Expansive soils are widely found in many parts of the world. Highly active smectite clay mineral Montmorillonite is the major constituent in these clays and can expand or contract up to 15 times of their original volume. Constrained swelling exert large amount of stress causing damage to structures, pavements etc. These clays are also used as barrier materials, Nano-materials in polymer clay Nano composites and drug delivery systems. Several factors influence the swelling potential such as water content, density, voids, electrolyte content and cation exchange capacity. However, molecular scale mechanisms that control swelling behavior in these clays need to be understood. Objectives of this research are to provide an insight into mechanisms that result in swelling of these clays. Molecular modeling is used to build and study solvation of Na-Montmorillonite system. Trajectories of water molecules are captured and the evolutions of interaction energies with swelling are calculated.