Effect of head group of surfactant on the self-assembly structures and aggregation transitions in a mixture of cationic surfactant and anionic surfactant-like ionic liquid

The effect of head group of surfactant on the self-assembly structures and aggregation transitions, which had rarely been reported in mixed aqueous solutions of cationic surfactant and anionic surfactant-like ionic liquid molecules, was studied in three different cases. We have investigated the effect of the methyl, ethyl and 2-phenoxy-ethyl on the aggregation by the methods of turbidity, dynamic light scattering (DLS), transmission electron microscopy (TEM), rheological measurements, and steady-state fluorescence measurements. It is found that the head group has an important effect on the self-assembly structures and phase transition behaviors of mixed aqueous solutions. The ethyl group introduces a more considerable steric hindrance compared with the methyl group, which leads to the smaller packing parameter. Therefore, the system containing the methyl group has a slightly larger vesicle phase range than the system containing the ethyl group. However, compared with the ethyl group, 2-phenoxy-ethyl introduces the non-covalent pi-pi interaction between the imidazolium ring and benzene ring. This interaction easily eliminates the steric hindrance. Therefore, it is more prone to form the wormlike micelles and vesicles for the system with the 2-phenoxy-ethyl group. We also find intriguing rheological behavior from the non-spherical vesicles. Moreover, for the wormlike micelles, we find it only form in aqueous surfactant mixtures with strong interactions. (C) 2020 Elsevier B.V. All rights reserved.