Atomistic simulations of 2:1 clay minerals based on parameterized forcefields have been applied successfully to provide a detailed description of the interfacial structure and dynamics of basal planes and interlayers, but have made limited progress in exploring the edge surfaces of these ubiquitous layer-type aluminosilicates. In the present study, molecular dynamics simulations and energy-minimization calculations of the edge surfaces using the fully flexible CLAYFF forcefield are reported. Pyrophyllite provides an ideal prototype for the 2:1 clay-mineral edge surface because it possesses no structural charge, thus rendering the basal planes inert, while crystal-growth theory can be applied to identify two major candidates for the structure of the edge surfaces. Models based on these candidate structures reproduced bulk crystal bond distances accurately when compared to X-ray data and ab initio molecular simulations, and the predicted edge surface bond distances were in agreement with those determined via ab initio simulation. The calculated surface free energy and surface stress led to an accurate prediction of pyrophyllite nanoparticle morphology, while surface excess energies calculated for the edge surfaces were always negative. These results are consistent with the observed pyrophyllite nanoparticle morphology, with the concept of negative interfacial energies, and conditions that may give rise to them including a role in the stabilization of layer-type nanoparticulate minerals. Molecular dynamics simulations of hydrated nanoparticle edge surfaces indicated five reactive surface oxygen sites on the dominant candidate edge, in agreement with a recent model of proton titration data for 2:1 clay minerals. These promising results illustrate the potential for classical mechanical atomistic simulations that explore edge surface phenomena at much greater length- and times-scales than are currently possible with computationally expensive ab initio methods.
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.
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