Physical properties of deep eutectic solvents formed by the sodium halide salts and ethylene glycol, and their mixtures with water

Deep eutectic solvents (DESs), with their attractive properties, are currently of great interest. Introducing new kinds of eutectic solvents with different components as their hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA), and investigating the (thermo-) physical properties of the new DESs can play an important role in promoting future research and use of such mixtures. In this study, we investigate the feasibility of the formation of new kinds of eutectic solvents based on the sodium halide salts as the hydrogen bond acceptors with ethylene glycol as the hydrogen bond donor. Different molar fractions of HBD/HBA were investigated. The results indicated that sodium fluoride does not form a DES throughout the whole concentration range investigated, while sodium chloride, sodium bromide, and sodium iodide do form eutectic mixtures at certain molar ratios. Differential scanning analysis was performed on the resulting eutectic solvents to measure the glass transition temperature. Other physical properties of the proposed DESs, namely density, viscosity, refractive index, and electric conductivity were also measured within the temperature range of 293–333 K. Furthermore, the effect of water in aqueous mixtures of the proposed DESs was investigated, with molar fractions of water ranging from 0.1 to 0.9. Based on the calculations of excess molar volume, the effect of water in the mixtures was investigated. Over the whole range of water concentrations in the mixture, the excess molar volumes had negative values, indicating intensive hydrogen-bonding between unlike molecules in the mixture and/or a void filling mechanism by the smaller molecules.