Fluctuations and Mixing State of an Aqueous Solution of the Ionic Liquid Tetrabutylphosphonium Trifluoroacetate around the Critical Point

Aqueous solutions of ionic liquids have unique mixing states. Fluctuations are useful for understanding the inhomogeneity of the mixing states. In this study, an aqueous solution of tetrabutylphosphonium trifluoroacetate, ([P-4,P-4,P-4,P-4]CF3COO), which exhibits a lower-critical-solution-temperature-type phase transition, was investigated. Focussing on the concentration and temperature range near the critical point, the fluctuations were evaluated by combining three kinds of experimentally obtained data: small-angle X-ray scattering intensity, partial molar volumes, and isothermal compressibility. Using Kirkwood-Buff integrals, individual density fluctuations of water and [P-4,P-4,P-4,P-4]CF3COO were calculated, and these suggested that a large number of water molecules hydrated [P-4,P-4,P-4,P-4]CF3COO ion pairs, and the hydrated ion pairs aggregated near the critical point. The relationship between the mesoscopic fluctuations and the macroscopic phase transition was clarified by drawing counter maps of the fluctuations in the phase diagrams.