Solvation of Anions in Water–Acetonitrile Mixtures

The enthalpies of transfer from water to mixtures with acetonitrile(ΔHt°) for various anions reach maxima at a mole fraction of acetonitrile(xAN)ca. 0.3, and decrease linearly and in parallel in the intermediate region. A linear relation is found between ΔHt° at xAN 0.3 and the hydration enthalpies for halide and symmetrical ions. This finding and the parallel xAN dependence of ΔHt° 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(ΔHINT) for halide ions in the mixtures is estimated based on a modified Eley and Evans hydration model. At xAN 0 to ca. 0.3, ΔHINT compared with that in water agree well with the observed ΔHt°, indicating that the increase in ΔHt° is due to the reduction in the outer hydration sphere interaction. However, they split’gradually at xAN 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.