(537a) Understanding the Mixing Behavior and Phase Separation Kinetics of Thermally Responsive Ionic Liquids in Water

Thermally responsive ionic liquids (ILs) exhibit a liquid-liquid phase separation in aqueous solutions when heated above a lower critical solution temperature (LCST). The phase separation results in the formation of a water-rich phase (WR) and an ionic liquid-rich phase (ILR). The chemical potential difference between the two phases can be leveraged for a wide range of applications from desalination to air conditioning and solvent extraction. With a focus on desalination as the application, desirable properties include a high osmotic strength (low water activity), tunable phase separation temperature, and fast phase separation kinetics. However, these properties are often inversely related as more hydrophilic ILs exhibit a higher osmotic strength at the cost of a higher phase separation temperature. To this end, we demonstrate that mixtures of two ILs exhibit a synergistic interaction, resulting in a higher osmolality and lower phase separation temperature than either constituent. We report the osmolality, phase diagram, and separation kinetics of N₄₄₄₄Sal (tetrabutylammonium-salicylate) and P₄₄₄₄TFA (tetrabutylphosphonium-2,4-trifluoroacetate) in different mixing ratios. This enables the selection of the ideal IL mixture composition for a given application depending on the imposed temperature and osmotic strength constraints. Next, we discuss the micro- to macro-scale phase separation behavior of the ILs at the critical temperature, which enables the development of a Stokes-Einstein-based kinetic model using experimental data for the nucleation behavior of the ILR phase (from microscopy). The phase separation time for a given IL mixture is measured using a light transmission technique that provides both spatial and temporal information. Results indicate that the time required for phase separation of a given IL mixture is analogous to the behavior of emulsions and obeys Stokes’ law for a given phase boundary height and ILR phase nucleate size distribution. Overall, this work establishes an improved understanding of thermally responsive ILs and mixtures thereof for applications in energy and sustainability.