Lower Critical Solution Temperature Phase Behavior and Water Activity of a Ternary Mixture of Oleic Acid, Lidocaine, and Water

Mixtures that possess a lower critical solution temperature (LCST) phase behavior form a homogeneous single phase at temperatures below the LCST and separate into two liquid phases (water-rich, WR, and water-scarce, WS) above the LCST. This unique thermoresponsive phase behavior can be leveraged in various thermodynamic cycles, which are used for applications such as desalination and dehumidification. In addition to their phase diagram, the performance of aqueous LCST mixtures is dictated by their water activity (<i>i.e</i>., the chemical potential of water in the mixture). Recently, a ternary mixture of oleic acid (OA), lidocaine (LD), and water was shown to possess an LCST of ∼298.15 K, but the phase diagram over the full range of concentrations and water activity has not been reported. In this work, we experimentally characterize the phase diagram (liquid-liquid equilibrium, LLE), water activity (vapor-liquid equilibrium, VLE), chemical potential of water, and osmotic pressure of OA/LD/H₂O under conditions that are relevant to the aforementioned applications. Our results suggest that OA/LD/H₂O can outperform other LCST mixtures (<i>e.g</i>., ionic liquids) given its broad phase diagram, low LCST, and purity of the two phases after separation.