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 of 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.