Ionic Solvation in Water4-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, ΔPa, of large molecules on transferring them from water into mixtures of water with methanol have been calculated. –ΔPa for anions and ΔPa 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 –ΔPa 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.