Recent progress in the assembly behavior of imidazolium-based ionic liquid surfactants

Abstract

Imidazolium-based surface active ionic liquids (Im-SAILs) are based on imidazolium salts with long hydrophobic alkyl chains. Due to the delocalized electrons on the imidazole ring and the hydrogen bonding between imidazole rings, Im-SAILs can exhibit novel aggregation behavior that differs from that of conventional surfactants. In both the absence and presence of a solvent (such as water, organic solvents, and ionic liquids), Im-SAILs exhibit rich self-aggregation behavior and form various types of assemblies, including spherical micelles, wormlike micelles, nanosheets, nanorods, nanospheres, nanotubes, nanoclusters, vesicles, vesicle clusters, liquid crystals, planar bilayers and gels. Additives (including conventional surfactants, polymers, metal oxides, amino acids, and proteins) enhance the aggregation of Im-SAILs through the synergistic effects of multiple intermolecular interactions, such as electrostatic, hydrophobic, hydrogen bonding, and cation-π interactions; in some cases, the resulting assemblies can even respond to external stimuli, such as pH, temperature, light, organic solvent, and others. By effectively utilizing the versatile “designability” of ionic liquids and/or choosing suitable additives, the controllable self-assembly of new ordered molecular aggregates can be realized. These strategies are expected to lead to the discovery of novel “soft” materials and the development of aggregates with important and/or novel applications. Therefore, in this review, we first summarize the recent advances related to the assembly behavior of Im-SAILs. The roles of the surfactant and counter-ion structures are analyzed. The assembly of Im-SAILs in the presence of additives, including conventional surfactants, polymers, metal oxides, amino acids, and proteins, is then reviewed. At the end of this review, a prospective future involving Im-SAILs are discussed.