Self-assembly of amphiphilic molecules
Background
Amphiphilic molecules such as surfactants and lipids are made up of a hydrophobic tail and a hydrophilic head group. Depending on the chemical structure of the molecule, as well as solution properties such as ionic strength, temperature and composition in mixed systems, they may self-assemble to form a variety of different structures with respect to size, shape, flexibility, polydispersity etc. The central problem of our research is to develop a theoretical model from which it is possible to rationalize and predict the structural behaviour of self-assembled structures (micelles, vesicles, lyotropic liquid crystalline phases, perforated bilayers, microemulsions etc) formed by amphiphilic molecules.
The scientific activities in this research programme combine theory and experimental investigations. The theory relates molecular properties such as surfactant tail length and volume, head group size and charge number etc with aggregate properties such as size, shape, flexibility and polydispersity etc. The experimental part consists of studies of the correlation between chemical architecture of the constituent amphiphilic molecules and various aggregate properties using small-angle neutron and x-ray scattering (SANS, SAXS) as well as static light scattering (SLS).
Experimental Investigations
This particular project consists of experimental SLS studies of the self-assembly in mixtures of an anionic surfactant (sodium dodecyl sulphate, SDS) and a cationic lipidlike amphiphilic molecule (didodecyldimethylammonium bromide, DDAB). Previous studies with SANS show that conspicuously small closed bilayer shells called vesicles (Radius ≲ 100 Å) are formed in dilute solutions with a composition [SDS]:[DDAB] = 20:1. By measuring at different concentrations of NaBr we want to study if these very small vesicles are stable or whether they behave as larger vesicles in many similar systems and grow into large bilayer sheets upon addition of salt. The results of this study are expected to shed considerable light on the still unresolved issue whether unilamellar bilayer vesicles may be thermodynamically stable or not.
Supervisor: Magnus Bergström, Surface and Corrosion Science
e-mail: magnus.bergstrom@surfchem.kth.se
