Weak Anion Hydration Drives LCST Behavior in Ionic Liquid–Water Mixtures: Insights from Calorimetry and Neutron Diffraction

Isothermal titration calorimetry (ITC) combined with neutron diffraction and isotopic substitution (NDIS) has been used to elucidate the molecular origins of lower critical solution temperature (LCST) behavior in the tetrabutylphosphonium 5-phenyltetrazolate/water ([P4444][Ph–tet]/H2O) system. Comparison with the structurally similar but fully water-miscible analogue, tetrabutylphosphonium benzoate ([P4444][Bz]) reveals a much smaller negative enthalpy of mixing with water. This is indicative that weaker solvation of the [Ph–tet]− anion drives an entropy-dominated desolvation process upon heating, leading to phase separation into IL-rich and water-rich domains and is supported by observation from NDIS that the [Ph–tet]− anion is only weakly hydrated by water.