INTERACTIONS BETWEEN ADSORBED LAYERS OF A LOW CHARGE-DENSITY CATIONICPOLYELECTROLYTE ON MICA IN THE ABSENCE AND PRESENCE OF ANIONIC SURFACTANT

Interactions between two negatively charged mica surfaces across aqueo us solutions containing various amounts of a 10% charged cationic poly electrolyte have been studied, It is found that the mica surface charg e is neutralized when the polyelectrolyte is adsorbed from a 10-50 ppm aqueous solution. Consequently no electrostatic double-layer force is observed. Instead an attractive force acts between the surfaces in th e distance regime 250-100 Angstrom. We suggest that this attraction is caused by bridging. Additional adsorption takes place when the polyel ectrolyte concentration is increased to 100 and 300 ppm, and a long-ra nge repulsion develops. This repulsive force is both of electrostatic and steric origin. The polyelectrolyte layer adsorbed from a 50 ppm so lution does not desorb when the polyelectrolyte solution is replaced w ith an aqueous polyelectrolyte-free solution. Injection of sodium dode cyl sulfate (SDS) into the measuring chamber to a concentration of abo ut 0.01 CMC (8.3 x 10(-5) M) does not affect the adsorbed layers or th e interaction forces. However, when the SDS concentration is increased to 0.02 CMC (0.166 mM) the adsorbed layer expands dramatically due to adsorption of SDS to the polyelectrolyte chains. The sudden swelling suggests a cooperative adsorption of SDS to the preadsorbed polyelectr olyte layer and that the critical aggregation concentration between th e polyelectrolyte and SDS at the surface is about 0.02 CMC. The floccu lation behavior of the polyelectrolyte in solution upon addition of SD S was also examined. It was found that 0.16-0.32 mol SDS/mol charged s egments on the polyelectrolyte is enough to make the solution slightly turbid. (C) 1997 Academic Press.