Homogeneous solutions of equimolar mixed cationic-anionic surfactants

Equimolar mixed cationic-anionic surfactants usually form a precipitate at very low concentration. However, we found that aqueous mixtures of alkyltriethylammonium bromide (CnH2n+1N(C2H5)(3)Br, CnNE) and sodium alkylsulfonate (CnH2n + 1SO3Na, CnSO3Na) could form homogeneous solutions even at high concentration. C8,10NE-C8,10,12SO3Na mixed systems and C12,14NE-C8SO3Na mixed systems formed homogeneous solutions at any compositions in the entire concentration range studied (up to 0.2 mol.L-1). Hydrodynamic radius R-h and polydispersity index PI of the mixed aggregates were measured by dynamic light scattering, and the effects of time, temperature, dilution, and added salt NaBr on that were studied. The mixed aggregates were observed by freeze-fracture transmission electron microscopy. The results showed that small and narrow distributed vesicles were formed in the mixed systems, and vesicle radii did not change with time, which indicates that the homogeneous solutions are stable. Besides, these mixed systems had high surface activity, and their viscosities are low.