Molecular dynamics simulation of hydrated Na-montmorillonite with inorganic salts addition at high temperature and high pressure

Abstract

The swelling behaviors of Na-montmorillonite (Na-Mt) in non-ambient environment have been the subject of considerable speculation due to the difficulties associated with their study. Inorganic salts are often used to minimize clay mineral hydration. The influence of inorganic salts addition (NaCl, CaCl2, KCl) on the hydration of Na-Mt at high temperature and high pressure (HTHP) was investigated by molecular dynamics simulation at 6GPa and different temperatures (200, 300, 400, 500 and 600K) in an isobaric isothermal ensemble (NPT). Water model and force field used in the clay-water system are SPC/E model and universal force field (UFF), respectively. The results show that after adding inorganic salts into Na-Mt, the mobility of the interlayer species increases, and the hydration shell of the interlayer cations decreases with a rise in temperature. The influence of temperature on the diffusion of water molecules is much greater than that of the ions present in the interlayer. Temperature increase and hydration degree play an important role on the diffusion behavior of the interlayer species. The size, valence, mass and hydration energy of cations present between the layers affect the structure of Na-Mt interlayer. The inorganic salts are able to inhibit the hydration of Na-Mt by reducing the mobility of the interlayer species and the basal spacing. After analyzing the effect of each salt on Na-Mt, it was found that, the most stable state of Na-Mt at HTHP is achieved with KCl addition, compared with NaCl and CaCl2. It is expected the results obtained from this study would help to understand the inhibition effect of inorganic salts on Na-Mt, and predict the swelling of Na-Mt at HTHP.