Salting constants of acetonitrile in aqueous electrolyte solutions and preferential solvation phenomenon

Abstract

The salting constant k_s of acetonitrile (AN) for inorganic electrolytes in water (NaCl, NaNO₃, NaClO₄, CsCl, CsNO₃, Na₂SO₄, BaCl₂, CuSO₄, LaCl₃, AgNO₃) has been determined using a precise static vapour pressure method. A very strong dependence of k_s on solute (AN) concentration was found down to very dilute AN concentrations (mAN < 0.3 mol kg^–1) in the case of BaCl₂, LaCl₃, CuSO₄ and AgNO₃, indicating that Henry's law, and hence the true salting constant, could not be experimentally attained for AN in the presence of these electrolytes. Such a dependency was not observed with the other electrolytes; a cavity term (using the scaled-particle theory) and an interaction term (using a Lennard-Jones 6–12 potential) describe most parts of the true k_s values. The small but systematic discrepancies between experimental and calculated salting constants for most polar solutes investigated with the present approach in aqueous inorganic electrolyte solutions can be accounted for by a preferential solvation effect of inorganic ions by water (in the case of the alcohols, tetrahydrofuran, acetone, acetonitrile); thus, permanent dipole–dipole forces between the polar solute and the water molecules do not necessarily have to be introduced in order to predict the salting effect of polar molecules by inorganic ions in water.