Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation

The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail.

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, ([P4,4,4,4]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 [P4,4,4,4]CF3COO were calculated, and these suggested that a large number of water molecules hydrated [P4,4,4,4]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.

Mixing scheme of an aqueous solution of tetrabutylphosphonium trifluoroacetate in the water-rich region

The enthalpic interaction of this particular ionic liquid is extremely high, 16 000 kJ mol⁻¹!