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 trace the phase behavior and microstructural evolution in mixtures of surfactants with symmetric tail groups: sodium dodecyl sulfate and dodecyltrimethylammonium bromide. Mixtures of single-chained cationic and anionic surfactants are studied using conductivity, cryo-transmission electron microscopy, video-enhanced microscopy, and time-resolved fluorescence quenching. We find that micelles of anionic surfactants grow upon addition of cationic surfactant. These rodlike micelles are transformed abruptly into vesicles over a very narrow composition range. For this symmetric system, formation of hydrated crystals of 1:1 anion/cation surfactant dominates the phase behavior. We develop a theoretical thermodynamic cell model to predict important properties of the mixed micellar solutions such as monomer and micellar composition and counterion binding as well as the equilibria between the crystalline and micellar phases. The cell model provides a good account of both the micelle/precipitate phase equilibria and the micellar solution properties as probed by electrical conductivity.