In the case of electrically charged porous media as clays, a precise definition of swelling pressure has already been given in our preceding works using the periodic homogenization medium for two scales (nano/macro) (Moyne and Murad, 2002, 2006). The modeling at the nanoscale is based on the Poisson-Boltzmann equation to describe the electrostatics. However, for montmorillonite clays, if for a monovalent compensating cation (eg Na), swelling is always observed. For divalent ions (eg Ca or ions of higher valence), on the contrary, shrinking phenomena can be observed. As the Poisson-Boltzmann equation always predicts swelling, a more precise description of the physics at the nanopore level is needed. Therefore, a pore-scale model is developed within the framework of Statistical Mechanics in conjunction with a thermodynamic approach based on Density Functional Theory.