A Grand Canonical Monte-Carlo Study of Argon Adsorption/Condensation in Mesoporous Silica Glasses: Application to the Ch
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A Grand Canonical Monte-Carlo Study of Argon Adsorption/Condensation in Mesoporous Silica Glasses: Application to the Characterization of Porous Materials R.J.-M. Pellenq*, S. Rodts, P. E. Levitz Centre de Recherche sur la Matière Divisée, CNRS et Université d'Orléans,1b rue de la Férollerie, 45071 Orléans, cedex 02, France. email: [email protected] ABSTRACT We have studied adsorption of argon in a mesoporous silica Controlled Porous Glass (CPG) by means of Grand Canonical Monte-Carlo (GCMC) simulation. A numerical sample of the CPG adsorbent has been obtained by using an off-lattice reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials. The off-lattice functional of (100 m2/g)-Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with an homothetic reduction of factor 2.5 so to obtain 30Å-CPG sample. A realistic surface chemistry is then obtained by saturating all oxygen dangling bonds with hydrogen. The Ar, Kr and Xe adsorption/desorption isotherms are calculated at different temperatures. At sufficiently low temperature, they exhibit a capillary condensation transition with a finite slope by contrast to that theoretically predicted for simple pore geometries such as slits and cylinders. In the low pressure and temperature domain, we have identified different adsorption scenarios, which can be interpreted on the basis of a Zisman-type of criterium for wetting. We demonstrate that the BET surface area is strongly related to this criterion. At higher pressure, we demonstrate that the pore size distribution obtained by using the standard BJH analysis applied to both adsorption and desorption data qualitatively reproduces the main features of the chord length distribution.
INTRODUCTION The vast majority of mesoporous materials are disordered structures. Silica controlled porous glasses constitute a large class of materials among which is Vycor, a porous silica glass widely used as a model structure to study the properties of confined fluids. In fact, there are two kinds of vycor (it is usually referred to as Vycor-7930 by the manufacturer Corning Inc.): a first type with a specific surface area around 100 m2/g and a second at around 200 m2/g (see Figure 1). The pores in the low-specific-surface-vycor have an average radius of about 35 Å (assuming a cylindrical geometry) [1]. Conversely, for the high-specific-surface material, the mean pore radius is around 22 Å [2]. It is known from theoretical and simulation studies on simple pore geometries (slits and cylinders) that confinement strongly influences the thermodynamics of confined fluid [3]. The effect of the matrix disorder in terms of pore morphology –the pore shape- and topology –the way the pores distribute and connect in space- on the thermodynamics of confined molecular fluid still remain to be clarified. This first raises the challenge of describing the morphology and the topology of these porous solids [4]. The aim of this work i
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