Synthesis and properties of tetrasiloxane Gemini imidazolium surfactants

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, γ _cmc, π _cmc, pc₂₀, Γ _max and A _min, 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 (ΔG _m ^θ), enthalpy (ΔH _m ^θ) and entropy (ΔS _m ^θ) 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).