Understanding the peculiar effect of water on the physicochemical properties of choline chloride based deep eutectic solvents theoretically and experimentally

Abstract

Deep Eutectic Solvents (DES), the novel eco-friendly solvents and are analogous to ionic liquids (IL) are also termed as the new age alternatives to the volatile organic solvents owing to their unique physicochemical properties similar to ILs. Water, either present as an unavoidable impurity or added purposefully to lower the cost and viscosities of DES/ILs have peculiar effect on their physicochemical properties. It is imperative to study the effect of water on these eco-friendly solvents. Herein, we report the experimentally determined densities (ρ) and speeds of sound (u) for aqueous DES solution over the entire range of composition and temperature ranges from 293.15 K to 323.15 K at atmospheric pressure. The equimolar ratio of choline chloride as the hydrogen bond acceptor (HBA) with malonic acid and glutaric acid as the hydrogen bond donors (HBD) was used to prepare the DESs. The dependence of DES and H2O composition, role of HBD and temperature is discussed on the excess molar volumes VmE and excess molar isentropic compressibilities Ks, mE. The negative VmE and Ks, mE indicate the dominance of intermolecular interactions over the intra-molecular interactions. The formation of H-bond between ion-water and HBD-water interactions is also discussed in terms of DES composition and temperature. We have used the Prigogine−Flory−Patterson (PFP) theory and for the first time extended real association model (ERAS) to evaluate the non-ideal behaviour of the studied systems. Results of highly unpredictable, distinct contours excess properties over the entire range of composition of DES could be helpful in designing various industrial processes and for the thermodynamic modeling of DES.