PHASE-BEHAVIOR AND PHASE-STRUCTURE FOR CATANIONIC SURFACTANT MIXTURES- DODECYLTRIMETHYLAMMONIUM CHLORIDE SODIUM NONANOATE WATER-SYSTEM

The phase behavior and phase structure of catanionic surfactant mixtur es of DoTAC (dodecyltrimethylammonium chloride) and SN (sodium nonanoa te) with water are studied by combined H-2 NMR, SAXS, and microscopy t echniques at 40 degrees C. The system forms a large isotropic micellar solution phase with excess water. As the concentration of total surfa ctant is increased, the solution phase coexists with different liquid crystalline phases-a lamellar phase at equimolar ratio of the two surf actants and hexagonal phases with excess DoTAC and excess SN. The lame llar and hexagonal liquid crystalline phases formed by the binary DoTA C system extensively swell with water on adding the anionic surfactant , and the swelling is more dramatic for the lamellar phase which exten ds 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 bila yer thickness and an unchanged average area per polar group on adding SN into the lamellar phase. For the DoTAC-rich hexagonal phase, the di ameter of the cylinder remains unchanged and the average area per pola r headgroup is decreased in catanionic mixtures. The H-2 NMR quadrupol ar 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 su rfactant systems. The H-2 splitting value in the lamellar phase first decreases, going through a zero splitting value, and then the splittin g increases again on a continuous decreasing of the total surfactant c oncentrations. Alkyl chain asymmetry is found to play a dominant role in the formation and stability of aggregates in catanionic surfactant mixtures.