Washburn numbers. Part 2.—Alkali metal chlorides in the methanol + water and dioxan + water systems; sodium halides in the methanol + water system. results, formal relationships, interpretation and preferential solvation

Abstract

New data for the Washburn numbers, w^°_w, of the sodium halides in water + methanol mixtures, and earlier values for alkali metal chlorides in the methanol + water and dioxan + water systems, are discussed in detail, particularly in relation to other experimental evidence on preferential solvation.

Certain formal relationships between the relative number of moles of one component transported by an ion, n, and the total number, N, are developed.

Solvent properties are assumed to be largely “averaged” over both components of the mixture; thus the equilibrium distribution of the molecules in the ionic field is perturbed at little energetic cost. This leads to a marked dependence of w^°_w on structural influences. An earlier explanation of this dependence in terms of changes in N is withdrawn.

Examination of the gross dependence of w^°_w on solvent composition eliminates strong specific solvation by methanol. Comparison with the raffinose + water system, which includes an attempt to find absolute values of n, eliminates strong specific solvation by water and provides evidence that both methanol and dioxan compete significantly with water for near neighbour solvation of at least Li⁺ and Na⁺. There is evidence for slight preferential solvation of halide ions by methanol and dioxan.