Like-charge attraction in a slit system: pressure components for the primitive model and molecular solvent simulations

We have recently reported Monte Carlo simulations for a system of two infinite like-charged plates in a dipolar fluid solvent (Pegado et al 2008 J. Chem. Phys. at press). The pressure as a function of plate separation qualitatively reproduces the ion-ion correlation attraction picture seen in primitive model studies, where the solvent only enters the picture implicitly through its dielectric constant epsilon(r), scaling all charge-charge interactions. Here we analyse in detail the different components of the pressure between the two plates. This shows that, by changing any of the relevant parameters (counterion valency, surface charge density or dielectric screening), the appearance or increase of a pressure minimum is connected to the same components in both the primitive model and the dipolar solvent model. Decomposing the pressure is helpful in distinguishing between solvent depletion or packing effects and the coexisting correlation attraction. Although the pressure can be evaluated at any plane parallel to the surfaces, the analysis of the pressure at the midplane provides the best physical insight.