Identification of Structural Relaxation in the Dielectric Response of Water (original) (raw)
Related papers
Dielectric Relaxation of Biological Water †
The Journal of Physical Chemistry B, 1997
Dielectric relaxation and NMR spectrum of water in biological systems such as proteins, DNA, and reverse micelles can often be described by two widely different time constants, one of which is in the picosecond while the other is in the nanosecond regime. Although it is widely believed that the bimodal relaxation arises from water at the hydration shell, a quantitative understanding of this important phenomenon is lacking. In this article we present a theory of dielectric relaxation of biological water. The time dependent relaxation of biological water is described in terms of a dynamic equilibrium between the free and bound water molecules. It is assumed that only the free water molecules undergo orientational motion; the bound water contribution enters only through the rotation of the biomolecule, which is also considered. The dielectric relaxation is then determined by the equilibrium constant between the two species and the rate of conversion from bound to free state and vice versa. However, the dielectric relaxation in such complex biomolecular systems depends on several parameters such as the rotational time constant of the protein molecule, the dimension of the hydration shell, the strength of the hydrogen bond, the static dielectric constant of the water bound to the biomolecule, etc. The present theory includes all these aspects in a consistent way. The results are shown to be in very good agreement with all the known results. The present study can be helpful in understanding the solvation of biomolecules such as proteins.
Nonlinear dielectric relaxation of polar liquids
Journal of Molecular Liquids
Molecular dynamics of two water models, SPC/E and TIP3P, at a number of temperatures is used to test the Kivelson-Madden equation connecting single-particle and collective dielectric relaxation times through the Kirkwood factor. The relation is confirmed by simulations and used to estimate the nonlinear effect of the electric field on the dielectric relaxation time. We show that the main effect of the field comes through slowing down of the single-particle rotational dynamics and the relative contribution of the field-induced alteration of the Kirkwood factor is insignificant for water. Theories of nonlinear dielectric relaxation need to mostly account for the effect of the field on rotations of a single dipole in a polar liquid.
Collective contributions to the dielectric relaxation of hydrogen-bonded liquids
The Journal of Chemical Physics, 2004
Dielectric relaxation times are often interpreted in terms of the reorientation of dipolar species or aggregates. The relevant time correlation function contains, however, cross terms between dipole moments of different particles. In the static case, these cross terms are accounted for by the Kirkwood factor g K. Theories and molecular dynamics simulations suggest that such cross correlations may also affect the time-dependent properties, as reflected in the dielectric spectra. We present an experimental method for detecting effects of such cross correlations in dielectric spectra by a comparative analysis of dielectric and magnetic relaxation data. We demonstrate that such collective contributions can substantially affect dielectric relaxation. Experiments for n-pentanol (g K ϭ3.06 at 298 K͒ and 2,2-dimethyl-3-ethyl-pentane-3-ol (g K ϭ0.59) and their solutions in carbon tetrachloride show that in systems with g K Ͼ1, the cross correlations slow down dielectric relaxation. In systems with g K Ͻ1, dielectric relaxation is enhanced. The results conform to theoretical predictions by Madden and Kivelson ͓Adv. Chem. Phys. 56, 467 ͑1984͔͒ and to results of molecular dynamics simulations. The relaxation enhancement by cross terms in the case of g K Ͻ1 is difficult to rationalize by conventional models of dielectric relaxation.
The Journal of Chemical Physics, 2004
We have measured the dielectric relaxation of several glass forming branched alkanes with very low dielectric loss in the frequency range 50 Hz-20 kHz. The molecular liquids of this study are 3-methylpentane, 3-methylheptane, 4-methylheptane, 2,3-dimethylpentane, and 2,4,6-trimethylheptane. All liquids display asymmetric loss peaks typical of supercooled liquids and slow  relaxations of similar amplitudes. As an unusual feature, deliberate doping with 2-ethyl-1-hexanol, 5-methyl-2-hexanol, 2-methyl-1-butanol, 1-propanol, or 2-methyltetrahydrofuran at the 1 wt % level generates additional relaxation peaks at frequencies below those of the ␣ relaxation. The relaxation times of these sub-␣-peaks increase systematically with the size of the dopant molecules. Because these features are spectrally separate from the bulk dynamics, the rotational behavior and effective dipole moments of the probes can be studied in detail. For the alcohol guest molecules, the large relative rotational time scales and small effective dipole moments are indicative of hydrogen bonded clusters instead of individual molecules.
Contribution to Understanding of the Molecular Dynamics in Liquids
The Journal of …, 2007
The dielectric relaxation spectroscopy is used for studying the orientational molecular dynamics in the isotropic (I) and nematic (N) phases of two mesogenic liquids composed of the molecules of similar structure and length, but of an essentially different polarity: n-heptylcyanobiphenyl, C 7 H 15 PhPhCN, 7CB (molecular dipole moment µ ≈ 5D) and 4-(trans-4′-n-hexylcyclohexyl)isothiocyanatobenzene, C 6 H 13 CyHxPhNCS, 6CHBT (µ ≈ 2.5D); advantageously, the temperatures of the I-N phase transition for the two compounds are very close to each other (T NI ) 316.6 ( 0.2 K). It is shown that regardless of the differences in polarity of 7CB and 6CHBT molecules and their abilities in dipolar aggregation, the values and temperature dependences of the relaxation time (corresponding to the rotational diffusion of the molecules around their short axis) are very close to each other, in both the isotropic and nematic phases of the liquids studied. Therefore, the data show that the dielectric relaxation processes occurring in dipolar liquids in the isotropic and nematic states lead through the rotational diffusion of individual molecules and the diffusion seems to be not influenced by the intermolecular interactions.
It is well established that many mono-hydroxy alcohols show an extra relaxation process of the Debye type in addition to the signatures of primary and secondary structural relaxations, which is observed only in dielectric spectroscopy and related techniques. In order to gain further insight into the nature of this Debye peak, we study the linear and nonlinear dielectric behavior of a series of isomeric octyl alcohols and of mixtures of n-propanol with one of the octanols. These samples display systematic variations of the Debye peak intensity and concomitant changes in the Kirkwood correlation factor g K from 0.1 to 4, indicative of different equilibrium constants, K c/r , that characterize the populations of non-polar ring and polar open chain structures. For cases where K c/r is not too far from unity, we find that a high electric field shifts K c/r towards more chains, and that the accompanying change in the end-to-end vector of hydrogen-bond connected structures occurs on the Debye time scale. The results suggest that g K is correlated with the spectral separation of the Debye and primary structural peaks, as both features depend on steric hindrance of chain flexibility or bond rotation barriers and on average chain lengths. Based on the complex dynamics of supercooled mono-hydroxy alcohols with three relaxation peaks that cover many orders of magnitude in frequency, it is argued that a frequency dependent g K may be required for assessing the average orientational correlations within hydrogenbonded structures correctly.
Relation between macroscopic and microscopic dielectric relaxation times in water dynamics
2003
A simplified derivation for the ratio of macroscopic to microscopic relaxation times of polar liquids is based on the Mori-Zwanzig projection-operator technique, with added statistical assumptions. We obtain several useful forms for the lifetime ratio, which we apply to the dynamics of liquid water. Our theoretical single-molecule relaxation times agree with the second Debye relaxation times as measured by frequency-domain dielectric spectroscopy of water and alcohols. From the theory, fast relaxation modes couple to the Debye relaxation time, τD, through very large water clusters, and their temperature dependence is similar to that of τ D . Slower modes are localized to smaller water clusters and exhibit weaker temperature dependence. This is exemplified by the lifetime ratios measured by time-domain dielectric spectroscopy and optical Kerr effect spectroscopy, respectively.
Microwave Dielectric Relaxation of Alcohols in non polar solutions
IOSR Journal of Applied Physics, 2014
The properties of the binary mixtures of 1-propanol and phenol have been studied at constant temperature 303K in dilute solutions of benzene using standard standing wave microwave X-band (9.4 GHz) Jband (7.4 GHz) technique. The values of different dielectric parameters ε 0 , ε', ε'', ε ∞ have been determined for five different mole fractions of 1-propanol and phenol. The values of permittivity and dielectric loss are used to evaluate relaxation time for overall molecular rotation (τ 1), relaxation time for intermolecular rotations (τ 2), most probable relaxation time (τ 0) and dipole moment (µ) at constant temperature 303K. The values of relaxation times and dipole moment are found to increases with the mole fraction of 1-propanol, phenol in all binary mixtures. The energy parameter (ΔFτ) for dielectric relaxation process of the mixtures is also calculated. It is found that the dielectric relaxation process can be treated as a rate process. The present investigation suggest that existence of both the intermolecular and intramolecular orientation takes place in both binary mixtures.