Ion-ion correlation attraction in a molecular solvent

We have performed Monte Carlo simulations for two like-charged infinite plates with counterions and solvent in between. The solvent is described at the primitive model (PM) level and also as a Stockmayer fluid. Both descriptions share the same qualitative ion correlation picture. At sufficiently high coupling, the pressure between the plates shows an attractive minimum, which is lost or reduced upon decreasing counterion valency, surface charge density, or increasing the screening. The pressure profiles become more oscillatory with higher density, which masks the ion-ion correlation attraction. An interpretation in terms of interaction free energies rather than pressures clarifies the picture. Analyzing the different contributions to the pressure gives further support to the conclusions; the same components are responsible for the same effect in the PM and in the molecular solvent.