|M.Sc Student||Davidovich Irina|
|Subject||The Study of Nanostructured Self-Aggregates|
by Cryogenic Electron Microscopy
|Department||Department of Nanoscience and Nanotechnology||Supervisor||PROFESSOR EMERITUS Yeshayahu Talmon|
|Full Thesis text|
Microemulsions are thermodynamically stable, optically isotropic solutions, of two immiscible phases, stabilized by surfactant. Microemulsions express unique properties that make them useful in various research and industrial fields. Despite the continuous growth of commercial microemulsion use, their phase behavior is still not fully understood. Investigation of microemulsion nanostructures and nanostructural transformations are of great importance.
The surfactant nature is one of the factors that determine the microemulsion phase behavior. One of the features that characterize microemulsions stabilized by a nonionic surfactant is a one-phase isotropic “channel”. In this phase region, the microemulsion nanostructure changes continuously from oil-in-water microemulsion to water-in-oil microemulsion through a bicontinuous structure as a function of temperature and composition.
During the last decades, the properties and phase behavior of nonionic microemulsions were well investigated by many indirect techniques, but there were no systematic studies of the structures formed in a single-phase channel using direct imaging.
In our work, we performed for the first the full nanostructural evolution of water/isooctane/C12E5 microemulsion system along the one-phase corridor in the “χ-cut” phase diagram, using direct imaging. We show the nanostructural transformation from water-continuous to bicontinuous and then to oil-continuous microemulsion, as the oil concentration and the temperature are increased. We used two complementary direct imaging techniques, cryogenic-temperature transmission electron microscopy (cryo-TEM), and cryogenic-temperature scanning electron microscopy (cryo-SEM).
One of the difficulties in the imaging of organic systems is their low contrast. To improve our cryo-TEM results, we took advantage of a new TEM, an FEI (now “Thermo Fisher Scientific”) Talos 200C, with the Volta phase plate (VPP). Like phase plates used in light microscopy, the VPP improves the image contrast by transforming image phase differences to amplitude differences. We applied this technique to evaluate the nanostructure of different weak phase systems that produce low contrast when imaged using conventional cryo-TEM. We directly imaged extracellular vesicles (EVs) and lipoprotein-based nanoparticles (Lipo-NPs) to determine their nanostructure. EVs and other biogenic nanoparticles (BiNPs) are subject to intensive research in medicine and biotechnology. The novel idea of this work is the use of BiNPs directly derived from the patient’s adipose tissue. This method has potential advantages, because of unique endogenous factors delivered by human tissue-derived EVs.
We also studied the nanostructure of methylcellulose (MC) fibrils formed in aqueous solutions upon heating. MC is a water-soluble cellulose derivative used as a rheology modifier in a wide range of industrial fields. At elevated temperatures, MC chains become less hydrophilic, and self-aggregate into long fibrils that form thermo-reversible gel. Although the wide commercial use of MC solutions, the internal fibril structure and the fibril formation process are still not well understood. With the VPP we were able to visualize the structural features that had been invisible using conventional cryo-TEM. Based on these and scattering results we proposed two models to describe the fibril nanostructure and their formation.