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Interaction regimes for oppositely charged plates with multivalent counterions

Within a mean field treatment of the interaction between two oppositely charged plates in a salt free solution, the distance at which a transition from an attractive to a repulsive regime appears can be computed analytically. The mean field description however breaks down under strong coulombic couplings, that can be achieved at room temperature with multivalent counter-ions and highly charged surfaces. Making use of the contact theorem and simple physical arguments, we propose explicit expressions for the equation of state in several situations at short distances. The possibility of Bjerrum pair formation is addressed and is shown to have profound consequences on the interactions. To complete the picture, we finally consider the large distance limit, from which schematic phase diagrams discriminating attractive from repulsive regions can be proposed.