The transport and mechanical properties of swelling clays play a vital role in their application as geological seals for nuclear waste disposal and subsurface CO2 storage. A molecular-level understanding of the interlayer and inter-tactoid forces causing swelling/collapse, influenced by the chemical composition and mobility of water and ions in the interlayers and pores, can provide valuable upscaling strategies to inform macroscopic models. Available experimental data suggest that most montmorillonite (MMT) clays have cis-vacant structure1 whereas atomistic models2-3 have traditionally imposed an assumption of centrosymmetry by modeling MMT as trans-vacant clays. In this study, we model cis-vacant MMT layers where structural hydroxyl groups occupy positions that are consistent with experimental data for Wy-MMT with a structural charge of -0.57 per O20(OH)4. This induces a relative shift of the TOT sheets4 and causes variations in the distribution of adsorbed cations in the interlayer. We also investigate the stacking behavior by performing MD simulations of Na-MMT in bulk water for energetically favorable rotational configurations. A grand canonical Monte Carlo approach is employed to obtain 1W hydrate between two layers of MMT. We employ OH, and OH2 terminations to the edges of the hexagonal top layer to obtain a neutral edge charge. Upon rotation of the layers relative to each other, the potential energy variations are monitored, and the positions of interlayer species are calculated as a function of the rotational angle between adjacent clay layers. Results indicate that the potential energy wells during rotation are freely accessible (&lt; 1kT) up to relative misalignment angles of 6-8°, consistent with observed turbostratic stacking in Na-MMT. The density distribution of cations along the axis perpendicular to the clay layers (z) does not significantly change during the rotation process. However, 2-dimensional density maps (x-y plane) exhibit the rearrangement of cationic species along hexagonal edges of the rotating clay layer. Insight into the fundamental interlayer forces that contribute to rotational disordering is useful to future models for swelling and dynamic exchange processes.

Atomistic simulations of rotational energetics in turbostratic Wy-montmorillonite clays

2019 Fall National Meeting

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