Density functional study of the adsorption of aspirin on the hydroxylated (0 0 1) a-quartz surface A. Abbasi a, * , E. Nadimi a , P. Plänitz b , C. Radehaus b,c a Institut für Physik, Technische Universität Chemnitz, D-09107 Chemnitz, Germany b GWT-TUD GmbH, Geschäftsstelle Chemnitz, Annaberger Str. 240, 09125 Chemnitz, Germany c Fakultät für Elektrotechnik und Inofrmationstechnik, Technische Universität Chemnitz, D-09107 Chemnitz, Germany article info Article history: Received 19 March 2009 Accepted for publication 8 June 2009 Available online 13 June 2009 Keywords: Density functional calculations Chemisorption Aspirin (001) a-quartz surface Drug stability abstract In this study the adsorption geometry of aspirin molecule on a hydroxylated (0 0 1) a-quartz surface has been investigated using DFT calculations. The optimized adsorption geometry indicates that both, adsorbed molecule and substrate are strongly deformed. Strong hydrogen bonding between aspirin and surface hydroxyls, leads to the breaking of the original hydroxyl–hydroxyl hydrogen bonds (Hydrog- enbridges) on the surface. In this case new hydrogen bonds on the hydroxylated (0 0 1) a-quartz surface appear which significantly differ from those at the clean surface. The 1.11 eV adsorption energy reveals that the interaction of aspirin with a-quartz is an exothermic chemical interaction. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The stability of drugs in the presence of solid additives have re- ceived considerable attention in the field of pharmaceutics [1,2]. Aspirin is a drug that hydrolyses to salicylic acid and acetic acid in the presence of moisture or water [3,4]. The effect of additives on the stability of aspirin has been widely studied. The degradation of aspirin in solid mixtures of silica [5], colloidal silica [6] and con- trolled pore glass (CPG) [7] have been also investigated. It was sta- ted that free silanol groups on the surface of silica influence the hydrolysis reaction rate of aspirin. However, the role of silanol groups in the hydrolysis of aspirin has not yet been explained on the basis of ab initio computations. The influence of the pore diameter on the stability of aspirin was investigated by Yonemochi et al. [7]. They concluded that in CPG mixtures with controlled pore diameters from 75 to 3000 Å the rate of hydrolyzing aspirin does not depend on the CPG pore diameters. However, in samples with pore diameters larger than 3000 Å, the rate of aspirin degradation decreases with increasing CPG pore diameter. On the other hand, in CPG samples with small pores (pore diameters less than 350 Å) the degradation rate of aspi- rin decreases by increasing the relative humidity (RH), contrary to general rule, that the rate of hydrolysis in the solid state increases with increasing humidity. Ager et al. [5] also studied the degrada- tion of aspirin in an aspirin–silica mixture. They concluded that aspirin is not stable at small aspirin:silica ratios, but it is stable at larger ratios. The adsorption of a molecule on a surface is the first step of every reaction that is catalyzed by the surface. The adsorption geometry is crucial in order to understand the details of such a reaction. Quantum chemical calculations are powerful tools to understand details of chemical reactions. Beside other information, they provide molecular geometries and energies in various confor- mations, helping to understand interface interactions better [8– 11]. Density functional theory (DFT) calculations have been widely used in modeling surface structures reactivity. Recently these methods have been successfully used to investigate interaction of carboxyl containing molecules with silica surface [12–15]. In this study we also apply DFT in the SIESTA [16] implementation to study the adsorption geometry and energy of aspirin adsorbed on a silica surface. This implementation uses the standard Kohn– Sham self-consistent density functional method in the local den- sity (LDA-LSD)- or the generalized gradient (GGA) approximation. As basis sets, very general and flexible linear combinations of numerical atomic orbitals (LCAO) are used. It allows for multiple- zeta, polarized and off-site orbitals so one can choose a suitable ba- sis set for the special interface study. In this study, we used a-quartz, which is the most stable crystalline phase of silica over a broad range of temperatures, pressures and ambient conditions, and its (001) surface to 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2009.06.004 * Corresponding author. Tel.: +49 3712352106; fax: +49 371531835794. E-mail addresses: afshin.abbasi@physik.tu-chemnitz.de (A. Abbasi), ebn@hrz. tu-chemnitz.de (E. Nadimi), plaenitz@matcalc.de (P. Plänitz), christian.radehaus@ zfm.tu-chemnitz.de (C. Radehaus). Surface Science 603 (2009) 2502–2506 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc