Vicinal water shell

The Cavity-vicinal water shell (CVS) model is shown diagramatically in Fig 4. It consists of a spherical cavity lined with a shell of vicinal water (p) enclosing a core of normal, unperturbed water (u). The real cavity in the gel... 

Figure 4.The cavity-vicinal water shell (CVS) model. See text.

The possible accumulation of I" in the vicinal water has a possible analogy in the observation made long ago (48,49) that the order of exclusion, in favour of bulk water, from another vicinal situation, the air-water interface, was as in Sephadex, F" > Cl" > Br" > I". If this is a valid analogy it seems that we might seek the mechanism of the halide selectivity in their interaction with vicinal water. Thus, the highly polarisable, poorly hydrated I" must be most easily accomodated there and the most hydrated F" (50) least. This may result from differences in the compatibility of the anion hydration shells with the vicinal water (51) or possibly be a question of solubility differences (52). 

In several systems, interfacial water, which is associated with the hydrophilic surfaces (polar groups and counterions) of surfactant microstructures, is present. This kind of water is also called bound water, hydration shell, hydration water, solvent shell [182], or vicinal water [171]. This water can be operationally defined as water detected by a certain technique as it had been influenced by the surface of the substrate in contact with the water [177]. The presence of the microstructure surface may alter the thermodynamic properties (such as melting point, melting enthalpy and entropy, and heat capacity) and the spectroscopic properties (such as IR absorption frequencies and band shapes) of water [61,214]. The chemical potential of bound water is different from that of bulk water [216]. Properties of bound water (viscosity, density, fl-eezing point, etc.) adsorbed on different surfaces of adsorbents differ from those of bulk water [216-223]. 

The first generation of antifoams were found to lose their effectiveness in wet diesel fuel, which may be explained by the interaction of water with the EO units of the attached polyether. Provided that there is a certain water concentration in the fuel, it is likely that a water shell forms in the vicinity of the oxygen atoms (as part of the polyether) by forming hydrogen bonds. Thus, the delicate HLB is influenced and consequently the antifoam is less effective. 

For Ca and Ba, whose n values are larger than 10, however, it is thought that some hydrated water molecules not only in the first hydration shell but also in the second hydration shell are cotransferred into NB. Accordingly, it can be supposed that some water molecules in the first hydration shell (i.e., in the vicinity of the ion) are covered with the second hydration shell, so that they cannot be associated with outer solvent... 

Some recent papers permit an exciting outlook on the degree of sophistication of experimental techniques and on the kind of data which may be available soon. In the field of NMR spectroscopy, a publication by Hertz and Raedle 172> deals with the hydration shell of the fluoride ion. From nuclear magnetic relaxation rates of 19F in 1M aqueous solutions of KF at room temperature, the authors were able to show that the orientation of the water molecules in the vicinity of fluoride ions is such that the two protons are non-equivalent. A geometry is proposed for the water coordination in the inner solvent shell of F corresponding to an almost linear H-bond and to an OF distance of approximately 2.76 A, at least under the conditions chosen. 

The Ion-Dipole Model. In this model ion-dipole forces are the principal forces in the ion-water interaction. The result of these forces is orientation of water molecules in the immediate vicinity of an ion (Fig. 2.11). One end of the water dipole is attached electrostatically to the oppositely charged ion. The result of this orienting force is that a certain number of water molecules in the immediate vicinity of the ion are preferentially oriented, forming a primary hydration shell of oriented water molecules. These water molecules do not move independently in the solution. Rather, the ion and its primary water sheath is a single entity that... 

Ions in aqueous solution are surrounded by a shell of water molecules in tetrahedral or octahedral co-ordination that are relatively immobile because of the intensity of the electric field in the vicinity of the ion. Three regions of solvent around an ion may be labelled. In region 1 (see Fig. 16), all the water molecules are aligned by the field of the central ion forming a solvation shell. Between the distances and R is region 2, known as the Gurney... 

The most inclnsive definition of hydration shell describes it as consisting of all thermodynamically altered water molecnles in the vicinity of a solnte. From a thermodynamic standpoint, hydration can be viewed as binding of water molecnles to the hydration sites of a solnte. The energetics of this association is modulated by the type of solute-solvent interactions (electrostatic, hydrogen bonding, van der Waals) and by solnte-indnced solvent reorganization. The latter occnrs even in the absence of appreciable solute-solvent interactions becanse the eqnUib-rium distribution of hydrogen-bonded water networks of the bulk becomes disrupted at the solute surface. 

Several authors reported measurements of the preferential binding parameter in the system water (l)/protein (2)/PEG (3) [10—14]. It was found that for various proteins, various PEGs molecular weights, and various PEG concentrations, the protein is preferentially hydrated and the PEG is excluded from the vicinity of the protein molecule. The prevalent viewpoint which explains such a behavior is based on the steric exclusion mechanism suggested by Kauzmann and cited in Ref. [15]. According to this mechanism [12,14], the deficit of PEG and the excess of water (in comparison with the bulk concentrations) are located in the shell (volume of exclusion) between the protein surface and a sphere of radius R (see Fig. 1) [12,14]. However, Lee and Lee [10,11] suggested that the preferential exclusion of the PEG from the protein surface also involves the protein hydrophobicity and charge. 

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