Transport processes at α-quartz-water interfaces:: Insights from first-principles molecular dynamics simulations

Car-Parrinello molecular dynamics(CP-MD) simulations are performed at high temperature and pressure to investigate chemical interactions and transport processes at the a-quartz-water interface. The model system initially consists of a periodically repeated quartz slab with O-terminated and Si-terminated (1000) surfaces sandwiching a film of liquid water. At a temperature of 7000 K and a pressure of 0.3 GPa, dissociation of H2O molecules into H+ and OH- is observed at the Si-terminated surface. The OH- fragments immediately bind chemically to the Si-terminated surface while Grotthus-type proton diffusion through the water film leads to protonation of the O-terminated surface. Eventually, both surfaces are fully hydroxylated and no further chemical reactions are observed. Due to the confinement between the two hydroxylated quartz surfaces, water diffusion is reduced by about one third in comparison to bulk water. Diffusion properties of dissolved SiO2 present as Si(OH)(4) in the water film are also studied. We do not observe strong interactions between the hydroxylated quartz surfaces and the Si(OH)4 molecule as would have been indicated by a substantial lowering of the Si(OH)4 diffusion coefficient along the surface. No spontaneous dissolution of quartz is observed. To study the mechanism of dissolution, constrained CP-MD simulations are done. The associated free energy profile is calculated by thermodynamic integration along the reaction coordinate. Dissolution is a stepwise process in which two Si-O bonds are successively broken. Each bond breaking between a silicon atom at the surface and an oxygen atom belonging to the quartz lattice is accompanied by the formation of a new Si-O bond between the silicon atom and a water molecule. The latter loses a proton in the process which eventually leads to protonation of the oxygen atom in the cleaved quartz Si-O bond. The final solute species is Si(OH)(4).