The surface energy of exchanged forms of montmorillonite has been computed using a partial charge model introduced in the calculation of the lattice energy. The results are in correct agreement with data on heats of adsorption. However, the difference between theoretical and experimental data increases from light cations to heavy cations associated with the clay network. This indicates that the heat of adsorption depends highly on the step hydration of the cations. This type of behavior was expected and thus the quantification seems realistic. (c) 2006 Published by Elsevier Inc.