In this work, we focus on the atomic structure of the water interlayer of Na-exchanged montmorillonite. For two different surface charge densities, namely -0.086 and -0.172 C/m(2), the adsorption process in the presence of water is described by first principles calculations. We describe the interactions and forces for every water molecule entering the interlayer during the swelling process. In particular, the dielectric permittivity of the water interlayer is calculated. Finally, we confirm our results performing ab initio thermodynamics calculations leading to a wide range of realistic experimental scenarios. (C) 2015 Elsevier Inc. All rights reserved.