Molecular Dynamics Simulations of Pyrophyllite Edge Surfaces: Structure, Surface Energies, and Solvent Accessibility

Abstract

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.

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

“…This contraction is attributed to the presence of trigonal bipyramidal Al at the edge. The Al sol -Oh distance on the Na-MMT B edge agrees well with the corresponding distance in the DFT model at low layer charge (x 1 = 0.25) and previously reported B edge Al-OH distance of pyrophyllite [28]. The Al sol -Ow distance of the classical mechanical model of the Na-MMT edge cannot be compared with the DFT models as the Al sol is hydrolyzed at a low layer charge (i.e., ≡Al-(OH) 2 ) or in five-fold coordination at a higher layer charge in the smaller DFT models.…”

“…On the AC edge, the pK a for the amphoteric Al site (i.e., ≡Al-OH 2 ) is 5.5 [26]. The AC edge also possesses a lower surface energy than the B edge and would be a greater fraction of the total edge surface area [19,28,31]. This difference in surface acidities and fraction of total surface area indicate that the reactive site density of the AC edge will be greater than that of the B edge.…”

“…The atomistic model of the Na-montmorillonite (Na-MMT) edge was derived from a previous model of the pyrophyllite edge, with the details of that model described elsewhere [28]. The B edge surface model was created from a 4 × 4 × 2 (a × b × c) expansion of the atomic coordinates of the pyrophyllite-1M unit cell [33].…”

“…While the development of reliable force fields for clay mineral edges is an ongoing research topic in the computational chemistry community, a recent classical molecular dynamics (MD) simulation study [28] demonstrated that the edge structures can be described within a reasonable accuracy by using the Clayff force field [29], a force field widely used in the research of interlayer or basal surfaces of clay minerals. The classical MD simulations not only reproduced the main edge structrual features obtained by QM-based MD simulations but also offered evidence for defect structures at the pyrophyllite AC edge, denoted as Edelman Favejee (EF) [30] defects by the authors who first proposed a similar structure for 2:1 smectites.…”

Molecular Dynamics Simulations of Anion Exclusion in Clay Interlayer Nanopores

“…Individual clay mineral layers (hereafter referred to as TOT layers) were truncated at the (110) and (11 0) structural planes in order to create edge surfaces, which were then healed by adding ÀOH or ÀH groups to under-coordinated edge atoms in order to obtain edge surfaces with zero net proton surface charge (Churakov, 2006). The resulting edge surface structures, which correspond to the AC chains of White and Zelazny (1988), are thought to be the most stable or one of the most stable smectite edge structures (White and Zelazny, 1988;Bickmore et al, 2003;Churakov, 2006;Liu et al, 2014;Newton and Sposito, 2015;Newton et al, 2016). Each TOT layer had a thickness of~10 Å (ambiguities associated with the definition of the TOT layer thickness are discussed below), a finite length of 45.7 Å in the direction normal to the edge surfaces, and a width of 63.4 Å (effectively an infinite width, because of the periodic boundary conditions of the simulation cell).…”