Forces between like-charged plates in electrolyte solution: Ion-solvent packing versus electrostatic effects

The anisotropic hypernetted-chain approximation is solved numerically for m ixtures of neutral hard-sphere solvent particles and divalent counterions b etween charged plates. A detailed analysis of the different components of f orce acting between the plates is given. At separations of a few solvent di ameters, it is shown that even at relatively high surface charge and modera te solvent density, the ionic contribution to the force tends to be dominat ed by the hard-core or packing component. If the ions and solvent particles are of equal size, then the net pressure between the plates can be reasona bly well approximated by adding the pressures of pure one-component ionic a nd solvent systems. However, if the ion and solvent diameters are significa ntly different the pressure curve is more complex, and the simple superposi tion of the ionic and solvent pressures no longer works. For this case, we show that to a good approximation it is still possible to divide the pressu re into electrostatic and hard-core components, but now the appropriate har d-core system must itself be a mixture of neutral hard spheres. [S1063-651X (99)08210-0].