Micelle formation of 1-alkyl-3-methylimidazolium bromide ionic liquids in aqueous solution

Association of 1-alkyl-3-methylimidazolium bromide (CnMImBr) ionic liquids has been studied in aqueous solution at 298 K. Micelle formation was proved by conductivity measurements and fluorescent probes. Mean aggregation number of micelles was determined by pyrene fluorescence quenching method. Linear relationship was found between the logarithm of the critical micelle concentration (cmc) and the number of carbon atoms in the surfactant hydrocarbon chain. The cmc values of ionic liquids were between those of alkyltrimethylammonium bromides (CnTAB) and sodium alkyl sulfates possessing the same alkyl tail. The fluorescent behavior of pyrene probe indicated higher polarity in CnMImBr micelles than in the corresponding CnTAB micelles. The interfacial layer of ionic liquid micelles was also characterized using the highly fluorescent, solvatochromic 2-hydroxy-substituted Nile Red dye.