Rate of Solvent Exchange Near Ions

The solvation of ions is a dynamic process, and solvent molecules in the solvation shells of ions do move out from them in exchange with molecules that come in. The rate of exchange of water molecules between the hydration shells of ions and bulk water [96] indicates the strength of the hydration and indirectly the effects of the ions on the water structure. The temperature coefficients of the self diffusion coefficient of water in the electrolyte solution, and of the ion mobility, m, (Section 2.3.2.1), yield the activation Gibbs energy of the exchange, 

Positive values of 0 characterized small and multivalent cations, such as LP, Na , Mg +, and Ca, and were called positively hydrated. Large univalent cations and most anions, such as K+, Cs+, CL, Br , and L, have negative AG 0 and were designated as negatively hydrated [96]. These terms are no longer in common use. 

The residence time of water molecules near each other in bulk water, that is, the average time it takes for a water molecule to diffuse away from its neighbors, is obtained from the diameter of a water molecule, d and the diffusion coefficient of neat water, = O.Sd lDl, = 17 ps at 25°C. The ratio of the average residence 

The unimolecular rate constants,, for water release from the hydration shells of cations [12] are expected to correspond with the values of deduced from Equation 4.46. These rate constants, obtained from ultrasound absorption [97,98], and NMR line widths [99-102] depend on the competition between water molecules and anions for sites in the coordination shell of the cations. These rate constants at 25°C span nearly 17 orders of magnitude (from K, the fastest to Rh, the slowest), and hence the logarithms log(fe/s ) are shown in Table 5.4. Considerably less experimental information is known for the rate of desolvation pertaining to the first solvation shell of cations in nonaqueous [12] solvents. Hardly any experimental rate constants regarding the rates of dehydration of anions are available. Computer simulations fill this gap concerning ions of both signs in aqueous solutions (Section 5.2.2). 

There exists an analogy between the transfer of ions from water to nonaqueous solvents (Section 4.3) and the corresponding transfer of non-electrolytes. The standard molar Gibbs energies of transfer of organic non-electrolytes N have been related to their properties and the properties of the target solvents by Marcus [103] by the expression  

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