Abstract
The edges of montmorillonite (MMT) react strongly with metals and organic matter, but the atomic structure of the edge and its surface complexes are not unambiguous since the experimental isolation of the edge is challenging. In this study, we introduce an atomistic model of a Na MMT edge that is suitable for classical molecular dynamics (MD) simulations, in particular for the B edge, a representative edge surface of 2:1 phyllosilicates. Our model possesses the surface groups identified through density functional theory (DFT) geometry optimizations performed with variation in the structural charge deficit and Mg substitution sites. The edge structure of the classical MD simulations agreed well with previous DFT-based MD simulation results. Our MD simulations revealed an extensive H-bond network stabilizing the Na-MMT edge surface, which required an extensive simulation trajectory. Some Na counter ions formed inner-sphere complexes at two edge sites. The stronger edge site coincided with the exposed vacancy in the dioctahedral sheet; a weaker site was associated with the cleaved hexagonal cavity of the tetrahedral sheet. The six-coordinate Na complexes were not directly associated with the Mg edge site. Our simulations have demonstrated the heterogeneous surface structures, the distribution of edge surface groups, and the reactivity of the MMT edge.
Highlights
Smectite minerals, common products of the weathering and low-temperature hydrothermal alteration of primary minerals, are omnipresent in the environment with an outsized role in the fate and transport of soil solution solutes
The results of our present atomistic simulations provide new insights when compared to previous simulations of the Na–MMT AC edge [32]
The density functional theory (DFT) geometry optimizations of the AC and B edges both demonstrated a relationship between the local charge deficits and structural disorder of the
Summary
Common products of the weathering and low-temperature hydrothermal alteration of primary minerals, are omnipresent in the environment with an outsized role in the fate and transport of soil solution solutes. This significant role is due to the highly specific surface area that is a consequence of the nanometer thickness (~1.0 nm) of these layered 2:1 phyllosilicate minerals. In MMT, more than 50% of the permanent negative charge (0.2–0.6 per formula unit) originates from substitutions of divalent cations (e.g., Mg2+ or Fe2+ ) for Al3+ in the octahedral sheet [1] Counter ions balance this permanent charge in the interlayer nanopores. These physicochemical properties of the MMT interlayer have been used to great advantage in engineered barriers [2,3,4], nanocomposite materials [5,6,7], and pharmaceutical and biomedical applications [8]
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