Transport processes at alpha-quartz-water interfaces: insights from first-principles molecular dynamics simulations

Area of Science:

• Geochemistry
• Materials Science
• Computational Chemistry

Background:

• Understanding the alpha-quartz-water interface is crucial for geological processes and material science applications.
• Previous studies have explored surface interactions, but atomistic details at high T/P remain less understood.

Purpose of the Study:

• To investigate chemical interactions and transport phenomena at the alpha-quartz-water interface under high temperature and pressure.
• To elucidate the mechanism of quartz dissolution at the molecular level.

Main Methods:

• Car-Parrinello molecular dynamics (CP-MD) simulations were employed.
• Simulations were conducted at 1000 K and 0.3 GPa using a model system of quartz slabs and liquid water.
• Constrained CP-MD simulations and thermodynamic integration were used to calculate the free energy of dissolution.

Main Results:

• Water dissociation (H2O -> H+ + OH-) was observed at the Si-terminated surface, leading to surface hydroxylation.
• Proton diffusion and surface hydroxylation occurred on both O- and Si-terminated surfaces.
• Water diffusion was reduced by approximately one-third due to confinement, and Si(OH)4 diffusion was not significantly hindered.
• Quartz dissolution was identified as a stepwise process involving sequential Si-O bond breaking and formation of Si-O bonds with water, ultimately forming Si(OH)4.

Conclusions:

• The alpha-quartz-water interface undergoes significant chemical changes, including surface hydroxylation, under high T/P conditions.
• Quartz dissolution is a multi-step process driven by water molecules, resulting in silicic acid as the final product.
• Simulations provide molecular-level insights into interfacial reactions relevant to geological and material science contexts.