Effect of polyelectrolyte adsorption on intercolloidal forces

The behavior of polyelectrolytes between charged surfaces immersed in semid ilute solutions is investigated theoretically. A continuum mean field appro ach is used for calculating numerically concentration profiles between two electrodes held at a constant potential. A generalized contact theorem rela tes the intersurface forces to the concentration profiles. The numerical re sults show that overcompensation of the surface charges by adsorbing polyel ectrolytes can lead to effective attraction between equally charged surface s. Simple scaling arguments enable us to characterize qualitatively the int ersurface interactions as a function of the fraction of charged monomers p and the salt concentration c(b). In the low-salt regime, Mie find strong re pulsion at short distances, where the polymers are depleted from the inters urface gap, followed by strong attraction when the two adsorbed layers over lap. The magnitude of this attraction scales as p(1/2) and its dominant len gth scale is proportional to a/p(1/2), where a is the monomer size. At larg er distances, the two adsorbing surfaces interact via a weak electrostatic repulsion. For strong polyelectrolytes at high salt concentration, the poly mer contribution to attraction at short distances scales as p/c(b)(1/2) and the length scale is proportional to kappa(s)a(2)/p, where kappa(s)(-1) is the Debye-Huckel screening length. For weak polyelectrolytes at high salt c oncentration, the interaction is repulsive for all surface separations and decays exponentially with a decay length equal to kappa(s)(-1). The effect of irreversible adsorption is discussed as well, and it is shown that inter surface attraction can be obtained in this case as well.