Composite ellipsoidal particles to simulate charge anisotropy in particle-scale simulations of kaolinite

Abstract

This paper outlines a new approach to use coarse-grained molecular dynamics (CGMD) and the Gay-Berne potential to simulate the compression of kaolinite saturated with water at an acidic pH (=4) in a low (1 mM) ion concentration solution. To overcome the limitations of the standard Gay-Berne potential and capture the charge heterogeneity on the surface of kaolinite particles under acidic pH conditions, each clay platelet is modelled using a two-ellipsoid composite particle. The molecular dynamics software LAMMPS was employed to generate virtual monodisperse samples containing 1,000 composite particles and to simulate isotropic compression to 100 kPa. The observed macro-scale response in void ratio – effective stress space lay above the response obtained in a simulation that used an equivalent CGMD model developed to simulate alkaline (pH=8) pore water conditions. This is in qualitative agreement with available experimental data for 1D compression. Post-compression qualitative observation of the two virtual samples revealed a book-house type fabric in the sample with acidic pore-fluid, while a turbostatic fabric was observed when the alkaline pore-fluid was simulated. These observations are also in qualitative agreement with SEM data reported in the literature.