(574h) Ionic Properties and Phase Behavior of Thermally Responsive Ionic Liquid - Water Mixtures

Thermally responsive ionic liquids (ILs) exhibit a liquid-liquid phase separation with water when heated above a lower critical solution temperature (LCST). Given that LCST phase separation leads to the formation of an ionic liquid-rich phase and a water-rich phase with different water activities, this property can be leveraged for applications ranging from forward osmosis desalination based on IL-water draw solutes, to novel cycles for refrigeration where the IL acts as a liquid dessicant. In both cases, desirable solution properties include tunable water activity and a low LCST to minimize the external energy required to induce the liquid-liquid phase separation. However, these properties are often interlinked with the chemistry of the IL comprising hydrophilic and hydrophobic moieties. For example, a handful of thermally responsive ILs have been identified and characterized in the literature, but a favorable water activity is also met with a high LCST. To address this, we develop and characterize the phase behavior and solution properties of binary IL mixtures with water. In this work, we present the water activity (osmolality), LCST, surface tension, viscosity, and phase density of two ILs with the same cation, namely P4444DMBS (tetrabutylphosphonium-2,4-dimethyl-benzenesulfonate) and P4444TFA (tetrabutylphosphonium-2,4-trifluoroacetate), when mixed in varying weight fractions. Results indicate a synergistic behavior between P4444TFA and P4444DMBS mixtures in water, achieving equivalent or improved osmolalities at a lower LCST when compared to individual IL solutions of the same weight fraction. Overall, this work lays the foundation for exploring aqueous mixtures of different ILs, and provides insight into the kinetics of phase separation.