Dynamics of glass-forming liquids. IX. Structural versus dielectric relaxation in monohydroxy alcohols

The prominent Debye-type but non-Arrhenius dielectric relaxation is a feature common to many monohydroxy alcohols in their liquid state. Although this exponential process is often considered to reflect the primary structural relaxation, only a faster, smaller, and nonexponential relaxation peak correlates with viscous flow and mechanical relaxation. We provide dielectric relaxation data for 2-methyl-1-butanol, 2-ethyl-1-hexanol, and 3,7-dimethyl-1-octanol across ten decades in time. Based on these and previous results, we show that there exists a variety of dielectric to mechanical relaxation time ratios in the viscous regime, but a universal value of 100 for that ratio appears to evolve in the high temperature limit. The temperature dependence for both the relaxation time and strength of the Debye peak differs from the typical behavior of structural dynamics in terms of the alpha process. The implications of these findings for rationalizing the Debye-type dielectric process of hydrogen-bonded liquids are discussed.     

Dielectric Dispersion and Electric Relaxation Processes Induced by Ionic Conduction in Formamide, 2-Aminoethanol and Their Binary Mixtures

The complex dielectric permittivity, ionic conductivity, electric modulus and impedance spectra of the dipolar molecules formamide (FA), 2-aminoethanol (AE) and their binary mixtures were investigated in the frequency range from 20 Hz to 1 MHz at 303.15 K. Debye-type distributions of the frequency dependent electric modulus and complex impedance were found, corresponding to an ionic conduction relaxation process in the upper frequency regime of the spectra, whereas a spike in the impedance spectra at low frequencies confirms the contribution of an electrode polarization (EP) relaxation process induced by ionic conduction. Due to the high static permittivity of FA, its ionic conductivity was found more than one order of magnitude higher than that of the AE, which is also shown by the comparative values of their EP and ionic conductivity relaxation times. The dependences of dc ionic conductivity values of the binary mixtures on their relaxation times and static permittivity were explored. The concentration dependent static permittivity and the relaxation times led us to infer the formation of a 1:1 H-bonded stable complex between FA and AE molecules with reduction in the number of effective parallel-aligned dipoles.     

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.     

Dielectric spectroscopy on aqueous solutions of pyridine and its derivatives

The complex (dielectric) permittivity has been measured as a function of frequency between 1 MHz and 40 GHz for aqueous solutions of pyridine, 2- and 3-methylpyridine, as well as 2,4- and 2,6-dimethylpyridine at various temperatures and solute concentrations. Different relaxation spectral functions are used to analytically represent the data, in particular the Cole-Cole function. The solute contribution to the extrapolated static permittivity has been calculated to show that, in correspondence with other aqueous solutions of organic molecules and ions, the permittivity of the solvent seems to be enhanced with respect to the pure water value. Also in accordance with other aqueous systems it is found that the principal dielectric relaxation time for equimolar solutions of stereo isomers at the same temperature may significantly differ from one another. A further result is the finding of an unusually strong temperature dependence in the relaxation time of the 1 molar solution of 2,6-dimethylpyridine.     

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.     

Ultrasonic absorption spectroscopy between 5 and 3100 MHz on some aqueous polyelectrolyte solutions

The ultrasonic absorption coefficient has been measured as a function of frequency between 5 MHz and 3.1 GHz for aqueous solutions of polyacrylic acid and of its sodium, potassium, and tetraethylammonium salts. Unlike an aqueous solution of propionic acid, all polymer solutions clearly exhibit excess absorption. Within the frequency range under consideration the excess absorption spectra can be analytically represented by two Debye-type relaxation terms. At 25°C the corresponding relaxation times adopt values between 3 and 12.4 ns, and between 0.12 and 0.22ns, respectively. The former process is discussed in accordance with previous models. The relaxation of the polyacrylic acid solutions is assumed to be related to the formation of hydrogen bonds of the polymeric molecules and that of the polyacrylate solutions may be due to interactions of counterions with chain segments. The latter process, the existence of which has been first proven in this study, is likely to reflect rotational motions of carboxyl groups.     

Hydrogen-bond configuration in hydrogen-bond solutions. 1. Alcohol/alcohol solutions

A theory based on the hydrogen-bond configuration is proposed and applied to alcohol/alcohol binary solutions. The theory leads explicit expressions for the mixing Gibbs energy and reproduces the experiments on the crystal–liquid phase-diagrams of pure crystals and co-crystals and mixing heats with the parameters common to these experiments. The mixing entropy arises from the increase in the hydrogen-bonding availability of proton donors to approach hydrogen-bond-free proton acceptors. The mixing heat arises from a balance between the contribution from maintaining the original associations in pure liquids and the contribution from a construction of hydrogen bonds freely to hydrogen-bond-free acceptors. When hydrogen-bond associations between component-1 and component-2 are distinguished statistically from associations in each pure component, we call the solution as a cooperative solution that has at least one stoichiometric cooperative concentration point. Some shorter alcohol/alcohol solutions and some aromatic alcohol/aromatic alcohol solutions, however, have no cooperative point and we call those solutions as the ideal hydrogen-bond solutions of which properties are mainly governed by the ideal-gas-like mixing hydrogen-bond entropy. The hydrogen-bond energies of various combinations of the proton acceptor and the proton donor have been estimated consistently from the fittings of the theory, the shifts by hydrogen bonding of the OH stretching in the Raman or IR spectroscopy, and the sublimation energy of crystals. The present theory reveals the characteristics of hydrogen-bond solutions and gives some predictions.     

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.     

Hopping rates and concentrations of mobile fluoride ions in Pb(1-x)Sn(x)F(2) solid solutions

In the present paper, the ion dynamics and relaxation of fluoride ions in Pb(1-x)Sn(x)F(2) (with x=0.2-0.6) solid solutions, prepared by mechanochemical milling, are studied in the conductivity formalism over wide ranges of frequencies and temperatures. The conductivity spectra of the investigated materials are analyzed by the Almond-West (AW) power-law model. The estimated values of the hopping rates and the dc conductivity of different compositions are thermally activated with almost the same activation energy. The calculated values of the concentration of mobile ions, n(c), are almost independent of temperature and composition for x=0.2-0.4. The maximum value of n(c) is obtained for the x=0.6 sample, although it does not show the maximum conductivity. Therefore, the composition dependence of the ionic conductivity of these solid solutions could be explained based on the extracted parameters. The results presented in the current work indicate that the AW model represents a reasonable approximation of the overall frequency-dependent conductivity behavior of the investigated materials. The conductivity spectra at different temperatures for each composition are successfully scaled to a single master curve, indicating a temperature-independent relaxation mechanism. For different compositions, however, the conductivity spectra cannot be scaled properly, indicating composition-dependent relaxation dynamics.     

Appearance of a Debye process at the conductivity relaxation frequency of a viscous liquid

The existence of a Debye-type ultraslow process in dielectric spectra of bulk polyalcohols and similar materials has been reported repeatedly in the recent literature. Its loss peak is observed at frequencies that are decades below those of the primary structural relaxation, in a range where the loss signal is usually dominated by dc-conductivity or even electrode polarization. We show that this peak originates from an incomplete filling of the capacitor volume, e.g., as a result of gas bubbles, a situation that gives rise to a Debye process at the conductivity relaxation frequency of the material, where the values of storage and loss components of permittivity are identical. The result implies that these peaks are not endemic to the liquid and can lead to various misinterpretations of the dielectric relaxation spectra. Techniques avoiding the occurrence of such artifacts are discussed.     

On the visco-elastic and dielectric behaviour of two alcohols and their binary mixture

Inter-molecular and intra-molecular interactions in liquids determine the physical properties of the systems. These interactions are understood through the measurement of these physical properties. These become especially important in the case of alcohols in view of the specific type of interactions involved. Study of the variation of dielectric relaxation time with the viscosity of the medium is relevant in drawing certain quantitative conclusions regarding molecular motion and the inter-molecular forces in liquids, liquid mixtures, dilute solutions and multi-component polar solutes in dilute solution. In the absence of a perfect empirical or theoretical equation for the variation of dielectric relaxation time with viscosity, the experimental investigations on different systems can only give an insight. In the present study, the results of dielectric measurements carried out on pure samples of methyl alcohol and propyl alcohol in dilute solutions in different mixed solvents (benzene?+?paraffin) and on binary mixture (1?:?1) of methyl alcohol?+?propyl alcohol are reported. Different parameters determined are presented and these studies indicate that the dielectric behaviour at microwave frequencies favour the concept of dynamic viscosity and a single visco-elastic relaxation time for the systems under study.     

Beyond the first hydration shell: dielectric study of LiCl aqueous solution

The dielectric constant for lithium chloride (LiCl) in aqueous solution with the entire concentration has been determined in the frequency range 0.5 GHz–50GHz at 298 K by dielectric relaxation spectroscopy (DRS). The system behaviour is described according to the Cole–Cole and two Debye-type relaxation functions whose evolution with composition is analysed. Combining the Cole–Cole and two Debye-type relaxation functions, the results of the dielectric properties of aqueous LiCl solution are presented and discussed. The maximum number of water molecules perturbed by ions in the hydration shell decays with its concentration. Using the extended Froehlich theory, it is concluded that the water structure is perturbed by ions beyond the first hydration shell in LiCl aqueous solution system.     

Temperature-dependent relaxation study of tertiary butyl alcohol–water mixtures using TDR technique

Using time-domain reflectometry (TDR) technique, we have measured the complex permittivity of tertiary butyl alcohol (TBA)–water mixtures in the frequency range of 10 MHz–30GHz, at temperatures 15°C, 20°C and 25°C. The complex permittivity of TBA–water mixture shows Debye-type behaviour. The dielectric parameters such as dielectric constant and relaxation time were obtained from the complex permittivity spectra. The Kirkwood correlation factor and Bruggeman factor have also been determined to investigate inter- and intramolecular interaction among associating liquids.     

Electro-optic,dielectric and optical studies of NiFe2O4-ferroelectric liquid crystal: a soft magnetoelectric material

In the present study, magnetic nanoparticles (NP, nickel ferrite) in different concentrations into ferroelectric liquid crystal (FLC) mixture have been prepared and studied. The effect of nickel ferrite concentration on the electro-optic, dielectric and optical properties of FLC mixture has been studied and discussed. An improvement in spontaneous polarization, response time in nickel ferrite-FLC-doped samples compared to FLC is observed and explained on the basis of dipole moment and anchoring phenomena. The Goldstone mode (GM) is detected in all samples and follows a Debye-type relaxation behaviour. A twofold increase in relaxation frequency for the doped sample rather than the pure sample has been observed. The band gap was found more or less independent of doping concentration. The activation energy (E_a) also decreases on increasing the doping amount.     

Dynamic polarization of solvent nuclei in solutions of diphenylpicrylhydrazyl in organic liquids

Measurements have been made of the dynamic polarization of nuclei of solvent in concentrated solutions of diphenylpicrylhydrazyl (DPPG) in a weak magnetic field of approximately 21 Oe. In proton-containing solutions, a negative dynamic polarization was observed, corresponding to dipole-dipole interaction of electronic and nuclear spins. In a hexafluorobenzene solution a considerable contribution of scalar interaction to the relaxation of the fluorine nucleus was observed. The temperature dependence of the dynamic polarization of the fluorine nucleus has been investigated, and an activation energy of 3.3 kcal/mole has been found for the bond between the molecules of hexafluorobenzene and the solvate shell of DPPH molecules.     

聚吡咯微粒掺入的聚苯乙烯膜在电解质溶液中的介电弛豫谱及其解析

在40 Hz～11 MHz频率范围测量了聚苯乙烯膜以及混入聚吡咯粒子的聚苯乙烯膜和电解质溶液构成的体系的介电谱, 发现了特异的弛豫现象: 纯的和掺入导电性聚吡咯后的聚苯乙烯膜分别显示出单一弛豫和双弛豫的不同模式的介电谱. 在Maxwell-Wagner界面极化概念基础上解释了该弛豫机制: 高、低频弛豫分别由膜-液界面极化和膜相本身的不均一性引起的. 将体系进行了模型化, 并利用Hanai理论方法对谱进行了解析, 获得了内部电性质的诸多参数. 对不同聚吡咯掺入量的膜/溶液体系的介电测量和解析结果表明, 电解质溶液的种类、浓度以及膜中混入聚吡咯的量都影响着膜相的介电响应. 这些结论为利用加入导电粒子改善绝缘高分子聚合物的电性质的研究以及制备既具有导电功能又使基体的力学性能得到提高的高分子复合物提供了重要的线索.     

Critical fluctuations near the consolute point of n-pentanol-nitromethane. An ultrasonic spectrometry, dynamic light scattering, and shear viscosity study

Ultrasonic attenuation spectra, the shear viscosity, and the mutual diffusion coefficient of the n-pentanol-nitromethane mixture of critical composition have been measured at different temperatures near the critical temperature. The noncritical background contribution, proportional to frequency, to the acoustical attenuation-per-wavelength spectra has been determined and subtracted from the total attenuation to yield the critical contribution. When plotted versus the reduced frequency, with the relaxation rate of order-parameter fluctuations from the shear viscosity and diffusion coefficient measurements, the critical part in the sonic attenuation coefficient displays a scaling function which nicely fits to the data for the critical system 3-methylpentane-nitromethane and also to the empirical scaling function of the Bhattacharjee-Ferrell dynamic scaling theory. The scaled half-attenuation frequency follows from the experimental data as Omega(1/2)emp= 1.8+/-0.1. The relaxation rate of order-parameter fluctuation shows power-law behavior with the theoretically predicted universal exponent and the extraordinary high amplitude Gammao= (187+/-2) x 10(9) s(-1). The amount of the adiabatic coupling constant /g/= 0.03, as estimated from the amplitude of the critical contribution to the acoustical spectra, is unusually small.     

Water dynamics in n-propylene glycol aqueous solutions

The relaxation dynamics of dipropylene glycol and tripropylene glycol (nPG-n=2,3) water solutions on the nPG-rich side has been studied by broadband dielectric spectroscopy and differential scanning calorimetry in the temperature range of 130-280 K. Two relaxation processes are observed for all the hydration levels; the slower process (I) is related to the alpha relaxation of the solution whereas the faster one (II) is associated with the reorientation of water molecules in the mixture. Dielectric data for process (II) at temperatures between 150 and 200 K indicate the existence of a critical water concentration (x(c)) below which water mobility is highly restricted. Below x(c), nPG-water domains drive the dielectric signal whereas above x(c), water-water domains dominate the dielectric response at low temperatures. The results also show that process (II) at low temperatures is due to local motions of water molecules in the glassy frozen matrix. Additionally, we will show that the glass transition temperatures (T(g)) for aqueous PG, 2PG, and 3PG solutions do not extrapolate to approximately 136 K, regardless of the extrapolation method. Instead, we find that the extrapolated T(g) value for water from these solutions lies in the neighborhood of 165 K.     

Nuclear magnetic resonance and dielectric noise study of spectral densities and correlation functions in the glass forming monoalcohol 2-ethyl-1-hexanol

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.     

Surface and confinement effects on the dielectric relaxation of a monohydroxy alcohol

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 alcohol's 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.