Ionic Liquids in Separation Techniques

Abstract

Recently, ionic liquids (ILs) have gained in popularity as unique solvents in different areas of separation techniques. Owing to tunable properties which can be selected by choosing appropriate cationic or anionic constituents, they can be applied in chromatographic and electro-chromatographic methods as mobile phase or stationary phase modifiers or in sample preparations processes as new extraction solvents. Their advantageous properties, from the point of view of separation mechanisms, such as good thermal stability, good extractability for various organic and inorganic ions and the possibility of choosing the optimal viscosity and miscibility with either water or organic solvents make this class of compounds an attractive alternative to classical organic solvents. Furthermore, replacing organic solvents with ILs appears to be much safer for environment regarding the nonflammable and non-volatile nature of these compounds. This review focuses on applications of ILs in separation techniques: extraction, capillary electrophoresis (CE) and liquid chromatography (LC). So far many studies have been published on the applications of ILs in different separation processes and their number is still growing and attracting great attention. With no doubt, this is connected with huge number of ILs consisting of numerous combinations of various bulky, nonsymmetrical organic cations and inorganic or organic anions. It seems to be obvious that new possibilities of ILs applications will also be discovered in future. ILs in the form of molten stearates of transition metals were applied firstly in gas chromatography as new liquid stationary phases by Baber et al. (Barber et al., 1959), but since alkylimidazolium-based ILs were introduced for modification of silica capillaries, GC systems have been achieving satisfactory efficiency (Armstrong et al., 1999; Berthod et al., 2001; Anderson & Armstrong, 2003). In liquid chromatography: either thin layer chromatography (TLC) or high performance liquid chromatography (HPLC), ILs are applied as mobile phase additives as they seem too viscous and non-transparent to UV light to be used as pure solvents. In diluted form, mixed with other low viscosity solvents, they revealed their dual nature responsible for unusual selectivity towards ionic and nonionic analytes. Among the whole group only pyridinium and imidazolium hexafluorophosphates or tetrafluoroborates in reversed-phase chromatographic systems are the most commonly applied for this purpose. ILs have been used also in capillary electrophoresis (CE) for modification of running buffer and for dynamic coating or covalent attachment to the capillary walls. The main aim of this modification is reverse the electroosmotic flow (EOF) provoked by silanol groups on the inner surface of silica capillaries.