Ion solvation in aqueous-organic mixtures

The importance of ion solvation in determining the properties of electrolyte solutions in aqueous-organic solvent mixtures is discussed. Solubility measurements are shown to he particularly useful for determining the Gibbs energies of transfer of ions between solvents, which reflect differences in the overall solvation of the ions in different solvent mixtures. Solubility measurements can also be used to determine the other thermodynamic parameters of transfer, but such quantities are usually better obtained by more direct methods. The inadequacy of current theories of ion solvation to quantitatively account for the thermodynamics of ion transfer is discussed by reference to measurements on some simple model systems. Although donor/acceptor interactions can explain many of the observed effects between Pure solvents, the situation is more complex in aqueous-organic mixtures because selective solvation and even solvent-solvent interactions may become significant. This is illustrated by consideration of ion transfer from water to water + t-butanol solutions, where spectacular effects are observed in the enthalpies and entropies and especially in the heat capacities and volumes.