DOI: 10.1002/adma.200801489 Realistic Models of Hydroxylated Amorphous Silica Surfaces and MCM-41 Mesoporous Material Simulated by Large-scale Periodic B3LYP Calculations** By P. Ugliengo, * M. Sodupe, * F. Musso, I. J. Bush, R. Orlando, and R. Dovesi Amorphous silica is a key inorganic material, with applications in many fields such as cromatography, microelec- tronics, and metal-supported catalysis. [1,2] Moreover, amor- phous silica is the basic constituent of mesoporous ordered materials such as the molecular sieve MCM-41. [3,4] These materials present high surface areas and high pore volumes, rendering them very attractive supports for adsorption or immobilization of biologically relevant molecules in confined spaces. [5] Indeed, due to their regular pore distribution, they are excellent candidates for controlled-drug-delivery sys- tems, [6] which was evidenced for the first time by the confinement of ibuprofen in MCM-41 matrices. [7] One important factor that determines the adsorption and subse- quent release of a drug is its interaction with the pore wall, which in this case contains large amounts of silanol groups. Thus, it is crucial to possess detailed structural information at a molecular level of the typology and distribution of the silanol groups at the wall interface. Unfortunately, due to the amorphous nature of the walls, long-range order among SiO 4 tetrahedra is lost, which means that no experimental structural data are available, and only average features are known, such as the radial distribution function and the density from X-ray diffraction, and the ring nuclearity distribution from NMR experiments. Structural information at the atomic level can be provided by modeling techniques based on molecular mechanics and dynamics, or via the more accurate ab initio methods. However, despite the huge progress in defining force fields accurately to simulate glass bulk silica, [8–10] almost no progress has been reported on the simulation of silica surfaces. The main reason for this is that terminal silanol groups (Si–OH) are deeply involved in mutual hydrogen bond interactions, which are extremely difficult to model accurately by means of classical simulations. Some progress in that direction has recently been provided by the new FFSiOH force field, developed by some of us. [11] The use of ab initio methods would be preferable, but because of both the noncrystalline nature of amorphous silica and the variety of surface sites (vicinal, geminal, or isolated, see ref. [1] for details), one needs to adopt large unit cells to model the material realistically, rendering the method very computationally demand- ing. For this reason, only few models for hydroxylated surfaces in amorphous silica materials have been proposed. [12,13] The use of crystalline edingtonite [14,15] to model surface properties of amorphous silica has been reported, and described the vibrational features of the different surface sites properly. [16] However, it is important to consider noncrystalline structures to improve the energetics of the adsorption processes, since distortion of the material is expected to be less costly for amorphous materials, especially when large biomolecules are adsorbed. Here, we report for the first time realistic models for amorphous silica surfaces. We have considered different degrees of hydroxylation, because the surface silanol content (usually expressed as the density r, i.e., the number of OH group per nm 2 ) determines the hydrophilic/hydrophobic character of the material and, thus, the adsorptive features of the surface. In particular, five surfaces with different OH densities have been simulated, namely 7.2, 5.4, 4.5, 2.4, and 1.5 OH nm 2 . We also report a realistic model for a completely hydroxylated MCM-41 material. Previous work on the simulation of MCM-41 [17,18] was limited to molecular mechanics techniques, and no explicit silanol termination was considered. Here, all the proposed models envisage silanol-terminated surfaces, and are simulated using a full ab COMMUNICATION [*] Prof. P. Ugliengo, Prof. R. Dovesi Dipartimento di Chimica IFM NIS Centre of Excellence and INSTM University of Torino Via P. Giuria, 7. Torino, I-10125 (Italy) E-mail: piero.ugliengo@unito.it Prof. M. Sodupe, Dr. F. Musso Departament de Quı ´mica, Universitat Auto`noma de Barcelona Bellaterra, Barcelona 08193 (Spain) E-mail: mariona@klingon.uab.es Dr. I. J. Bush Science and Technology Facilities Council Daresbury Laboratory – Daresbury Science and Innovation Campus Warrington, Cheshire WA4 4AD (UK) Prof. R. Orlando Dipartimento di Scienze e Tecnologie Avanzate University of Piemonte Orientale ‘‘Amedeo Avogadro’’ Via Bellini, 25G. Alessandria, I-15100 (Italy) [**] Financial support from MCYT and DURSI, through projects CTQ2005-08797-C02-02/BQU and SGR2005-00244, and allowance of computer resources from the BSC Mare Nostrum Supercomputing Centre are gratefully acknowledged. F. M. gratefully acknowledges the Ministerio de Educacio ´n y Ciencia for a FPU scholarship. The authors are grateful to A. Pedone (Univ. Modena and Reggio Emilia) for providing the starting bulk structures, to V. Bolis and C. Busco (University of Piemonte Orientale) for discussions, to G. Magnacca (Univ. Torino) for kindly providing the A300 IR spectrum, and to B. Onida (Politecnico Torino) for kindly providing the MTS IR spectrum and for discussions. Supporting Information is available online from Wiley InterScience or from the author. Adv. Mater. 2008, 20, 4579–4583 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4579