Self-assembly of an imidazolium surfactant in aprotic ionic liquids. 2. More than solvents

Abstract

As tailorable solvents, the physiochemical properties of ionic liquids can be tuned by the structure of ions. Herein, we investigate the structural effects of ILs on the self-assembly of surfactants. It has been confirmed that the cationic surfactant 1-hexadecyl-3-methylimidazolium bromide (C₁₆mimBr) can self-assemble into micellar and lamellar lyotropic liquid crystal phases in the aprotic ionic liquid (AIL) 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF₄). In this work, we explore the aggregation behaviours in AILs with different alkyl chains on the imidazolium group, i.e., 1-propyl-3-methylimidazolium tetrafluoroborate ([Pmim]BF₄), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄), 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim]BF₄) and 1-octyl-3-methylimidazolium tetrafluoroborate ([Omim]BF₄). With the increase of the cation chain length, AILs have better solubility of the solvophobic part of the surfactants and hence a weaker driving force for self-assembly. Therefore, the critical micellization concentration of C₁₆mimBr in AILs increases as confirmed by the surface tension and small angle X-ray scattering characterizations. More interesting things happen to the phase behaviours. Besides the micellar and lamellar lyotropic liquid crystal phases, a hexagonal lyotropic liquid crystal phase is formed in [Pmim]BF₄ while hexagonal and bicontinuous cubic lyotropic liquid crystal phases are formed in [Bmim]BF₄, [Hmim]BF₄ and [Omim]BF₄. It is surprising to observe richer phase behaviours in solvents of lower cohesive energy. The detailed structural information of various aggregates has been obtained by small-angle X-ray scattering. It is demonstrated that AILs work as not only solvents but also co-surfactants.