Molecular dynamics simulations of microstructure and dynamic shearing behaviors of kaolinite-water-salt system

The electrical double layer (EDL) plays an essential role in the microfabric development and macroscale mechanical behaviors of clayey soils. However, how salt concentration and cation valence affect the shearing behaviors of EDL at nanoscale is still unclear. Through molecular dynamics (MD) simulations, this study systematically investigated the microstructure and the shearing properties of the kaolinite-water-salt system. During the shearing process, ions showed two kinds of movement patterns, including the restricted movement pattern and the free movement pattern. In addition, the fluidity of the strongly bound water on gibbsite surfaces, the strongly bound water on siloxane surfaces, the weakly bound water and the bulk water decreased gradually. Furthermore, with the increase of salt concentration and cation valence, some water molecules in the diffusion layer escaped into the bulk water, which finally resulted in a compression of the EDL and a decrease of the solution viscosity. By measuring the displacement of water molecules around clay platelets, the essence of the EDL compression was verified and explained for the first time. The simulation results offered a microscopic insight into the shearing behaviors of clay-water-salt systems and would promote further investigations of clay-water-salt interactions under various environments.