The Structure of Solvated Ions

The solvation (including hydration) of ions in the gas phase is described in Section 2.1.2 in terms of the stepwise Gibbs energies and enthalpies of the formation of ion/solvent clusters. The absolute values of these quantities diminish as the number 

Ions with hydrophobic groups in their periphery around a buried charge, such as or (C4Hg)4N, are generally only poorly solvated. However, (QH5)4N and larger tetraalkylanunonium ions may have the water in clathrate-like or enhanced tetrahedral ice-like structures around them as for nonionic hydrophobic solutes. 

Consider an ion placed at the origin of coordinates with solvent species S (or water, W) surrounding it. The number of particles of these species in a spherical shell of thickness dr at a distance r from the center of the ion is  

The study of the constitution of the solvation shells of ions can be made along three lines. (1) Pair correlation functions and coordination numbers are obtained from diffraction of x-rays or neutrons. These refer to the geometry of solvent 

FIGURE 4.2 Coordination numbers obtained from the pair correlation function g r) the dashed curve represents the continuous thick curve is 4%pg(r)t. The cross-hatched area, symmetric with respect to of the first peak, and the area with vertical lines up to of the first trough are two versions of the coordination number of a solvent around an ion (Taken from Ref. 52, with permission from the publisher, WUey). 

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This distinction is meaningful if the resultant distribution function is of the type shown in Figure 4.7 (Szwarc, 1965). This figure shows that there is a high probability that the cation and anion are either in contact, separated by a solvent molecule or far apart (Szwarc, 1965). Intermediate positions are improbable. The structure of solvated ion-pairs has been studied by Grunwald (1979) using dipole measurements. 

The dissolution of ionic or polar solutes is explained. by the polarity and the high dielectric constant of water. The solvation of ions allows their separation in solution. The structure of solvated ions is investigated using spectroscopy techniques (EXAFS, XANES, NMR, infrared, Raman), or diffraction and diffusion techniques (X-ray, neutrons) [11,12]. Such techniques have shown that solvated cations and anions are linked to water molecules through the oxygen and hydrogen atoms respectively (Figure 1.4a). 

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