Molecular simulation of the adsorption and structure of benzene confined in mesoporous silicas
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Molecular simulation of the adsorption and structure of benzene confined in mesoporous silicas Benoit Coasne · Christiane Alba-Simionesco · Fabrice Audonnet · Gilberte Dosseh · Keith E. Gubbins
Received: 30 April 2007 / Revised: 4 September 2007 / Accepted: 11 September 2007 / Published online: 6 October 2007 © Springer Science+Business Media, LLC 2007
Abstract Grand Canonical Monte Carlo simulations are used to study the adsorption of benzene at 298 K in an atomistic cylindrical silica nanopore of a diameter 3.6 nm. The adsorption involves a transition from a partially filled pore (a two layers thick film at the pore surface) to a completely filled pore configuration. Strong layering of the benzene molecules at the pore surface is observed. It is found that the layering decays as the distance to the pore surface increases. The position of the peaks for the density of the C, H atoms and the center of mass of the molecules shows that benzene molecules prefer an orientation in which their ring is perpendicular to the pore surface. This result is corroborated by calculating orientational order parameters and examining the distribution of the distances between the H and C atoms of the benzene molecules and the H and O atoms of the silica substrate. Keywords Benzene · Capillary condensation · Monte Carlo simulation · Nanoporous material
B. Coasne () Institut Charles Gerhardt Montpellier, CNRS (UMR 5253) and Université Montpellier 2, Montpellier, France e-mail: [email protected] C. Alba-Simionesco · F. Audonnet · G. Dosseh Laboratoire de Chimie Physique, CNRS (UMR 8000) and Université Orsay, Orsay, France K.E. Gubbins Center for High Performance Simulation and Department of Chemical and Biomolecular Engineering, North Carolina Sate University, Raleigh, NC 27695-7905, USA
1 Introduction The behavior of fluids confined within nanometric pores (size of a few molecular diameters) significantly differs from that of the bulk. In particular, the effect of confinement, surface forces, and reduced dimension affects the phase transitions (condensation, freezing, etc.) (for reviews, see Gelb et al. 1999 and Alba-Simionesco et al. 2006). Significant shifts in transitions (e.g. pressure, temperature) are observed and, in some cases, new types of phase transitions (layering, wetting, etc.) can also be found for these inhomogeneous systems. Understanding such confinement and surface effects on the thermodynamics of fluids is of crucial interest for both fundamental research and potential applications. Among nanoporous solids, the siliceous MCM-41 and SBA15 are important materials because of their possible uses as adsorbents or catalytic supports for gas adsorption, phase separation, catalysis, preparation of nano-structured materials, drug delivery, etc. (Soler-Illia et al. 2002). The pore diameter distribution is narrow with an average value that can be varied from 2 up to 20 nm, depending on the synthesis conditions. From a fundamental point of view, MCM-41 and SBA-15 are considered as model materials to investigate the
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