Structural and thermodynamic investigation of the hydration-dehydration process of Na +-Montmorillonite using DFT calculations

This work is mainly focused on investigating the atomic structure of the interlayer water molecules inside Na+-Montmorillonite, their key interactions in the structure and the thermodynamics of the dehydration process. A theoretical study of the different degrees of hydration was performed using calculations based on DFT with several exchange-correlation functionals. The weak layer-water molecules interactions suggested the need of dispersion corrections to simulate correctly the basal spacing and the thermodynamic potentials for the dehydration process. The existence of the structures with sodium cation in planar geometry, representative of the one-water layer model and in octahedral geometry in a two-water layer model was confirmed. The stages of the dehydration of Na+-Mt were evaluated by thermodynamics calculations leading to a good description of the results from TGA/DTG studies. The thermodynamic data suggest also that the one-water layer structure as an intermediate with a metastable configuration.