A quasi-lattice quasi-chemical theory of preferential solvation of ions in mixed solvents

Abstract

The quasi-lattice quasi-chemical theory is used with a single fitting parameter (Z, the number of nearest neighbours) and data independent of the transfer to solvent mixtures, to describe quantitatively the standard molar Gibbs free energy of transfer of ions from a reference solvent to solvent mixtures, ΔG_tº (X, W → S₁ + S₂), as a function of the composition (mole fraction, x). The independent data include the ΔG_tº to the two pure solvents and the excess Gibbs free energy of mixing of these solvents. A defined preferential solvation parameter, g(x), is a convenient measure that is obtained from this treatment. The advantages of employing volume fractions, ø, and g(ø) rather than mole fractions, x, and g(x), are examined. The theory is applied to the transfer from water of Cl^- to ethanol, of Ag⁺ to acetonitrile or dimethyl sulfoxide, and of NaCl to methanol, and of Na⁺ from acetonitrile to dimethyl sulfoxide as illustrative examples, comparing the calculated values to experimental data.