|M.Sc Student||Oss-Ronen Liat|
|Subject||Nanoscopic Characterization of Self-Assembled Nanotubules|
of Lithocholic Acid in Water
|Department||Department of Chemical Engineering||Supervisor||Professor Emeritus Yeshayahu Talmon|
Different kinds of synthetic and natural materials are
known to form nanotubules, which have numerous potential applications. In this
research we studied the dispersions of lithocholic acid (LCA) in alkaline
solutions, which produce monodispersed nanotubules, with an outer diameter of
52 nm and a wall thickness of 1.5 nm.
The main experimental method we used was cryogenic transmission electron microscopy (cryo-TEM), combined with freeze-fracture replication TEM, digital light microscopy and rheological measurements.
We examined sodium lithocholate (SLC) to study the formation mechanism of the nanotubules, and showed that the process is divided into two main time scales. The first step takes less than two minutes, during which multi- and single-walled nanotubules with different diameters are formed. In the second step, taking a few hours, the tubules develop into their “mature” form. The tubules were found to be formed by helical structures, and stay stable at least one month after preparation.
The behavior of the SLC system was compared with that of LCA dispersions in alkaline solutions of different counteions. Electrostatic interactions are the important factor when comparing systems of different alkali counterions; we found strong dependence on the hydrated ion size. In the ammonium system we were able to obtain nanotubules in solutions with a pH value lower than that of the other systems. This could be important for different applications. The tetramethylammonium counterion gives only twisted ribbons and large helices, no tubules. This could be explained by the hydrophobic interactions between the counterion and the LCA molecules.
We also evaluated the thermal stability of the system, and found that the tubules are stable when the dispersion is heated to 50oC, but twisted ribbons and large fibers are the dominant structure at 62oC.