|Ph.D Student||Abezgauz Ludmila|
|Subject||Structure of Viscous Micellar Solutions|
|Department||Department of Biotechnology and Food Engineering||Supervisor||Professor Dganit Danino|
|Full Thesis text|
Amphiphilic molecules spontaneously self-assemble into various aggregates in solution, as a result of their dual hydrophilic-hydrophobic nature. The assemblies’ morphology depends on a delicate balance of forces determined by the chemical structure and the solution conditions. Due to their advantageous thermodynamic and rheological properties, self-assembled structures, especially viscous micellar solutions, attract great interest.
The main goal of my work was to get deeper insight to the dependency between the solution properties and micellar morphology, with focus on the relations between micellar structure and the solution viscosity. Several pure and mixed surfactant micellar systems were studied. All those systems display similar characteristic rheological behavior: rapid increase in the viscosity upon change in the solution conditions (composition or temperature), and one or two viscosity maximum. Rheology, calorimetry, FTIR, scattering, and primarily cryo-TEM were applied to unravel the self-assemblies structures at the nanoscale.
The nonionic surfactants we studied belong to the ChEOm family. They display unique assembly characteristics, and close to the CMC organize into micelles, discs and vesicles, due to their structure comprising a cholesteryl unit as a hydrophobic group, and an ethylene oxide chain as headgroup. We show that as the headgroup chain gets smaller, the assemblies become bigger in size and of lower curvature. We demonstrate that micellization of these amphiphiles is a slow, complex process compared to that of regular surfactants. We found a new micellar growth mechanism where discs transit into flat ribbons, a saturated ribbon network and vesicles. This mechanism is similar to the sphere-to-rod transition and network formation occurring in ionic micellar systems.
In the ionic systems we focused on the salt effect on micellization, micellar behavior and changes in micellar structure that control the viscosity. We show that salts affect the viscosity according their location in the lyotropic series. For the first time, we provided by cryo-TEM information about the shape and length of micelles at high salt content. For the systems with two viscosity peaks we found two micellar mechanisms which influence the viscosity decrease: the formation of branching points and a micellar network after the first viscosity maximum, and shortening of the micellar length after the second viscosity maximum. We propose that adsorption and penetration of hydrotrope’ ions on or into micelle at their high concentration promotes micelle charge inversion and shortening of the micellar length.
In our study excellent correlation is obtained between the microscopy findings and our rheological measurements.