Aggregation behaviors of dodecyl sulfate-based anionic surface active ionic liquids in water.

Halogen-free, low-cost alkyl sulfate-based surface active ionic liquids (SAILs), 1-butyl-3-methylimidazolium dodecyl sulfate ([C(4)mim][C(12)SO(4)]), and N-butyl-N-methylpyrrolidinium dodecyl sulfate ([C(4)MP][C(12)SO(4)]) were easily synthesized through ion exchange reaction. The aggregation behaviors of [C(4)mim][C(12)SO(4)] and [C(4)MP][C(12)SO(4)] in aqueous solution were investigated by surface tension, electric conductivity, and static fluorescence quenching. Both [C(4)mim][C(12)SO(4)] and [C(4)MP][C(12)SO(4)] have rather lower cmc, γ(cmc) values and higher pC(20), π(cmc) values than those reported for the traditional ionic surfactant, sodium dodecyl sulfate (SDS), and imidazolium-based SAIL, 1-dodecyl-3-methylimidazolium bromide ([C(12)mim]Br), with the same hydrocarbon chain length. The thermodynamic parameters evaluated from electric conductivity measurements show that the micelle formation of [C(4)mim][C(12)SO(4)] and [C(4)MP][C(12)SO(4)] is entropy-driven in the temperature range investigated. Lower average aggregation number indicates that the micelles of two SAILs present much looser structure. It is found that both the nature and the ring type of counterions can affect the aggregation behavior in aqueous solution. (1)H NMR results of [C(4)mim][C(12)SO(4)] were used to further verify the mechanism of micelle formation. Hydration ability and steric hindrance of the imidazolium or pyrrolidinium counterion as well as the cooperative hydrophobic interaction of longer alkyl chain of [C(12)SO(4)] anion and comparatively shorter alkyl chain of [C(4)mim] or [C(4)MP] cation are proposed to play critical roles in the aggregation of [C(4)mim][C(12)SO(4)] and [C(4)MP][C(12)SO(4)].