Vesicle Formation and General Phase Behavior in the Catanionic Mixture SDS-DDAB-Water. The Cationic-Rich Side Eduardo F. Marques, †,§ Oren Regev, ‡ Ali Khan,* ,† Maria da Grac ¸ a Miguel, § and Bjo 1 rn Lindman † Physical Chemistry 1, Center for Chemistry and Chemical Engineering, P.O. Box 124, Lund UniVersity, Lund S-221 00, Sweden, Department of Chemical Engineering, Ben-Gurion UniVersity, P.O.Box 653, 84105 Beer-SheVa, Israel, and Departamento de Quı ´mica, UniVersidade de Coimbra, 3049 Coimbra, Portugal ReceiVed: March 10, 1999; In Final Form: June 14, 1999 The phase behavior in the cationic-rich side of the phase diagram of the mixed system sodium dodecyl sulfate (SDS)-didodecyldimethylammonium bromide (DDAB)-water at 25 °C is presented. DDAB is a double- chained surfactant and thus it tends to self-assemble in water into bilayer structures-vesicles 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 surfactant NMR self-diffusion and light microscopy are shown. The structural and phase behavior effects resulting from the addition of SDS are then investigated by complementary microscopy and NMR methods. Upon adding SDS to DDAB dispersions, the area for which a single phase of vesicles occurs is largely extended and a lobe is defined in the phase diagram. The DDAB-rich vesicles are essentially unilamellar and characterized by large sizes (range 0.1-5 μm) and high polydispersity, as probed by combined cryo-TEM and light microscopy. Self-diffusion measurements show a nonmonotonic variation of water self-diffusion coefficients with the molar fraction of SDS in the mixture, which is correlated to a nonmonotonic variation of mean vesicle size. Microscopy results support this picture. The trends are qualitatively reproduced if initially sonicated (nonequilibrium) DDAB vesicles are used to prepare the catanionic mixtures. The observations are rationalized in terms of an interplay between two opposing effects associated with the presence of SDS in the bilayer-electrostatic effects and packing effects. I. Introduction The formation of stable vesicle phases in aqueous mixtures of cationic and anionic surfactants (catanionic mixtures), without recourse to highly energetic methods such as sonication and extrusion, is perhaps the most outstanding feature of the phase behavior of these systems, in the dilute regime. In the past decade several authors have presented experimental evidence for vesicle formation in catanionic systems 1-7 and theoretical modeling has also been put forth, 8-10 in close connection with the experimental observations. In a previous work, 7 a global view of the phase behavior of the catanionic system sodium dodecyl sulfate (SDS)-didodecyldimethylammonium bromide (DDAB)-water was presented at 25 °C in the very dilute region. Emphasis was placed in the detailed characterization of phase microstructure in the SDS-rich side of the system. In the current work, the complementary investigation of the cationic surfactant- rich side is presented. The system of interest here can be seen as a case study for catanionic mixtures, since both electrostatic and geometric packing effects are at play in the dictation of self-assembly. Because of its strong hydrophobicity, the DDAB molecule has very low solubility in water and its packing parameter dictates a preference to assemble into bilayer-based structures. 11,12 Thus, the surfactant readily displays lamellar liquid crystal formation in water 13-16 and, at low concentrations, complex equilibria involving giant onion structures and uni- and bilamellar vesicles. 17-21 The formation of vesicular structures also in the DDAB-rich area of the catanionic system is thus expected, since they occur already in the binary system DDAB-water. Less predictable is the effect of adding the anionic single-chain amphiphile on the particular sequence of structures forming (shape, size, polydispersity) and, consequently, in the complete phase behavior picture. While electrostatic interactions are the main driving force for the association between the two surfac- tants, they are modulated by the packing requirements derived from the different molecular geometry of the two amphiphiles. The concentration of salt (NaBr) in the system, which varies as the mixing ratio between the surfactants is varied, may also influence phase stability and structure. The very low concentration of monomeric surfactant in the present type of mixtures implies that equilibration rates, which undergo via monomer exchange, may be rather slow. Days, weeks, or months may be necessary before a bilayer-based system reaches equilibrium, in contrast to the labile micellar system. In such cases, time and the historical treatment of the sample may be of importance in terms of phase behavior determination and also of the structural details of the phases, as will be shown further. Finally, we note that there is an analogy between the current work and studies on the solubilization or disintegration process of vesicles prepared from phospholipids (which are commonly double-chained amphiphiles) by the addition of single-chain surfactants. 22-24 These studies are directly relevant to protocols for membrane reconstitution of proteins in biological research and pharmacological applications. * Email: ali.khan@fkem1.lu.se Fax: +46 46 222 4413 tel: +46 46 222 3247 † Lund University. ‡ Ben-Gurion University. § Universidade de Coimbra. 8353 J. Phys. Chem. B 1999, 103, 8353-8363 10.1021/jp990852p CCC: $18.00 © 1999 American Chemical Society Published on Web 09/15/1999