Using ionic liquids to formulate microemulsions: Current state of affairs

Microemulsions are stable mixtures of a polar solvent, surfactant and an unpolar solvent. Ionic liquids (ILs, i.e. salts with melting points below 100 °C) are a huge class of potentially promising solvents. We discuss here published structural or thermodynamic investigations concerning microemulsions in which one or more of the three classical components are ILs.In microemulsions IL can replace respectively the “oil”, the “surfactant” and the “water” phase. Experimental proofs of the existence and stability of microemulsions are given as well as hints at their microstructure. While the four regimes initially defined by Winsor are all accessible, most of the examples of microemulsions containing ionic liquids belong to the class of “rigid” microemulsions. Since additional solutes have characteristic distribution coefficients for each pseudo phase, IL based microemulsions may provide a useful tool for solubilization (reaction medium) and separation, thus allowing the recovery of a large variety of reaction products, but also waste. Further to a discussion of phase diagrams and thermodynamics, we will show some application examples and propose challenges for future studies, in this vast but only emerging domain.