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 relate d microemulsions by small angle neutron scattering (SANS). The emulsions ha ve 90% internal phase micron-scale water droplets in a continuous hexadecan e microemulsion. The surfactants have polyisobutylene oligomer tails with a cid-amide headgroups. Dilution experiments with surfactant concentration va rying over a 75-fold range confirm that the oil phase component of the emul sion contains reverse spherical micelles. We have produced single phase sam ples of microemulsions designed to have the same composition and same high Q scattering as the oil phase within our emulsions. SANS data from these fi t to a model with a compound micelle in which a core region of radius a lit tle less than 15 Angstrom is surrounded by a shell of ca. 20 Angstrom thick ness. There is no hexadecane in the core and no water in the shell. The ove rall volume percentages in the surfactant concentrated microemulsions of wa ter, hexadecane, and surfactant are 6%, 31%, and 64%, while for the more di lute 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 a t the emulsion droplet interface, the rest being in the form of micelles. T he headgroup area per molecule at the interface is 140 Angstrom (2) and cor responds 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.