Ionic solvation in water + co-solvent mixtures. Part 5.—Free energies of transfer of large single ions from water into water + methanol with the “neutral” component removed

Abstract

Values for the change in proton affinity, ΔP_a, of large molecules on transferring them from water into mixtures of water with methanol have been calculated. –ΔP_a for anions and ΔP_a for cations represent an elimination of the non-electrostatic or “neutral” component from the free energy of transfer of the ion i, ΔG^°_t(i). Although plots of –ΔP_a against reciprocal of dielectric constant are linear for the anions, the values of their slopes show that there is a structural component of this “electrostatic” free energy, ΔG^°_t(i)_e, in addition to the Born contribution. ΔG^°_t(F^–) is calculated from data newly available, and ΔG^°_t(halide) values are compared with ΔG^°_t(i)_e values for the large ions. The negative values for ΔG^°_t(i)_e for large cations confirm that there is a contribution from structural changes in the solvent; these are compared with the negative values for ΔG^°_t(i) for small cations.