PNNL

Abstract

Clay minerals – layered silicate nanoparticles that constitute roughly half of the mineral mass in soils, sediments, and sedimentary rocks – are a critical building block of the Earth’s subsurface. A key aspect of these materials is their control over fluid flow, solute migration, and solid mechanics in many systems. The aggregation of clay platelets strongly impacts associated transport and mechanical properties. Unfortunately, experimental and computational characterization of clay aggregation is inhibited by the delicate water-mediated nature of the process and by the wide range of spatial scales involved, from 1-nm-thick platelets to flocs with dimensions up to micrometers or more. In this work, we utilize a new coarse-grained molecular dynamics (CGMD) model to predict the microstructure, dynamics, and rheology of hydrated smectite clay assemblages as a function of particle diameter (6 to 25 nm) and the proportion of Na vs Ca exchangeable cations. Simulated systems have dimensions up to 0.1 µm and contain up to 2,000 clay particles. We monitor the assembly and growth of clay tactoids (i.e., stacks of parallel clay platelets) and aggregates (assemblages of tactoids) as a function of simulation time, exchangeable cation type, and clay platelet diameter. Equilibrated configurations are analyzed to predict structural properties including tactoid size and size distribution, basal spacing, counterion distribution in the electrical double layer (EDL) around tactoids, and the modes of clay association. Finally, non-equilibrium simulations are carried out at different shear rates to predict the rheological properties of smectite gels. Our results demonstrate new potential to characterize and understand clay aggregation in dilute suspensions and gels on a scale of thousands of particles with explicit representation of counterion clouds and with accuracy approaching that of all-atom molecular dynamics (MD) simulations.