Synthesis and properties of tetrasiloxane Gemini imidazolium surfactants

Two tetrasiloxane Gemini imidazolium surfactants with methylene spacer groups ([Si-4-s-Si(4)im]Cl-2, s = 4, 6) were synthesized and characterized by H-1 NMR and ESI-MS spectrum. The surface activity and thermodynamic properties in aqueous solution among three categories of surfactants, including [Si-4-s-Si(4)im]Cl-2, the corresponding monomer ([Si(4)mim]Cl) and hydrocarbon-based Gemini imidazolium surfactant ([C-14-4-C(14)im]Cl-2), were compared by surface tension and electrical conductivity methods. A series of surface activity parameters, including cmc, gamma (cmc), pi (cmc), pc(20), I" (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 [Si-4-s-Si(4)im]Cl-2 was about one order of magnitude lower than that of [Si(4)mim]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 (Delta G (m) (theta)), enthalpy (Delta H (m) (theta)) and entropy (Delta S (m) (theta)) originated from the electrical conductivity measurements at five different temperatures, suggested that the micellization of [Si-4-s-Si(4)im]Cl-2 and [C-14-4-C(14)im]Cl-2 is entropy-driven process whereas aggregation of [Si(4)mim]Cl is enthalpy-driven process at the whole investigated temperatures. The dynamic light scattering results indicate that the aggregation size of [Si-4-4-Si(4)im]Cl-2 (113.6 nm) is larger than [Si-4-6-Si(4)im]Cl-2 (101.7 nm).