Two groups of disymmetric Gemini imidazolium surfactants, [C14C4Cmim]Br2 (m = 10, 12, 14) and [CmC4Cnim]Br2 (m + n = 24, m = 12, 14, 16, 18) surfactants, were synthesized and their structures were confirmed by 1H 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, γcmc, πcmc, pC20, cmc/C20, Γmax and Amin, were obtained from surface tension measurements. The results revealed that the overall hydrophobic chain length (Nc) for [C14C4Cmim]Br2 and the disymmetry (m/n) for [CmC4Cnim]Br2 had a significant effect on the surface activity. The cmc values decreased with an increase of Nc or m/n. The thermodynamic parameters of micellization (ΔGmθ, ΔHmθ, ΔSmθ) derived from the electrical conductivity indicated that the micellization process of [C14C4Cmim]Br2 and [CmC4Cnim]Br2 was entropy-driven at different temperatures, but the contribution of ΔHmθ to ΔGmθ was enhanced by increasing Nc or m/n. The micropolarity and micellar aggregation number (Nagg) were estimated by steady-state fluorescence measurements. The results showed that the surfactant with higher Nc or m/n can form larger micelles, due to a tighter micellar structure.

Two tetrasiloxane Gemini imidazolium surfactants with methylene spacer groups ([Si4-s-Si4im]Cl2, s = 4, 6) were synthesized and characterized by 1H NMR and ESI-MS spectrum. The surface activity and thermodynamic properties in aqueous solution among three categories of surfactants, including [Si4-s-Si4im]Cl2, the corresponding monomer ([Si4mim]Cl) and hydrocarbon-based Gemini imidazolium surfactant ([C14-4-C14im]Cl2), were compared by surface tension and electrical conductivity methods. A series of surface activity parameters, including cmc, γcmc, πcmc, pc20, Γmax and Amin, and the adsorption isotherms were obtained from the surface tension plots. The results indicated that the tetrasiloxane Gemini imidazolium surfactant with the longer spacer group has the higher capacity to form micelles but lower efficiency to reduce surface tension. Besides, the cmc value of [Si4-s-Si4im]Cl2 was about one order of magnitude lower than that of [Si4mim]Cl. The tetrasiloxane-based surfactants have the higher capacity to low the surface tension than the hydrocarbon-based surfactant. The adsorption isotherms of the tetrasiloxane-based surfactants are similar to those of conventional hydrocarbon-based surfactants. The thermodynamic parameters of micellization process, namely, the standard Gibbs free energy (ΔGmθ), enthalpy (ΔHmθ) and entropy (ΔSmθ) originated from the electrical conductivity measurements at five different temperatures, suggested that the micellization of [Si4-s-Si4im]Cl2 and [C14-4-C14im]Cl2 is entropy-driven process whereas aggregation of [Si4mim]Cl is enthalpy-driven process at the whole investigated temperatures. The dynamic light scattering results indicate that the aggregation size of [Si4-4-Si4im]Cl2 (113.6 nm) is larger than [Si4-6-Si4im]Cl2 (101.7 nm).