Experimental Study on the Solvation Behavior in Aqueous Choline Chloride-based Deep Eutectic Solutions by Solvatochromic Dyes

The preferential solvation behavior of three solvatochromic indicators (4-Nitroaniline, N, N-dimethyl-4-nitroaniline, and 2,6-diphenyl-4-(2,4,6-tri phenyl pyridinium-1-yl)phenolate in some deep eutectic solvents (DESs) including choline chloride + glycerol (1:2), choline chloride + ethylene glycol (1:2) and choline chloride + lactic acid (1:2), and their mixtures with water was investigated at constant temperature 298.15±0.1 K by using UV–vis absorption measurements. The solvent exchange model was successfully utilized to analyze the spectral shifts of these dyes to make a description of the preferential solvation behavior of the solute in mixed solvents under study. It was found that the solvation shell of the dyes is mainly affected by the solvent-solvent and solvent-solute interactions, especially hydrogen bonding. The preferential solvation parameter shows that in all aqueous mixtures of DES, the order of preferential solvation for the solvatochromic dyes is as DES (water-DES) complex water. The contribution of water to the preferential solvation of dyes is low. This is probably due to the great affinity of water molecules for the self-association and the strong interaction between DES and the dyes.