Solvation of Anions in Water-Acetonitrile Mixtures

Article abstract of DOI:10.1246/bcsj.59.1695

The enthalpies of transfer from water to mixtures with acetonitrile (ΔH⁰_t) for various anions reach maxima at a mole fraction of acetonitrile (χ_AN) ca. 0.3, and decrease linearly and in parallel in the intermediate region.A linear relation is found between ΔH⁰_t at χ_AN 0.3 and the hydration enthalpies for halide and symmetrical ions.This finding and the parallel χ_AN dependence of ΔH⁰_t in the intermediate region are interpreted on the basis of the linear relation between the solvation enthalpies and the entropies for the halide ions.The reduction in the hydrogen-bonding interaction extending beyond the primary hydration sphere (ΔH_INT) for halide ions in the mixtures is estimated based on a modified Eley and Evans hydration model.At χ_AN 0 to ca. 0.3, ΔH_INT compared with that in water agree well with the observed ΔH⁰_t, indicating that the increase in ΔH⁰_t is due to the reduction in the outer hydration sphere interaction.However, they split gradually at χ_AN above 0.3.The adequacy of the modifications in the treatment of the solvation enthalpy cycle in the mixtures is re-evaluated from the structural viewpoint.The exothermic change in the intermediate region may be attributable to the reduction in the cavity formation energy.