High Internal Phase Water-in-Oil Emulsions Studied by Small-Angle Neutron Scattering

Abstract

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) Å) aqueous−oil interface, with the remainder as spherical 26−30 Å 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) Å) aqueous interface, and larger micelles containing more waterall 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.