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Forces between like-charged walls immersed in electrolyte solution Otto, Frank

Abstract

The effects of a molecular solvent on the forces between two infinite like-charged walls immersed in aqueous electrolyte solution are investigated. The solution models are chosen such that the numerical solution of accurate anisotropic integral equation theories is possible, and the anisotropic hypernetted-chain (AHNC) approximation is used in the present calculations. The resulting average particle densities and the pair correlation functions are used to analyze the force acting between the walls in detail. Mixtures of neutral hard-sphere solvent particles and counterions are employed to investigate the influence of the finite size of the solvent molecules. At wall separations of a few solvent diameters, even at relatively high surface charge and moderate solvent density, the ionic contribution to the force tends to be dominated by the hard-core or packing component. If the ions and solvent particles are of equal size, then the net pressure between the walls can be reasonably well approximated by adding the pressures of pure one-component ionic and solvent systems. However, if the ion and solvent diameters are significantly different, the pressure curve is more complex, and the hard-core component must be evaluated for a mixture of neutral hard spheres. The interaction of the walls is also investigated at the McMillan-Mayer (MM) level of description. In these models, the solvent is not represented by discrete particles but exerts its influence through solvent-averaged ion-ion potentials of mean force which serve as effective potentials. The approximations involved in applying MM theory to the inhomogeneous slit system are discussed. The wall-wall interactions obtained can differ dramatically from the primitive model (dielectric continuum solvent) case. Most interestingly, at the MM level, the force between like-charged walls at small separations and with realistic surface charges can be attractive for monovalent counterions, which is due to solvent effects on the effective counterion-counterion interaction. Several ion-ion potentials of mean force for "realistic" models for Na+ and CI- in water are available in the literature. We find that the forces strongly depend on the model employed and different models for the same ion can give qualitatively different results. To construct a self-consistent model, ion-wall potentials of mean force must also be considered. Typical model ion-wall potentials show an additional counterion attraction towards the walls. Incorporation of these wall potentials yields an overall increase of the attraction between the walls. The possible relevance of our observations in the interpretation of experimental force measurements is briefly discussed.

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