Ab Initio Study of Sorption on Pyrophyllite:  Structure and Acidity of the Edge Sites

Pyrophyllite, Al2[Si4O10](OH)2, is the simplest structural prototype for 2:1 dioctahedral phyllosilicates. Since it does not possess permanent structural charge as other clay minerals do, it is used to investigate sorption properties of the clay surfaces not related to the permanent structural charge. The bulk structure and surface geometries of pyrophyllite have been modeled using an orthorhombic constrained supercell. The results of the calculations are in excellent agreement with available experimental data and earlier ab initio simulations. It is shown that the symmetry-constrained model is able to accurately reproduce the basic structural characteristic of pyrophyllite. The electrostatic potential near the (001) surface was used to analyze a possible scenario for the water sorption on the basal plane of pyrophyllite. The calculations predict a slightly hydrophobic behavior of the basal plane. The hydronium ion was found to form a strongly bonded conformation in the siloxane cavity. The relative stability and composition of lateral facets of pyrophyllite have been studied using the supercell approach. The crystals of pyrophyllite are predicted to have a prismatic habit dominated by (110) and (-110) edge facets and basal plane. On the basis of the Fukui functions and the relative protonation/deprotonation enthalpies, the relative acidity and density of the reactive surface sites have been predicted. The triple bond Al-O-Si triple bond sites have the highest proton affinity on the (100), (110), and (130) facets and three line sign Al-OH groups on the (010) edges. The deprotonation of the triple bond Al-OH2 sites is followed by triple bond Al-OH and triple bond Si-OH groups. The calculations suggest a new scale for acidities of edge sites in pyrophyllite that should facilitate the thermodynamic modeling of the sorption processes in compacted clays.