Micellization and interfacial behavior of imidazolium-based ionic liquids in organic solvent-water mixtures

The surface and micellar properties of aqueous solutions of two imidazolium-based ionic liquids (ILs), 1-hexadecyl-3-butylimidazolium bromide (HDBIm-Br) and 1,3-didodecylimidazolium bromide (DDDDIm-Br), are examined in the presence of several organic solvents by surface tensiometry. The organic solvents studied include methanol, 1-propanol, 1-butanol, 1-pentanol, and acetonitrile. Increases in the critical micelle concentration (cmc) values were obtained for both ILs when increasing the organic solvent content with a more significant increase observed for the DDDDIm-Br IL. For both ILs, decreases in the maximum surface excess concentration (Gamma(max)), increases in the minimum surface area per surfactant molecule (A(min)), decreases in the adsorption efficiency (pC(20)), and decreases in the effectiveness of surface tension reduction (Pi(cmc)) were obtained when increasing the organic solvent content. However, the studied organic solvents affect the surface tension at the cmc (gamma(cmc)) differently; generating increases for DDDDIm-Br and decreases for HDBIm-Br. These changes can be linked to the different water-air interface orientation of both ILs in aqueous solutions free of organic solvents. Linear correlations between the extent of the change in these parameters when increasing the alkyl-chain of the alcohol modifier were also observed. A preliminary study of the utilization of HDBIm-Br in micellar-liquid chromatography (MLC) is also presented, demonstrating the applicability of the IL-aggregates in this analytical technique.