Mechanical properties and pore network connectivity of sodium montmorillonite as predicted by a coarse-grained molecular model

Abstract

The study of ionic transport through hydrated sodium montmorillonite (Na-MMT)—the main swelling component of bentonite—is of significant interest to better understand its ability to contain contaminants. From a macroscopic viewpoint, porosity and tortuosity can be viewed as scaling factors connecting diffusion coefficients in the clay material to free diffusion coefficients of ions in water. In this work, the mechanical properties, porosity and tortuosity of Na-MMT were calculated using a bottom-up approach. First, the constructed coarse-grained mesoscopic models of Na-MMT were fitted to all-atom molecular dynamics simulations. Thirty different models—each containing one thousand 120 Å-wide hexagonal Na-MMT platelets—were generated. The dry densities considered herein range from 0.8 to 1.3 g/cm³. Second, calculated the models' elastic constants were in excellent agreement with experimental values reported in the literature. Third, each system's porosity, pore size distribution and pore network were analysed. Fourth, the pore network information was used to create a 3D image of hydrated Na-MMT, and random walk simulations were employed to evaluate its tortuosity. Finally, the porosity and tortuosity values estimated using the models were compared to macroscopic experimental values describing tritium and iodide diffusion in Na-MMT dominant bentonite. The values obtained using the models were fairly consistent with experimental values reported in the literature.