The solvation number of ions obtained from their entropies of solvation

The entropy of solvation of an ion contains contributions from i) the change of the volume at its disposal, ii) long-range electrostatic effects, iii) immobilization of solvent molecules in the first solvation shell, and iv) effects on the structure of the solvent. The last item is important in water, but can be ignored in less structured solvents. Standard ionic entropies of transfer from water to a dozen solvents are used for the estimation of the entropy of solvent immobilization, and the (extrapolated) entropy of freezing of the solvent is then used to estimate the number of solvent molecules immobilized.