Designing imidazole-based ionic liquids and ionic liquid monomers for emerging technologies

Imidazolium-based ionic liquids and ionic liquid monomers are becoming increasingly popular in a variety of areas including biphasic reaction catalysis, electromechanical actuator membranes and diluents, separation science membranes, and water purification agents. Ionic liquids first incorporated the imidazole ring in 1984 and this heterocyclic ring has emerged as the focal point of the ionic liquid field. Imidazole was targeted for its ability to form cationic compounds, which are molten salts at low molar mass. Ionic liquids offer several beneficial attributes including fixed charge, potential as green solvents, and relatively high thermal stability. Due to an ionic liquid's ability to facilitate electron or ion motion, they are now enabling electroactive devices. Commercially available conductive membranes are swollen with ionic liquids to enhance their conductivity; alternatively, conductive membranes are synthesized from novel ionic liquid monomers, also termed polymerizable ionic liquids. The imidazole ring has gained much attention for its ability to tune the properties of the resulting ionic liquid. Careful selection of substituents on any of the positions in the ring and exchange of the counteranion influences many physical properties such as the melting point, the boiling point, and the viscosity. Finally, imidazolium ionic liquids utilize two of their unique properties in biphasic catalysis, i.e. their ability to coordinate transition metals and their hydrophilic ionic nature. Several imidazolium-based ionic liquid molecules have displayed the ability to catalyze atom transfer radical polymerization and facilitate the synthesis of polymers with narrow molecular weight distributions. This manuscript reviews some of the more recent advances that are associated with these unusual liquids.