A cryogenic-electron microscopy study of the one-phase corridor in the phase diagram of a nonionic surfactant-based microemulsion system

We describe full nanostructural characterization of the “one-phase corridor” in the “χ-cut” phase diagram of the isooctane-water-C₁₂E₅ system. We followed the nanostructural development from oil-in-water microemulsion to a bicontinuous microemulsion, and to water-in-oil microemulsion, as the oil content is increased, and temperature is raised accordingly. We used cryogenic-temperature transmission electron microscopy (cryo-TEM) and cryogenic-temperature scanning electron microscopy (cryo-SEM). In most of the composition range studied, we were able to apply both methodologies with similar results. This is a first report of nanostructural mapping of the entire composition range, from water-rich to oil-rich, of a microemulsion system, using well-controlled cryo-specimen preparation. The nanostructural sequence directly imaged here agrees well with theory and previous experimental work by nonimaging techniques, and some, partial electron microscopy.