High internal phase water-in-oil emulsions studied by small-angle neutron scattering

We present a preliminary examination of three isotopically substituted series of concentrated emulsions by small-angle neutron scattering (SANS). These have 90% internal phase water or salt solution droplets in continuous hexadecane. The surfactants have polyisobutylene oligomer tails with mainly acid-amide headgroups. The emulsion structure is well approximated by a polydisperse system of micrometer scale aqueous spheres surrounded by a continuous, surfactant/hexadecane phase L₂ microemulsion. Even though the aqueous volume fraction in the whole emulsion is ca. 90%, we see no evidence for nonsphericality of aqueous droplets, i.e., long-scale planarity of the aqueous-hexadecane boundary. The salt emulsion data fit well to a model in which there is 12-16% of the surfactant absorbed as a monolayer at a flat (0(3) angstroms) aqueous-oil interface, with the remainder as spherical 26-30 angstroms radius reverse micelles in the hexadecane continuous oil phase. The micelles contain 8-10% water and a large fraction of hexadecane as well as the surfactant. The water emulsion has less surfactant absorbed at a much rougher (62(1) angstroms) aqueous interface, and larger micelles containing more water - all reflecting less tightly held water in the aqueous as opposed to salt solution droplets. The structure is insensitive to heating from 20 to 70 °C, but cooling to 5 °C precipitates large surfactant aggregates, giving three phases. The three possible relative specific surface areas (aqueous/aggregate, aggregate/hexadecane, and aqueous/hexadecane) show increasing intrusion of the surfactant aggregate into the aqueous droplets as the aggregate increases in size.