Molecular dynamics simulations of hydration, dissolution and nucleation processes at the alpha-quartz (0001) surface in liquid water.

Molecular dynamics simulations have been employed to investigate the hydration and dissolution of alpha-quartz (0001) surfaces in a liquid water environment. Our study indicates that the structure of the water layers near the surfaces is affected by the nature of the substrate surface and by temperature. Ordered mono-layers of interfacial water molecules form in the region of the substrate where the surface is highly charged and built up of Si-O-Si bridges. As the temperature is increased this ordered mono-layer structure is gradually lost. When the surface is terminated by silanol groups, the water retains liquid-like properties even at low temperature and the molecules are distributed in a random manner, without the formation of distinct ordered mono-layers of water molecules near the surface. Taking into account the entropy of the system, the calculated energies of stepwise dissolution of a silicon species from the surface suggest that on thermodynamic grounds the complete dissolution of silicon atoms from the quartz surfaces in a liquid water environment is an endothermic process, but that the formation of a -Si(OH)3 species at the surface would be possible. In addition, if the Si(OH)(4) species were to be dissolved, it would remain near the surface, and re-deposition at the defect-free surface is thermodynamically preferred, although there is an activation enthalpy to the first step in the process of nucleation of Si(OH)4 at the perfect surface.