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.