Effects of Salts on Thickness of Diffused Double Layer around Clay Particles Using Molecular Dynamics

This study uses molecular dynamics (MD) simulations to evaluate the effect of salt concentration on the thickness of the diffused double layer (DDL) around clay particles. The electrolyte system for the model used in this study is built by implementing the kaolinite system suggested by Bish combined with the SPE/C water system and sodium chloride (NaCl) in LAMMPS (large-scale atomic/molecular massively parallel simulator). Effects on the basal surface of kaolinite clay particles are studied due to its larger surface area compared to the clay edges. CLAYFF intermolecular force fields, including typical 12-6 Lennard-Jones and coulomb interactions, are used for the potential setup with the PPPM long-range solver. Simulations proceeded under different NaCl concentrations ranging from 1% to 10% by weight to capture the range of both low and high salinities. Quantitative analyses, including thermodynamics, radial distribution function (RDF), and coordination number (CN), are applied at different salt concentrations to capture the interactions within the electrolyte system and DDL’s thickness variations. To observe the interactions among clay, water, salt, and the evolution of DDL, visual molecular dynamics (VMD) is used as a visualization tool for this study. The results show that salt presence within the saline water decreases the peak of the RDF and the number of nearest neighbor lists. In addition, DDL thickness around the clay surface tends to decrease because of increasing water salinity with higher reference ion concentrations. The key contribution of this study is a robust evaluation of the change in the DDL thickness under various salinity concentrations and an understanding of the underlying physics and interactions between clay, water, and salt from a molecular viewpoint.