DEPLETION ZONES IN POLYELECTROLYTE SYSTEMS - POLYDISPERSITY EFFECTS AND COLLOIDAL STABILITY

We have used the Scheutjens-Fleer theory for polymers at interfaces to study depletion zone effects in polyelectrolyte systems. The segment density profiles of depletion regions are independent of the chain len gth, N, and the depletion interaction is often fully repulsive. In pol yelectrolyte systems, a depletion zone develops both at adsorbing and at nonadsorbing interfaces, especially at low ionic strengths. Interac tions which are repulsive at some surface separations and attractive a t others can also be found, eg., for intermediate ionic strength condi tions or for very low charge density in the chain. At very high ionic strength or very low charge density, the classical neutral polymer dep letion layers develop, causing attraction between particles in solutio n. Effects of polydispersity of the polyelectrolytes on the segment de nsity profiles are very small. There is a minor effect on the depletio n region at low ionic strengths, provided that there is not a large fr action of very short chains present. Only depletion zones near adsorbi ng surfaces tend to be preferentially populated by low molecular weigh t polymer. Under these conditions, long chains are still depleted from this filled-up depletion region, which has immediate implications for the equilibration of the adsorbed layer: The diffusion of long chains through this region to the surface is slow. At an adsorbing surface, preferential adsorption of the longer polyelectrolytes over the shorte r ones results in a fractionation of the chains at the adsorbing inter face, if equilibrium can be reached.