Polyelectrolyte-surfactant association at solid surfaces

The interferometric surface force technique has been utilized for studying the interaction between negatively charged surfaces coated with a cationic polyelectrolyte across solutions of anionic surfactant, sodium dodecyl sulphate (SDS). The polyelectrolyte used was poly ([2-(propionploxy)ethyl]trimethylammonium chloride), PCMA, which has one positive charge per segment. At low ionic strength the polyelectrolyte adsorbs in a flat conformation and neutralizes the negative mica surface charge. The interaction forces between the polyelectrolyte-coated surfaces are dominated by a strongly attractive force at distances shorter than about 150 Angstrom. Addition of SDS into the measuring chamber to a concentration of about 0.1 cmc changes the interaction forces dramatically. The long-range forces are now repulsive due to a recharging of the surfaces. The polyelectrolyte layer also swells considerably and, more surprisingly, the force versus distance profile displays clear oscillations. We interpret these oscillations as being caused by the spatial arrangement of SDS micelles stabilized by the polyelectrolyte. The oscillations in the force curve remain as the SDS concentration is increased to cmc. The interaction forces and the layer structure for the PCMA/SDS system are very different compared to those observed for weakly charged polyelectrolyte/SDS and lysozyme/SDS systems. The differences can be rationalized by considering that low charge density polyelectrolytes and lysozyme, a compact cationic globular protein, cannot equally efficiently stabilize SDS micelles as a flexible high charge density cationic polyelectrolyte such as PCMA.