Influence of Water on the Structure and Solvation Properties of Deep Eutectic Solvents Based on Choline Chloride and Carboxylic Acids

Abstract The aim of this work is to investigate the influence of water on the structure and solvation properties of deep eutectic solvents (DES) based on choline chloride and carboxylic acids (formic, acetic, lactic, oxalic, malonic, malic). The mechanism of structure formation in the DES-water system has been established using ATR-IR-spectroscopy: the role of carboxyl groups of acids in hydrogen bond formation has been confirmed; the energy, length and relative contribution of various types of hydrogen bonds has been calculated. Based on the refraction index and density values, the free molar volume of the studied DESs and their mixtures with water has been determined. By UV-visible spectroscopy, the values of the solvatochromic parameters (Nile red solvation energy ENR, Kamlet–Taft parameters of polarity/polarizability π*, basicity β and acidity α) of the studied solvents have been determined. For the DES-water systems, the solvation shell composition has been characterized according to the preferential solvation model, the boundaries of the formation of the DES–DES aquacomplex-water ternary system have been determined. It has been established that at a content of up to 50 wt % water does not lead to a radical change in the structure and solvation properties of the solvents under study, being incorporated into the DES structure and forming an aquacomplex with them, and at a higher content it destroys the hydrogen bond network between the DES components, forming their aqueous solution, which is reflected by a sharp change in the energy of hydrogen bonds with carboxyl groups, solvatochromic parameters and the solvation shell composition.