Hydrate structures around cations

The increase of the activation energy with increasing water content is a common phenomenon in zeolites (5, 6, 7,10). Although there is a strong discrepancy among the values given in the literature (7, 10), this increase is generally ascribed to the formation of hydrate structures around the cations. 

Generally, the mechanism proposed for structure direction and self-assembly in the synthesis of Si-ZSM-5 involves the formation of an ordered, hydrophobic hydration sphere around the TPA cation [11]. The assembly process of MCM-41 is controlled by electrostatic interaction between silicate species and charged surfactant head groups [12]. 

Anions have a much weaker tendency to coordinate water molecules than cations and precise values for hydration numbers are difficult to obtain from X-ray data. Although a hydration sphere around an anion is often included in the least-squares analysis of an intensity curve for a metal salt solution, meaningful values are usually obtained only for the bond lengths. Coordination numbers and rms variations are often kept constant at assumed values. For the halide ions the bond lengths found show no significant deviations from values in crystal structures. 

Thus, cation water clusters favour internal structures in contrast to the surface strucmres favoured by anionic water clusters. This critical difference in the structural preferences of hydrated cation and anion clusters provides important cues for the design of cation- and anion-specific ionophores and receptors. Indeed, we note that most cation receptors have spherical structures, while almost all anion receptors do not have compact spherical structures but have a vacant space around the anion binding site without full coordination (which might be exceptional for the F ion with strong electronegativity for which the excess electron is strongly bound to F due to its small ion radius). However, as the temperature increases, the hydration structure tends to be more spherical due to entropy effects. 

Proton chemical shifts in aqueous solutions of Al i nitrate, perchlorate, and sulphate have been measured over a range of temperatures, and the effects of hydrolysis and addition of acid have been studied in detail. These arise from changes in the interactions between the bulk water and three different environments, viz. [Al(OH2)e] + itself, the second hydration sphere of this cation, and the broken-water structure around the anion. ... 

Fig. 3.6 Possible structure (optimized via periodic density functional calculations) of a Zn cation surrounded by six H2O molecules forming a hydration sphere around the cation, with the color convention for atoms green for zinc, red for oxygen and grey for hydrogen...

For both the cation and anion in NaCl, there are six H2O molecules in the primary hydration shell (Fig. 7.5). Spectroscopic studies suggest that the hydration of other halide irais is similar to that of CP. Experimental techniques that are used to investigate the hydration shells around metal icMis include X-ray and neutron diffraction extended X-ray absorption fine structure (EXAFS) spectroscopy and NMR (particularly O NMR) spectroscopy. Modem computatiOTial methods (e.g. molecular dynamics) are also invaluable. ... 

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... 

Even greater disruption is encountered in the case of trivalent cations (Figures 4.9,4.10). They completely penetrate both hydration regions and destroy the structure of water around the polyion. This amounts to complete desolvation. The same is true of bound hydrogen ions which are localized. 

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