1998/03/26 by I. Borukhov, D. Andelman, H. Orland
Physics and Astronomy · #cond-mat.soft #physics.chem-ph
published as J. Phys. Chem. B 103, p. 5042 (1999) · 35 pages, 16 figures, submitted to J. Phys. Chem. B
arxiv created 1998/03/26 · arxiv updated 2009/11/30
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 cb. 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 p1/2 and its dominant length scale is proportional to a/p1/2, 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/cb1/2 and the length scale is proportional to kappa*a2/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.