Study on phase behavior of [C16mim] [br]/CAHS/n-butanol/n-hexane/water microemulsions: synergistic effect of ionic liquid-zwitterionic surfactants

The synergistic effect of ionic liquids (ILs) and anionic surfactants, ILs and cationic surfactants, and ILs and nonionic surfactants to form microemulsions has been studied by other researchers, while the synergistic effect of ionic liquids and zwitterionic surfactants to form microemulsions has not been reported. Here, we report the synergistic effect of imidazolium ionic liquid, 1-hexadecyl-3-methylimidazolium bromide [C(16)mim] [Br], and betaine-type zwitterionic surfactant, cocamidopropyl hydroxysultaine (CAHS), as surfactants, to form microemulsions with n-butanol, n-hexane and water. Phase behavior of the systems with different mass ratios of two surfactants, varied mass ratios of surfactant to cosurfactant (Km), and different temperatures are investigated by pseudo-ternary phase diagrams. The single-phase region of the microemulsion is maximized when the ratio of CAHS to [C(16)mim] [Br] is 1:4 and Km is 1.5, with little temperature dependence. The size and the polarity of microemulsions are explored. The synergistic effect of [C(16)mim] [Br] and CAHS is confirmed by surface tension and interface tension. A universal preparation method to form microemulsions by combining ionic liquids with zwitterionic betaine-type surfactants is provided and it play a pivotal role in broadening the potential uses of ILs in microemulsions. {Graphical Abstract}