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, ΔG0t(H+), and thereby to separate ΔG0t(HX) to produce ΔG0t(X–), has now been applied to water + dioxan mixtures. Values of ΔG0t(X–) are then used to produce values for ΔG0t(M+) and ΔG0t(M2+) from ΔG0t(MX) and ΔG0t(MX2). To determine ΔG0t(H+), values for Kc=[(dioxan, H+)solv]/[H+solv][(dioxan)solv] are measured using the previously described spectrophotometric technique. All values for ΔG0t(M+) and ΔG0t(M2+) are negative in water-rich conditions; values for ΔG0t(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–, ΔG0t(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 ΔG0t 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.