Aggregation behavior of dodecylsulfonate-based surface active ionic liquids in water

Halogen-free, low-cost alkylsulfonate-based surface active ionic liquid (SAIL), 1-butyl-3-methylimidazolium dodecylsulfonate ([C4mim][C12H25SO3]) was easily synthesized through ion exchange reaction. Its self-aggregation behavior was systematically investigated by surface tension and electric conductivity measurements. [C4mim][C12H25SO3] has rather lower cmc than the traditional ionic surfactant, sodium dodecylsulfonate (SDDS), while it has higher cmc than the anionic SAIL, 1-butyl-3-methylimidazolium dodecylsulfate ([C4mim][C12H25SO4]), with the same hydrocarbon chain length. The negative ΔGmθ values, mainly result from more negative − TΔSmθ, which demonstrates that the micellization process is spontaneous and entropy-driven in the temperature range investigated. Furthermore, influence of four organic electrolytes, viz. tetraalkylammonium bromides ((CH3)4NBr, (C2H5)4NBr, (C3H7)4NBr, and (C4H9)4NBr), on the aggregation behavior of [C4mim][C12H25SO3] in aqueous solution was studied. The results show that all the electrolytes investigated promote aggregates formation of [C4mim][C12H25SO3]. In addition, to comprehend the discrepancy in the surface activity of alkylsulfate- and alkylsulfonate-based SAILs, theoretical calculations were performed to evaluate the electrostatic potential of the corresponding anion. Compared to [C4mim][C12H25SO4], the inferior surface activity of [C4mim][C12H25SO3] mainly originates from the less electronegativity of its anion. This work illustrates the self-assembly of the environmentally friendly alkylsulfonate-based SAILs and is expected to be useful in the applications of ionic liquids.