High internal phase water-in-oil emulsions and related microemulsions studied by small angle neutron scattering. 2: The distribution of surfactant

We have examined isotopically substituted concentrated emulsions and related microemulsions by small angle neutron scattering (SANS). The emulsions have 90% internal phase micron-scale water droplets in a continuous hexadecane microemulsion. The surfactants have polyisobutylene oligomer tails with acid-amide headgroups. Dilution experiments with surfactant concentration varying over a 75-fold range confirm that the oil phase component of the emulsion contains reverse spherical micelles. We have produced single phase samples of microemulsions designed to have the same composition and same high Q scattering as the oil phase within our emulsions. SANS data from these fit to a model with a compound micelle in which a core region of radius a little less than 15 Å is surrounded by a shell of ca. 20 Å thickness. There is no hexadecane in the core and no water in the shell. The overall volume percentages in the surfactant concentrated microemulsions of water, hexadecane, and surfactant are 6%, 31%, and 64%, while for the more dilute microemulsions we obtain 3%, 37%, and 60%. The dilution data show that the surfactant loading at the oil-water interface is almost independent of dilution, and at the highest concentrations only 5% of the surfactant is at the emulsion droplet interface, the rest being in the form of micelles. The headgroup area per molecule at the interface is 140 2̊² and corresponds well with that expected for a monolayer of surfactant. The aqueous-oil interface is rough, with the water-surfactant interface smoother than the very rough surfactant-oil interface.