Energetics of colloids: Do oppositely charged particles necessarily attract each other?

The electrostatic free energy of two oppositely charged macroions, suspended in ionic solution is calculated for different distances between the macroions. This is done by using a lattice field theory formulation of the statistical mechanics of Coulomb gas particles of finite size interacting with a fixed charge distribution (J. Chem. Phys. 1995, 102, 4584). In many cases, it is found that, regardless of the size of the mobile ions, the minimum of the free energy is at a separation between the macroions corresponding to a noncontact configuration. Illustrative examples, which emphasize the source of the repulsion at near-touching configurations are presented and discussed.