Preferential solvation in mixed binary solvents: ultraviolet–visible spectroscopy of N-alkylpyridinium lodides in mixed solvents containing cyclic ethers

Preferential solvation of N-ethyl-4-cyanopyridinium iodide and N-ethylpyrazinium iodide in binary solvent mixtures of cyclic ethers (1,4-dioxane and tetrahydrofuran) and cosolvents (water, monohydroxy alkanols and acetone) has been studied by monitoring the solvatochromic charge-transfer band of the solute. Protic solvents are preferred near the solute except in regions rich in water, methanol or ethanol where molecules of cyclic ethers are preferred over the protic component. This effect has been explained in terms of self-association of the protic solvent through hydrogen bonding. No preferential solvation of N-ethyl-4-cyanopyridinium iodide has been found to occur in 1,4-dioxane–propan-1-oI, 1,4-dioxane-propan-2-oI and 1,4-dioxane–tert-butyl alcohol. This has been explained in terms of specific solute–solvent interactions.