Micropores, fluid behavior

M. Schoen, D. J. Diestler, J. H. Cushman. Fluids in micropores. IV. The behavior of molecularly thin confined films in the grand isostress ensemble. J Chem Phys 700 7707-7717, 1994. 

Some current research areas using the methods described in the previous section are shown in Table 2. Many of these involve molecular simulations that exploit the limit of speed and storage of currently available supercomputers [6]. In this section I shall consider two examples from among these (1) the determination of phase equilibria by computer simulation and (2) the behavior of fluids in microporous materials. 

Gas flow processes through microporous materials are important to many industrial applications involving membrane gas separations. Permeability measurements through mesoporous media have been published exhibiting a maximum at some relative pressure, a fact that has been attributed to the occurrence of capillary condensation and the menisci formed at the gas-liquid interface [1,2]. Although, similar results, implying a transition in the adsorbed phase, have been reported for microporous media [3] and several theoretical studies [4-6] have been carried out, a comprehensive explanation of the static and dynamic behavior of fluids in micropores is yet to be given, especially when supercritical conditions are considered. Supercritical fluids attract, nowadays, both industrial and scientific interest, due to their unique thermodynamic properties at the vicinity of the critical point. For example supercritical CO2 is widely used in industry as an extraction solvent as well as for chemical... 

Molecular dynamics has been successful in revealing preferred adsorption sites within microporous materials diffusion paths in microporous materials and on single crystals, e.g., Refs. calculation of sticking coefficients of small adsorbates, e.g., Ref etc. It is in the calculation of dynamics though, such as sticking coefficients and diffusion paths, where the real merit of MD lies. It works well when there is no large activation barrier compared to the thermal energy, i.e., a fluid-like behavior. The small reachable scales have limited direct comparison of MD simulation results with data from most-far-from-equilibrium systems. 

In recent years, the behavior of fluid molecules in small pores has been well studied by computer simulation [136,137]. Computer simulation can provide us with valuable information about the microscopic behavior of adsorbate molecules confined in small pores in terms of intermolecular and surface forces, thus enabling us to understand the fundamental behaviors of adsorbates in the potential field of the model pores. The current trend in the literature suggests that for physical adsorption in activated carbon, the adsorbate-adsorbate interaction and adsorbate-pore interaction are well represented by the LJ potential theory while the model micropore is a slit-shaped channel of infinite extent. This forms the basis for the appliction of the statistical method in adsorption processes. 

Analysis of low-temperature thermal events such as freezing and supercooling is important for understanding the behavior of water in microporous materials, gels, biological tissues, foods, and other microstructured fluids at subzero temperatures [8],... 

It is worth noticing that QSNLDFT provides a unified model of adsorption on both smooth and heterogeneous surfaces. QSNLDFT describes the behavior of LJ fluid near a smooth hard wall by employing a uniform solid density distribution with a packing density approaching unity. The behavior of LJ fluid near a smooth attractive wall is approximated by using an appropriate division of the solid-fluid potential into repulsive and attractive parts. Below we consider several prominent examples, which demonstrate that QSNLDFT is capable of describing experimental adsorption isotherms on amorphous and microporous solids. 

Let us illustrate the above with a specific example. Although this example is strongly application driven, the conclusions are equally valid for more fundamental problems in simulations of complex fluids. There is considerable interest in studying the adsorption of alkanes in the pores of a zeolite. Zeolites are microporous materials which are used as catalysts in petrochemical applications (see Zeolites Applications of Computational Methods). A prerequisite for an understanding of the catalytic activity of these zeolites is a knowledge of the behavior of the molecules adsorbed in their narrow pores. Since this type of information is extremely difficult to obtain experimentally, simulations appear to be an attractive alternative. Indeed, over the last decade many simulation studies on the behavior of molecules in zeolites have been published (for a review see Ref. 3). A more careful look at these studies reveals that most simulations concern the adsorption of noble gases or methane, only a few studies of ethane or propane have been published. In petrochemical applications of zeolites, however, we are interested in the behavior of much longer alkanes such as octane and decane. 

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