Structural characterization of glass-forming oil/water microemulsions by neutron scattering

Small angle neutron scattering (SANS) has been used to establish the dispersed droplet character of a class of pseudo-three-component oil/water (o/w) microemulsions which can be cooled continuously into the glassy state without either crystallization or microemulsion destabilization. SANS of toluene microemulsions at low volume fractions ( ~0.5%) of toluene indicate a droplet radius of 270 ±10 Å. At normal concentrations ( ~ 33 vol % oil), the droplet size cannot be determined precisely but the presence of a droplet state of small polydispersity is established for toluene, p-xylene, benzene, CS₂, and CC1₄ by the presence of a sharp peak in the scattered neutron intensity vs Q plots. Average interdropiet separations range from 200 to 300 Å, implying average radii of ~ 125 Å. Such radii are consistent with freeze-fracture electron microscope studies of the same or similar systems. Variable temperature studies on toluene microemulsions during both cooling and warming cycles in the temperature range ambient to 100 K, show only minor ( ~ 10% ) changes in the interdropiet separation down to 220 K at which temperature diffusion thrpugh the viscous matrix phase becomes too slow to permit further emulsion structure changes on the time scale of the cooling experiment. The origin, and some possible exploitations, of this unusual stability against temperature change are discussed.