Recent Progress in Molecular Modeling of Adsorption and Hysteresis in Mesoporous Materials
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Recent Progress in Molecular Modeling of Adsorption and Hysteresis in Mesoporous Materials PETER A. MONSON Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA [email protected]
Abstract. We review some recent progress in molecular modeling of the behavior of fluids confined in mesoporous materials. We address three issues. The first is the applicability of the grand canonical ensemble for studying adsorption and hysteresis in porous materials. Next we discuss models of complex pore structure and how these can be coarse grained using a lattice model. Then we consider the question of how hysteresis is related to a vapor-liquid phase transition for disordered mesoporous materials. In concluding we assess the prospects of developing a unified description of adsorption and hysteresis valid over the range of mesoporous materials. Keywords: adsorption, hysteresis, mesoporous materials, molecular modeling, phase transitions
Introduction A major challenge in the use of adsorption for characterization of porous materials has been to understand the precise influence of porous material microstructure upon the properties of confined fluids. Porous materials frequently have a complex interconnected void space, yet traditional approaches that engineers use to relate the adsorption behavior to the porous material structure largely ignore this fact, and seek to describe the microstructure in terms of a distribution of independent pores. In principle statistical thermodynamics offers the solution to this problem, provided that a sufficiently realistic model of the microstructure can be developed. However, implementation of this approach has been hampered by the computational expense of making Monte Carlo simulations on complex models or applying theories like density functional theory when the density distribution in the system is fully threedimensional. Moreover the use of the grand canonical ensemble in modeling adsorption/desorption hysteresis has been subject to question because of uncertainty over the role of dynamics in these phenomena. This paper gives a short review of some recent progress in developing the statistical thermodynamic theory of adsorption and desorption in mesoporous materials. We first con-
sider the applicability of the grand canonical ensemble for studying adsorption and hysteresis in porous materials. We also discuss the development of models of complex pore structure and how coarse-graining of these models using a lattice model leads to a computationally efficient theoretical and simulation methods for studying these systems. We consider the question of how hysteresis is related to a vapor-liquid phase transition for disordered mesoporous materials and also the role of dynamics in hysteresis. In our concluding section we assess the prospects of developing a unified description of adsorption and hysteresis valid over the range of mesoporous materials.
Can We Use the Grand Canonical Ensemble to Study Adsorption and Hysteresis? As is very well known, the grand canonic
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