Nanostructure formation in the lecithin/isooctane/water system.

We present here for the first time a study of the self-assembled nanostructures in the lecithin/isooctane/water system by direct-imaging techniques, namely, cryogenic transmission electron microscopy (cryo-TEM) and cryogenic scanning electron microscopy (cryo-SEM). Along the dilution line [water]/[lecithin] = 5, we identified a nanostructural development with the increase of lecithin concentration. The system changes from a single reverse micellar phase, through a reverse micellar phase coexisting with a lamellar phase, and finally to a reverse liquid crystalline cubic phase and a lamellar phase. We compared the nanostructures formed when phosphatidylcholine rather than naturally occurring lecithin is used and found that both phase behavior and nanostructure are significantly different. The use of the two complementary cryo-EM techniques proved very efficient in the nanostructural characterization of the system. We also performed small-angle X-ray scattering to confirm our findings. Since the system is very sensitive to changes in composition, the cryo-EM specimens were prepared in a Controlled Environment Vitrification System (CEVS) that has been modified for our specimen preparation needs. We were able to overcome the challenges involved in directly imaging this nonaqueous (oil-rich), concentrated complex liquid systems, thus extending the usefulness of those characterization techniques.