Surface Adsorption and Micelle Formation of Surface Active Ionic Liquids in Aqueous Solution

TLDR

Surface tension and electrical conductivity measurements at 298 K were used to determine critical micelle concentration, surface tension at the cmc, adsorption efficiency, surface tension reduction, maximum surface excess, and minimum surface area per molecule for three 1‑alkyl‑3‑methylimidazolium ionic liquids, and the influence of NaCl, NaBr, and NaI was also examined. The long‑chained imidazolium ionic liquids exhibited higher adsorption efficiency and surface tension reduction than conventional ionic surfactants, with critical micelle concentrations lower than typical cationic surfactants and comparable to anionic surfactants, indicating superior surface activity.

Abstract

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 (γcmc), adsorption efficiency (pC20), and effectiveness of surface tension reduction (Πcmc). In addition, with application of the Gibbs adsorption isotherm, maximum surface excess concentration (Γmax) and minimum surface area/molecule (Amin) 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 pC20 and the Πcmc 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.