Molecular Thermodynamics of Lower Critical Solution Temperature Ionic Liquid/Water Mixtures: Tetrabutylphosphonium Trifluoroacetate

Lower critical solution temperature (LCST) ionic liquids (ILs) are thermoresponsive materials that undergo liquid–liquid demixing with water into water-rich (WR) and IL-rich (ILR) phases upon heating. Despite the vast combinatorial space of cation–anion pairings, LCST behavior is observed in only a handful of aqueous ILs. To uncover the enthalpic and entropic driving forces that govern this unique phase behavior among molecular liquids, we investigate an exemplary LCST IL, tetrabutylphosphonium trifluoroacetate ([P4444][TFA]), by pairing equilibrium molecular dynamics and finite-concentration thermodynamic integration with chilled-mirror dew point water activity experimental measurements. Computationally predicted excess chemical potentials of water (μH2OE) show excellent agreement with experimental measurements over IL weight fractions wIL = 85–95% at T = 300 and 323 K, with absolute deviations ranging from 0.04 to 1.2 kJ mol–1, establishing accurate resolution of the non-ideal solvation contribution. μH2OE is counterbalanced by a strongly favorable ideal entropy of mixing, governed by the large (∼19:1) IL/water molar volume ratio, and a uniformly unfavorable solvation free energy (relative to bulk water). The excess partial molar enthalpy of water (hH2OE) is marked by strong temperature sensitivity, undergoing an exothermic to endothermic shift of ∼5–6 kJ mol–1 from 300–373 K. On the other hand, the non-ideal entropy of solvation (sH2OE,n.id.) is uniformly unfavorable. Mechanistically, this enthalpy/entropy temperature dependence is traced to electrostatic screening probed by the charge–correlation structure factor SZZ(k)/k2, the competing water–water and water–anion interactions, and the structural organization of water imposed by the hydrophobic P4444+ cation.