The spectrophotometric solvent sorting method for the determination of free energies of transfer of individual ions—a critical appraisal

Abstract

The spectrophotometric method for determining values for the free energy of transfer of the proton from water into water + co-solvent mixtures at mole fractions of co-solvent up to x₂ ≈ 0.3 is examined critically from first principles. It is found that some corrections become significant at the higher x₂ in this range and these are applied to all the co-solvents used. The new values for ΔG_tº(H+) are then used to calculate new values for ΔG_tº (X-) from ΔG_tº(HX) and new values for ΔG_tº(M+) and (M²⁺)from ΔG_tº(MX) and ΔGTº(MX₂). New electrochemical, solubility and pK data are incorporated into these calculations, resulting in ΔG_tº values for some additional ions for several co-solvents. The ΔG_tº values for all co-solvents are compared and contrasted, and it is concluded that structural changes in the solvent have a dominant effect in determining these values. The evidence for molecular rearrangements involving solvent molecules in the neighbourhood of the ions resulting from the transfer is examined.