Dielectric behavior hydrates

Dipolar Absorption. When the dielectric behavior of hydrated Na+-zeolites is investigated between two electrodes blocked from the sample with thin mica sheets, the conduction process is eliminated and two absorp-... 

The effect of "residual water" on either protein stability or enzyme activity continues to be a topic of great interest. For example, several properties of lysozyme (e.g., heat capacity, diamagnetic susceptibility (Hageman, 1988), and dielectric behavior (Bone and Pethig, 1985 Bone, 1996)) show an inflection point at the hydration limit. Detailed studies on the direct current protonic conductivity of lysozyme powders at various levels of hydration have suggested that the onset of hydration-induced protonic conduction (and quite possibly for the onset of enzymatic activity) occurs at the hydration limit. It was hypothesized that this threshold corresponds to the formation of a percolation network of absorbed water molecules on the surface of the protein (Careri et al., 1988). More recently. Smith et al., (2002) have shown that, beyond the hydration limit, the heat of interaction of water with the amorphous solid approaches the heat of condensation of water, as we have shown to be the case for amorphous sugars. 

The dielectric behavior on water adsorption is heterogeneous. First, a regular small increase in conductivity occurs up to about three water molecules per cation in the large cavities. Adsorption of more water results in a sudden increase in conductivity, probably because of the high mobility of the exchangeable cations in the hydrated state. 

While the local-density perturbation is common to electrolyte and nonelectrolyte solutions, it becomes rather large for ion solvation, due to the resultant increased dipole concentration in the region of highest field-strength. Typically, the perturbation process takes place within the first hydration shells [36], affecting the solvent s electric field, and consequently, its dielectric behavior. Perhaps for this reason most attention has been often focussed on the behavior of the first solvation shell of species in solution, determined either by experimental or theoretical means [258-260,263]. Yet, molecular simulation offers the best tool to probe the properties of the solvent in the vicinity of the ions, because the method provides total control and full manipulation of the variables involved, and thus allows us to make precise cause-effect coi... 

With water, nitrogen trifluoride forms a clathrate hydrate when ice at -25 to -40°C is exposed to pressurized NF3 (9 to 23 atm). Hydrate formation was found complete within 24 h by the fall of gas pressure. X-ray analysis indicated the clathrate hydrate to be of structure 1, space group Pm3n. The dielectric behavior and the F NMR spectrum of that hydrate as well of others containing tetrahydrofuran, p-dioxane, or sulfur hexafluoride as additional hosts, were analyzed to study the site distribution and motional dynamics of NF3 [1 ]. A crystalline clathrate... 

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. 

We showed previously that a simple model for the ion-hydration interactions, which separates the ion-hydration forces in a long-range term due to the behavior of water as a continuous dielectric (the screened image force) and a short-range term due to the discreetness of the water molecules (SM/SB), can explain almost quantitatively a number of phenomena related to the electrolyte interfaces.6 In this article, we examined the limitations of the model in predicting the distributions of ions near the air/water interface, by comparison with molecular dynamics simulations. It is clear that the real ion-hydration forces are more complicated than the simple model employed here however, the interfacia] phenomena (including specific ionic effects) can be understood, at least qualitatively, in terms of this simple approach. 

Recent molecular dynamics simulations of water between two surfactant (sodium dodecyl sulfate) layers, reported by Faraudo and Bresme,14 revealed oscillatory behaviors for both the polarization and the electric fields near a surface and that the two fields are not proportional to each other. While the nonmonotonic behavior again invalidated the Gruen—Marcelja model for the polarization, the nonproportionality suggested that a more complex dielectric response of water might, be at the origin of the hydration force. The latter conclusion was also supported by recent molecular dynamics simulations of Far audo and Bresme, who reported interactions between surfactant surfaces with a nonmonotonic dependence on distance.15... 

The main purpose of this section is to give the basis of how measurements of the dielectric constants of ionic solutions can give information on solvation, particularly primary hydration numbers. However, dielectric measurements as a function of frequency also give information on the dynamic behavior of water by allowing us to determine the relaxation time of water in ionic solutions and expressing the changes in terms of the number of water molecules bound to the ion. 

The low temperature properties of a dodecane-hexanol-K.oleate w/o microemulsion from 20°C to -190°C were studied vs. increasing water content (C,mass fraction) in the interval 0.021+-0.1+, by Differential Scanning Calorimetry and dielectric analysis (5 Hz-100 MHz). A differentiation between w/o dispersions is obtained depending on whether they possess a "free water" fraction. Polydispersity is evidenced by means of dielectric loss analysis. Hydration processes occurring, at constant surface tension, on the hydrophilic groups of the amphiphiles, at the expenses of the free water fraction of the droplets, are shown to develop "on ageing" of samples exhibiting a time dependent behavior. 

Dielectric relaxation results are proven to be the most definitive to infer the distinctly different dynamic behavior of the hydration layer compared to bulk water. However, it is also important to understand the contributions that give rise to such an anomalous spectrum in the protein hydration layer, and in this context MD simulation has proven to be useful. The calculated frequency-dependent dielectric properties of an ubiquitin solution showed a significant dielectric increment for the static dielectric constant at low frequencies but a decrement at high frequencies [8]. When the overall dielectric response was decomposed into protein-protein, water-water, and water-protein cross-terms, the most important contribution was found to arise from the self-term of water. The simulations beautifully captured the bimodal shape of the dielectric response function, as often observed in experiments. 

As these cosolvents contain both hydrophilic and hydrophobic groups, the same molecule can induce opposite effects in water. The hydrophilic part can interact with water to form strong HBs, while the hydrophobic part may induce cooperative ordering in the system by a hydrophobic hydration effect. These two effects combine together to regulate the extensive HB network of water in their aqueous binary mixtures that is reflected in strong, often anomalous non-ideal behavior in many physical properties such as viscosity, density, dielectric constant, excess mixing volume, surface tension, heat of formation, etc. 

The electrostatic interactions compete with the thermal movement of all the particles in the solution, ions and water molecules, and are screened by the high dielectric permittivity of the water. The overall interactions, involving ion hydration in addition to ion-ion interactions and the hydrogen bonded network of water are quite complicated. Approximations have to be applied in order to handle the resulting behavior of the ions theoretically. 

Dielectric decrement has been observed in bulk electrolytes and reflects structural rearrangement of water due to the presence of salt. For salt concentrations between 0 and 1.5 M, the dielectric constant was found to depend linearly on the salt concentration, e (c ) = e + ac [5,6, 31]. The orientation of water dipoles within the hydration shell around a dissolved ion is fixed by field lines originating from ion centers, so that these dipoles respond poorly to an external field. This behavior can be quantified with a crude model. Because the tightly bound dipoles within the hydration shell are excluded from screening an external electrostatic field, the effective density of free water dipoles becomes reduced, - (M c + M c ), where is the solvation number of water molecules in a hydration shell around either a cation or an anion. In the linear regime the dielectric constant of water is g = -i- Pc pll3. After the addition of salt the effective... 

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