Microemulsions with Didodecyldimethylammonium Bromide Studied by Neutron Contrast Variation

Abstract

The structure of water-in-oil microemulsion droplets, stabilized by didodecyldimethylammonium bromide (DDAB), has been investigated by small-angle neutron scattering (SANS). Detailed information about the curved surfactant film has been obtained by selectively deuterating the water, DDAB, and cyclohexane components. For each surfactanth-DDAB andd-DDAB and concentration, three sets of complementary neutron contrast data were analyzed together in terms of a Schultz distribution of core–shell particles. The modeling was consistent with a simple liquid-like surfactant layer, of density 0.80 g cm−3, with no evidence for any solvent penetration. This film thickness was found to be 11–12 Å, about 70% of an all-transC12chain length. At the water interface the area per head was 56–61 Å2, while for the alkyl chains at the outer surface it was 90–125 Å2(15–30% lower than that for a truncated cone molecular configuration). The cyclohexane–water interfacial tensions γo/w, measured by surface light scattering, were used along with the droplet polydispersities to find that the rigidity of the DDAB film, 2K+K̄is close to 1.0kBT. This means that rather than acting as an effective parameter in the SANS analysis, the polydispersity is a natural consequence of the film rigidity. These results show that the film bending energy model accounts well for the behavior of such DDAB microemulsions.