Published: January 18, 2011 r2011 American Chemical Society 2076 dx.doi.org/10.1021/jp109446d | J. Phys. Chem. C 2011, 115, 2076–2088 ARTICLE pubs.acs.org/JPCC Simulations of the Quartz(10 11)/Water Interface: A Comparison of Classical Force Fields, Ab Initio Molecular Dynamics, and X-ray Reflectivity Experiments A. A. Skelton,* ,† P. Fenter, ‡ J. D. Kubicki, § D. J. Wesolowski, || and P. T. Cummings ||,† † Department of Chemical Engineering, Vanderbilt University, Nashville, Tennessee 37240, United States ‡ Chemical Sciences and Engineering, Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ) Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States b S Supporting Information ABSTRACT: Classical molecular dynamics (CMD) simulations of the (10 11) surface of quartz interacting with bulk liquid water are performed using three different classical force fields, Lopes et al., ClayFF, and CHARMM water contact angle (CWCA), and com- pared to ab initio molecular dynamics (AIMD) and X-ray reflectivity (XR) results. The axial densities of the water and surface atoms normal to the surface are calculated and compared to previous XR experiments. Favorable agreement is shown for all the force fields with respect to the position of the water atoms. Analyses such as the radial distribution functions between water and hydroxyl atoms and the average cosine of the angle between the water dipole vector and the normal of the surface are also calculated for each force field. Significant differences are found between the different force fields from such analyses, indicating differing descriptions of the structured water in the near vicinity of the surface. AIMD simulations are also performed to obtain the water and hydroxyl structure for comparison among the predictions of the three classical force fields to better understand which force field is most accurate. It is shown that ClayFF exhibits the best agreement with the AIMD simulations for water hydroxyl radial distribution functions, suggesting that ClayFF treats the hydrogen bonding more accurately. ’ INTRODUCTION The interaction of quartz (R-SiO 2 ) with water is of interest in geological, 1-4 biological, 5,6 and technological 7,8 contexts. Quartz is a significant component of soils, clays, and rocks constituting about 20% of the Earth’s exposed crust. 9,10 Moreover, the proper- ties of amorphous silicas involving high surface area gels and colloidal particles 11,12 depend on the silica surface chemistry. 13 The interaction of quartz and other silica phases with a range of biomolecules is relevant to diseases such as silicosis 14 and in the controlled assembly of nanocomposites. 15 Water is important as it is ubiquitous in natural environments, and it mediates the interac- tion of biomolecules with surfaces. 16 Quartz is challenging to study both experimentally and theoretically because, unlike many other metal oxides, it exhibits significant solubility in water. To add to the complexity, it has been found that the dissolution rate of quartz depends on the solution pH and dissolved salt concentration in aqueous solutions. 17-21 Quartz also exhibits surface acid/base properties; it has been suggested that the pH at the point of zero charge (PZC) for quartz is approximately 2. 22 At higher pH, deproto- naton of surface silanol groups (dSiOH) leads to the develop- ment of negative surface charge. 23-27 In the present study, classical molecular dynamics (CMD) simulations will be compared to previous X-ray reflectivity (XR) experiments 28 as well as ab initio molecular dynamics (AIMD). In the aforementioned XR study, two surfaces of R-quartz, (10 10) and (10 11), were investigated. The study provided information about the atomic-scale structure of the quartz- water interface, wherein the electron density normal to the surface in the interfacial region is converted to the axial density of atoms at the surface and the distribution of water in direct contact with the surface, which indicates interfacial water layering (we use the term “axial” to refer to profiles normal to the surface). The atomic distribution and electron density can be extracted Received: October 1, 2010 Revised: December 10, 2010