Simulation Study of the Silicon Oxide and Water Interface

Classical and ab-initio molecular dynamics simulations were carried out to study the structural properties of water near a silica interface for three different silica substrates: alpha-quartz, beta-cristobalite, and amorphous silica. For the classical simulations, both the OPLSTTIP3P and the COMPASS/SPCE force fields were used to study the silica/water interface. The quantum simulations used the VASP plane-wave pseudopotential code that treats the valence electrons in the system explicitly by using the generalized gradient approximation (GGA) of Perdew and Wang and represents the inner core electrons by the Vanderbilt ultrasoft pseudopotential scheme. The orientation of the water molecules at the interface was determined and in general both of the classical force fields and the quantum simulations show that near the interface the water molecules are oriented such that at least one of the hydrogens are nearer the silica than the oxygen of the water molecule. The hydrogen bond network of water molecules with other water molecules and with the silanol (Si-O-H) groups on the silica surface was determined. The water molecules form a significant number of hydrogen bonds with the silanol groups resulting in a reduction in mobility of the water near the interface as compared to water molecules in the bulk.