SALT EFFECTS ON THE INTERACTION BETWEEN ADSORBED CATIONIC POLYELECTROLYTE LAYERS - THEORY AND EXPERIMENT

The effect of inert salt concentration on polyelectrolyte adsorption f rom aqueous solutions onto oppositely charged surfaces and the interac tions between such surfaces were studied experimentally using a surfac e force technique and compared to theoretical predictions from Monte C arlo simulations. At a polyelectrolyte concentration of 10 ppm and a l ow inert salt concentration (10(-4) M), the polyelectrolytes adsorb in a flat conformation and the force acting between the surfaces is clos e to zero down to a separation of 10(-15) nm, where the surfaces jump inward. The attractive force observed at separations below 10 nm is st ronger than the expected van der Waals force. The magnitude and range of the attraction agree with forces obtained from Monte Carlo simulati on, and we interpret the attraction as being due to bridging polyelect rolytes. When the salt concentration is increased, additional polyelec trolyte adsorption takes place. This again gives rise to a repulsive f orce, which is significantly larger than what is observed between bare surfaces. The extra repulsion is due to adsorbed polyelectrolytes str etching out from the surfaces and is also predicted from simulations.