Nanoscale simulation of Na-Montmorillonite hydrate under basin conditions, application of CLAYFF force field in parallel GCMC

The swelling of Na-Montmorillonite is simulated by Monte Carlo molecular simulations in NPzzT and VT ensembles, in two sedimentary basin conditions of 423K at 750 atm and 483K at 1050 atm and by using the CLAYFF force field [J. Phys. Chem. B 108, 1255-1266 (2004)]. The CLAYFF force field predicts a one, two-layer stable hydrate under 423K at 750 atm and a one-layer stable hydrate under 483K at 1050 atm with the stable basal spacings of 12.54 and 12.44 angstrom. The adsorbed water molecules are 45.22 and 41.92 per clay layer, with water densities of 0.28 and 0.26g/cm(3) and total energy of 2.11 and 3.65 kcal/mol of water. The predicted structure of Na-Montmorillonite are consistent with those reported in literature for high temperature and pressure conditions, however the results did not indicate a one-layer hydrate as the stable phase under basin conditions of 423K at 750 atm [Clays Clay Miner. 34, 385 (1986)]. This work allows addressing the issue of the effect of basin condition on the structure of Na-Montmorillonite hydrate.