Phase behavior and phase structure for catanionic surfactant mixtures: Dodecyltrimethylammonium chloride sodium nonanoate water system

The phase behavior and phase structure of catanionic surfactant mixtures of DoTAC (dodecyltrimethylammonium chloride) and SN (sodium nonanoate) with water are studied by combined H-2 NMR, SAXS, and microscopy techniques at 40 degrees C. The system forms a large isotropic micellar solution phase with excess water. As the concentration of total surfactant is increased, the solution phase coexists with different liquid crystalline phases-a lamellar phase at equimolar ratio of the two surfactants and hexagonal phases with excess DoTAC and excess SN. The lamellar and hexagonal liquid crystalline phases formed by the binary DoTAC system extensively swell with water on adding the anionic surfactant, and the swelling is more dramatic for the lamellar phase which extends to an equimolar ratio of the two surfactants. The mesophase of the short alkyl chain is incapable of solubilizing any substantial amounts of the long chain DoTAC molecules. SAXS data shows a decrease in bilayer thickness and an unchanged average area per polar group on adding SN into the lamellar phase. For the DoTAC-rich hexagonal phase, the diameter of the cylinder remains unchanged and the average area per polar headgroup is decreased in catanionic mixtures. The H-2 NMR quadrupolar splitting values in the hexagonal liquid crystalline phase indicate that the polar headgroups are less extensively hydrated in catanionic mixtures compared to the hydration of the headgroups in the single surfactant systems. The H-2 splitting value in the lamellar phase first decreases, going through a zero splitting value, and then the splitting increases again on a continuous decreasing of the total surfactant concentrations. Alkyl chain asymmetry is found to play a dominant role in the formation and stability of aggregates in catanionic surfactant mixtures.