Effects of ions on the dielectric permittivity and relaxation rate and the decoupling of ionic diffusion from dielectric relaxation in supercooled liquid and glassy 1-propanol

A comparative study of ibuprofen and ketoprofen glass-forming liquids by molecular dynamics simulations

In this paper, structural and dynamical properties of ibuprofen and ketoprofen glass-forming liquids have been investigated by means of molecular dynamics simulations. Molecular mobility of both materials is analyzed with respect to the different inter-molecular linear/cyclic hydrogen bonding associations. For ibuprofen, the dominant organization is found to be composed of small hydrogen bonding aggregates corresponding to cyclic dimers through the carboxyl group. For ketoprofen, the propensity of cyclic dimers is significantly reduced by the formation of hydrogen bonds with the ketone oxygen of the molecule altering the hydrogen bond (HB) associating structures that can be formed and thus molecular dynamics. The issue of the presence/absence of the peculiar low frequency Debye-type process in dielectric relaxation spectroscopy (DRS) data in these materials is addressed. Results obtained from simulations confirm that the Debye process originates from the internal cis-trans conversion of the -COOH carboxyl group. It is shown that the specific intermolecular HB structures associated to a given profen control the main dynamical features of this conversion, in particular its separation from the α-process, which make it detectable or not from DRS. For ibuprofen, the possible role of the -CCCO torsion motion, more "local" than the -COOH motion since it is less influenced by the intermolecular HBs, is suggested in the microscopic origin of the quite intense secondary γ-relaxation process detected from DRS.

Liquid 1-propanol studied by neutron scattering, near-infrared, and dielectric spectroscopy

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.

Comparative dielectric studies on two hydrogen-bonded and van der Waals liquids

Broadband dielectric measurements were performed in a wide range of temperatures on glucose, maltose, and their acetyl derivatives. We have indicated that molecular dynamics above and below the glass transition temperature differ considerably for the hydrogen-bonded and van der Waals systems. We have shown that structural relaxation dispersions of D-glucose and maltose are broader than those obtained for peracetyl carbohydrates. Moreover, glass transition temperatures of the former systems are much higher than for the latter ones. In the glassy state of both glucose and its acetyl derivatives only one well-separated secondary relaxation process was identified. In the case of maltose and peracetyl maltose a completely different situation was observed. In the former carbohydrate two secondary modes were detected, whereas in the latter one only a faster relaxation process was visible in the glassy state. This finding is discussed in greater detail on the basis of density functional theory calculations.

Dynamics of the slow mode in the family of six-carbon monosaccharides monitored by dielectric spectroscopy

Broadband dielectric measurements performed on D-glucose, L-sorbose, D-fructose and D-galactose revealed that, except for the structural relaxation process, one can detect in the liquid phase of these carbohydrates a much slower relaxation mode. Recently we have demonstrated that in D-glucose this relaxation mode might be related to the long range correlation of density fluctuations (LRCDF), also called Fischer clusters (FC). Based on the dielectric data obtained for the four monosaccharides we were able to make a more general conclusion about the characteristic dielectric features of the slow mode in the whole family of carbohydrates. We found out that the timescale separation between structural and considered relaxation reaches up to six decades at the glass transition temperature and the dielectric strength decreases significantly with lowering temperature. Another very interesting feature of the slow process is that it can be described by an almost exponential response function. We have found out that the fragility of the slow process lies within the range m = 44-50. Finally, we have also shown that there is a close link between structural and slow relaxation.

Dielectric relaxation and crystallization of nanophase separated 1-propanol-isoamylbromide mixture

The effects of liquid-liquid phase separation on molecular relaxation of an apparently homogeneous mixture of 1-propanol and isoamylbromide has been studied by dielectric spectroscopy over a broad frequency and temperature range, and its crystallization kinetics investigated in real time. The mixture shows two widely separated relaxation processes, as before, with the faster relaxation due to the orientational diffusion of isoamylbromide and the slower due to that of 1-propanol. In the mixture, the scaled contribution to permittivity from orientation polarization, Deltaepsilon, of isoamylbromide is about the same as in the pure state, but that of 1-propanol decreases by a factor of approximately 3 at 120 K. As the temperature is decreased, this difference remains constant. The relaxation time, tau, of isoamylbromide and its distribution parameter remains the same as for the pure liquid, but that of 1-propanol is longer and increases with decrease in T, becoming approximately 130 times the pure liquid's value at 119 K. This is in contrast to the finding for an isomeric heptanol, whose tau had decreased. Extrapolation suggests that at T>151 K, tau of 1-propanol in the mixture may become less than that in the pure liquid (the isoamylbromide component crystallizes before this temperature could be reached). This indicates that Tg corresponding to tau of 10(3) s for 1-propanol in the mixture would be higher than in the pure liquid. Crystallization of the two components in the mixture occurs at different rates and 1-propanol remains partially uncrystallized while isoamylbromide completely crystallizes. tau of any remaining liquid isoamylbromide does not change in the presence of crystallized states while tau of residual liquid 1-propanol in the mixture is reduced. The mixture phase separates in submicron or nanosize aggregates of the alcohol in isoamylbromide, without affecting the latter's relaxation kinetics, while its own epsilon(s) decreases and tau increases. Consequences of the finding for various relaxation mechanisms are briefly described.

Structure-Dependent DC Conductivity and Relaxation Time in the Debye−Stokes−Einstein Equation

The basis for a modification of the Debye-Stokes-Einstein (DSE) equation between the dc conductivity, sigma(dc), and dielectric relaxation time, tau, has been examined by using broad-band dielectric spectroscopy of LiClO4 solutions in 5-methyl-2-hexanol and 1-propanol and of pure liquids. According to the DSE equation, the log sigma(dc)-log tau plots should have a slope of -1. We find that sigma(dc) begins to depend upon the structure of an electrolytic solution when a variation of solvent's equilibrium dielectric permittivity, epsilon(s), with temperature causes the ion population to vary. As a consequence of this intrinsic dependence, the log sigma(dc)-log tau plots do not obey the DSE equation. Inclusion of the effect of change in epsilon(s) on the DSE equation may be useful in analyzing the measured quantities in terms of Brownian diffusion of both ions and molecules in ultraviscous liquids. Proton translocation along a hydrogen bond contributes little to sigma(dc), which appears to be predominantly determined by the ion population in the two alcohols and the solutions. The effect is briefly discussed in the potential energy landscape paradigm of structure fluctuations, and it is suggested that the high-frequency shear modulus measurements of ionic solutions would help reveal the temperature-dependent deviation from the DSE equation.

Comparing calorimetric and dielectric polarization modes in viscous 2-ethyl-1-hexanol

Dielectric relaxation and dynamic heat capacity measurements are compared for 2-ethyl-1-hexanol near its glass transition temperature Tg in order to further clarify the origin of the prominent Debye-type loss peak observed in many monohydroxy alcohols and other hydrogen-bonding liquids. While the dielectric spectrum epsilon" displays two distinct polarization processes that are separated by a factor of 2000 in terms of the peak frequency, the heat capacity cp" shows only a single peak. The dielectric process with lower amplitude and higher peak frequency coincides with the calorimetric signal, whereas the large dielectric Debye signal is not associated with calorimetric modes. The authors conclude that the Debye process corresponds to a transition among states which differ in energy only in the case of an external electric field.

Relaxations and nano-phase-separation in ultraviscous heptanol-alkyl halide mixture

To gain insight into the effects of liquid-liquid phase separation on molecular relaxation behavior we have studied an apparently homogeneous mixture of 5-methyl-2-hexanol and isoamylbromide by dielectric spectroscopy over a broad temperature range. It shows two relaxation regions, widely separated in frequency and temperature, with the low-frequency relaxation due to the alcohol and the high-frequency relaxation due to the halide. In the mixture, the equilibrium dielectric permittivity epsilon(s) of the alcohol is 41% of the pure state at 155.7 K and epsilon(s) of isoamylbromide is approximately 86% of the pure state at 128.7 K. The difference decreases for the alcohol component with decreasing temperature and increases for the isoamylbromide component. The relaxation time tau of 5-methyl-2-hexanol in the mixture at 155.7 K is over five orders of magnitude less than in the pure state, and this difference increases with decreasing temperature, but tau of isoamylbromide in the mixture is marginally higher than in the pure liquid. This shows that the mixture would have two T(g)'s corresponding to its tau of 10(3) s, with values of approximately 121 K for its 5-methyl-2-hexanol component and approximately 108 K for its isoamylbromide component. It is concluded that the mixture phase separates in submicron or nanometer-size aggregates of the alcohol in isoamylbromide, without affecting the latter's relaxation kinetics, while its own epsilon(s) and tau decrease markedly.

Diluent Effects on the Debye-Type Dielectric Relaxation in Viscous Monohydroxy Alcohols

With the recognition that the Debye-type dielectric relaxation of liquid monohydroxy alcohols does not reflect the structural relaxation dynamics associated with the viscous flow and the glass transition, its behavior upon dilution is expected to differ from that of real alpha-processes. We have investigated the Debye-type dielectric relaxation of binary alcohol/alkane mixtures across the entire concentration range in the supercooled regimes. The focus is on 2-ethyl-1-hexanol in two nonpolar liquids, 3-methylpentane and squalane, which are more fluid and more viscous than the alcohol, respectively. The Debye relaxation is found to occur only for alcohol mole fractions x > 0.2 and is always accompanied by a non-Debye relaxation originating from the alcohol component. Prior to its complete disappearance, the Debye relaxation is subject to broadening. We observe that the Debye dynamics of 2-ethyl-1-hexanol is accelerated in the more fluid 3-methylpentane, while the more viscous squalane leads to longer Debye relaxation times. The present experiments also provide evidence that the breakdown of the Debye relaxation amplitude does not imply the absence of hydrogen-bonded structures.

Debye Type Dielectric Relaxation and the Glass Transition of Alcohols

Many hydrogen-bonded liquids, especially glass-forming cases, display a dielectric relaxation behavior that differs qualitatively from that of other simple liquids. The majority of models aimed at explaining this unusual dielectric behavior associate the prominent Debye process with structural relaxation, viscous flow, and the glass transition. We perform dielectric and calorimetric studies of glass-forming mixtures of 2-ethylhexylamine and 2-ethyl-1-hexanol across the entire composition range. The kinetic glass transition temperature derived from the large dielectric Debye peak decreases, whereas that of the much smaller and asymmetrically broadened peak increases upon addition of amine. Only the latter feature coincides with the calorimetric glass transition results, implying that molecular structure and dielectric polarization fluctuate on time scales that can differ by orders of magnitude in many hydrogen-bonding liquids.