Phase equilibria of mixtures of mutually immiscible ionic liquids

In the last decade, the scientific community has witnessed a burgeoning research in ionic liquids. Numerous and diverse applications for them have been pointed out in the literature, with some processes based on ionic liquid technology already working in real plants. Among the possible fields of application, that of solvent extraction involving ionic liquids as solvents holds an important position. The opportunities of ionic liquids as alternative solvents for these kind of processes could be improved by the recent discovery of mutually immiscible ionic liquids. The use of ionic liquids presenting mutual immiscibility can generate stable multi-layer solvent systems, which, combined with the typical advantages offered by ionic liquids (good solvation properties, task-specific tuned structures, etc.), can mean a powerful medium for multi-component separations.To get a deeper knowledge of the behaviour of mutually immiscible ionic liquids, the phase equilibria of binary and ternary mixtures of ionic liquids were investigated. Specifically, hydrophobic ionic liquids with the common anion bis{(trifluoromethyl)sulfonyl}amide were selected. Pairs of ionic liquids with largely different cations were chosen, looking for the existence of mutual immiscibility between them. It was found that ionic liquids with a tetraalkylphosphonium cation are largely immiscible at ambient temperature with some other ionic liquids including 1,3-dialkylimidazolium or 1-alkylpyridinium cations. The miscibility of such pairs of ionic liquids was temperature dependent, following an upper critical solution temperature (UCST) behaviour. It was also found that small changes in the structure of an ionic liquid can drastically affect the mutual solubility with another ionic liquid. The liquid–liquid equlibrium (LLE) data of these systems were acceptably correlated by means of the classical non-random two-liquid (NRTL) equation.