Vesicle formation and general phase behavior in the catanionic mixture SDS-DDAB-water. The cationic-rich side

The phase behavior in the cationic-rich side of the phase diagram of the mi xed system sodium dodecyl sulfate (SDS)-didodecyldimethylammonium bromide ( DDAB)-water at 25 degrees C is presented. DDAB is a double-chained surfacta nt and thus it tends to self-assemble in water into bilayer structures-vesi cles and lamellar phases. The phase diagram of the binary system DDAB-water has been studied, and some features of the diluted region as revealed by s urfactant NMR self-diffusion and light microscopy are shown. The structural and phase behavior effects resulting from the addition of SDS are then inv estigated by complementary microscopy and NMR methods. Upon adding SDS to D DAB dispersions, the area for which a single phase of vesicles occurs is la rgely extended and a lobe is defined in the phase diagram. The DDAB-rich ve sicles are essentially unilamellar and characterized by large sizes (range 0.1-5 mu m) and high polydispersity, as probed by combined cryo-TEM and lig ht microscopy. Self-diffusion measurements show a nonmonotonic variation of water self-diffusion coefficients with the molar fraction of SDS in the mi xture, which is correlated to a nonmonotonic variation of mean vesicle size . Microscopy results support this picture. The trends are qualitatively rep roduced if initially sonicated (nonequilibrium) DDAB vesicles are used to p repare the catanionic mixtures. The observations are rationalized in terms of an interplay between two opposing effects associated with the presence o f SDS in the bilayer-electrostatic effects and packing effects.