Study of a Family of 40 Hydroxylated -Cristobalite Surfaces Using Empirical Potential Energy Functions Shikha Nangia, ² Nancy M. Washton, ² Karl T. Mueller, ² James D. Kubicki, ‡ and Barbara J. Garrison* ,² 104 Chemistry Building, Department of Chemistry, Department of Geosciences and The Earth and EnVironmental Systems Institute, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: NoVember 27, 2006; In Final Form: January 31, 2007 We present a study of a family of 40 unique hydroxylated -cristobalite surfaces generated by cleaving the -cristobalite unit cell along crystallographic planes to include a combination of several low Miller index surfaces. The surface silicon atoms are quantified as percentages of Q 2 and Q 3 centers based on their polymeric state. We find that Q 3 centers are, on average, three times more abundant than Q 2 centers. To study the surface properties, we use two different empirical potential energy functions: the multibody potential proposed by Fueston and Garofalini (J. Phys. Chem. 1990, 94, 5351) and the newly developed CHARMM potential by Lopes et al. (J. Phys. Chem. B 2006, 110, 2782). Our results for the surface water interactions are in good agreement with previous ab initio theoretical studies by Yang et al. (Phys. ReV.B 2006, 73, 146102) for the (100) surface. We find that the most commonly studied family of {100} surfaces is unique and is the only surface with 100% abundance of Q 2 centers, whereas there are nine examples of surfaces with 100% Q 3 centers. The predominantly pure Q 3 surfaces show no hydrogen bonding with the neighboring surface hydroxyl groups and weakly adsorb water overlayers. This is markedly different from the {100} pure Q 2 surface that shows strong hydrogen bonding within the surface groups and with water. As compared to all the surfaces studied in this work, we find that the {100} surfaces are not true representations of the overall -cristobalite surfaces and their properties. We find that the two main factors that characterize the physical properties of silica surfaces are the polymeric state of the silicon atom and surface topography. Two types of pure Q 3 crystallographic planes have been identified and are labeled as Q 3A and Q 3B based on the differences in their topological features. Using the {111} and {011} surfaces as examples, we show that the Q 3A surface adsorbs H 2 O that forms a stable monolayer, but the Q 3B surface fails to form a stable H 2 O overlayer. Other crystallographic planes with different ratios of Q 2 to Q 3 centers are contrasted by the differences in the hydrogen- bonding network and their ability to form ordered H 2 O overlayers. I. Introduction Silica (SiO 2 ) with its myriad industrial uses and ubiquitous presence in nature is necessarily one of the most studied oxide surfaces in both crystalline and amorphous forms. 1-7 Silica surfaces are relevant in studies of glass corrosion, self-assembled monolayers, processing silicon semiconductors, and human health to name a few examples. Recent work has demonstrated that oxide surface properties can vary dramatically from crystal face to crystal face. 8 Hence, it is necessary to have a reasonably efficient method for mapping out all the potential crystal faces of an oxide particle and predicting their surface chemistries. Experiments and electronic structure calculations can then focus on the most representative crystal faces, and the behavior of the particle can be determined based on summation of all the contributing facets. Silica is also an environmentally relevant mineral. Dissolution of silica during the weathering of rocks is an important component of the global carbon cycle and thus affects predic- tions of future climate change related to increasing greenhouse gas concentration in the Earth’s atmosphere (i.e., global warm- ing). Although there have been advances in this field, the microscopic details of silica dissolution are yet to be understood completely. Dissolution occurs at the silica-water interface at which the hydroxylated surfaces can undergo hydrolysis cata- lyzed by hydronium or hydroxide ions. 9,10 These studies have not taken into account, however, the long-range structure of the large variety of silica surfaces and the effect this structure has on the interaction with water. From studies on R-TiO 2 , 11-13 oxide-water interfaces can vary significantly from crystal face to crystal face. These differences affect the isoelectric point of the crystal faces 14 that strongly influences the sorption behavior of the surface. Adsorption is another important environmental process in the transport of toxic metals and agro-chemicals in soils and groundwater. 15,16 Considering these variations, a more general view of possibilities for the silica-water interface is warranted. Silica surfaces with different topologies can also result in different dissolution and adsorption properties. 17-21 Gratz et al. 19 have shown that the rate of dissolution can differ by an order of magnitude depending on the organization of surface into steps (roughness) or straightening of steps (smooth- ness). Another area where silica-water interactions are important is human health. The relationship of surface properties of * To whom correspondence should be addressed. E-mail: bjg@ psu.edu. ² Department of Chemistry. ‡ Department of Geosciences and The Earth and Environmental Systems Institute. 5169 J. Phys. Chem. C 2007, 111, 5169-5177 10.1021/jp0678608 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007