Aggregation behavior of long-chain imidazolium ionic liquids in aqueous solution: Micellization and characterization of micelle microenvironment

Aqueous solutions of long-chain imidazolium ionic liquids have been investigated by surface tension and steady-state fluorescence measurements at room temperature (298 K). The micelle aggregation number (Nagg) was obtained by pyrene fluorescence quenching method. From the surface tension data, critical micelle concentration (cmc), surface tension at the cmc (γcmc), adsorption efficiency (pC20), and effectiveness of surface tension reduction (Πcmc), were determined. Moreover, applying the Gibbs adsorption isotherm, maximum surface excess concentration (Γmax) and minimum surface area per 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. The addition of salts decreases significantly both cmc and γcmc, and the dependence of the salt effect on the anion species is analogous for the case of conventional ionic surfactants. Due to the bulkiness of the imidazolium head group, the microenvironment in long-chain imidazolium ionic liquid micelles exhibits higher polarity compared with the corresponding CnTAB micelles, and the micelle aggregation numbers (Nagg) are smaller than those of CnTAB.