Preferential Solvation of Ni(II) and Mg(II) Ions in Water–Acetonitrile Mixtures. Enthalpies of Transfer and the Electronic Spectra

Abstract

Abstract The enthalpies of transfer (ΔHt°) from water to water-acetonitrile (W–AN) mixtures for Ni2+ and Mg2+ ions were estimated using the enthalpies of dilution of the concentrated aqueous electrolyte solutions into water and mixtures, and the enthalpies of solution of water in the mixtures. The ΔHt° values for these ions decreased with an increase in the mole fraction of AN (xAN) in the range of xAN<ca. 0.9, however, they increased rapidly in the range xAN>ca. 0.9. The values of ΔHt° for the Ni2+ ion were more negative than those for the Mg2+ ion throughout the xAN regions, especially in the extremely AN-rich region and in pure AN. The average number of coordinated water molecules, N, for the [Ni(AN)6−N(W)N]2+ ion in the mixtures (xAN=0.2, 0.6, and 0.9), as estimated from the electronic absorption spectra, was always greater than that calculated on the basis of the bulk-solvent composition, indicating that the Ni2+ ion was preferentially solvated by water. These results, including the conductance data, were discussed in terms of the principle of hard and soft acid and base.