Surface adsorption and micelle formation of imidazolium-based zwitterionic surface active ionic liquids in aqueous solution

A novel class of zwitterionic surface active ionic liquids (SAILs), N-alkyl-N'-carboxymethyl imidazolium inner salts ([N-C(n), N'-CO(2)-Im], n=10, 12, 14), was synthesized. Their aggregation behavior in aqueous solution was investigated by surface tension, isothermal titration calorimetry, and steady-state fluorescence. Compared with the reported imidazolium-based cationic SAILs, 1-alkyl-3-methylimidazolium bromide ([C(n)mim]Br) and zwitterionic betaine surfactants, (C(n)H(2)(n)(+1)N(CH(3))(2)CH(2)COO(-)), [N-C(n), N'-CO(2)-Im] exhibits significantly lower critical micelle concentration (cmc) and surface tension at cmc (γ(cmc)) values. It is attributed to the incorporation of a deprotonated carboxylic group into the head group, which weakens the electrostatic repulsion between head groups and favors micellization. The micellar aggregation number of [N-C(n), N'-CO(2)-Im] is larger than that of [C(n)mim]Br, while less than that of C(n)H(2)(n)(+1)N(CH(3))(2)CH(2)COO(-). Similar to the traditional zwitterionic surfactants, the surface activity and adsorption properties of [N-C(12), N'-CO(2)-Im] at air/water interface have a slight variation with temperature, pH, and ionic strength. This indicates that the present zwitterionic SAILs display the aggregation behavior much similar to zwitterionic surfactants, distinctly different from imidazolium-based cationic SAILs. Sets in low sensitivity to the environmental conditions, superior surface activity and unique physicochemical properties of ionic liquids, [N-C(n), N'-CO(2)-Im] can be exploited for utilizing as a potential substitute for conventional surfactants in certain fields.