Structure of Interfacial Water on Fluorapatite (100) Surface Aparna Pareek,* ,² Xavier Torrelles, ‡ Klaus Angermund, § Jordi Rius, ‡ Uta Magdans, ² and Hermann Gies ² Faculty of Geosciences, Department of Geology, Mineralogy and Geophysics, Ruhr-UniVersitaet Bochum, UniVersitaetsstrasse 150, 44780 Bochum, Germany, Institut de Ciencia de Materials de Barcelona, Campus de la UniVersitat Autonoma de Barcelona, 08193 Bellaterra, Spain, and Max-Planck-Institut fuer Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Muelheim a. d. Ruhr, Germany ReceiVed June 28, 2007. In Final Form: October 16, 2007 The structure relaxation mechanism of the fluorapatite (100) surface under completely hydrated ambient conditions has been investigated with the grazing incidence X-ray diffraction (GIXRD) technique. Detailed information on lateral as well as perpendicular ordering corresponding to the water molecules and atomic relaxations of the (100) surface of fluorapatite (FAp) crystal was obtained from the experimental analysis of the CTR intensities. Two laterally ordered water layers are present at the water/mineral interface. The first layer consists of four water molecules located at 1.6(1) Å above the relaxed fluorapatite (100) surface while the second shows the presence of only two water molecules at a distance of 3.18(10) Å from the mineral surface. Thus, the first layer water molecules complete the truncated coordination sites of the topmost surface Ca atoms, while the second water layer molecules remain bonded by means of H-bonding to the first layer molecules and the surface phosphate groups. Molecular mechanics simulations using force field techniques are in good agreement with this general structural behavior determined from the experiment. Introduction Recently, apatite minerals have gained much attention because of their biological importance. Hydroxyapatite is the major constituent of mammalian bones and tooth enamel. In addition to bone mineral and organic matrix, water is an abundant component of bone, accounting for up to ∼25% by weight. 1 Attempts have been made to define and understand the role of water present in pore spaces of bones. 2-4 Simultaneous derivative thermogravimetric analysis and variable temperature X-ray diffraction studies revealed the presence of structural water on carbonated apatite, a synthetic apatite used as a model of bone mineral that contains carbonate ions and is deficient in hydroxide, phosphate, and calcium ions. 5 A recent solid-state NMR investigation on hydrogen-bearing species in bone mineral environment revealed three structural roles of water in bone. One of the roles of water was determined in mediating mineral- organic matrix interaction at the bone mineral surface. 6 Various methods have been formulated to get an insight into the spatial arrangement of water, to understand its structural behavior and interaction on to the surface of bone mineral. To understand the structure of interfacial water in the apatite- water system, X-ray diffraction methods were employed using natural fluorapatite crystals. 7,8 Also, various theoretical calculation methods were used to investigate the nature of the hydrated apatite surface. 9,10 Molecular scale investigation of the fluorapatite (100)-water interface structure by high-resolution X-ray re- flectivity showed the presence of two distinct water layers, however, does not reveal the lateral arrangement of water molecules. 7 In our previous study on the fluorapatite (100) surface in humid atmospheric conditions using grazing incidence X-ray diffraction (GIXRD), we revealed the structure of a single layer of adsorbed water with lateral order and the nature of surface relaxations at atomic scale. 8 GIXRD has been used for investigating lateral ordering of atomic structures on surfaces and interfaces at atomic scale. 11 Information about the electron density profile perpendicular to the surface can be derived from reflectivity measurements, whereas the details of the lateral surface structure are obtained from nonspecular diffraction patterns called crystal truncation rods (CTRs). The intensity distribution along these reflections depends on the structural features occurring at the surface at atomic scale, such as atomic relaxations but also on long range order given by surface flatness 12,13 and domain size. Rough surfaces mainly annihilate the intensity in the most surface sensitive regions of the CTRs, which are close to anti-Bragg conditions. X-ray scattering based on synchrotron sources has been used as tool to probe solid-liquid interfaces in liquid and non-UHV environments. 14-22 In recent years, the lateral and perpendicular ordering of water layers in several systems was * E-mail: aparna.pareek@ruhr-uni-bochum.de. ² Ruhr-Universitaet Bochum. ‡ Institut de Ciencia de Materials de Barcelona. § Max-Planck-Institut fuer Kohlenforschung. (1) http://depts.washington.edu/bonebio/ASBMRed/ASBMRed.html [online]. (2) Fernandez-Seara, M. A.; Wehrli, S. L.; Wehrli, F. W. Biophys. J. 2002, 82, 522-529. (3) Cowin, S. C. J. Biomech. 1999, 32, 217-238. (4) Wilson, E.; Awonusi, A.; Morris, M. D.; Kohn, D. H.; Tecklenburg, M. M. J.; Beck, L. W. J. Bone Miner. Res. 2005, 20, 625-634. (5) Ivanova, T. I.; Frank-Kamenetskaya, O. V.; Kol’tsov, A. B.; Ugolkov, V. L. J. Solid State Chem. 2001, 160, 340-349. (6) Wilson, E. E.; Awonusi, A.; Morris, M. D.; Kohn, D. H.; Tecklenburg, M. M. J.; Beck, L. W. Biophys. J. 2006, 90, 3722-3731. (7) Park, C.; Fenter, P.; Zhang, Z.; Cheng, L.; Sturchio, N. C. Am. Mineralogist 2004, 89, 1647-1654. (8) Pareek, A.; Torrelles, X.; Rius, J.; Magdans, U.; Gies, H. Phys. ReV.B 2007, 75, 035418. (9) Mkhonto, D.; de Leeuw, N. J. Mat. Chem. 2002, 12, 2633-2642. (10) Zahn, D.; Hochrein, O. Phys. Chem. Chem. Phys. 2003, 5, 4004-4007. (11) Feidenhans’l, R. Surf. Sci. Rep. 1989, 10, 105-188. (12) Robinson, I. K. Acta Crystallogr, Sect A: Found. Crystallogr. 1998, A54, 772-778 (and cited references). (13) Als-Nielson, J.; McMorrow Des Elements of modern X-ray physics; John Wiley & Sons Ltd.: New York, 2001; p 135. (14) Arsic, J.; Kaminski, D. M.; Poodt, P.; Vlieg, E. Phys. ReV.B 2004, 69, 245406. (15) Reedijk, M. F.; Arsic, J.; Hollander, F. F. A.; de Vries, S. A.; Vlieg E. Phys. ReV. Lett. 2003, 90, 066103. (16) de Vries, S. A.; Goedtkindt, P.; Bennett, S. L.; Huisman, W. J.; Zwanenburg, M. J.; Smilgies, D. M.; De Yoreo, J. J.; van Enckevort, W. J. P.; Bennema, P.; Vlieg, E. Phys. ReV. Lett. 1998, 80, 2229-2232. (17) Arsic, J.; Kaminski, D. M.; Radenovic, N.; Poodt, P.; Graswinckel, W. S.; Cuppen, H. M.; Vlieg, E. J. Chem. Phys. 2004, 120, 9720-9724. 2459 Langmuir 2008, 24, 2459-2464 10.1021/la701929p CCC: $40.75 © 2008 American Chemical Society Published on Web 02/16/2008