Salt-induced aqueous two-phase systems of oppositely charged surfactants with excess anionic surfactant

The salt effect on the phase behavior of gemini cationic surfactant 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (abbreviated as 12-3-12) and conventional anionic surfactant sodium dodecyl sulfonate (abbreviated as AS) aqueous mixtures with excess AS has been investigated. Salt-induced aqueous two-phase systems with anionic surfactant in excess (abbreviated as ATPS-A) were obtained. Two kinds of salt-induced ATPS-A were distinguished by their origin. The first kind salt-induced ATPS-A originated from the solid suspension and turbid emulsion coexisted aqueous systems. Birefringence was observed at all of the top phases in this kind of ATPS-A. The volume ratio of the top phase to the bottom phase VT/VB decreased as the molar ratio of AS to 12-3-12 MRAS/12-3-12 increased. The second kind of salt-induced ATPS-A originated from the isotropic micellar aqueous solutions, there are two kinds of top phases, one is isotropic and the other is birefringent. Phase volume ratio VT/VB increased with the increase of MRAS/12-3-12. At lower salt concentration, the salt effect on ATPS-A was mainly dependent on the counterions. However, at higher salt concentration, in addition to counterion effect, coions also played a role. For the birefringent top phase in the two kinds of salt-induced ATPS-A, TEM results indicate that vesicles and lamellar microstructures were observed. For the isotropic top phase, TEM result illustrates that only vesicles or the fusion of vesicles were observed. All the bottom phases in the two kinds of ATPS-A are water-like solutions.