The Effect of Polyelectrolyte Adsorption on Inter-Colloidal Forces

TL;DR

This paper theoretically investigates how polyelectrolyte adsorption influences inter-colloidal forces, revealing conditions under which surfaces attract or repel based on charge, salt concentration, and adsorption strength.

Contribution

It introduces a mean field model linking polyelectrolyte adsorption to inter-surface forces, including effects of irreversible adsorption and various salt conditions.

Findings

Over-compensation of surface charges can cause attraction between like-charged surfaces.

At low salt, short-range repulsion transitions to attraction as layers overlap.

At high salt, attraction scales with p/c_b^{1/2} and decay length depends on screening length.

Abstract

The behavior of polyelectrolytes between charged surfaces immersed in semi-dilute solutions is investigated theoretically. A continuum mean field approach is used for calculating numerically concentration profiles between two electrodes held at a constant potential. A generalized contact theorem relates the inter-surface forces to the concentration profiles. The numerical results show that over-compensation of the surface charges by adsorbing polyelectrolytes can lead to effective attraction between equally charged surfaces. Simple scaling arguments enable us to characterize qualitatively the inter-surface interactions as function of the fraction of charged monomers p and the salt concentration c_b. In the low salt regime we find strong repulsion at short distances, where the polymers are depleted from the inter-surface gap, followed by strong attraction when the two adsorbed layers overlap. The magnitude of this attraction scales as p^{1/2} and its dominant length scale is proportional to a/p^{1/2}, where a is the monomer size. At larger distances the two adsorbing surfaces interact via a weak electrostatic repulsion. For strong polyelectrolytes at high salt concentration the polymer contribution to attraction at short distances scales as p/c_b^{1/2} and the length scale is proportional to kappa*a^2/p, where kappa^{-1} is the Debye-Huckel screening length. For weak polyelectrolytes at high salt concentration the interaction is repulsive for all surface separations and decays exponentially with a decay length equal to kappa^{-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.