Study of mixed aggregates in aqueous solutions of sodium dodecyl sulfate and dodecyltrimethylammonium bromide

Mixed solutions containing sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) have been investigated by tracer self-diffusion (Na⁺ and Cl⁻ ion diffusion) and potentiometry with Br⁻- and DTA⁺-specific electrodes, with particular emphasis on the binding of counter-ions to the mixed micelles in the SDS-rich and DTAB-rich isotropic systems. At DTAB mole fractions (X) between 0 and 0.38 it has been shown that no Br⁻ ion binding occurs, whereas all added DTA⁺ ions are incorporated in the mixed micelles. In the DTAB-rich systems, all added DS⁻ ions are incorporated into mixed micelles, and the degree of Br⁻ ion binding decreases upon increasing added SDS. However, it remains well above zero at the upper precipitation boundary (X = 0.88), in contrast to the degree of Na⁺ ion binding which tends toward zero at the lower precipitation boundary (X = 0.38). This difference in counter-ion binding in the two composition ranges correlates well with the reported changes of micelle size in these ranges. Finally, in the range 0.32 < X < 0.38, which corresponds to turbid but stable systems, the self-diffusion data suggest the existence of vesicles and/or more-complex surfactant assemblies. Direct examination of the solutions by phase-contrast light microscopy and cryo-transmission electron microscopy (cryo-TEM) has confirmed this conclusion, and has shown the existence or highly polydisperse, sometimes open, vesicles which are spontaneously formed in solutions of mixtures of SDS and DTAB at X = 0.34. Cryo-TEM also shows that some thread-like micelles coexist with the vesicles at X = 0.34. At X = 0.30 the micelles are mostly spheroidal or slightly elongated. The micrographs also show some open vesicles.