Self-assembly of imidazolium-based surface active ionic liquids in aqueous solution: The role of different substituent group on aromatic counterions

Abstract

Self-assembly behavior of 1-dodecyl-3-methylimidazolium o-anisate (C12mimA) and 1-dodecyl-3-methylimidazolium o-bromobenzoate (C12mimB) in aqueous solution was systematically explored, respectively. Combined with the concerning results of 1-dodecyl-3-methylimidazolium salicylate (C12mimSal) reported previously, impact of aromatic counterions with various substituent group on the aggregation behavior of surface active ionic liquids (SAILs) was demonstrated. Compared with C12mimSal, inferior surface activity of C12mimA and C12mimB can be attributed to deficiency of H-bonding. Lower critical micelle concentration (cmc) of C12mimB than C12mimA may result from the less negative interaction energy between it and water molecule, testified by density functional theory (DFT) calculations. An investigation of temperature effect on cmc reveals that, micellization processes of these two SAILs are entropic-driven within the range of temperature studied. Polarized optical microscopy (POM) and small angle X-ray scattering (SAXS) were employed to determine the generation of lyotropic liquid crystals (LLCs). Similar to C12mimSal, a hexagonal liquid crystalline phase (H1) and a cubic liquid crystalline phase (V2) appear successively with the increasing concentration of C12mimA and C12mimB. While only at higher concentration than C12mimSal, they can exhibit V2 phase. Structural parameters calculated from SAXS patterns suggest that higher concentration or temperature can lead to a closer alignment of SAIL molecules. Rheological results of LLC formed by the two SAILs show traits of general H1 phase analogous to C12mimSal. This work can provide insights into the self-assembly behavior of SAILs and facilitate their potential applications in some fields, e.g. nanomaterial preparation and drug delivery, etc.