Polyelectrolytes on Surfaces and their Complexes with Surfactants, Thermodynamics versus kinetics

Sammanfattning: The interfacial properties of polyelectrolytes, mainly cationic with various content of amphiphilic groups, and their complexes with an oppositely charged surfactant, sodium dodecyl sulfate (SDS), have been investigated using time-resolved ellipsometry. This thesis focuses on the kinetics and reversibility aspects of polymer adsorption and the control of the adsorption by regulating the electrostatic interactions. The formed structure of adsorbed layer was found to be dependent not only on the bulk conditions, but also on the pre-applied conditions, that is the path used to obtain a particular solution condition (e.g. by changing pH and concentration of salt, surfactant or polymer). The results indicate that polyelectrolyte adsorption appears only partially reversible, due to the high affinity to the surface, which slows down the rearrangement process. In general relaxation occurs more easily if the direction of the process is from low to the higher surface coverage. By pre-applying certain bulk conditions, the desired conformation and enhanced (or reduced) concentration of the polymer in the interfacial layer can be achieved. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. In all cases, the maximum adsorption was obtained at a SDS concentration just before the expected phase separation region, while the complex in some cases could desorb from the surface at high SDS concentration (above the cmc). Different results were obtained for coadsoption of amphiphilic polyelectrolytes when surfactant SDS was added to the preadsorbed layers and when complexes were preformed in the solution prior the adsorption.