A SMALL-ANGLE NEUTRON-SCATTERING (SANS) CONTRAST VARIATION INVESTIGATION OF AGGREGATE COMPOSITION IN CATANIONIC SURFACTANT MIXTURES

Mixtures of the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octylsulfate (SOS) combine to form an array of microstructures at various mixing ratios. At certain compositions, thermodynamically stable unilamellar vesicles form. To determine the c omposition of these vesicles, we have performed small-angel neutron sc attering (SANS) experiments, using a contrast variation technique, on samples containing 2.0 wt % 3/7 (weight ratio) CTAB/SOS, which is a bu lk composition of 21 mol % CTAB. The scattering contrast between the a ggregates and the solvent was varied internally by substituting varyin g amounts of the SOS with deuterated SOS. Using a modified Guinier ana lysis of the data, we infer a vesicle composition that is 45 mol % CTA B, which is much more nearly equimolar than the bulk mixing ratio, and we find a bilayer thickness of 22 Angstrom. In parallel to the SANS e xperiments, we have developed a thermodynamic cell model to predict mi crostructure properties. The model results for composition and bilayer thickness compare very well with SANS results. When sodium bromide (N aBr) is added to solutions at these bulk compositions, a transition to micelles occurs, and samples transform from bluish to clear. SANS exp eriments with samples containing 5.0 wt % (0.5 M) NaBr yield the expec ted result that the micelles have a composition equal to that of the b ulk and are ellipsoidal or cylindrical in shape. Thus, the addition of NaBr at this surfactant concentration favors the formation of increas ingly charged microstructure, and simple packing arguments suggest tha t the resulting increase in head-group electrostatic repulsions favors a smaller, more highly curved microstructure, i.e., micelles, over a flatter bilayer vesicle phase.