Molar enthalpy of mixing and refractive indices of choline chloride-based deep eutectic solvents with water

Abstract

The molar enthalpies of mixing were measured for binary systems of choline chloride-based deep eutectic solvents (glycerol, ethylene glycol and malonic acid) with water at 298.15K and 308.15K, and atmospheric pressure with an isothermal calorimeter. Refractive indices were also measured at 303.15K and atmospheric pressure. The binary mixtures of {chcl/glycerol (1:2)+water, chcl/ethylene glycol (1:2)+water} showed exothermic behaviour over the entire range of composition, while the binary mixture of {chcl/malonic acid (1:1)+water} showed endothermic behaviour at first and then changed to be exothermic with the increasing content of chcl/malonic acid (1:1). Experimental refractive indices were fitted with the Redlich–Kister equation, and experimental molar enthalpies of mixing were correlated with the Redlich–Kister equation and the non-random two-liquid (NRTL) model. The NRTL model with the fitted parameters was used to predict the vapour pressures of these three mixtures. For mixtures of {chcl/glycerol (1:2)+water} and {chcl/ethylene glycol (1:2)+water}, the predicted vapour pressures agreed well with the experimental results from the literature. While for mixture of {chcl/malonic acid (1:1)+water}, the predicted vapour pressures showed deviation at the high concentration of chcl/malonic acid (1:1), and this was probably because of the complex molecular interaction between chcl/malonic acid (1:1) and water.