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 ([NCn, N′CO2Im], 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 ([Cnmim]Br) and zwitterionic betaine surfactants, (CnH2n+1N(CH3)2CH2COO−), [NCn, N′CO2Im] exhibits significantly lower critical micelle concentration (cmc) and surface tension at cmc (γcmc) 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 [NCn, N′CO2Im] is larger than that of [Cnmim]Br, while less than that of CnH2n+1N(CH3)2CH2COO−. Similar to the traditional zwitterionic surfactants, the surface activity and adsorption properties of [NC12, N′CO2Im] 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, [NCn, N′CO2Im] can be exploited for utilizing as a potential substitute for conventional surfactants in certain fields.

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► A novel class of zwitterionic surface active ionic liquids was synthesized.
► Their aggregation behavior in aqueous media was investigated systematically.
► They exhibit superior surface activity.
► They are stable against the variation of temperature, pH, and ionic strength.