Dielectric relaxation, hydration

It takes 10-u s, the normal dielectric relaxation time for water, to form the hydrated electron, and -10-9 s for the electron to disappear by reacting with the water molecule (the former is an overestimate, the latter an underestimate). 

Migus et al. s (1987) delineation of the formation of a primary species absorbing in the IR, which develops in -110 fs and which transforms to the well-known spectrum of the hydrated electron in -240 fs, which is consistent with the longitudinal dielectric relaxation time of water (Mozumder, 1969a, b). 

Paddison et al. performed high frequency (4 dielectric relaxation studies, in the Gig ertz range, of hydrated Nafion 117 for the purpose of understanding fundamental mechanisms, for example, water molecule rotation and other possible processes that are involved in charge transport. Pure, bulk, liquid water is known to exhibit a distinct dielectric relaxation in the range 10—100 GHz in the form of an e" versus /peak and a sharp drop in the real part of the dielectric permittivity at high / A network analyzer was used for data acquisition, and measurements were taken in reflection mode. 

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

The properties of zeolitic water and the behavior of the exchangeable cations can be studied simultaneously by dielectric measurements (5, 6). In X-type zeolites Schirmer et al. (7) interpreted the dielectric relaxation as a jump of cations from sites II to III or from sites II to II. Jansen and Schoonheydt found only relaxations of cations on sites III in the dehydrated zeolites (8) as well as in the hydrated samples (9). Matron et al. (10) found three relaxations, a, (, and 7, in partially hydrated and hydrated NaX. They ascribed them respectively to cations on sites I and II, on sites III, and to water molecules. 

Davidson and Ripmeester (1984) discuss the mobility of water molecules in the host lattices, on the basis of NMR and dielectric experiments. Water mobility comes from molecular reorientation and diffusion, with the former being substantially faster than the water mobility in ice. Dielectric relaxation data suggest that Bjerrum defects in the hydrate lattice, caused by guest dipoles, may enhance water diffusion rates. 

For si and sll, Davidson et al. (1977a, 1981) performed NMR spectroscopy and dielectric relaxation measurements where applicable, in order to estimate the barriers to molecular reorientation for simple hydrates of natural gas components, except carbon dioxide. Substantial barriers to rotation should also affect such properties as hydrate heat capacity. 

Neutron-scattering and dielectric relaxation studies [23] both indicate that the water molecules solvating monovalent exchangeable cations on montmorillonite are a little less mobile, in respect to translational and reorientational motion, than are water molecules in the bulk liquid. For example, as with vermiculite, neutron-scattering data show that no water molecule is stationary on the neutron-scattering time scale. In the one-layer hydrate of Li-montmorillonite, the residence time of a water molecules is about six times longer than in the bulk liquid, with a diffusive jump distance of about 0.35 nm, and a water molecules reorients its dipole axis about half... 

In order to understand the influence of alcohol on the zeolitization process, it is useful to summarize the structural aspects of alcohol-water mixtures. Considerable work has been done in this area. It is well-recognized that at low alcohol concentrations the viscosity, reciprocal self-diffusion coefficient, the dielectric relaxation time and NMR relaxation times of the water molecules are all greater than that of pure water.(21-241 These observations indicate that addition of alcohol to water at low levels leads to an increased structure of water.(25) This concept is also supported by X-ray diffraction studies(26) and is commonly referred to as hydrophobic hydration.(27) On a molecular level, this effect... 

Wachter, W., Buchner, R., and Hefter, G. Hydration of tetraphenylphosphonium and tetraphenylborate ions by dielectric relaxation spectroscopy. 7. Phys. Chem. B. 2006, 110,5147-5154. 

Fig. 13. Hydration dependence of protonic conduction. The dielectric relaxation time, Ts, is shown versus hydration, h, for lysozyme powders. The relaxation time is proportional to the reciprocal of the conductivity. (A) H20-hydrated samples solid curve, lysozyme without substrate , lysozyme with equimolar (GlcNAc)< at pH 7.0 , with 3x molar (G1cNAc)4 at pH 6.5. The relaxation time is nearly constant between pH 5.0 and 7.0. (B) HjO-hydrated samples solid curve, lysozyme without substrate 9, lysozyme with equimolar (GlcNAcb at pH 7.0. From Careri etal. (1985). 

The quantitative interpretation of the dielectric relaxation times is still not on a satisfactory basis. The earliest attempt in this direction was made on the basis of an ion-oriented hydration sheath, for the formation of which a calculated number of hydrogen-bonds must be broken. This breakage changes the equilibrium of species in the liquid, and statistical relationships connect the proportion of bonds broken, the equilibrium populations, and the relaxation time. From the observed shift of relaxation time one can calculate the number of molecules in the sheath, and show that for temperatures between 276 and 298 K it is approximately the same as the number calculated from the depression of the static permittivity (comparison in Figure 4 of ref. 54). This treatment is open to criticism on the following grounds ... 

Figure 3 (Left) Calorimetric and dielectric relaxation time of pure and KOH-doped THF hydrate crystal. (Right) A Cole-Cole plot of KOH-doped THF hydrate crystal.

The situation is quite similar to that of ice. A dielectric measurement on a KOH-doped THE showed that the relaxation time t for the reorientational motion was dramatically shortened by the dopant, possibly by creating a pair of the orientational defects proposed by Bjemim. Not only the absolute value of t but also the activation energy for the process decreased by the dopant, as shown in Fig. 3. The value of x at 70 K is 10- times smaller than that for pure (undoped) sample. This is the reason why the ordering transition has escaped from observation for a pure sample by a kinetic reason appeared now at 62 K in the doped sample by a catalytic action of the dopant within a reasonable time. Also given in the figure is a Cole-Cole plot of the dielectric permittivity of the KOH-doped THF hydrate. The distribution of dielectric relaxation times is much wider in the doped sample than in the pure sample. 

To summarize, three conclusions transpire from the nanoscale thermodynamics results (a) The interfacial tension between protein and water is patchy and the result of both nanoscale confinement of interfacial water and local redshifts in dielectric relaxation (b) the poor hydration of polar groups (a curvature-dependent phenomenon) generates interfacial tension, a property previously attributed only to hydrophobic patches and (c) because of its higher occurrence at protein-water interfaces, the poorly hydrated dehydrons become collectively bigger contributors to the interfacial tension than the rarer nonpolar patches on the protein surface. 

Figure 1 shows the dielectric relaxation properties of the Tween microemulsions plotted on the complex permittivity plane (from Foster et al ( 1). The mean relaxation frequency (corresponding to the peak of each semicircle) decreases gradually from 20 GHz for pure water at 25°C to ca. 2 GHz for a concentrated microemulsion containing 20% water. Since the permittivity of the suspended oil/ emulsifier is 6 or less at frequencies above 1 GHz, this relaxation principally arises from the dipolar relaxation of the water in the system. Therefore, the data shown in Figure 1 clearly show that the dielectric relaxation times of the water in the microemulsions are slower on the average than those of the pure liquid. The depressed semicircles indicate a distribution of relaxation times (9), and were analyzed assuming the presence of two water components (free and hydration) in our previous studies. 

Physical Mechanisms. The simplest interpretation of these results is that the transport coefficients, other than the thermal conductivity, of the water are decreased by the hydration interaction. The changes in these transport properties are correlated the microemulsion with compositional phase volume 0.4 (i.e. 60% water) exhibits a mean dielectric relaxation frequency one-half that of the pure liquid water, and ionic conductivity and water selfdiffusion coefficient one half that of the bulk liquid. In bulk solutions, the dielectric relaxation frequency, ionic conductivity, and self-diffusion coefficient are all inversely proportional to the viscosity there is no such relation for the thermal conductivity. The transport properties of the microemulsions thus vary as expected from simple changes in "viscosity" of the aqueous phase. (This is quite different from the bulk viscosity of the microemulsion.)... 

Relaxation Measurements. NMR measurements have been used to examine water-protein interactions in solution and in hydrated powders (2). Hilton et al (24) have shown that the motional properties of water in partially hydrated powders of lysozyme are best characterized as those of a viscous liquid. Dielectric relaxation spectra of water in lysozyme powders (28) distinguish... 

In fact, it is remarkable that the agreement between the dynamic and static measurements is so close in defining the sequence of events in the hydration process as noted above, NMR, dielectric relaxation, ESR, and enzymatic measurements each define one or more of the steps in the hydration process seen in static measurements. 

Again, so long as one speaks qualitatively, it is well established that solute protein is hydrated in water solution (cf. D. Results from measurements of the hydrodynamic properties of protein solutions and the hydrodynamic properties of proteins molecules in solution, from the early work on viscosity, flow birefringence, and dielectric relaxation (cf. O to the more modem work on translational and rotational diffusion measured by inelastic... 

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