PNNL

Abstract

The aggregation of clay mineral nanoparticles is a paradigm of colloidal self-assembly in natural systems. These charged anisotropic particles interact through a combination of at least six mechanisms—contact repulsion, Van der Waals attraction, hydration repulsion, specific Coulomb interactions in overlapping electrical double layers (EDLs), osmotic repulsion, and excluded volume effects—with different sensitivities to particle shape, surface charge, aqueous chemistry, and interparticle distance, which gives rise to complex aggregation structures and dynamics. Experiments have difficulty resolving individual clay particles, while all-atom molecular dynamics (MD) simulations can examine the interactions of only a few particles with dimensions up to about 10 nm. We develop a new coarse-grained (CG) model capable of representing assemblages of hundreds of clay particles with dimensions matching those of real clay particles ( 25 nm). Our CG model is parameterized based on MD simulations of a pair of sodium-exchanged smectite clay particles in liquid water. It explicitly represents the EDL formed by charge-compensating cations at the mineral-water interface. It has the potential to reveal how collective structure and dynamics emerge from the atomistic-level characteristics of charged clay particles and their EDLs. For example, it predicts the coexistence of crystalline and osmotic hydrates in compacted hydrated Na-smectite.