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 ([NC n, N'CO 2Im], 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 nmim]Br) and zwitterionic betaine surfactants, (C nH 2n+1N(CH 3) 2CH 2COO -), [NC n, N'CO 2Im] 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 [NC n, N'CO 2Im] is larger than that of [C nmim]Br, while less than that of C nH 2n+1N(CH 3) 2CH 2COO -. Similar to the traditional zwitterionic surfactants, the surface activity and adsorption properties of [NC 12, N'CO 2Im] 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, [NC n, N'CO 2Im] can be exploited for utilizing as a potential substitute for conventional surfactants in certain fields. © 2012 Elsevier Inc.
Wang, X; Liu, J; Yu, L; Jiao, J; Wang, R; Sun, L
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