Preferential solvation of ions in mixed solvents: Part 4. - Comparison of the Kirkwood-Buff and quasi-lattice quasi-chemical approaches

The relation of the Kirkwood-Buff integrals G_ij = ∫₀^x4πr²[g_ij(r)-1]dr where i is an ion solute and j one of two solvents in their mixture, to measurable quantitites is examined in the light of recent work of Ben-Naim and Newman. The derived local excess of one of the solvents δx_ij is compared with the same quantity calculated for the author's quasi-lattice quasi-chemical mode. The results for three aqueous solvent mixtures, involving a protic solvent (methanol), a strong donor aprotic solvent (dimethyl sulphoxide) and a weak donor aprotic solvent (acetonitrile) and several ions are compared and discussed. It is concluded that the need for the derivative of the standard Gibbs free energy of transfer of the ion with respect to the solvent composition limits the reliability of the Kirkwood-Buff approach.