POLYELECTROLYTE-SURFACTANT ASSOCIATION AT SOLID-SURFACES

The interferometric surface force technique has been utilized for stud ying the interaction between negatively charged surfaces coated with a cationic polyelectrolyte across solutions of anionic surfactant, sodi um dodecyl sulphate (SDS). The polyelectrolyte used was poly ([2-(prop ionploxy)ethyl]trimethylammonium chloride), PCMA, which has one positi ve charge per segment. At low ionic strength the polyelectrolyte adsor bs in a flat conformation and neutralizes the negative mica surface ch arge. The interaction forces between the polyelectrolyte-coated surfac es are dominated by a strongly attractive force at distances shorter t han about 150 Angstrom. Addition of SDS into the measuring chamber to a concentration of about 0.1 cmc changes the interaction forces dramat ically. 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 o scillations. We interpret these oscillations as being caused by the sp atial arrangement of SDS micelles stabilized by the polyelectrolyte. T he oscillations in the force curve remain as the SDS concentration is increased to cmc. The interaction forces and the layer structure for t he PCMA/SDS system are very different compared to those observed for w eakly charged polyelectrolyte/SDS and lysozyme/SDS systems. The differ ences can be rationalized by considering that low charge density polye lectrolytes and lysozyme, a compact cationic globular protein, cannot equally efficiently stabilize SDS micelles as a flexible high charge d ensity cationic polyelectrolyte such as PCMA.