Washburn numbers. Part 5.—Relative solvent transport numbers for ion constituents in the dioxan + water and dimethylsulphoxide + water systems

Abstract

Washburn numbers w^°_w=(n₊)^°_wt₊–(n_–)^°_wt_– for the alkali-metal chlorides in the dioxan + water system are reported. These and values previously determined for the dimethylsulphoxide (DMSO)+water system have been combined with the Erdey-Grúz numbers Σn^°_w=(n₊)^°_w+(n_–)^°_w for these systems, also determined in this work, to give the relative solvent transport numbers n_w for alkali-metal and chloride ions.

The variation of n_w with solvent composition in the range 0–40%(w/w)(x₂= 0–0.09) of organic solvent shows quite different patterns for each solvent mixture. The main features can be explained on the basis that small concentrations of DMSO enhance the three-dimensional structure of water, whilst dioxan breaks it down at all concentrations; that the behaviour of the ions is influenced by changes in both the total dynamic solvation number and the degree of preferential solvation; and that the smaller ion–solvent complexes tend to carry the current preferentially.