The First-Principles Molecular Dynamics Study of Quartz– Water Interface Mia Ledyastuti, Yunfeng Liang,* and Toshifumi Matsuoka* By first-principles molecular dynamics, we have investigated the interaction of quartz (0001) surface with liquid water for two different fresh silica cleavages. The first type of cleavage is terminated by ¼¼Si(O) 2 and ¼¼Si for two surfaces, respectively, whereas the second type cleavage has the same termination on both surfaces that consists of ¼¼SiAO. The water molecules were found to be decomposed and spontaneously form silanol group on both silica surfaces. After 4.0 ps, the chemical reactions were almost saturated. It was found that the silanol groups dominated on both surfaces. For the first type of cleavage, the final structure contains geminal, triple, and single silanols together with peroxy ASiAOAOASiA and ASiAOAOAH defects. For the second type of cleavage, the final structure contains geminal silanol, single silanol, and siloxane bridge. A small amount of unsaturated sites were also found on both surfaces. Further, we have shown that the newly formed rings at quartz surface range from four- to seven-member ring for both surfaces. All the findings are in good consistent with the recent experimental observations and theoretical respects. VC 2012 Wiley Periodicals, Inc. DOI: 10.1002/qua.24138 Introduction Quartz is a ubiquitous chemical compound in the Earth’s crust. The interaction with water becomes crucial due to the role in many environmental and geochemical processes. The rapid hydroxylation of fresh silica surface, refer to quartz, by water has already been observed in the experiment [1–7] and simula- tions. [8–14] The experimental information of silica surface struc- ture is limited and still in debate. [3,5,7] The structure of quartz (101  0) and (101  1)–water interface was investigated by X-ray reflectivity, which indicates that surface silica is likely fully hydroxylated. [3] The two modes at 880 and 980 cm 1 were identified as SiAOASi and SiAOH vibration of quartz (0001) surface in sum-frequency generation spectroscopy. [5] The sur- face composition of a quartz surface reacted with various elec- trolyte solutions was evaluated by using X-ray photoelectron spectroscopy (XPS), where three different surface species have been quantitatively verified. [7] A deeper insight into the quartz–water interface, toward a consistent interpretation on experiment data, is therefore desirable. The structure of the different pristine and hydrolyzed silica surfaces has been investigated in previous works. [8–14] The for- mation of SiAOASi bridges, two-membered rings, and three- membered rings are some features in the relaxation of fresh silica surface. [8,10,12,14] The other defects may also occur on the silica surface or bulk that have been detected by experiment. The two-membered rings, peroxy links, and double bonds were the surface defects that have been observed by infrared spectroscopy. [15–19] The two-membered ring silica surface has been detected when silica was dehydroxylated at tempera- tures above 800 K. [15] The presence of peroxy link ¼¼SiAOAOASi¼¼ has been identified as the defect in silica glass via electron spin resonance measurement. [16] This defect is believed to be the precursor for other defect formation such as E 0 centers (BSi), nonbridging-oxygen hole centers (BSiAO), peroxy radicals (BSiAOAO), and self-trapped holes. [17] The two-coordinated Si atom was also supposed to be the defect of silica surface, which was reported by the luminescence polarization. [18,19] In the Earth sciences, peroxy link is indicated as the nonseismic pre-earthquake signal. [20] Every igneous and high-grade metamorphic rock in the Earth’s crust carries a non-zero complement of peroxy, which can be activated to generate electric current when rocks are subjected to mechan- ical stress. This electric current is detectable and has been regarded as one pre-earthquake signal. [20–24] If the electromag- netic signals that are ‘reported’ by the earth can be under- stood well, we would be able to predict the major earthquake. It is, therefore, interesting to understand the formation mecha- nism at molecular scale for all relevant defects in crystalline and amorphous silica. Previously, the silica surface structure formed in aqueous environment was investigated mainly on the basis of the com- bination of fresh silica surface and water molecules. [8–13] This method has not dealt with the full solid–liquid interaction, but rather individual water molecules with silica surface. The other simulations presented the silica surface as fully hydroxylated structure with higher density (10 silanol groups/nm 2 ). [25,26] In comparison, the experimental average surface density of SiAOH of the various silica surfaces is around 4.5–6.2 silanol groups/nm 2 under ambient condition. [27,28] To examine the effect of the polarity of quartz surface, different degrees of hydroxylation of silica surface have also been used. [29,30] How- ever, the above simulation work could not answer the real structure of silica surface as revealed by the XPS measure- ments, that is, with a significant amount of SiAO. [7] M. Ledyastuti, Y. Liang, T. Matsuoka Department of Urban Management, Kyoto University, Kyoto 615-8540, Japan E-mail: y_liang@earth.kumst.kyoto-u.ac.jp matsuoka@earth.kumst.kyoto-u.ac.jp VC 2012 Wiley Periodicals, Inc. International Journal of Quantum Chemistry 2012, DOI: 10.1002/qua.24138 1 WWW.Q-CHEM.ORG FULL PAPER