A Comprehensive Study on the Synthesis and Micellization of Disymmetric Gemini Imidazolium Surfactants

Two groups of disymmetric Gemini imidazolium surfactants, [C 14 C 4 C m im]Br 2 ( m  = 10, 12, 14) and [C m C 4 C n im]Br 2 ( m  +  n  = 24, m  = 12, 14, 16, 18) surfactants, were synthesized and their structures were confirmed by 1 H NMR and ESI–MS spectroscopy. Their adsorption at the air/water interface, thermodynamic parameters and aggregation behavior were explored by means of surface tension, electrical conductivity and steady-state fluorescence. A series of surface activity parameters, including cmc, γ cmc , π cmc , p C 20 , cmc/ C 20 , Γ max and A min , were obtained from surface tension measurements. The results revealed that the overall hydrophobic chain length ( N c ) for [C 14 C 4 C m im]Br 2 and the disymmetry ( m/n ) for [C m C 4 C n im]Br 2 had a significant effect on the surface activity. The cmc values decreased with an increase of N c or m/n . The thermodynamic parameters of micellization (Δ G θ m , Δ H θ m , Δ S θ m ) derived from the electrical conductivity indicated that the micellization process of [C 14 C 4 C m im]Br 2 and [C m C 4 C n im]Br 2 was entropy-driven at different temperatures, but the contribution of Δ H θ m to Δ G θ m was enhanced by increasing N c or m/n . The micropolarity and micellar aggregation number ( N agg ) were estimated by steady-state fluorescence measurements. The results showed that the surfactant with higher N c or m/n can form larger micelles, due to a tighter micellar structure.