Edge Structure of Montmorillonite from Atomistic Simulations

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.

Na-Montmorillonite Edge Structure and Surface Complexes: An Atomistic Perspective

“…The results of our present atomistic simulations provide new insights when compared to previous simulations of the Na-MMT AC edge [32]. The DFT geometry optimizations of the AC and B edges both demonstrated a relationship between the local charge deficits and structural disorder of the Na-MMT edge.…”

“…The relative stability of octahedral substitutions in the edge PBC, however, was different for the two stoichiometrically-equivalent edge structures. The AC edge favored substitutions in the solvent-inaccessible positions (Mg pbc ) at both low and high layer-charges [32]. In contrast, on the B edge, the favorable site changed from Mg sol at low a layer-charge to Mg pbc at a high layer-charge.…”

“…The simulation algorithm has been described in detail elsewhere [32]. In brief, the solution phase was initiated with a series of short MD simulations in the microcanonical (NVE, T = 298.15 K) ensemble, where the number of atoms (N), simulation cell volume (V), and energy (E) were held constant, with the temperature (T) controlled with a direct temperature-rescaling thermostat and the SHAKE algorithm [47] for water molecule bonds and angles.…”

“…Isomorphic substitutions of Al 3+ by Mg 2+ in the octahedral layer were assigned at random and were subject to the two constraints described previously [32]. The random assignment of substitutions and the first constraint that prohibits the presence of Mg-(OH) 2 -Mg pairs in the octahedral sheet are based on 1 H magic angle spinning nuclear magnetic resonance (MAS NMR) and FT-IR spectra from MMTs that show no contributions from Mg-(OH) 2 -Mg moieties and QM simulations that demonstrated that Mg 2+ substitutions are randomly distributed within the octahedral layer [35][36][37].…”

“…Our atomistic thermodynamics study based on density functional theory (DFT) [31] tested the thermodynamics stability of the EF defects and showed that the defects on the pyrophyllite AC edge are feasible at water chemical potentials and temperatures at which the pyrophyllite B edge would exist. Recently, our classical MD simulations [32] using Clayff explored the effects of layer charge arising from isomorphic substitutions in the octahedral layer on the AC edge of MMT by employing DFT models with different layer charges. The simulations demonstrated the correlation between local charge deficits in the octahedral layer and coordination of Al and Mg at the AC edge.…”