How to select the best ionic liquid for a given application?

Ionic liquids are considered as designer solvents because their properties can be tuned by an independent choice of the cation and anion. Given the huge number of possible cation-anion combinations, an astronomical number of ionic liquids can be prepared, not to speak about the number of binary or ternary mixtures. Therefore, it is very surprising that the majority of researchers are using only a very small number of ionic liquids. For instance 1-ethyl-3-methylimidazolium acetate is very popular for cellulose processing, whereas 1-butyl-1-methylimidazolium bis(trifluoromethylsulfonyl)imide is often the ionic liquid of choice for the electrodeposition of metals. However, by restricting the choice of the ionic liquid to a very small set of compounds, one will not take advantage of the full potential of ionic liquids. The choice of the right ionic liquid does matter for many applications, and some ionic liquids are not suitable at all for a given application. The imidazolium ionic liquids are a good example. Whereas imidazolium salts often have a low melting point and a low viscosity in comparison with other classes of ionic liquids, they cannot be used in the presence of strong bases or for the electrodeposition of very reactive metals. Imidazolium and pyridinium ionic liquids are not stable towards ozone. Metal salts have a poor solubility in ionic liquids with weakly coordinating anions such as bis(trifluoromethylsulfonyl)imide ionic liquids, whereas they are well soluble in ionic liquids with coordinating anions (e.g. chloride, acetate or thiocyanate). Also in the case of biphasic catalysis the choice of the ionic liquids is of prime importance. In the ideal case, the product spontaneously separate from the ionic liquids and the catalyst remains dissolved in the ionic liquid phase. This talk will give an overview of the most important rules to be followed for the selection of an ionic liquid for a given application, with emphasis on separation processes.Keynote lecture by Koen Binnemansstatus: publishe