Ionic solvation in methanol + water mixtures. Free energies of transfer from water

Abstract

The free energy of transfer ΔG^°_t(H⁺) of the solvated proton from water to methanol + water mixtures up to 50 % w/w methanol is considered as the sum of two processes, the transfer of a tetrahedral aquo-proton from water into the mixture using the Born expression and the subsequent replacement of an H₂O molecule in the tetrahedron by a methanol molecule according to the equilibrium, H⁺(H₂O)_4solv+ CH₃OH_solv⇌ H⁺(CH₃OH)(H₂O)_3solv+ water. These values for ΔG^°_t(H⁺) are subsequently used to derive values for ΔG^°_t(halide ion) from the ΔG^°_t for the appropriate hydrogen halides, and ΔG^°_t(Cl^–) then used to produce values of ΔG^°_t for other cations from ΔG^°_t for simple salts. These values for ΔG^°_t for the single ions are discussed with respect to solvation of ions by H₂O and CH₃OH molecules and structural effects in the bulk solvent.