PHASE-BEHAVIOR OF AQUEOUS MIXTURES OF DODECYLTRIMETHYLAMMONIUM BROMIDE (DTAB) AND SODIUM DODECYL-SULFATE (SDS)

Strong electrostatic interactions between the oppositely charged head groups of cationic and anionic surfactants in aqueous mixtures provide an added degree of flexibility in tailoring microstructure. Here, we track the phase behavior and microstructural evolution in mixtures of surfactants with symmetric tail groups: sodium dodecyl sulfate and dod ecyltrimethylammonium bromide. Mixtures of single-chained cationic and anionic surfactants are studied using conductivity, cryo-transmission electron microscopy, video-enhanced microscopy, and time resolved flu orescence quenching. We find that micelles of anionic surfactants grow upon addition of cationic surfactant. These rodlike micelles are tran sformed abruptly into vesicles over a very narrow composition range. F or this symmetric system, formation of hydrated crystals of 1:1 anion/ cation surfactant dominates the phase behavior. We develop a theoretic al thermodynamic cell model to predict important properties of the mix ed micellar solutions such as monomer and micellar composition and cou nterion binding as well as the equilibria between the crystalline and micellar phases. The cell model provides a good account of both the mi celle/ precipitate phase equilibria and the micellar solution properti es as probed by electrical conductivity.