Relationship between the enthalpy of transfer of a solute and the thermodynamic mixing functions of mixed solvents

Abstract

The entry of a solute into solution must affect the interactions between the solvent molecules by making, breaking, strengthening or weakening intermolecular bonds. In a binary mixed solvent the enthalpy of mixing ΔH^E and relative partial molal enthalpies L_i are also related to these forces. Thus a relationship between the enthalpy of transfer, ΔH^⊖_t, of the solute from one pure solvent component to a mixture and these thermodynamic functions might be expected.

A general relationship is developed and successfully applied to solutions of alkali-metal halides in the methanol + water system, where any preferential solvation is unimportant, and in the acetonitrile + water system, where there is strong preferential hydration of the ions.

The characteristic shape of the ΔH^⊖_t against x₂ profile in the former case with the maximum in ΔH^⊖_t at low x₂ can be accounted for completely in this way and offers no independent evidence for a maximum in mixed-solvent structure.