Microemulsions with Didodecyldimethylammonium Bromide Studied by Neutron Contrast Variation

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 surfactant h -DDAB and d -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 A, about 70% of an all-trans C12 chain length. At the water interface the area per head was 56-61 A2 , while for the alkyl chains at the outer surface it was 90-125 A2 (15-30% lower than that for a truncated cone molecular configuration). The cyclohexane-water interfacial tensions gammao/w , measured by surface light scattering, were used along with the droplet polydispersities to find that the rigidity of the DDAB film, 2K + K ; is close to 1.0k B T . 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.