Aggregation Behavior of Surface Active Dialkylimidazolium Ionic Liquids [C12Cnim]Br (n=1-4) in Aqueous Solutions

Four diakylimidazolium ionic liquids, namely 1-alkyl-3-dodecylimidazolium bromides ([C(12)C(n)im]Br) with the same dodecyl long-chain tail (C-12) and the short alkyl side chain (C-n, n = 1-4), were synthesized, and their molecule structures were confirmed by ESI-MS, H-1-NMR and elemental analysis. The physicochemical properties of [C(12)C(n)im]Br (n = 1-4) were determined by means of surface tension and fluorescence probe methods, respectively. It was found that elongation of the side chain length will bring about an enhancement of surface activity. Along with the side chain length increasing, the critical micelle concentration (CMC), surface tension at CMC (gamma(CMC)), the maximum surface excess (Gamma(m)), micellar aggregation number (N-m) and micellar microenvironment polarity of [C(12)C(n)im]Br decrease, while adsorption efficiency (pC(20)), surface pressure at CMC (Pi(CMC)), the minimum molecular cross-sectional area (A(min)) at air-solution interfaces and CMC/C-20 ratio increase.