Synthesis and properties of novel ester-containing gemini imidazolium surfactants

Novel ester-containing gemini imidazolium surfactants were synthesized from various aliphatic acids (C₁₂, C₁₄, C₁₆), 1-methylimidazol, and 2,2-bis (bromomethyl)-1,3-propanediol with a two step reaction. The surfactants are abbreviated as [Cnim]2 (n = 12, 14, 16). The chemical structures of the prepared compounds were confirmed by ¹H NMR, IR spectra, and elemental analysis. Thermal properties of these new surfactants were studied with thermogravimetry (TG) and differential scanning calorimetry (DSC). Their critical micelle concentrations (cmc) in the aqueous solutions were determined by surface tension and electrical conductivity methods. Surface tension parameters including T_cmc, A_min, and γ_cmc were obtained. The parameters β (degree of counterion binding to micelles), ΔG⁰_ads (Gibbs free energy of adsorption), and ΔG⁰_mic (Gibbs free energy change of micellization) were also derived. The results indicated that these novel gemini surfactants exhibited very low cmcs and a good efficiency in lowering the surface tension of water. Dynamic light scattering (DLS) measurements showed that [C₁₂im]₂ and [C₁₄im]₂ can easily form vesicles at very low concentration, and that the self-assembly of [C₁₄im]₂ is more well-organized than for [C₁₂im]₂. The use of DOSY and DLS as a complementary technique shows that a dynamic micelle-vesicle equilibrium exists in solution.