|M.Sc Student||Naama Koifman|
|Subject||Nanostructure in Non-Aqueous Solutions|
|Department||Department of Nanoscience and Nanotechnology||Supervisor||Professor Emeritus Talmon Yeshayahu|
|Full Thesis text|
Non-aqueous complex liquid systems include systems in which the continuous phase is organic, or where water is not present at all. Such systems can exhibit self-assembly into a variety of nanostructures. The investigation by direct imaging methods of such systems has been fairly limited compared to the corresponding aqueous systems, although they are interesting both for scientific reasons and for potential practical applications.
The main aim of this work was to investigate by direct-imaging, namely, cryogenic-temperature transmission electron microscopy (cryo-TEM) and cryogenic-temperature scanning electron microscopy (cryo-SEM), the nanostructure of two non-aqueous systems. The first system was composed of lecithin, isooctane and water, and was investigated mainly in the oil-rich corner of the phase diagram. The second system was of ethylammonium nitrate (EAN), isooctane and tetraethylene glycol monotetradecyl ether (C14E4). Another study aim was to improve the methodology of cryo-SEM, a technique that only recently has been made applicable to the study of labile liquids. Cryo-EM was the main technique used in this study; it was sometimes complemented by small-angle x-ray scattering (SAXS), rheological properties measurements, and polarized light microscopy.
We investigated two types of lecithin systems, a naturally-occurring lecithin, and purified phosphatidylcholine (PC). The former system developed from an inverse spherical micellar solution into a cubic close-packed arrangement of inverse micelles coexisting with a lamellar phase. At the water-rich corner of the phase diagram, we identified another discontinuous cubic phase, whose symmetry was clearly visible in cryo-SEM images. In the PC system, the system formed reverse threadlike micelles (rTLMs) upon the addition of minute amounts of water to an isooctane/PC mixture. With the increase of lecithin concentration the rTLMS formed an entangled network that made the system viscoelastic, similarly to some polymeric solutions.
The ionic liquid-based system showed unusual nanostructures that were imaged for the very first time in this study. At high surfactant concentrations we identified perforated regions among continuous channels. SAXS indicated a reversible transformation from a lamellar phase to a microemulsion phase with increasing temperature. In some cases specimen preparation technique was altered to improve heat transfer rate to the specimen surface and eliminate the formation of crystallization artifacts in the system.
The combination of cryo-TEM, cryo-SEM, and several indirect techniques, allowed for a more complete characterization of the investigated systems. We show that direct imaging techniques can be applied to non-aqueous systems, and recognize the challenges and advantages of working with such systems.