(480f) Modeling the Effect of Hydrogen Bonding for Understanding the Dissolution of α-Quartz Surfaces in Water

The interaction between water and two surfaces relating to different crystallographic faces of α-quartz have been atomistically modeled using the force-field of Lopes et al. (1). Comparisons with X-ray reflectivity experiments (2) have shown agreement between simulation and experiment for the 10-10 and α-quartz surfaces, supporting the accuracy of this model.

The focus of this study is to understand the dissolution of quartz and it is thought that the dissolution process involves both {1} hydrogen bonding between water hydrogen and quartz siloxane oxygen, and {2} close contact of water oxygen to the adjacent silicon (3,4). Different spatial arrangements of atoms on different surfaces/terminations mean that the access of silicon atoms and siloxane oxygens to water is different. The statistics of these close contacts/hydrogen bonds are analyzed and it is considered that atomistic simulations can give us an estimate of the probability of water molecules attacking (i.e. dissolving) quartz surfaces.

Although it has previously been reported that there are two possible terminations for the 10-10 α-quartz surface, only one of these surfaces shows agreement between experiment and simulation. It is postulated that the surface termination that is observed experimentally is the one more resistant to dissolution by water (2). It is suggested that a concerted mechanism involving water, hydroxyls and siloxanes is responsible for the dissolution of the top surface layer thereby exposing the dominant termination. This is supported by the greater number of water-siloxane and hydroxyl-siloxane H-bonds for the less dominant surface than for the more dominant surface.

Step edges for the α-quartz (the dominant surface) and the 10-11 surface have also been simulated in order to further illuminate the mechanism of dissolution. Insight into dissolution has been gained by rationalizing H-bonding statistics with previous AFM results (3), which show that steps form only in particular directions.

(1) P.E.M. Lopes et al . J. Phys. Chem. B 2006, 110, 2782

(2) M.L. Schlegel et al Geochimica et Cosmochimica Acta 2002 66, 3037

(3) L. J. Crisenti, J. D. Kubicki and Susan L. Brantley J. Phys. Chem. A 2006, 110, 198

(4) A. Pelmenschikov, J. Leszczynski and L. G. M. Pettersson, J. Phys. Chem. A 2001, 105, 9528