Nuclear magnetic resonance studies of preferential solvation. Part 4.—Thermodynamic treatment involving non-statistical distribution of solvated species

Abstract

The solvation of an ion in a binary solvent mixture has been treated in terms of consecutive stepwise equilibria with the solvent exchange process becoming more, or less, energetically favourable as the solvation shell becomes successively richer in the second component. Equations developed can be used to fit a wide range of observed n.m.r. chemical, and u.v. peak maximum, shift results in mixed solvents, such as dimethyl sulphoxide + H₂O, acetonitrile + H₂O, ethylenediamine + water. A classification of the observed behaviour is presented in terms of two parameters, K^1/n(K^1/n > 1 preferential solvation by second component, where n is the solvation number) and k(1.5 > k > 0.5). It is concluded that Cs⁺, NO^–₃, I^– are preferentially solvated by DMSO and Li⁺ slightly preferentially solvated by H₂O, in DMSO + H₂O mixtures. In acetonitrile + H₂O mixtures, Na⁺, Cl^–, Br^–, I^– are preferentially hydrated.