A study in preferential solvation using a solvatochromic pyridinium betaine and its relationship with reaction rates in mixed solvents

Abstract

The E_T polarity values for binary liquid mixtures, derived from the solvatochromic behaviour of a pyridinium betaine, have been used to investigate the preferential solvation of the betaine by the components of 17 binary solvent mixtures. In systems involving two organic liquids the E_T values have been interpreted in terms of the solvation of the betaine by the more polar component and some agreement with previous n.m.r. studies was obtained. In aqueous–organic solvent mixtures, however, the betaine was preferentially solvated by the organic component. The findings have been related to the rates of solvolysis in mixed-solvent systems, and preferential solvation is suggested as a reason for the solvent dependence of the rates of reaction.