Liquid-liquid equilibria of mutually immiscible ionic liquids with a common anion of basic character

• Several pairs of ionic liquids with chloride or acetate as common anion yield liquid-liquid biphases.
• Experimentally determined liquid-liquid equilibria show different evolutions of the mutual solubility with temperature.
• Little or no tetraalkylpnictogenium-based ionic liquid enters the dialkylimidazolium-rich ionic liquid phase.
• The apparent enthalpy and entropy of solution may be constant or variable with temperature, depending on the system.
• Either the solubility behaviours are entropically driven, or they switch between enthalpically and entropically driven.

Pairs of ionic liquids with the chloride or acetate anion as common ion and with largely different cations (namely a 1-alkyl-3-methylimidazolium cation and either methyltrioctylammonium or trihexyl(tetradecyl)phosphonium) were selected to investigate their mutual miscibility. Most of the pairs investigated were found to give rise to biphasic, non-volatile, totally liquid systems over wide temperature and composition ranges. An accurate determination of the liquid-liquid equilibria was carried out as a function of temperature, finding LCST-like behaviours for the systems with the phosphonium cation, and hourglass-shaped liquid-liquid domains for the systems with the ammonium cation. Enthalpies and entropies of mixing, calculated from the liquid-liquid equilibrium values, revealed that the first type of liquid-liquid equilibria was entropically driven, whereas for the second type a transition from enthalpically to entropically driven occurred with an increase in temperature.

Figure optionsDownload as PowerPoint slide