Cleavage and Recovery of Molecular Water in Silica Rene ´ e M. Van Ginhoven, ²,§,# Hannes Jo ´ nsson, ‡,§,| Byeongwon Park, ⊥,# and L. Rene ´ Corrales* ,# Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195, Faculty of Science VR-II, UniVersity of Iceland, 107 ReykjaVik, Iceland, Korean Institute of S&T EValuation and Planning, Seoul, 137-130 Korea, and Pacific Northwest National Laboratory, PO Box 999, Richland, Washington 99352 ReceiVed: NoVember 3, 2004; In Final Form: April 5, 2005 The network response associated with the incorporation and reactivity of water molecules in bulk phases of amorphous and crystalline silica are investigated using density functional theory. The extent of network relaxation is found to change the relative stabilities of the reactant and product states. A highly reactive site, with a low activation barrier, is associated with a highly strained site in which network relaxation significantly stabilizes the silanol state by effectively annealing the local structure. Diffusion and exchange reaction paths are found to likely be associated with minimum energy paths in which the stability of the product and reactant states are equal. These latter paths are associated with minimal network response, although the ability of the silanol groups to take on several conformations has an overall effect of changing the stability along a given reaction path. 1. Introduction The reaction of molecular water with silica and its reverse reaction play key roles in the diffusion of water in silica, 2 in the processes that lead to dissolution or corrosion, 3 in the exchange of oxygen atoms with the network backbone, 4,5 in sol-gel processing, 6 and in controlling the phase stability of surfactant/silica composites. 7 The presence of water, either in the reacted form as silanol groups (Si-OH) or in its molecular form, has strong effects on the physical properties of silica. At elevated temper- atures, the presence of water reduces the viscosity. 2 At lower temperatures, the presence of water can affect properties such as the optical absorption. 2 Experiments have characterized water reaction and diffusion into silica glass by determining the buildup of silanol groups and the extent of penetration, 1 the exchange of oxygen isotopes between labeled water and the silica network, 8,9 the solubility of water in silica and its dependence on vapor pressure and temperature of the system, 10 and by glass dissolution methods. 2 Other recent experiments include studies in which significant network relaxation or modification ensue from the reaction of water with silica at the surface as well as in the bulk 11,12 and when molecular water is solvated in silica. 13 Silica glass is known to contain different concentrations of water in the form of silanol groups or whole molecular water. The concentration and distribution of water or silanol groups in a particular glass depends on the sample history. In fact, it is very difficult to make a dry glass without the use of additives such as fluorine. The mechanism for trapping reacted water in silica is difficult to determine experimentally because the reactions generally occur during processing and the water cannot be subsequently removed. In contrast, water can diffuse from the surface of a glass into its interior, and vice versa, and continue diffusing if it does not trap. The mechanism for diffusion consists of a mix of the water-silica reaction and its reverse and whole molecular water molecules diffusing through the open network structure without reactions. Experiments based on diffusion studies predict average reaction activation barriers of around 18 kcal/mol (0.78 eV). 1 The relationship between “trapped” water and diffusive water is not well known, and it is difficult to determine the mechanism of trapping experimen- tally. In this work, density functional theory (DFT) calculations are used to show that nondiffusive silanol groups are a result of an irreversible reaction that anneals the glass network to a significantly lower energy configuration. A comparison is made with a glass system in which diffusive behavior by a reaction mechanism is observed. Network relaxation is found to play a key role in controlling the height of the activation barriers by varying the degree of stability between the reactant and product states. Additionally, the possibility that the silanol state has multiple configurations caused by network response can lead to stabilizing that state, an effect not seen in pristine crystals. The characterization of the reactivity of water with silica has received considerable theoretical attention. Previous theoretical studies of water interacting with silica focused on gas-phase cluster calculations that are unable to capture the effects of network response. 14-16 Earlier bulk-like calculations focused on the diffusion of water in large quartz-like clusters that allowed only the relaxation of the water molecules. 17 Additional studies of water binding in quartz have considered the importance of network response, 18 albeit still in large quartz-like clusters. Recent work has addressed key issues in diffusion and reactions of water with respect to different ring sizes and defects in silica. 19,20 That work mentioned the existence of a range of activation barriers and a range of relative stability between the molecular water and silanol states. The intent of this study is * Corresponding author. E-mail: rene.corrales@pnl.gov. ² E-mail: rmvangi@sandia.gov. ‡ E-mail: hannes@u.washington.edu. § University of Washington. | University of Iceland. ⊥ Korean Institute of S&T Evaluation and Planning; e-mail: bpark@ kistep.re.kr. # Pacific Northwest National Laboratory. SiOSi + H 2 O T 2SiOH (1) 10936 J. Phys. Chem. B 2005, 109, 10936-10945 10.1021/jp044973n CCC: $30.25 © 2005 American Chemical Society Published on Web 05/06/2005