Thermo-solvatochromism in binary mixtures of water and ionic liquids: on the relative importance of solvophobic interactions

Abstract

The thermo-solvatochromism of 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr₂, has been studied in mixtures of water, W, with ionic liquids, ILs, in the temperature range of 10 to 60 °C, where feasible. The objectives of the study were to test the applicability of a recently introduced solvation model, and to assess the relative importance of solute–solvent solvophobic interactions. The ILs were 1-allyl-3-alkylimidazolium chlorides, where the alkyl groups are methyl, 1-butyl, and 1-hexyl, respectively. The equilibrium constants for the interaction of W and the ILs were calculated from density data; they were found to be linearly dependent on N_C, the number of carbon atoms of the alkyl group; van’t Hoff equation (log K versus 1/T) applied satisfactorily. Plots of the empirical solvent polarities, E_T (MePMBr₂) in kcal mol⁻¹, versus the mole fraction of water in the binary mixture, χ_w, showed non-linear, i.e., non-ideal behavior. The dependence of E_T (MePMBr₂) on χ_w, has been conveniently quantified in terms of solvation by W, IL, and the “complex” solvent IL-W. The non-ideal behavior is due to preferential solvation by the IL and, more efficiently, by IL-W. The deviation from linearity increases as a function of increasing N_C of the IL, and is stronger than that observed for solvation of MePMBr₂ by aqueous 1-propanol, a solvent whose lipophilicity is 12.8 to 52.1 times larger than those of the ILs investigated. The dependence on N_C is attributed to solute–solvent solvophobic interactions, whose relative contribution to solvation are presumably greater than that in mixtures of water and 1-propanol.