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The behavior of electrolyte solutions close to a charged surface is studied theoretically. A modified Poisson-Boltzmann equation which takes into account the volume excluded by the ions in addition to the electrostatic interactions is presented. In a formal lattice gas formalism the modified Poisson-Boltzmann equation can be obtained from a mean-field approximation of the partition function. In an alternative phenomenological approach, the same equation can be derived by including the entropy of the solvent molecules in the free energy. In order to visualize the effect of the steric repulsion, a simple case of a single, highly charged, flat surface is discussed. This situation resembles recent adsorption experiments of large ions onto a charged monolayer. A simple criterion for the importance of the steric effects is expressed in terms of the surface charge density and the size of the ions. It is shown that when these effects are important a saturated layer is formed near the surface. A modified Grahame equation relating the ion concentration at the surface to the surface charge density is obtained., Comment: 18 pages, 7 postscript figures included. Submitted to Electrochimica Acta

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., Comment: 35 pages, 16 figures, submitted to J. Phys. Chem. B