Interfacial and micellar properties of imidazolium-based monocationic and dicationic ionic liquids

The critical micelle concentration (CMC) values of an extensive class of 18 different monocationic and dicationic imidazolium-based ionic liquids (IL) were determined using surface tensiometry. An impressive correlation between the headgroup substituent chain length and the CMC value was obtained for a variety of structurally-related monocationic ILs. In one homologous series of ILs, a 3400-fold decrease in the CMC value was obtained by simply increasing the chain length of a fixed butylimidazolium headgroup. The CMC values of different classes of dicationic ionic liquids resembling gemini and bolaform surfactants were found to be highly dependent on the length of the employed linkage chain connecting the two headgroups as well as the nature of the free alkyl side chain. Dicationic ILs studied in this work containing longer alkyl linkage chains were found to produce modest decreases in the CMC whereas free alkyl chains appended onto the imidazolium cation provided over a 100-fold decrease in the CMC value. Dicationic ILs possessing hydrophilic and hydrophobic moieties analogous to structurally-related monocationic ILs exhibited much higher efficiencies at lowering the surface tension.