Atomic-scale structures of interfaces between phyllosilicate edges and water

摘要

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 (010)-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 (110)-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., MgOSi). The free energy results indicate that there should be a considerable number of 5-fold coordinated octahedral sites (i.e., MgOH/AlOH) at the interfaces. The interfacial structures and acid/base groups were determined by detailed H-bonding analyses. (1) Bridging oxygen sites. For (010) edges, the bridging oxygen atoms are not effective proton-accepting sites because they are inaccessible from the solvent. For (110) edges, the oxygen atoms of neutral and octahedrally substituted frameworks (i.e., MOSi for MAl/Mg) are proton-accepting sites. For T-sheet substituted cases, the bridging oxygen site becomes a proton-donating group (i.e., AlOHSi) through the capture of a proton. (2) T-sheet groups. All T-sheet edge groups are MOH (MSi/Al) and act as both proton donors and acceptors. (3) O-sheet groups. For (010) edges with 6-fold Al, the active surface groups include Al(OH)(H _2O) and Al(OH) _2, whereas for Mg cations, the edge group is Mg(OH _2) _2. For 5-fold coordination, the active groups are AlOH. At (110) edges, proton-donating sites include AlOH, AlOH _2 and MgOH _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.