  The aggregation of clay particles in aqueous solution is a ubiquitous
everyday process of broad environmental and technological importance. However,
it is poorly understood at the all-important atomistic level since it depends
on a complex and dynamic interplay of solvent-mediated electrostatic,
hydrogen-bonding, and dispersion interactions. With this in mind we have
performed an extensive set of classical molecular dynamics simulations
(included enhanced sampling simulations) on the interactions between model
kaolinite nanoparticles in pure and salty water. Our simulations reveal highly
anisotropic behaviour in which the interaction between the nanoparticles varies
from attractive to repulsive depending on the relative orientation of the
nanoparticles. Detailed analysis reveals that at large separation (>1.5 nm)
this interaction is dominated by electrostatic effects whereas at smaller
separations the nature of the water hydration structure becomes critical. This
study highlights an incredible richness in how clay nanoparticles interact,
which should be accounted for in e.g. coarse grained models of clay
nanoparticle aggregation.
