are carried out and are combined with selective-inversion spin alignment as well as two-dimensional chemical exchange spectroscopy to track the temperature-dependent Li ion motion. From symmetry considerations the latter is clearly identified as interchannel hopping. By combining the present results with those from the published, yet so far not fully analyzed, dielectric loss spectra, it is shown how seeming differences in energy barriers hindering the ion motion and in the evolution of the distribution of correlation times can be reconciled. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.6b01347
  • Positive and Negative Mixed Glass Former Effects in Sodium Borosilicate and Borophosphate Glasses Studied by 23Na NMR
    Storek, M. and Adjei-Acheamfour, M. and Christensen, R. and Martin, S.W. and Böhmer, R.
    Journal of Physical Chemistry B 120 (2016)
    Glasses with varying compositions of constituent network formers but constant mobile ion content can display minima or maxima in their ion transport which are known as the negative or the positive mixed glass former effect, MGFE, respectively. Various nuclear magnetic resonance (NMR) techniques are used to probe the ion hopping dynamics via the 23Na nucleus on the microscopic level, and the results are compared with those from conductivity spectroscopy, which are more sensitive to the macroscopic charge carrier mobility. In this way, the current work examines two series of sodium borosilicate and sodium borophosphate glasses that display positive and negative MGFEs, respectively, in the composition dependence of their Na+ ion conductivities at intermediate compositions of boron oxide substitution for silicon oxide and phosphorus oxide, respectively. A coherent theoretical analysis is performed for these glasses which jointly captures the results from measurements of spin relaxation and central-transition line shapes. On this basis and including new information from 11B magic-angle spinning NMR regarding the speciation in the sodium borosilicate glasses, a comparison is carried out with predictions from theoretical approaches, notably from the network unit trap model. This comparison yields detailed insights into how a variation of the boron oxide content and thus of either the population of silicon or phosphorus containing network-forming units with different charge-trapping capabilities leads to nonlinear changes of the microscopic transport properties. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcb.6b00482
  • Deuteron magnetic resonance and dielectric studies of guest reorientation and water dynamics in six clathrate hydrates containing ring-type guests
    Nelson, H. and Ihrig, A. and Kahlau, R. and Kibies, P. and Kast, S.M. and Böhmer, R.
    Journal of Non-Crystalline Solids 407 (2015)
    Using deuteron nuclear magnetic resonance and high-resolution dielectric spectroscopy the guest dynamics of tetrahydropyran, cyclopentane, trimethylene oxide, 1,3-dioxolane, and 1,4-dioxane clathrate hydrates is studied. By investigating lattice-deuterated as well as guest-deuterated crystals, the anisotropic guest reorientation is scrutinized and compared with previous results for tetrahydrofuran clathrate hydrate. The reorientational energy barriers depend linearly on the size of the guest molecules except for the clathrate hydrate of cyclopentane, a molecule which exhibits a dipole moment of the order of 0.01 D. The ether oxygens of the other guests can induce Bjerrum L defects on the hydrate lattice. Their concentration is examined for ammonia-doped tetrahydrofuran clathrate hydrate. Covering a wide range of NH3 concentrations x, a minimal overall Bjerrum defect concentration is observed that leads to a maximum mobility on the hydrate lattice at x ≈ 0.03%. To examine guest-induced Bjerrum L defects further, the translational motion on the undoped hydrate lattices is studied using stimulated-echo spectroscopy: The proton dynamics of CP clathrate hydrate is virtually identical to that of hexagonal ice and the water motion of THP clathrate hydrate closely matches that of the tetrahydrofuran compound. © 2014 Elsevier B.V.
    view abstract10.1016/j.jnoncrysol.2014.08.059
  • Dynamics enhanced by HCl doping triggers full Pauling entropy release at the ice XII-XIV transition
    Köster, K.W. and Fuentes-Landete, V. and Raidt, A. and Seidl, M. and Gainaru, C. and Loerting, T. and Böhmer, R.
    Nature Communications 6 (2015)
    The pressure-temperature phase diagram of ice displays a perplexing variety of structurally distinct phases. In the century-long history of scientific research on ice, the proton-ordered ice phases numbered XIII through XV were discovered only recently. Despite considerable effort, none of the transitions leading from the low-temperature ordered ices VIII, IX, XI, XIII, XIV and XV to their high-temperature disordered counterparts were experimentally found to display the full Pauling entropy. Here we report calorimetric measurements on suitably high-pressure-treated, hydrogen chloride-doped ice XIV that demonstrate just this at the transition to ice XII. Dielectric spectroscopy on undoped and on variously doped ice XII crystals reveals that addition of hydrogen chloride, the agent triggering complete proton order in ice XIV, enhances the precursor dynamics strongest. These discoveries provide new insights into the puzzling observation that different dopants trigger the formation of different proton-ordered ice phases. © 2015 Macmillan Publishers Limited.
    view abstract10.1038/ncomms8349
  • Dynamics in Supercooled Secondary Amide Mixtures: Dielectric and Hydrogen Bond Specific Spectroscopies
    Gainaru, C. and Bauer, S. and Vynokur, E. and Wittkamp, H. and Hiller, W. and Richert, R. and Böhmer, R.
    Journal of Physical Chemistry B 119 (2015)
    Alkylacetamide-based model peptides display an intense Debye-type dielectric relaxation. In order to explore the extent to which this feature has to be regarded analogous to that in other supramolecular liquids, notably the monohydroxy alcohols, we applied broadband dielectric, time-dependent solvation, and near-infrared spectroscopies as well as shear rheology and various nuclear magnetic resonance techniques to mixtures of N-methylacetamide (NMA) or N-ethylacetamide (NEA) with N-methylformamide. Compared in the modulus format, dielectric relaxation, solvation dynamics, and mechanical response indicate a common global and local dynamics. The present spin-relaxation measurements reflect motional processes which are significantly faster than the dominant Debye dielectric response, and a similar conclusion is drawn from measurements of the shear viscosity. The NH overtone stretching vibrations reveal a temperature-dependent hydrogen-bond equilibrium that changes its characteristics near temperatures of 325 K. Finally, dielectric low-temperature data recorded for (NEA)0.4(NMF)0.6 mixed with 2-picoline indicate the existence of a critical concentration akin to the situation in various monohydroxy alcohol mixtures. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcb.5b10034
  • Experimental evidence for two distinct deeply supercooled liquid states of water - Response to "comment on 'Water's second glass transition"', by G.P. Johari, Thermochim. Acta (2015)
    Stern, J. and Seidl, M. and Gainaru, C. and Fuentes-Landete, V. and Amann-Winkel, K. and Handle, P.H. and Köster, K.W. and Nelson, H. and Böhmer, R. and Loerting, T.
    Thermochimica Acta 617 (2015)
    Recently, our earlier data which led us to conclude that deeply supercooled water displays a second glass transition (Amann-Winkel et al., 2013) was reinterpreted (Johari, 2015). In particular, the increase in heat capacity observed for high-density amorphous ice (HDA) samples at 116 K was reinterpreted to indicate sub-T<inf>g</inf> features of low-density amorphous ice's (LDA's) glass transition. We reply to the criticism in detail and report an experiment triggered by the comment on our work. This experiment unequivocally confirms our original interpretation of the observations and reinforces the case for water's second glass transition, its polyamorphism, and the observation of two distinct ultraviscous states of water differing by about 25% in density. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tca.2015.08.030
  • Mixed Debye-type liquids studied by dielectric, shear mechanical, nuclear magnetic resonance, and near-infrared spectroscopy
    Bauer, S. and Moch, K. and Münzner, P. and Schildmann, S. and Gainaru, C. and Böhmer, R.
    Journal of Non-Crystalline Solids 407 (2015)
    Monohydroxy alcohols have been in the focus of scientific research for a long time and their Debye (-like) process was studied predominantly using dielectric spectroscopy. However, a number of other techniques are useful to unravel the dynamics of these supramolecular liquids. For a recent review on neat monohydroxy alcohols, see R. Böhmer, C. Gainaru, R. Richert, Phys. Rep. (accepted). On this background the present article deals mostly with mixtures involving monohydroxy alcohols using various experimental methods. Examples given include dielectric spectroscopy on a mixture which shows a small Debye process well separated from the structural relaxation. For another mixture it is demonstrated that the time scales of the dielectric and the rheological signatures of the Debye process coincide. Isotope labeling is exploited to map out the rotational dynamics of both components in binary mixtures of 1-butanol (BuOH) and 1-bromobutane (BuBr) using spin-lattice relaxation nuclear magnetic resonance (NMR) spectroscopy. While the hydroxyl motion in BuOH becomes faster upon admixture of BuBr, the alkyl bromide dynamics is virtually independent of composition. Two-time and four-time stimulated-echo NMR experiments show that the dynamic exchange in a BuOH-BuBr mixture is similar to that of supercooled liquids devoid of a Debye process and hence it does not provide a rationale to understand the symmetric broadening of the structural dielectric loss peak in this and related mixtures. Finally, a wavelength dependent derivative analysis of near-infrared spectra recorded for pure and mixed monohydroxy alcohols over wide temperature ranges shows that the rearrangement of the hydrogen network differs below and above about 250 K. © 2014 Elsevier B.V.
    view abstract10.1016/j.jnoncrysol.2014.07.018
  • Molecular motions in supercooled and glassy ibuprofen: Deuteron magnetic resonance and high-resolution rheology study
    Bauer, S. and Storek, M. and Gainaru, C. and Zimmermann, H.b and Böhmer, R.
    Journal of Physical Chemistry B 119 (2015)
    Using deuteron nuclear magnetic resonance, the molecular motions of specifically isotope-labeled ibuprofen were probed at the carboxylic group and at the methin group next to it. Spin relaxometry revealed slight differences between the molecular motions of the two isotopomers that are rationalized with reference to the hydrogen bonding of the COOH moiety. In the glassy state, a small-angle jump process among about four sites, related to the so-called γ-process, was identified using stimulated-echo spectroscopy. Indications for a Debye-like process, previously found to leave a weak signature in the dielectric loss, could not unambiguously be detected in magnetic resonance or shear mechanical experiments carried out for supercooled liquid ibuprofen. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcb.5b01072
  • Quadrupolar transients, cosine correlation functions, and two-dimensional exchange spectra of non-selectively excited spin-3/2 nuclei: A 7Li NMR study of the superionic conductor lithium indium phosphate
    Storek, M. and Böhmer, R.
    Journal of Magnetic Resonance 260 (2015)
    Cos-cos stimulated echoes of non-selectively excited spin-3/2 nuclei were not exploited in studies of slow motional processes in solids and solid-like samples, so far. Based on a theoretical analysis of the quadrupolar transients which hitherto obviously precluded the application of such echoes, their utility is demonstrated for the example of 7Li NMR on the polycrystalline fast ion conductor lithium indium phosphate. Quadrupolar transients can adversely affect the shape of two- and three-pulse echo spectra and strategies are successfully tested that mitigate their impact. Furthermore, by means of suitably adapted cos-cos echo sequences an effective suppression of central-line contributions to the NMR spectra is achieved. By combining cos-cos and sin-sin datasets static two-dimensional exchange spectra were recorded that display quadrupolarly modulated off-diagonal intensity indicative of ionic motion. © 2015 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jmr.2015.09.009
  • The glass transition in high-density amorphous ice
    Loerting, T. and Fuentes-Landete, V. and Handle, P.H. and Seidl, M. and Amann-Winkel, K. and Gainaru, C. and Böhmer, R.
    Journal of Non-Crystalline Solids 407 (2015)
    There has been a long controversy regarding the glass transition in low-density amorphous ice (LDA). The central question is whether or not it transforms to an ultraviscous liquid state above 136 K at ambient pressure prior to crystallization. Currently, the most widespread interpretation of the experimental findings is in terms of a transformation to a superstrong liquid above 136 K. In the last decade some work has also been devoted to the study of the glass transition in high-density amorphous ice (HDA) which is in the focus of the present review. At ambient pressure HDA is metastable against both ice I and LDA, whereas at > 0.2 GPa HDA is no longer metastable against LDA, but merely against high-pressure forms of crystalline ice. The first experimental observation interpreted as the glass transition of HDA was made using in situ methods by Mishima, who reported a glass transition temperature Tinfg/inf of 160 K at 0.40 GPa. Soon thereafter Andersson and Inaba reported a much lower glass transition temperature of 122 K at 1.0 GPa. Based on the pressure dependence of HDA's Tinfg/inf measured in Innsbruck, we suggest that they were in fact probing the distinct glass transition of very high-density amorphous ice (VHDA). Very recently the glass transition in HDA was also observed at ambient pressure at 116 K. That is, LDA and HDA show two distinct glass transitions, clearly separated by about 20 K at ambient pressure. In summary, this suggests that three glass transition lines can be defined in the p-T plane for LDA, HDA, and VHDA. © 2015 Published by Elsevier Inc.
    view abstract10.1016/j.jnoncrysol.2014.09.003
  • Two-dimensional second-order quadrupolar exchange powder spectra for nuclei with half-integer spins. Calculations and an experimental example using oxygen NMR
    Adjei-Acheamfour, M. and Storek, M. and Beerwerth, J. and Böhmer, R.
    Solid State Nuclear Magnetic Resonance 71 (2015)
    Two-dimensional chemical exchange spectra are determined for powders containing half-integer quadrupolar nuclei that evolve under the second-order quadrupolar interaction. For simple exchange processes it is shown that the calculated exchange patterns depend sensitively on the jump angles, the tensor parameters, and the rotation axes characterizing the geometry of the underlying molecular motion. The inclusion of other interactions is discussed. To demonstrate the feasibility of the experimental method a two-dimensional second-order quadrupolar exchange spectrum was recorded for a clathrate hydrate using oxygen NMR. Simulations based on a motional model involving a six-site jump capture the essential features of the experimental results. © 2015 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.ssnmr.2015.05.004
  • Vibrational study of anharmonicity, supramolecular structure, and hydrogen bonding in two octanol isomers
    Bauer, S. and Stern, J. and Böhm, F. and Gainaru, C. and Havenith, M. and Loerting, T. and Böhmer, R.
    Vibrational Spectroscopy 79 (2015)
    Results from far-, mid-, and near-infrared spectroscopy are presented and combined with previous dielectric data on 2-ethyl-1-hexanol and 4-methyl-3-heptanol. This yields the electromagnetic absorption spectrum of the two monohydroxy alcohols, presented in terms of the imaginary part of the complex refractive index in a very wide frequency range. Low-frequency Raman measurements and far-infrared spectra hint at differences in the supramolecular structure of the two alcohols. By comparing the wavenumbers and amplitudes of the fundamental and the first overtone of the hydroxyl group's stretching vibration the temperature dependence of anharmonic effects and hydrogen bond cooperativity is studied. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.vibspec.2015.05.001
  • Water dynamics on ice and hydrate lattices studied by second-order central-line stimulated-echo oxygen-17 nuclear magnetic resonance
    Adjei-Acheamfour, M. and Tilly, J.F. and Beerwerth, J. and Böhmer, R.
    Journal of Chemical Physics 143 (2015)
    Oxygen-17 stimulated-echo spectroscopy is a novel nuclear magnetic resonance (NMR) technique that allows one to investigate the time scale and geometry of ultraslow molecular motions in materials containing oxygen. The method is based on detecting orientationally encoded frequency changes within oxygen's central-transition NMR line that are caused by second-order quadrupolar interactions. In addition to the latter, the present theoretical analysis of various two-pulse echo and stimulated-echo pulse sequences takes also heteronuclear dipolar interactions into account. As an experimental example, the ultraslow water motion in polycrystals of tetrahydrofuran clathrate hydrate is studied via two-time oxygen-17 stimulated-echo correlation functions. The resulting correlation times and those of hexagonal ice are similar to those from previous deuteron NMR measurements. Calculations of the echo functions' final-state correlations for various motional models are compared with the experimental data of the clathrate hydrate. It is found that a six-site model including the oxygen-proton dipolar interaction describes the present results. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4936416
  • Anomalously large isotope effect in the glass transition of water
    Gainaru, C. and Agapov, A.L. and Fuentes-Landete, V. and Amann-Winkel, K. and Nelson, H. and Köster, K.W. and Kolesnikov, A.I. and Novikov, V.N. and Richert, R. and Böhmer, R. and Loerting, T. and Sokolov, A.
    Proceedings of the National Academy of Sciences of the United States of America 111 (2014)
    We present the discovery of an unusually large isotope effect in the structural relaxation and the glass transition temperature Tg of water. Dielectric relaxation spectroscopy of low-density as well as of vapor-deposited amorphous water reveal Tg differences of 10 ± 2 K between H2O and D2O, sharply contrasting with other hydrogen- bonded liquids for which H/D exchange increases Tg by typically less than 1 K. We show that the large isotope effect and the unusual variation of relaxation times in water at low temperatures can be explained in terms of quantum effects. Thus, our findings shed new light on water's peculiar low-temperature dynamics and the possible role of quantum effects in its structural relaxation, and possibly in dynamics of other low-molecularweight liquids.
    view abstract10.1073/pnas.1411620111
  • Liquid 1-propanol studied by neutron scattering, near-infrared, and dielectric spectroscopy
    Sillrén, P. and Matic, A. and Karlsson, M. and Koza, M. and MacCarini, M. and Fouquet, P. and Götz, M. and Bauer, T. and Gulich, R. and Lunkenheimer, P. and Loidl, A. and Mattsson, J. and Gainaru, C. and Vynokur, E. and Schildmann, S. and Bauer, S. and Böhmer, R.
    Journal of Chemical Physics 140 (2014)
    Liquid monohydroxy alcohols exhibit unusual dynamics related to their hydrogen bonding induced structures. The connection between structure and dynamics is studied for liquid 1-propanol using quasi-elastic neutron scattering, combining time-of-flight and neutron spin-echo techniques, with a focus on the dynamics at length scales corresponding to the main peak and the pre-peak of the structure factor. At the main peak, the structural relaxation times are probed. These correspond well to mechanical relaxation times calculated from literature data. At the pre-peak, corresponding to length scales related to H-bonded structures, the relaxation times are almost an order of magnitude longer. According to previous work [C. Gainaru, R. Meier, S. Schildmann, C. Lederle, W. Hiller, E. Rössler, and R. Böhmer, Phys. Rev. Lett. 105, 258303 (2010)] this time scale difference is connected to the average size of H-bonded clusters. The relation between the relaxation times from neutron scattering and those determined from dielectric spectroscopy is discussed on the basis of broad-band permittivity data of 1-propanol. Moreover, in 1-propanol the dielectric relaxation strength as well as the near-infrared absorbance reveal anomalous behavior below ambient temperature. A corresponding feature could not be found in the polyalcohols propylene glycol and glycerol. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4868556
  • Local-field approximation of homonuclear dipolar interactions in 7Li-NMR: Density-matrix calculations and random-walk simulations tested by echo experiments on borate glasses
    Storek, M. and Jeffrey, K.R. and Böhmer, R.
    Solid State Nuclear Magnetic Resonance 59-60 (2014)
    NMR echo techniques have proven to be important to study dynamics in ion conductors and other solid materials. Using the spin-3/2 nucleus 7Li as a probe, both the quadrupolar and the often neglected homonuclear dipolar interactions modulate the NMR frequency as the ion performs jump processes. Retaining only the local-field term of the many-body Hamiltonian, the impact of the dipolar interaction on various echo experiments was studied using spin dynamics calculations yielding products of dipolar and quadrupolar correlation functions. Using a simple stochastic model these functions were simulated with particular emphasis on the impact of ionic motions and on the conditions under which the dipolar and quadrupolar contributions factorize. The results of the computations and of the random-walk simulations are compared with experimental data obtained for various lithium borate and lithium borophosphate glasses. It is concluded that the local-field approximation is a useful means of treating the Li-Li dipole interactions and that the simple model that we introduce is capable of describing many experimentally observed features. Furthermore, because the dipolar and quadrupolar contributions essentially factorize, a selective determination of the corresponding correlation functions becomes possible. © 2014 Elsevier Inc.
    view abstract10.1016/j.ssnmr.2014.01.002
  • Oscillatory shear and high-pressure dielectric study of 5-methyl-3-heptanol
    Gainaru, C. and Wikarek, M. and Pawlus, S. and Paluch, M. and Figuli, R. and Wilhelm, M. and Hecksher, T. and Jakobsen, B. and Dyre, J.C. and Böhmer, R.
    Colloid and Polymer Science 292 (2014)
    The monohydroxy alcohol 5-methyl-3-heptanol is studied using rheology at ambient pressure and using dielectric spectroscopy at elevated pressures up to 1.03 GPa. Both experimental techniques reveal that the relaxational behavior of this liquid is intermediate between those that show a large Debye process, such as 2-ethyl-1-hexanol, or a small Debye-like feature, such as 4-methyl-3-heptanol, with which comparisons are made. Various phenomenological approaches assigning a time scale for the rheological signature of supramolecular dynamics in monohydroxy alcohols are discussed. © 2014 Springer-Verlag.
    view abstract10.1007/s00396-014-3274-0
  • Second-order quadrupole interaction based detection of ultra-slow motions: Tensor operator framework for central-transition spectroscopy and the dynamics in hexagonal ice as an experimental example
    Adjei-Acheamfour, M. and Böhmer, R.
    Journal of Magnetic Resonance 249 (2014)
    The second-order quadrupolar broadening of the central transition of nuclear probes with half-integer spins I is demonstrated to be useful to detect ultraslow molecular motions. On the basis of density matrix calculations explicit expressions are derived for quadrupolarly modulated sin-sin and cos-cos signals of selectively excited nuclei with I = 3/2, 5/2, 7/2, and 9/2. These correlation functions are suitable for implementation in two-dimensional exchange spectroscopy as well as for stimulated-echo experiments. As an application, 17O measurements of the reorientational correlation function of water molecules in hexagonal ice are presented. © 2014 Elsevier Inc. All rights reserved.
    view abstract10.1016/j.jmr.2014.09.019
  • Shear-Modulus investigations of monohydroxy alcohols: Evidence for a short-chain-polymer rheological response
    Gainaru, C. and Figuli, R. and Hecksher, T. and Jakobsen, B. and Dyre, J.C. and Wilhelm, M. and Böhmer, R.
    Physical Review Letters 112 (2014)
    Liquids composed of small-molecule monohydroxy alcohols are demonstrated to display rheological behavior typical for oligomeric chains. This observation was made possible by rheological experiments in which more than seven decades in frequency and more than five decades on the mechanical modulus scale are covered. The singly hydrogen-bonded monohydroxy alcohols were chosen because they display significant, but surprisingly poorly understood effects of intermolecular association. Based on the present shear study, one can apply theoretical concepts of polymer science to understand the anomalous physical behavior of a wide range of hydrogen-bonded liquids. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.112.098301
  • Structure and dynamics of monohydroxy alcohols-Milestones towards their microscopic understanding, 100 years after Debye
    Böhmer, R. and Gainaru, C. and Richert, R.
    Physics Reports 545 (2014)
    In 1913 Debye devised a relaxation model for application to the dielectric properties of water and alcohols. These hydrogen-bonded liquids continue to be studied extensively because they are vital for biophysical processes, of fundamental importance as solvents in industrial processes, and in every-day use. Nevertheless, the way to a microscopic understanding of their properties has been beset with apparently conflicting observations and conceptual difficulties. Much of this remains true for water, but fortunately the situation for monohydroxy alcohols is different. Here, with the experimental progress witnessed in recent years and with the growing recognition of the importance of specific supramolecular structures, a coherent microscopic understanding of the structure and dynamics of these hydrogen-bonded liquids is within reach. © 2014 Elsevier B.V.
    view abstract10.1016/j.physrep.2014.07.005
  • Supramolecular x-ray signature of susceptibility amplification in hydrogen-bonded liquids
    Bierwirth, S.P. and Büning, T. and Gainaru, C. and Sternemann, C. and Tolan, M. and Böhmer, R.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 90 (2014)
    Mixing two nonconducting hydrogen-bonded liquids, each exhibiting a low dielectric relaxation strength, can result in a highly electrically absorbing fluid. This susceptibility amplification effect is demonstrated for mixtures of monohydroxy alcohols. Whereas in the pure liquids a tendency to form ringlike low-dipole moment clusters prevails, in the mixtures such supramolecular structures are disfavored leading to an up to tenfold enhancement of the dielectric loss. The compositional evolution of density and mean cluster-cluster separation is traced using x-ray scattering and indicates deviations from ideal mixing with decreased C-C but simultaneously increased O-O correlation lengths. Thus, the variation in the supramolecular absorption strength could be tracked using a static scattering technique. These observations are in harmony with volume exclusion and ring open effects that predict an optimized susceptibility amplification for mixtures in which the two components occupy equal volume fractions as experimentally observed. © 2014 American Physical Society.
    view abstract10.1103/PhysRevE.90.052807
  • Broadband dynamics in neat 4-methyl-3-heptanol and in mixtures with 2-ethyl-1-hexanol
    Bauer, S. and Wittkamp, H. and Schildmann, S. and Frey, M. and Hiller, W. and Hecksher, T. and Olsen, N.B. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 139 (2013)
    The relatively small dielectric Debye-like process of the monohydroxy alcohol 4-methyl-3-heptanol (4M3H) was found to depend slightly on the intramolecular conformation. Proton and deuteron nuclear magnetic resonance demonstrate that the hydroxyl dynamics and the overall molecular dynamics take place on similar time scales in contrast to the situation for the structural isomer 2-ethyl-1-hexanol (2E1H) [S. Schildmann, J. Chem. Phys. 135, 174511 (2011)]. This indicates a very weak decoupling of Debye-like and structural relaxation which was further probed using volume expansivity experiments. Shear viscosity as well as diffusometry measurements were performed and the data were analyzed in terms of the Debye-Stokes-Einstein equations. In mixtures of 4M3H with 2E1H the Debye-like process becomes much stronger and for 2E1H mole fraction of more than 25% the behavior of this alcohol is rapidly approached. This finding is interpreted to indicate that the ring-like supramolecular structures in 4M3H become energetically unfavorable when adding 2E1H, an alcohol that tends to form chain-like molecular aggregates. The concentration dependence of the Kirkwood factor in these mixtures displays a high degree of similarity with experimental results on monohydroxy alcohols in which the pressure or the location of the OH group within the molecular structure is varied. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4821229
  • Debye relaxation and 250 K anomaly in glass forming monohydroxy alcohols
    Bauer, S. and Burlafinger, K. and Gainaru, C. and Lunkenheimer, P. and Hiller, W. and Loidl, A. and Böhmer, R.
    Journal of Chemical Physics 138 (2013)
    A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance study on the hydrogen-bonded liquid 2-ethyl-1-hexanol C. Gainaru, Phys. Rev. Lett. 107, 118304 (2011)10.1103/PhysRevLett.107.118304 revealed anomalous behavior in various static quantities near 250 K. To check whether corresponding observations can be made for other monohydroxy alcohols as well, these experimental methods were applied to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular backbone. All studied liquids exhibit a change of behavior near 250 K, which is tentatively ascribed to effects of hydrogen bond cooperativity. By analyzing the NIR band intensities, a linear cluster size is derived that agrees with estimates from dielectric spectroscopy. All studied alcohols, except 4-methyl-3-heptanol, display a dominant Debye-like peak. Furthermore, neat 2-ethyl-1-butanol exhibits a well resolved structural relaxation in its dielectric loss spectrum, which so far has only been observed for diluted monohydroxy alcohols. © 2013 American Institute of Physics.
    view abstract10.1063/1.4793469
  • Dynamics in glass forming sulfuric and nitric acid hydrates
    Frey, M. and Didzoleit, H. and Gainaru, C. and Böhmer, R.
    Journal of Physical Chemistry B 117 (2013)
    Deuteron nuclear magnetic resonance (NMR) and dielectric spectroscopy are utilized to investigate the dynamics in sulfuric acid hydrates as well as in nitric acid hydrates for various degrees of hydration. Near the glass transition temperature the electrical response is up to four decades faster than the calorimetric one, a feature found also for several other inorganic ionic liquids. The acid hydrates display pronounced super-Arrhenius behavior with fragility indices of the order of 100. The relaxation strength of the acid hydrates increases with increasing temperature, an observation that was rationalized with reference to the degree of molecular dissociation. Spin relaxometry and stimulated-echo spectroscopy revealed an overall isotropic reorientation process featuring a jump angle of about 30. Finally, the implications of the present results for the understanding of the glass transition of pure ultraviscous water are discussed. © 2013 American Chemical Society.
    view abstract10.1021/jp407588j
  • Dynamics of glass forming ammonia hydrates
    Didzoleit, H. and Storek, M. and Gainaru, C. and Geil, B. and Böhmer, R.
    Journal of Physical Chemistry B 117 (2013)
    The dielectric relaxation of ammonia-water mixtures was studied for a range of NH3 mole fractions x. For 0.01 ≤ x &lt; 0.33, the samples can be supercooled relatively easily. In this composition range, the relaxation strength is proportional to x. The dielectric relaxation times display a super-Arrhenius behavior, are independent of the NH3 content in the specified range, and, for T &gt; 250 K, line up smoothly with those of pure water. The relaxation behavior of glass forming ammonia hydrates was also investigated using nuclear magnetic resonance techniques including deuteron relaxometry and stimulated-echo spectroscopy, as well as static field-gradient proton diffusometry. These experiments yielded additional insights into the rotational and translational dynamics of ammonia hydrates. © 2013 American Chemical Society.
    view abstract10.1021/jp407581q
  • How do high pressures change the Debye process of 4-methyl-3-heptanol?
    Pawlus, S. and Wikarek, M. and Gainaru, C. and Paluch, M. and Böhmer, R.
    Journal of Chemical Physics 139 (2013)
    4-methyl-3-heptanol, a monohydroxy alcohol with a relatively small dielectric Debye process, is studied in wide ranges of temperature (143 K < T < 308 K) and pressure (0.1 MPa < p < 864 MPa). When monitored under isochronous conditions, i.e., focusing on constant relaxation times, as well as under isothermal conditions, the Debye process gains significant intensity upon pressure application. This behavior contrasts with that of the previously studied octanol 2-ethyl-1-hexanol, which features a large Debye process. These experimentally observed, clearly distinguishable pressure evolutions are discussed to reflect differences in the formation of hydrogen-bonded supramolecular structures. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4816364
  • Salty Water in KOH-Doped Hexagonal Ice: A Proton and Deuteron NMR Study
    Schildmann, S.a and Schöler, A.a and Nowaczyk, A.a and Geil, B.b and Böhmer, R.a
    Applied Magnetic Resonance 44 (2013)
    Water doped with 10-2 mol of KOH was cooled to temperatures at which most of the solution freezes to form hexagonal ice. Using proton and deuteron spin-lattice relaxometry as well as static field gradient diffusometry, it was found that a liquid-like phase coexists with the crystal down to below 200 K. The ionic dopants are expelled from the crystalline phase and form a KOH-enriched aqueous solution probably in the form of inclusions within the ice crystal. Its self-diffusion coefficient is only slightly smaller than that of nominally pure water. Motional correlation times were determined on the basis of spin-lattice relaxation times and compared with previous electrical conductivity and calorimetry results. © 2012 Springer-Verlag Wien.
    view abstract10.1007/s00723-012-0414-x
  • Small-angle water reorientations in KOH doped hexagonal ice and clathrate hydrates
    Nelson, H. and Schildmann, S. and Nowaczyk, A. and Gainaru, C. and Geil, B. and Böhmer, R.
    Physical Chemistry Chemical Physics 15 (2013)
    Using deuteron nuclear magnetic resonance and dielectric spectroscopy KOH doped tetrahydrofuran clathrate hydrates and KOH doped hexagonal ice are studied at temperatures above 60 and 72 K, respectively. Below these temperatures proton order is established on the lattice formed by the water molecules. In the clathrate hydrate a new type of small-angle motion is discovered using deuteron spin-spin relaxation, line-shape analysis, and stimulated-echo experiments. Based on the latter results a model is developed for the local proton motion that could successfully be tested using random-walk simulations. It is argued that the newly identified small-angle motion, obviously absent in undoped samples, is an important feature of the mechanism which accompanies the establishment of proton order not only in doped clathrate hydrates but also in doped hexagonal ice. Specific motions of OH- defects are demonstrated to explain the experimentally observed behavior. The relative importance of localized versus delocalized OH- defect motions is discussed. © 2013 the Owner Societies.
    view abstract10.1039/c3cp00139c
  • Water's second glass transition
    Amann-Winkel, K. and Gainaru, C. and Handle, P.H. and Seidl, M. and Nelson, H. and Böhmer, R. and Loerting, T.
    Proceedings of the National Academy of Sciences of the United States of America 110 (2013)
    The glassy states of water are of common interest as the majority of H2O in space is in the glassy state and especially because a proper description of this phenomenon is considered to be the key to our understanding why liquid water shows exceptional properties, different from all other liquids. The occurrence of water's calorimetric glass transition of low-density amorphous ice at 136 K has been discussed controversially for many years because its calorimetric signature is very feeble. Here, we report that high-density amorphous ice at ambient pressure shows a distinct calorimetric glass transitions at 116 K and present evidence that this second glass transition involves liquid-like translational mobility of water molecules. This "double Tg scenario" is related to the coexistence of two liquid phases. The calorimetric signature of the second glass transition is much less feeble, with a heat capacity increase at Tg,2 about five times as large as at Tg,1. By using broadband- dielectric spectroscopy we resolve loss peaks yielding relaxation times near 100 s at 126 K for low-density amorphous ice and at 110 K for high-density amorphous ice as signatures of these two distinct glass transitions. Temperature-dependent dielectric data and heating-rate-dependent calorimetric data allow us to construct the relaxation map for the two distinct phases of water and to extract fragility indices m = 14 for the low-density and m = 20-25 for the high-density liquid. Thus, low-density liquid is classified as the strongest of all liquids known ("superstrong"), and also high-density liquid is classified as a strong liquid.
    view abstract10.1073/pnas.1311718110
  • Deuteron magnetic resonance studies of anhydrous caffeine
    Nowaczyk, A. and Nath, R.C. and Zimmermann, H. and Böhmer, R.
    Zeitschrift fur Physikalische Chemie 226 (2012)
    Methyl deuterated caffeine powder was studied using 2H-NMR relaxometry and line shape analysis. In caffeine's low-temperature phase the spin-lattice relaxation times indicate a thermally activated CD3 reorientation (activation energy ≈ 4 kJ/mol). Below the T1 minimum near 35 K the quadrupolar echo spectra develop features indicative for quantum mechanical tunneling. Dielectric measurements indicate the presence of dipole moment fluctuations with times scales that follow an Arrhenius law (energy barrier ≈ 107 kJ/mol). It is argued that the underlying motion involves small-angle molecular excursions and therefore remains undetected via stimulated-echo spectroscopy. The hysteresis accompanying the order/disorder transition taking place above 400 K was monitored using solid-echo line shapes as well as via spin relaxation times. In the high-temperature phase indications for the onset of anisotropic large-angle motions were obtained. © by Oldenbourg Wissenschaftsverlag, München.
    view abstract10.1524/zpch.2012.0333
  • Experimental studies of Debye-like process and structural relaxation in mixtures of 2-ethyl-1-hexanol and 2-ethyl-1-hexyl bromide
    Preu, M. and Gainaru, C. and Hecksher, T. and Bauer, S. and Dyre, J.C. and Richert, R. and Böhmer, R.
    Journal of Chemical Physics 137 (2012)
    Binary solutions of 2-ethyl-1-hexanol (2E1H) with 2-ethyl-1-hexyl bromide (2E1Br) are investigated by means of dielectric, shear mechanical, near-infrared, and solvation spectroscopy as well as dielectrically monitored physical aging. For moderately diluted 2E1H the slow Debye-like process, which dominates the dielectric spectra of the neat monohydroxy alcohol, separates significantly from the α-relaxation. For example, the separation in equimolar mixtures amounts to four decades in frequency. This situation of highly resolved processes allows one to demonstrate unambiguously that physical aging is governed by the α-process, but even under these ideal conditions the Debye process remains undetectable in shear mechanical experiments. Furthermore, the solvation experiments show that under constant charge conditions the microscopic polarization fluctuations take place on the time scale of the structural process. The hydrogen-bond populations monitored via near-infrared spectroscopy indicate the presence of a critical alcohol concentration, x c ≈ 0.5-0.6, thereby confirming the dielectric data. In the pure bromide a slow dielectric process of reduced intensity is present in addition to the main relaxation. This is taken as a sign of intermolecular cooperativity probably mediated via halogen bonds. © 2012 American Institute of Physics.
    view abstract10.1063/1.4755754
  • NMR and conductivity studies of the mixed glass former effect in lithium borophosphate glasses
    Storek, M. and Böhmer, R. and Martin, S.W. and Larink, D. and Eckert, H.
    Journal of Chemical Physics 137 (2012)
    Alkali ion charge transport has been studied in a series of mixed glass former lithium borophosphate glasses of composition 0.33Li 2O 0.67xB 2O 3 (1 - x)P 2O 5. The entire concentration range, 0.0 x 1.0, from pure glassy Li 2P 4O 11 to pure glassy Li 2B 4O 7 has been examined while keeping the molar fraction of Li 2O constant. Electrical conductivity measurements and nuclear magnetic resonance techniques such as spin relaxometry, line shape analysis, and stimulated-echo spectroscopy were used to examine the temperature and frequency dependence of the Li ion motion over wide ranges of time scale and temperature. By accurately determining motional time scales and activation energies over the entire composition range the ion dynamics and the charge transport are found to be fastest if the borate and the phosphate fractions are similar. The nonlinear variation of the charge conduction, the most notable feature of the mixed glass former effect, is discussed in terms of the composition dependence of network former units which determine the local glass structure. © 2012 American Institute of Physics.
    view abstract10.1063/1.4754664
  • Shear and dielectric responses of propylene carbonate, tripropylene glycol, and a mixture of two secondary amides
    Gainaru, C. and Hecksher, T. and Olsen, N.B. and Böhmer, R. and Dyre, J.C.
    Journal of Chemical Physics 137 (2012)
    Propylene carbonate and a mixture of two secondary amides, N-methylformamide and N-ethylacetamide, are investigated by means of broadband dielectric and mechanical shear spectroscopy. The similarities between the rheological and the dielectric responses of these liquids and of the previously investigated tripropylene glycol are discussed within a simple approach that employs an electrical circuit for describing the frequency-dependent behavior of viscous materials. The circuit is equivalent to the Gemant-DiMarzio-Bishop model, but allows for a negative capacitive element. The circuit can be used to calculate the dielectric from the mechanical response and vice versa. Using a single parameter for a given system, good agreement between model calculations and experimental data is achieved for the entire relaxation spectra, including secondary relaxations and the Debye-like dielectric peak in the secondary amides. In addition, the predictions of the shoving model are confirmed for the investigated liquids. © 2012 American Institute of Physics.
    view abstract10.1063/1.4740236
  • Diluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane: II. A comparison of rotational and translational motions
    Lederle, C. and Hiller, W. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 134 (2011)
    Mixtures of the monohydroxy alcohol n-butanol with n-bromobutane are investigated via dielectric and nuclear magnetic resonance (NMR) techniques. Static- and pulsed-field gradient proton NMR yielded self-diffusion coefficients as a function of concentration and temperature. To monitor reorientational motions, broadband dielectric and 13C-spin relaxation time measurements were carried out. The latter demonstrate that the structural relaxation stems from the motion of the alkyl chains. By combining data from translational diffusion coefficients with published shear viscosities, hydrodynamic radii were determined that compare favorably with the van der Waals radii of single molecules. The results for the neat alcohol and for the binary mixtures are discussed with respect to a recent transient chain model. The approach of Debye and structural relaxation times at high temperatures, identified as a general feature of monohydroxy alcohols, is also discussed within that framework. © 2011 American Institute of Physics.
    view abstract10.1063/1.3549123
  • Hydrogen-bond equilibria and lifetimes in a monohydroxy alcohol
    Gainaru, C. and Kastner, S. and Mayr, F. and Lunkenheimer, P. and Schildmann, S. and Weber, H.J. and Hiller, W.c and Loidl, A. and Böhmer, R.
    Physical Review Letters 107 (2011)
    Dielectric loss spectra covering 13 decades in frequency were collected for 2-ethyl-1-hexanol, a monohydroxy alcohol that exhibits a prominent Debye-like relaxation, typical for several classes of hydrogen-bonded liquids. The thermal variation of the dielectric absorption amplitude agrees well with that of the hydrogen-bond equilibrium population, experimentally mapped out using near infrared (NIR) and nuclear magnetic resonance (NMR) measurements. Despite this agreement, temperature-jump NIR spectroscopy reveals that the hydrogen-bond switching rate does not define the frequency position of the prominent absorption peak. This contrasts with widespread notions and models based thereon, but is consistent with a recent approach. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.107.118304
  • Mixed-cation LixAg1-xPO3 glasses studied by Li6, Li7, and Ag109 stimulated-echo NMR spectroscopy
    Faske, S. and Koch, B. and Murawski, S. and Küchler, R. and Böhmer, R. and Melchior, J. and Vogel, M.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We show that NMR stimulated-echo experiments provide detailed information about the jump dynamics of each of the ionic species in mixed mobile ion glasses. The potential of this technique is exploited to measure two-time correlation functions of the lithium and silver ionic hopping motions in Li xAg1-xPO3 glasses. Comparison of stimulated-echo decays from Li6 or Li7 NMR with that from Ag109 NMR shows that the residence times at the ionic sites are significantly longer for the respective minority component than for the majority component at both ends of the composition range, while lithium and silver ions exhibit similar jump rates for x=0.5. Substitution of silver by lithium results in a strong and continuous slowdown of the silver ionic jumps, whereas the lithium ionic jumps show a weaker dependence on the glass composition. In the vicinity of the conductivity minimum, the activation energies obtained from the stimulated-echo studies for both lithium and silver ionic jumps are significantly smaller than that obtained from the dc conductivity. This suggests that mixing of cation species promotes differences between short-range and long-range ionic motions. For all studied glass compositions and for both lithium and silver, we can rule out the existence of a significant fraction of truly immobile cations. However, broad distributions of jump rates lead to strongly nonexponential correlation functions of the ionic hopping motion. Interestingly, the correlation functions become more and more exponential when the observed cation species is successively replaced by the unobserved cation species. The present results suggest that dynamical heterogeneities and correlations of ionic motions, which involve like and unlike ions and length scales of several interatomic distances, are important aspects of ion dynamics in mixed mobile ion glasses. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.024202
  • NMR and dielectric studies of hydrated collagen and elastin: Evidence for a delocalized secondary relaxation
    Lusceac, S.A. and Rosenstihl, M. and Vogel, M. and Gainaru, C. and Fillmer, A. and Böhmer, R.
    Journal of Non-Crystalline Solids 357 (2011)
    Using a combination of dielectric spectroscopy and solid-state deuteron NMR, the hydration water dynamics of connective tissue proteins is studied at sub-ambient temperatures. In this range, the water dynamics follows an Arrhenius law. A scaling analysis of dielectric losses, 'two-phase' NMR spectra, and spin-lattice relaxation times consistently yield evidence for a Gaussian distribution of energy barriers. With the dielectric data as input, random-walk simulations of a large-angle, water reorientation provide an approximate description of stimulated-echo data on hydrated elastin. This secondary process is quasi-isotropic and delocalized. The delocalization is inferred from previous NMR diffusometry experiments. It is emphasized that the phenomenology of this process is shared by many non-aqueous binary glasses in which the constituent components exhibit a sufficient dynamical contrast. © 2010 Published by Elsevier B.V.
    view abstract10.1016/j.jnoncrysol.2010.07.035
  • Nuclear magnetic resonance and dielectric noise study of spectral densities and correlation functions in the glass forming monoalcohol 2-ethyl-1-hexanol
    Schildmann, S. and Reiser, A. and Gainaru, R. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 135 (2011)
    The spectral densities related to various relaxation processes of the glass former 2-ethyl-1-hexanol (2E1H), a monohydroxy alcohol, are probed using several nuclear magnetic resonance (NMR) experiments as well as via dielectric noise spectroscopy (DNS). On the basis of the spectral density relating to voltage fluctuations, i.e., without the application of external electrical fields, DNS enables the detection of the structural relaxation and of the prominent, about two decades slower Debye process. The NMR-detected spectral density, sensitive to the orientational fluctuations of the hydroxyl deuteron, also reveals dynamics slower than the structural relaxation, but not as slow as the Debye process. Rotational and translational correlation functions of 2E1H are probed using stimulated-echo NMR techniques which could only resolve the structural dynamics or faster processes. The experimental results are discussed with reference to models that were suggested to describe the dynamics in supercooled alcohols. © 2011 American Institute of Physics.
    view abstract10.1063/1.3647954
  • Surface and confinement effects on the dielectric relaxation of a monohydroxy alcohol
    Gainaru, C. and Schildmann, S. and Böhmer, R.
    Journal of Chemical Physics 135 (2011)
    2-ethyl-1-hexanol (2E1H) was confined to the surface of a collagen matrix at various concentration levels c. Dielectric spectroscopy revealed that upon decreasing c, the alcohols prominent hydrogen-bond mediated Debye-like relaxation broadens and turns nonexponential. This destabilization of the supramolecular association is accompanied by an increasing relative strength of the structural relaxation in 2E1H up to a point beyond which the two processes are merged when the solvent molecules are sufficiently diluted. These results demonstrate that the contribution of the Debye-like relaxation can be completely suppressed and concomitantly the limit of a simple, nonassociating liquid is reached. Confinement of the alcohol in a monolithic glass with nanoscopic pores subjected to different internal surface treatments is also demonstrated to bear a large impact on the relative strengths of the two processes. © 2011 American Institute of Physics.
    view abstract10.1063/1.3655356
  • A dielectric study of oligo- and poly(propylene glycol)
    Gainaru, C. and Hiller, W. and Böhmer, R.
    Macromolecules 43 (2010)
    The segmental and normal mode dynamics of poly(propylene glycol) (PPG) were studied using dielectric spectroscopy for a wide range of molecular weights. For intermediate chain lengths the normal mode spectra, unmasked by electrical purging procedures, were described quantitatively using the Rouse model. Based on the ratio of the dispersion strengths of the normal and of the segmental mode, the characteristic ratio C∞, was determined. The spectral width of the segmental mode evolves smoothly with the number of repeat units N and, except for dipropylene glycol, so does the mean time scale. The exceptional behavior was confirmed by 13C NMR relaxation experiments. For small N the segmental and the normal modes tend to merge near the glass transformation range. Exploiting the polymer chains as "molecular rulers", the growth of the characteristic length scale associated with the vitrification process is estimated for PPG. In the glassy state two dipolarly active relaxation processes were resolved. The slower one is characterized by an activation energy typical for hydrated systems, and its dispersion strength can be modified by appropriate heat treatment. © 2010 American Chemical Society.
    view abstract10.1021/ma9026383
  • Broadened dielectric loss spectra and reduced dispersion strength of viscous glycerol in a connective tissue protein
    Fillmer, A. and Gainaru, C. and Böhmer, R.
    Journal of Non-Crystalline Solids 356 (2010)
    Dielectric spectroscopy is used to study collagen-glycerol systems at different concentration levels for temperatures above the glass transition of the solvent. The mean dielectric relaxation times of the mixed systems exhibit essentially the same temperature dependence as that of pure glycerol. The presence of collagen does, however, broaden the distribution of relaxation times significantly. The static dielectric constant of the solvated samples is considerably smaller than that of neat glycerol. This is explained semi-quantitatively by a simple approach. Phenomenological analyses indicate a weak, but not completely unambiguous decoupling of conductivity from structural relaxation times. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jnoncrysol.2009.04.078
  • Comment on "hidden slow dynamics in water"
    Gainaru, C.P. and Böhmer, R.
    Physical Review Letters 104 (2010)
    view abstract10.1103/PhysRevLett.104.249803
  • Communications: High-pressure dielectric scaling study of a monohydroxy alcohol
    Reiser, A. and Kasper, G. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 132 (2010)
    Glass forming monohydroxy alcohols show an exponential Debye-type dielectric relaxation, which proceeds slower than the structural relaxation. Dielectric high-pressure measurements of 2-ethyl-1-hexanol reveal that isochronal scaling is violated. Temperature-volume scaling was found to be valid separately for the Debye-type and for the structural relaxation, yielding a scaling exponent of γ≈1.8. © 2010 American Institute of Physics.
    view abstract10.1063/1.3421555
  • Coupling of the electrical conductivity to the structural relaxation, absence of physical aging on the time scale of the Debye process, and number of correlated molecules in the supercooled monohydroxy alcohol 2-ethylhexanol
    Gainaru, C. and Böhmer, R.
    Journal of Non-Crystalline Solids 356 (2010)
    Using dielectric experiments the structural, the Debye-like, and the conductivity relaxation times of 2-ethyl-1-hexanol are measured in a wide temperature range. It is found that the electrical transport scales with the temperature dependence of the structural relaxation but that it is decoupled from that of the Debye-like response. Temperature-jump experiments were carried out, and signs of a physical aging on the time scale of the Debye-like response could not be detected. The minimum number of correlated molecules contributing to the structural and to the Debye-like responses was estimated on the basis of the temperature derivative of the susceptibility as expressed in terms of the fragility index and of the spectral line shape parameters. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jnoncrysol.2009.07.030
  • Deuteron nuclear magnetic resonance and dielectric study of host and guest dynamics in KOH-doped tetrahydrofuran clathrate hydrate
    Nelson, H. and Nowaczyk, A. and Gainaru, C. and Schildmann, S. and Geil, B. and Böhmer, R.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    The host as well as the guest dynamics in ion doped clathrate hydrates were studied via several deuteron nuclear magnetic resonance techniques and using broadband dielectric spectroscopy in conjunction with the application of large electrical fields. At a given temperature evidence for up to three relaxation processes was found for samples with mole fractions larger than 10-4 KOH. The two slower processes, unraveled via an electrical cleaning procedure, are similar to those detected on undoped samples in which they proceed on slightly longer time scales. The fastest process exhibits a weak temperature dependence except close to the transition into the low-temperature phase. Here, an incomplete proton order is established on the hydrate lattice and a residual orientational motion of the guest molecules could be detected at low temperatures. These results demonstrate the large degree of coupling between host and guest motions. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.224206
  • Energy landscape in molecular glasses probed by high-resolution dielectric experiments
    Gainaru, C. and Böhmer, R. and Kahlau, R. and Rössler, E.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    Dielectric measurements between the glass transition temperature Tg and about 3 K are reported for a supercooled plastic crystal and several organic molecular glasses that exhibit a secondary loss peak (β process) close to Tg. Above about 10 K and up to Tg /2, the dynamics can be described as a thermally activated process taking place within an energy landscape characterized by a bimodal distribution of barrier heights. This energy landscape is revealed by subjecting the dielectric loss data to an appropriate scaling involving a single free parameter. At temperatures above about Tg /2, the scaling deteriorates for most glasses due to the emergence of the so-called excess wing. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.104205
  • Lon sweeping in conducting dielectric materials
    Gainaru, C. and Bohmer, R. and Williams, G.
    European Physical Journal B 75 (2010)
    Conductivity-related low-frequency dielectric losses frequently obscure loss peaks arising from dipole relaxations in dielectric materials. The application of moderately large electrical fields to ion containing liquids and solids in combination with temperature cycling enables one to reduce the contribution of conductivity to dielectric loss spectra significantly. Details of this electrical cleaning method are given. Its application is demonstrated and discussed for a diverse array of materials ranging from polymeric and small-molecule supercooled liquids to hydrated proteins and ice-like crystals. The suppression of conductivity-related losses allows one to gain insights into the low-frequency dynamics of such materials. The mobility of the ionic impurities at the base temperature and at the 'cleaning' temperature are briefly discussed. © EDP Sciences, Societa Italiana di Fisica, Springer-Verlag 2010.
    view abstract10.1140/epjb/e2010-00142-2
  • Nuclear-magnetic-resonance measurements reveal the origin of the debye process in monohydroxy alcohols
    Gainaru, C. and Meier, R. and Schildmann, S. and Lederle, C. and Hiller, W. and Rössler, E.A. and Böhmer, R.
    Physical Review Letters 105 (2010)
    Monohydroxy alcohols show a structural relaxation and at longer time scales a Debye-type dielectric peak. From spin-lattice relaxation experiments using different nuclear probes, an intermediate, slower-than-structural dynamics is identified for n-butanol. Based on these findings and on translational diffusion measurements, a model of self-restructuring, transient chains is proposed. The model is demonstrated to explain consistently the so-far puzzling observations made for this class of hydrogen-bonded glass forming liquids. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.105.258303
  • dielectric liquids

  • glass

  • hydrogen bonds

  • nuclear magnetic resonance

  • solid body electrolyte

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