ALL-ATOM AND COARSE-GRAINED MOLECULAR DYNAMICS SIMULATIONS OF THE COLLOIDAL INTERACTION BETWEEN CHARGED CLAY PARTICLES IN WATER

Abstract

Colloidal interactions between clay particles have a strong influence on the hydrology and mechanics of soil and sedimentary media. This phenomenon is grounded in complex multiscale couplings between aqueous chemistry, mechanics, and transport in nanoporous clay assemblages, for which predictive models remain elusive. Simulation predictions of clay aggregation and swelling, to date, have relied almost exclusively on coarse-grained simulation techniques that rely on effective inter-particle potential models. Most such models are based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of long-range colloidal interactions, which breaks down at interparticle distances below ~3 nm, whereas important stable swelling states are observed at interparticle distances of 0.3, 0.6, and 0.9 nm (the so-called one-, two-, and three-layer hydrates). All-atom molecular dynamics (MD) simulations have the potential to help inform existing models but have focused almost exclusively on crystalline hydrates. This thesis presents a first molecular dynamics (MD) simulation study of the free energy of interaction of parallel smectite clay particles over a wide range of interparticle distances (0.3 to 3 nm) and salinities (0.0 to 1.0 M NaCl). The work is then extended to characterize the sensitivity of smectite inter-particle interactions to counterion type (Na, K, Ca). In the process, a detailed picture emerges of key interactions that are not predicted by the DLVO model. Finally, a new coarse-grained MD simulation model is developed and parameterized based on all-atom simulations predictions of free energy of interaction of a pair of sodium clay nanoparticles as a function of inter-particle distance and salinity. This coarse-grained model is used to examine the chemo-mechanical coupling in assemblages of clay nanoparticles.