Atomic-scale structures of interfaces between phyllosilicate edges and water

Abstract

We report first-principles molecular dynamics (FPMD) studies on the structures of interfaces between phyllosilicate edges and water. Using FPMD, the substrates and solvents are simulated at the same first-principles level, and the thermal motions are sampled via molecular dynamics. Both the neutral and charged silicate frameworks are considered, and for charged cases, the octahedral (Mg for Al) and tetrahedral (Al for Si) substitutions are taken into account. For all frameworks, we focus on the commonly occurring (0 1 0)-and (1 1 0)-type edge surfaces.With constrained FPMD, we calculated the free energy of the leaving processes of coordinated water of octahedral cations; therefore, the coordination states of those edge cations are determined. For (0 1 0)-type edges, both the 5-and 6-fold coordination states of Al are stable and occur with a similar probability, whereas only the 5-fold coordination is stable for Mg cations. For (1 1 0)-type edges, only the 6-fold states of Al cations are stable. However, for Mg cations, both coordination states are stable. In the 5-fold case, the solvent water molecules form H-bonds with the bridging oxygen atoms (i.e., "MgAOASi"). The free energy results indicate that there should be a considerable number of 5-fold coordinated octahedral sites (i.e., "MgAOH/ "AlAOH) at the interfaces. The interfacial structures and acid/base groups were determined by detailed H-bonding analyses.(1) Bridging oxygen sites. For (0 1 0) edges, the bridging oxygen atoms are not effective proton-accepting sites because they are inaccessible from the solvent. For (1 1 0) edges, the oxygen atoms of neutral and octahedrally substituted frameworks (i.e., "MAOASi" for M@Al/Mg) are proton-accepting sites. For T-sheet substituted cases, the bridging oxygen site becomes a proton-donating group (i.e., "AlAOHASi") through the capture of a proton. (2) T-sheet groups. All T-sheet edge groups are "MAOH (M@Si/Al) and act as both proton donors and acceptors. (3)O-sheet groups. For (0 1 0) edges with 6-fold Al, the active surface groups include "AlA(OH)(H 2 O) and "AlA(OH) 2 , whereas for Mg cations, the edge group is "MgA(OH 2 ) 2 . For 5-fold coordination, the active groups are "AlAOH. At (1 1 0) edges, proton-donating sites include "AlAOH, "AlAOH 2 and "MgAOH 2 groups. Overall, our results reveal the significant effects of isomorphic substitutions on the interfacial structures, and the models provide a molecular-level basis for understanding relevant interfacial processes.

Interlayer Structures and Dynamics of Arsenate and Arsenite Intercalated Layered Double Hydroxides: A First Principles Study

“…Recently, precise information of the speciation, structures, and acidities of As in aqueous solutions has been obtained by using FPMD [54], indicating that FPMD can provide precise information in As-bearing systems. The efforts to explore the structural and thermodynamic properties of layered minerals such as gibbsite [55] and clay minerals [56][57][58][59] also obtained results consistent with experiments. In this study, we employ first principles simulation techniques to investigate basal spacings and interlayer properties of arsenate and arsenite intercalated LDHs.…”

Acidity of edge surface sites of montmorillonite and kaolinite

“…the edge site is ≡Mg-(OH 2 ) 2 (Liu et al, 2012a). For Al at the edges of the two clay minerals, both the 114 6-fold and 5-fold coordinations are possible with the former being more stable, and therefore, only the 115 6-fold cases are investigated in this study.…”

“…Siloxane surfaces normally 46 behave as adsorbing region for foreign molecules such as waters and cations, e.g. In previous studies, FPMD simulations were employed to investigate the structures of edge-water 81 interfaces for kaolinite and montmorillonite (Churakov, 2007;Liu et al, 2012a;Liu et al, 2012b). 82…”

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

“…The unsaturated octahedron of each TOT unit is healed with one physisorbed water molecule. Unlike the DFT-based molecular dynamics (DFT-MD), wherein energetically unfavorable initial distributions of physi-and chemi-sorbed water molecules may result in a re-distribution of protons amongst the surface oxygen [20,24,34], classical mechanical simulations must proceed from surface structures chosen a priori; the initial surface configuration was created based on our DFT geometry optimization results. The force fields used in classical mechanical simulation assign an atom type to each atom in the molecular structure based on the chemical element of the atom and its coordination environment.…”

“…These edge structures have been the initial configuration for atomistic simulations of the 2:1 clay edge at both the quantum mechanical (QM) and molecular mechanical (MM) levels of theory. Many QM simulations have been used for a detailed characterization of the 2:1 edge structures, surface energies, acid-base reactivity, and cationic surface complexes [18][19][20][21][22][23][24][25][26][27]. These simulations are the most theoretically rigorous available, but the theoretical rigor comes with high computational costs that limit the model size and the duration of any QM-based molecular dynamics (MD) simulations.…”