Comparative study of the thermophysical properties of octanoic acid and [Emim][Tf2N] for low-temperature directional solvent extraction desalination

• Directional solvent extraction (DSE) is an emerging desalination process. • Two potential solvents investigated: octanoic acid and [Emim][Tf 2 N]. • Octanoic acid performed better in terms of freshwater yield, solvent loss and TDS rejection than [Emim][Tf 2 N]. • [Emim][Tf 2 N] is more viscous, which may increase the required pumping power. • [Emim][Tf 2 N] exhibited much faster phase separation of recovered water, compared to Octanoic acid. Directional solvent extraction (DSE) is a promising desalination technology valued for its low temperature energy requirements and potential for resource recycling. However, the development of an optimal solvent in terms of freshwater solubility and yield is an ongoing challenge. Although various solvents have been investigated, reported results have discrepancies due to differences in methodologies, equipment, and sample sizes. In this study, experiments were conducted to investigate and compare the thermophysical properties and solubility behavior of two promising solvents for DSE: octanoic acid (OA) and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf 2 N]). For both solvents, freshwater solubility increased with temperature, as did their solubility in water. In contrast, solubility decreased with increasing total dissolved solids (TDS) concentration. Of the two solvents examined, OA consistently outperformed [Emim][Tf 2 N] across many critical metrics, including higher TDS rejection, greater water yield, and lower solvent loss. In addition, [Emim][Tf 2 N] exhibited a viscosity approximately 4.5 times higher than that of OA, which would increase the power demand for pumping in continuous DSE systems. OA also offered a substantial economic advantage, being considerably less expensive. Nevertheless, OA exhibited much slower phase separation of recovered water upon cooling, compared to [Emim][Tf 2 N]. For both solvents, the desalinated water required additional processing to reduce the TDS to levels suitable for optimum drinking water quality.