The Surface Coulomb Energy and Proton Coulomb Potentials of Pyrophyllite {010}, {110}, {100}, and {130} EDGES

Abstract

Abstract--This paper describes structural models of four pyrophyllite edge faces: {010}, { 110}, { 100}, and { 130}. Water molecules chemisorbed to Lewis acid sites stabilize edge faces both crystallochemically and electrostatically. The detailed assignment of protons to surface oxygens and the orientation of OH bond-vectors both influence the surface Coulomb energy.The geometry chosen for the electrostatic calculations was infinite pyrophyllite ribbon the thickness of a single phyllosilicate layer and the width of 50 to 70 unit cells. Such a phyllosilicate ribbon has only two edges, a top and bottom, which were simulated using the edge-face models mentioned above. About 94% of the surface Coulomb energy is confined to the edge-face repeat unit. The surface Coulomb energies of the four edge faces are on the order of 2-3 n J/m, varying + 1 nJ/m with proton assignment. The Coulomb potential, measured either within the layer or parallel to the layer, has a distinct negative trend near the edge face that can be traced to chemisorbed water molecules. Finally, the correlation between proton Coulomb potentials at the edge face and the coordination environment of the protons is poor, obscured by long-range interactions.

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

“…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]. The octahedral sheet of this 2:1 phyllosilicate unit cell is trans-vacant (i.e., the two cis-sites, where the anionic positions are shared along an octahedral edge, are occupied; Figure 1).…”

“…A periodic slab model of the B edge surface for DFT calculations ( Figure 2) was created based on a 2 × 2 × 1 supercell of a geometry optimized pyrophyllite-1M unit cell from Bleam, et al [33]. This slab model has two symmetric edge surfaces at the left and right of Figure 2.…”

EMSP Project 70070: Reactivity of Primary Soil Minerals and Secondary Precipitates Beneath Leaking Hanford Waste Tanks - Final Report

“…For example, edge surfaces exhibit pH-dependent surface charging behavior, whereas siloxane-terminated basal surfaces do not (White and Zelazny, 1988;Bleam, 1993;Bleam et al, 1993). Furthermore, acid dissolution of phyllosilicates proceeds almost exclusively at edge surfaces (Kaviratna and Pinnavaia, 1994;Turpault and Trotignon, 1994;Rufe and Hochella, 1999;Bosbach et al, 2000;Bickmore et al, 2001).…”

“…First attempts included schematic representations of surface site types (Schofield and Samson, 1953;Muljadi et al, 1966), but did not address site density or distribution. White and Zelazny (1988), Bleam (1993), Bleam et al (1993), Brady et al (1996), and Bickmore et al (2001) used crystal chemical methods to predict site type, distribution, and density. The precise termination of each edge surface is determined by cutting the crystal structure parallel to the plane of interest in such a way as to break the weakest bonds and preserve stoichiometry .…”

“…Another complication is surface relaxation (i.e. shifts in electron density, and hence bond lengths, with respect to the bulk structure), which Bleam et al (1993) did not consider. White and Zelazny (1988) proposed a protonation scheme for the (110) surface based on relaxation patterns observed for related minerals (Fig.…”