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Minerals 2016, 6(2), 25; https://doi.org/10.3390/min6020025

Edge Structure of Montmorillonite from Atomistic Simulations

1
Department of Geology, Kangwon National University, Chuncheon 24341, Korea
2
Division of Energy and Environmental Systems, Faculty of Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo 060-8628, Japan
*
Author to whom correspondence should be addressed.
Academic Editor: Athanasios Godelitsas
Received: 23 February 2016 / Revised: 18 March 2016 / Accepted: 18 March 2016 / Published: 25 March 2016
(This article belongs to the Special Issue Mineral Surface Science and Nanogeoscience)
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Abstract

Classical molecular dynamics (MD) simulations have been performed to investigate the effects of substitutions in the octahedral sheet (Mg for Al) and layer charge on an atomistic model of the montmorillonite edge. The edge models considered substitutions in both the solvent accessible and inaccessible octahedral positions of the edge bond chain for a representative edge surface. The MD simulations based on CLAYFF, a fully-flexible forcefield widely used in the MD simulations of bulk clay minerals, predicted Mg–O bond distances at the edge and in bulk that agreed with those of the density functional theory (DFT) geometry optimizations and available experimental data. The DFT results for the edge surfaces indicated that substitutions in the solvent inaccessible positions of the edge bond chain are energetically favorable and an increase in layer charge and local substitution density coincided with the occurrence of five-coordinate, square pyramidal Mg and Al edge structures. Both computational methods predicted these square pyramidal structures, which are stabilized by water bridging H-bonds between the unsaturated bridging oxygen [(Al or Mg)–O–Si] and other surface O atoms. The MD simulations predict that the presence of Mg substitutions in the edge bond chain results in increased disorder of the edge Al polyhedra relative to the unsubstituted edge. In addition to the square pyramidal Al, these disordered structures include trigonal bipyramidal and tetrahedral Al at the edge and inverted Si tetrahedra. These simulation results represent the first test of the fully-flexible CLAYFF forcefield for classical MD simulations of the Na-monmorillonite edge and demonstrate the potential of combined classical MD simulations and DFT geometry-optimizations to elucidate the edge structure of 2:1 phyllosilicate minerals. View Full-Text
Keywords: clay edge; mineral surfaces; Na-montmorillonite; nanoporous minerals; atomistic simulation; molecular dynamics; density functional theory clay edge; mineral surfaces; Na-montmorillonite; nanoporous minerals; atomistic simulation; molecular dynamics; density functional theory
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Newton, A.G.; Kwon, K.D.; Cheong, D.-K. Edge Structure of Montmorillonite from Atomistic Simulations. Minerals 2016, 6, 25.

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