Volumetric and compressibility studies on aqueous mixtures of deep eutectic solvents based on choline chloride and carboxylic acids at different temperatures: Experimental, theoretical and computational approach

Abstract

Deep eutectic solvents (DESs) constitute a new class of green solvents, the studies on which are going at a fast pace. The binary mixtures of DESs with water are attracting a great deal of interest for various industrial applications owing to their unique properties not exhibited by the neat components. Therefore, thermodynamic properties of DESs-water mixtures should be explored to better understand the prevailing intermolecular interactions for enhancing the application arena of such binary mixtures. Herein, the density (ρ) and speed of sound (u) of binary mixtures of DESs (choline chloride: acetic acid; 1: 2; and choline chloride: L-(+)-lactic acid; 1: 2) with water has been measured over whole composition range at different temperatures (T = 298.15 to 313.15 K) and atmospheric pressure. The values of various thermodynamic parameters have been derived and discussed in terms of the varying extent of intermolecular interactions governed by the components of DESs. The negative values of excess molar volumes (VmE) are ascribed to the stronger hydrogen bond interactions and efficient molecular packing in the binary mixtures over repulsive interactions. The VmE values have been analyzed using Prigogine-Flory-Patterson (PFP) theory, which satisfactorily explained the volumetric behaviour of the binary mixtures. Density functional theory (DFT) calculations have been made to support the variation in VmE and the underlying interactions prevailing between the components. Further, isentropic compressibility (ks) and excess isentropic compressibility (ksE) have been evaluated from the speed of sound data. It is expected that such studies will be beneficial from the academic as well as industrial point of view considering the increasing utilization of DESs for various applications.