Phase behavior of catanionic surfactant mixtures: Sodium bis(2-ethylhexyl)sulfosuccinate-didocyldimethylammonium bromide-water system

The phase equilibria of the catanionic surfactant mixtures sodium bis(2-ethylhexyl)sulfosuccinate (AOT)-didodecyldimethyl ammonium bromide (DDAB)-water has been studied by combined NMR, polarizing microscopy, and cryo-TEM techniques at 303 K. Equimolar mixtures of AOT and DDAB form a reverse hexagonal liquid crystalline phase in the water-poor part of the phase diagram. This hexagonal phase exists in equilibrium with another reverse hexagonal phase which originates from the binary AOT-water system. Lamellar liquid crystalline phases formed by the binary AOT-water and DDAB-water systems cannot solubilize any significant amounts of the other parent surfactant. The bicontinuous-type cubic phase of the aqueous AOT system is found to swell substantially with water by the addition of small amounts DDAB. A L(3) solution phase, a cubic phase and a viscous liquid phase are also characterized. The former phase is formed for the AOT-rich area with small amounts of DDAB, and the latter two phases exist for the DDAB-rich area containing small amounts of both AOT and water. Stable polydispersed vesicles are detected in the very dilute part (>98% water) of the phase diagram for both the catanionic pairs (equimolar) as well as the catanionic pairs containing an excess of one of the two parent surfactants. The formation and the stability of the different phases are discussed in terms of surfactant molecular packing constraints and electrostatic effects.