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)-didodecyldime thylammonium bromide (DDAB)-water system have been studied by water de uteron NMR and polarizing microscopy methods at 40-degrees-C. H-1 NMR relaxation, PGSE FT-NMR self-diffusion, and dynamic light scattering a re used to study aggregate structures of isotropic phases. A pseudoter nary representation of the four-component phase diagram contains a lar ge number of regions of homogeneous solutions and liquid crystalline p hases, as well as multiphase regions. Isotropic solution regions conta in SDS-rich micellar aggregates or (spontaneously forming) vesicles, r ich in either surfactant; the vesicles form at very high dilution (<0. 1% surfactant). Addition of DDAB to solutions of spherical SDS micelle s may induce a micellar growth to rodlike aggregates. There is a large region of bicontinuous cubic liquid crystalline phase, with high conc entrations of both surfactants. The phase diagram contains several reg ions of lamellar liquid crystalline phase, and under certain condition s two lamellar phases may coexist. The two lamellar phases of the bina ry DDAB-water system can incorporate very different amounts of SDS, th e one dilute in DDAB only small amounts, but the concentrated one larg e amounts. In addition, the lamellar phase may form for concentrated r oughly equimolar mixtures of the two surfactants and for mixtures cont aining high concentrations of SDS. The possible connection between dif ferent lamellar regions in an appropriate three (or higher) dimensiona l phase diagram is discussed as well as the interactions, in terms of electrostatic and molecular packing effects, giving rise to the observ ed phase behavior and solution microstructure.