Thermophysical properties of novel ammonium-based eutectic solvents with ethane-1,2-diol and ZnCl₂

Background: Deep eutectic solvents (DESs) are new types of tunable solvents that have attracted a large scientific community due to their versatile applications and attractive physical properties. Ammonium-based solvents are widely used in desulfurization processes, but they are toxic and expensive. Thus, to replace the conventional ammonium-based solvents, this work aims to synthesize novel ammonium–ethylene glycol (EG)-based ternary DESs (TDESs). The thermophysical properties, such as viscosity, density, acidity, stability, degradation point, melting point, conductivity, refractivity and miscibility, were measured in this study. Results: The TDESs were prepared using varying molar ratios ranging from 1:3 to 1:5 by combining a hydrogen bond donor (EG) with ZnCl₂ and three ammonium-based hydrogen bond acceptors, namely ethylammonium chloride, diethylammonium chloride and diethylethanolammonium chloride. It was found that most properties are significantly superior to those of the constituent components of the TDESs. Arrhenius and linear equation fittings with respect to temperatures ranging from 20 to 100 °C predict R² values close to unity. The viscosity, pH and density decrease while electrical conductivity increases with increasing temperature. Conclusion: Such interesting observations are mainly caused by the particles’ excitation phenomena that increase their kinetic energy and volume. Furthermore, the lower melting point of TDESs compared to the constituent components is attributed to the charge delocalization through hydrogen bonding between donor and acceptor molecules.