ADSORPTION OF WEAK POLYELECTROLYTES ON SURFACES WITH A VARIABLE CHARGE - SELF-CONSISTENT-FIELD CALCULATIONS

A self-consistent-field model for the adsorption of flexible weak poly electrolytes onto surfaces with a variable charge is developed. The ch ain statistics and the short and long range interactions are described using a lattice model. The degree of dissociation of the chargeable p olyelectrolyte segments is allowed to vary with the distance from the surface. Electrostatic potential profiles as well as volume density pr ofiles are evaluated numerically as a function of pH, ionic strength, and segment-solvent/segment-surface interaction parameters. For the ca se of pure electrosorption of a polyelectrolyte from an athermal aqueo us electrolyte solution the adsorbed amount of polyelectrolyte is main ly determined by the compensation of the surface charge. The surface c harge is affected slightly by the adsorbed polyelectrolyte, which is c aused by the higher effective valence of the polyelectrolyte chain as compared to that of the background electrolyte. In absence of specific interactions, no point of zero charge shift is observed. Specific int eractions between polyelectrolyte chains and the surface lead to an in crease in the adsorbed amount. Further, depending on the conditions, t he changes associated with the adsorbed polyelectrolytes compensate or even overcompensate the surface charge. This affects the local electr ostatic potential, and thus both components adjust their initial charg e significantly. The component that, at a given pH and salt concentrat ion, has the highest initial charge dominates the local electrostatic potential and dictates to a large extent the degree of charging of the other component. A decrease in solvent quality enhances the effects o f specific adsorption. Due to increasing overcompensation of the surfa ce charge by the adsorbed polyelectrolyte charge, the fraction of trai n segments increases with increasing pH, decreasing salt concentration , and increasing adsorption energy.