SELF-ASSEMBLY IN MIXTURES OF A CATIONIC AND AN ANIONIC SURFACTANT - THE SODIUM DODECYL-SULFATE DIDODECYLDIMETHYLAMMONIUM BROMIDE WATER-SYSTEM

The phase equilibria of the sodium dodecyl sulfate (SDS)-didodecyldimethylammonium bromide (DDAB)-water system have been studied by water deuteron NMR and polarizing microscopy methods at 40-degrees-C. H-1 NMR relaxation, PGSE FT-NMR self-diffusion, and dynamic light scattering are used to study aggregate structures of isotropic phases. A pseudoternary representation of the four-component phase diagram contains a large number of regions of homogeneous solutions and liquid crystalline phases, as well as multiphase regions. Isotropic solution regions contain SDS-rich micellar aggregates or (spontaneously forming) vesicles, rich in either surfactant; the vesicles form at very high dilution (<0.1% surfactant). Addition of DDAB to solutions of spherical SDS micelles may induce a micellar growth to rodlike aggregates. There is a large region of bicontinuous cubic liquid crystalline phase, with high concentrations of both surfactants. The phase diagram contains several regions of lamellar liquid crystalline phase, and under certain conditions two lamellar phases may coexist. The two lamellar phases of the binary DDAB-water system can incorporate very different amounts of SDS, the one dilute in DDAB only small amounts, but the concentrated one large amounts. In addition, the lamellar phase may form for concentrated roughly equimolar mixtures of the two surfactants and for mixtures containing high concentrations of SDS. The possible connection between different lamellar regions in an appropriate three (or higher) dimensional phase diagram is discussed as well as the interactions, in terms of electrostatic and molecular packing effects, giving rise to the observed phase behavior and solution microstructure.