Ion-ion correlations in liquid dispersions

Ion-ion correlations play an important role in liquid dispersion with, strong eleetrostatic inlet-actions. Examples can be found in very diverse areas with the setting of cement paste as one extreme and the compactation, of DNA as another. One particularly spectacular effect of ion-ion correlations is that the traditional double layer repulsion sometimes can be converted into a net attraction. This typically takes place in the presence of multivalent counterions and/or in solutions with low dielectric permittivity. The attractive forces are driven by the energy while the repulsive are mainly of entropic origin, and the final outcome is a delicate balance of these contributions. Here we present two simple models, which give a conceptually simple description of this balance.