Ionic solvation in water + co-solvent mixtures. Part 6.—Free energies of transfer of single ions from water into water + dioxan mixtures

Abstract

The method used earlier to calculate the free energy of transfer of the proton from water into aqueous mixtures, ΔG⁰_t(H⁺), and thereby to separate ΔG⁰_t(HX) to produce ΔG⁰_t(X^–), has now been applied to water + dioxan mixtures. Values of ΔG⁰_t(X^–) are then used to produce values for ΔG⁰_t(M⁺) and ΔG⁰_t(M²⁺) from ΔG⁰_t(MX) and ΔG⁰_t(MX₂). To determine ΔG⁰_t(H⁺), values for K_c=[(dioxan, H⁺)_solv]/[H⁺_solv][(dioxan)_solv] are measured using the previously described spectrophotometric technique. All values for ΔG⁰_t(M⁺) and ΔG⁰_t(M²⁺) are negative in water-rich conditions; values for ΔG⁰_t(X^–) for most X^– are positive, increasing with increasing [dioxan]. Contrary to the results obtained with water + alcohol mixtures where alkoxide ions are produced from added OH^–, ΔG⁰_t(OH^–) in water + dioxan is in the place expected in the order of X^– on structure breaking and forming properties alone. The change in the proton affinity of conjugate bases is computed for the transfer of a range of acid + base systems from water into water + dioxan; this is used to separate ΔG⁰_t for the ion into “non-electronic” and “electronic” components. The variation of the latter with solvent composition suggests that dielectric effects of the solvent make only minor contributions compared with the structural effects.