Surface adsorption and micelle formation of surface active ionic liquids in aqueous solution

Aqueous solutions of three kinds of surface active ionic liquids composed of the 1-alkyl-3-methylimidazolium cation have been investigated by means of surface tension and electrical conductivity measurements at room temperature (298 K). The surface tension measurements provided a series of parameters, including critical micelle concentration (cmc), surface tension at the cmc (gamma(cmc)), adsorption efficiency (pC(20)), and effectiveness of surface tension reduction (Pi(cmc)). In addition, with application of the Gibbs adsorption isotherm, maximum surface excess concentration (Gamma(max)) and minimum surface area/molecule (A(min)) at the air-water interface were estimated. The effect of sodium halides, NaCl, NaBr, and NaI, on the surface activity was also investigated. It was found that both the pC(20) and the Pi(cmc) were rather larger than those reported for traditional ionic surfactants and the cmc values were somewhat lower than those for typical cationic surfactants, alkyltrimethylammonium bromides, and comparable to typical anionic surfactants, sodium alkyl sulfates. These results demonstrate that the surface activity of long-chained imidazolium IL is somewhat superior to that of conventional ionic surfactants.