Study of preferential solvation in mixed binary solvents by ultraviolet–visible spectroscopy

Abstract

The longest-wavelength band of N-alkylpyridinium iodide has been studied as a function of the solvent composition in various mixed binary solvents. It has been observed that the quantity [E₁₂–(x₁E₁+x₂E₂)] serves as a measure of the extent of preferential solvation, where E and x stand for the position of band maximum and the mole fraction values, respectively, and the subscripts 1, 2 and 12 indicate the pure solvents and their binary mixtures. The local mole fractions of the component solvents have been calculated from the E₁₂vs. bulk mole fraction curves. The nature of the preferential solvation agrees well with the quasi-lattice quasi-chemical theoretical result. Stepwise solvent exchange equilibria, e.g. SA_iB_j+ B ⇌ SA_i–1B_j+1+ A, have been used as a chemical model to explain the experimental findings. The influence of component solvents on the extent of preferential solvation has also been discussed.