|M.Sc Student||Maya Schnabel Lubovsky|
|Subject||Direct Imaging of Reverse Threadlike Micelles in Non-Aqueous|
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Talmon Yeshayahu|
|Full Thesis text|
When certain surfactants are dissolved in organic solvents, reverse micelles are formed spontaneously. Those are thermodynamically stable surfactant aggregates with a polar core and a hydrophobic corona. One of these shapes is reverse threadlike micelles (rTLMs). The hydrophobic tails of the surfactant point out into the continuous oil medium, and the hydrophilic head groups point into the aqueous environment.
Until now no systematic electron microscopy data of rTLMs in non-aqueous solutions have been published, because of the difficulties associated with cryogenic transmission electron microscopy (cryo-TEM) of organic solvents.
The main objective of this work was to investigate rTLMs in an organic medium, and to show for the first time images of rTLMs as function of the system parameters. We used two methods of cryo-TEM to characterize the rTLMs. One was direct- imaging cryogenic transmission electron microscopy (DICT), and the other was freeze-fracture-replication (FFR). We also used rheometry to understand the rheological properties of the rTLMs, and to support the microscopy results.
We first studied by cryo-TEM the rTLMs in the lecithin/isooctane/water system. We imaged many different composition of the system, and shoed that time has a dramatic effect on the system nanostructures, and that the system reaches equilibrium very slowly, probably in more than a year. The system reveals various rTLMs morphologies: branched, entangled, and networks, similar to those expected for polymers.
The second system, AOT/isooctane/water, was difficult to study by cryo-TEM, due to very low contrast between the bulk and the reverse micelles. We used uranyl acetate and sodium iodide to try enhance the contrast. The additives helped only slightly to enhence the system contrast. We did succeed to visualize the reverse micelles, but we could not determine whether the reverse micelles were spherical or threadlike.
We preformed rheological measurements to characterize the rTLMs in the lecithin/isooctane/water system. We found that the system has very low viscosity, similar to that of water, and that we can distinguish between the different structures of the rTLMs through rheological measurements. Systems of connected long rTLMs exhibited Newtonian behavior, while systems of short and entangled rTLMs exhibited shear- thinning behavior. As far as we know, this is the first research to combine TEM and rheology to characterize rTLMs in non-aqueous systems.