Monte Carlo molecular simulation of the Na-, Mg-, and mixtures of Na/Mg-montmorillonites systems, in function of the pressure

Abstract

In this paper, Monte Carlo (MC) simulation has been used to study the swelling pattern of Na-montmorillonite (Na-Mnt), Mg-montmorillonite (Mg-Mnt), and Na/Mg-mixture montmorillonite (4NaMg-Mnt; 2Na2Mg-Mnt). The molecular simulation was performed in the NVT (number of molecules, volume and temperature are constant) ensemble at normal temperature (300 K) and 225, 300, and 340 bar over an H2O content 147, 196, and 294 mg g−1 of clay. The simulations reproduce the swelling pattern of Na-Mnt and Mg-Mnt. The predicted spacing of the Na/Mg-Mnt mixtures is closely related to that of Mg-Mnt and confirms the results reported in the literature for Na-rich/Mg-poor Mnt. The results of the water adsorption and the swelling properties on the system Na-Mnt, Mg-Mnt, and the Na/Mg-Mnt mixtures are reflected with a transformation to two-hydrate stages. The probability of the coordination number of Na+, Mg2+, and mixtures tends to increase with an increasing amount of H2O molecules, but decreases with increasing pressure. The cation–oxygen distances (Na–O or Mg–O) show two signals, corresponding to the first and second coordination shells, which indicates that the ions behave as in bulk water.