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

Abstract

The entry of a solute into a solvent will affect the interactions between solvent molecules by making, breaking, strengthening or weakening solvent–solvent bonds. In a binary mixed solvent the relative partial molar entropies, S_i, and excess entropies of mixing, ΔS^E, are related to these interactions. Thus a relationship between the entropy of transfer, ΔS^⊖_t, of a solute and these thermodynamic parameters might be expected. A general relationship is developed and successfully applied to the ΔS^⊖_t for alkali-metal halides in methanol–water mixtures. The results of this analysis show that, for this system at least, ΔS^⊖_t is dominated by the effects of changes in solvent–solvent interactions, in contrast to the enthalpy of transfer, ΔH^⊖_t. Combination of the results of this analysis with those previously reported for ΔH^⊖_t allows a complete, quantitative explanation of the thermodynamics of solvation in the alkali-metal halides in methanol–water mixtures.