Self-assembly in mixtures of sodium alkyl sulfates and alkyltrimethylammonium bromides:Aggregation behavior and catalytic properties

Two aqueous mixtures of cationic and anionic surfactants have been studied by means of conductometry, transmission electron microscopy, and microcalorimetry. Their catalytic effects on the decarboxylation of the kinetic probe 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) were also examined in some detail. The mixtures differ profoundly in the hydrophobic match between both surfactant tails, which is perfect for dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS) and poor for hexadecyltrimethylammonium bromide (CTAB) and sodium heptyl sulfate(SHS). This difference is reflected in the more pronounced synergism in critical aggregation concentration and catalytic efficiency of the DTAB/SDS mixture and in the phase behavior of the mixtures. CTAB and SHS can be mixed in a 1:1 ratio without precipitation, forming both small, unilamellar and large, multilamellar vesicles. In DTAB/SDS mixtures, however, precipitation of the catanionic surfactant occurs for a mole fraction of DTAB (x) between 0.3 and 0.7, while both vesicles and large bilayer fragments are formed for x = 0.8. The excess of DTAB in the x = 0.8 mixture results in the solubilization of the Vesicles by DTAB micelles close to the cmc of pure DTAB. A network of interconnected threadlike cylindrical micelles was found as an intermediate stage of aggregation between the vesicles and the mixed micelles. These cylindrical micelles are formed exclusively on further increasing the surfactant concentration.