Quasi-universality in mixed salt-free counterion systems

The screening of plate-plate interactions by counterions is an age-old problem. We revisit this classic question when counterions exhibit a distribution of charges. While it is expected that the long-distance regime of interactions is universal, the behaviour of the inter-plate pressure at smaller distances should a priori depend rather severely on the nature of the ionic mixture screening the plate charges. We show that this is not the case, and that for comparable Coulombic couplings, different systems exhibit a quasi-universal equation of state.