Effect of the reservoir size on gas adsorption in inhomogeneous porous media (original) (raw)
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Journal of Physics: Condensed Matter, 2002
We present a theoretical study of capillary condensation of fluids adsorbed in mesoporous disordered media. Combining mean-field density functional theory with a coarse-grained description in terms of a lattice-gas model allows us to investigate both the out-of-equilibrium (hysteresis) and the equilibrium behavior. We show that the main features of capillary condensation in disordered solids result from the appearance of a complex free-energy landscape with a large number of metastable states. We detail the numerical procedures for finding these states, and the presence or absence of transitions in the thermodynamic limit is determined by careful finite-size studies.
Physical Review E, 2009
Using a lattice-gas model in mean-field theory, we discuss the problem of how adsorption and desorption of fluids in independent cylinderlike pores is influenced by variations in the pore diameter along the length of the pore, surface roughness of the pore walls, and chemical heterogeneity. We also consider the impact of contact with the bulk phase via the pore opening and the possibility of interactions between neighboring pores via a liquid film on the external surface of the material. We find that a combination of pore size variation along the length of the pore and surface roughness yields sorption hysteresis similar to that found in systems with three-dimensional disordered pore networks such as porous glasses. Our results are especially relevant to adsorption and desorption in porous silicon materials with independent linear pores and apparently anomalous features of the behavior in these systems can be accounted for within the context of the present model.
Exploration of molecular dynamics during transient sorption of fluids in mesoporous materials
Nature, 2006
In recent years, considerable progress has been made in the development of novel porous materials with controlled architectures and pore sizes in the mesoporous range 1-4 . An important feature of these materials is the phenomenon of adsorption hysteresis: for certain ranges of applied pressure, the amount of a molecular species adsorbed by the mesoporous host is higher on desorption than on adsorption, indicating a failure of the system to equilibrate. Although this phenomenon has been known for over a century, the underlying internal dynamics responsible for the hysteresis remain poorly understood 5-9 . Here we present a combined experimental and theoretical study in which microscopic and macroscopic aspects of the relaxation dynamics associated with hysteresis are quantified by direct measurement and computer simulations of molecular models. Using nuclear magnetic resonance techniques 10-14 and Vycor porous glass 15,16 as a model mesoporous system, we have explored the relationship between molecular self-diffusion and global uptake dynamics. For states outside the hysteresis region, the relaxation process is found to be essentially diffusive in character; within the hysteresis region, the dynamics slow down dramatically and, at long times, are dominated by activated rearrangement of the adsorbate density within the host material.
Molecular simulation of phase coexistence in adsorption in porous solids
Molecular Physics, 2002
In this work a recently proposed method, the gauge-cell Gibbs ensemble Monte Carlo, is extended to deal with polar substances. The behaviour of water, a hydrogen bonding, weakly adsorbing¯uid, is compared with that of methane, a strongly adsorbing, non-polar¯uid, in the vicinity of the phase transition. The mechanisms of condensation for the two species are seen to be signi®cantly di erent in nature. A systematic study of the e ect of the pore width on the phase equilibrium of water is also performed. Our results show that the narrowing of the pore shifts the equilibrium transition pressure to lower values and reduces the extent of vapour metastability, but exerts little in¯uence on the stability of the liquid phase.
Physical Chemistry Chemical Physics, 2001
We study the relation between out-of-equilibrium (hysteretic) and equilibrium behavior in the capillary condensation of Ñuids in disordered mesoporous solids. Using mean-Ðeld density functional theory, we show that a simple lattice-gas model can reproduce the major experimental observations and that the classical van der Waals picture of metastability fails due the appearance of many metastable states. We Ðnd that (i) a true equilibrium capillary phase transition may occur when the perturbation induced by the solid is sufficiently small ; (ii) hysteresis does not necessarily imply the existence of this phase transition ; (iii) the disappearance of the hysteresis loop is not associated with capillary criticality ; and (iv) thermodynamic consistency is violated along the adsorptionÈdesorption isotherms.
In this work, nitrogen adsorption and desorption onto solid surfaces were studied using computer simulations of the three-dimensional Ising model, for systems with complex porous structures at the mesoscopic and microscopic levels. A hysteresis cycle between the adsorption and desorption processes appears and it was found that its characteristics are dependent on the geometry of the pore and on the strength of the surface–fluid interaction. We also obtained an average adsorption isotherm, which represents a combination of differently shaped pores, and shows robust jumps at certain values of the chemical potential as a consequence of the shape of the pores. A comparison of the results of this study with experimental data is also made. In addition, we report the filling of microscopic pores connected with mesopores. It is argued that these predictions are useful for researchers working on the enhanced recovery of oil and for the design of new nanomaterials, among others.
The Journal of Physical Chemistry B, 1998
A two-dimensional (2D) model for a simple fluid adsorbed in a disordered porous medium is investigated by means of the replica Ornstein-Zernike (ROZ) equations with the hypernetted chain (HNC) and Percus-Yevick (PY) approximations. In addition, Monte Carlo simulations for this model are presented. The structural properties of adsorbed monolayer resulting from the theory agree well with the computer simulation data. Also we obtain the fluid compressibility from the ROZ equations and the adsorption isotherms. Besides the hard core repulsion, a long-range interaction between the fluid species in the form of an attractive square well is included. The contribution of this long-range term is considered using a mean-field approximation. This permits us to analyze the qualitative dependence of the behavior of the critical density and temperature on the matrix parameters. We find that the critical temperature of the hard discs with a square well attraction decreases with an increasing density of the quenched component. The critical density is less sensitive to these changes.
On the universal behavior of sorption isotherms in disordered mesoporous solids
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004
Adsorption-desorption isotherms in disordered mesoporous solids, described by the Dual Site-Bond Model, are obtained through Monte Carlo simulations and their behavior is correlated to the topological properties of the porous networks and to their percolation properties, extending previous results to the general case of variable connectivity networks. A quasi-universal curve is found which may be useful in the problem of obtaining pore size distributions from the analysis of experimental Adsorption-desorption isotherms.
Influence of reservoir size on the adsorption path in an ideal pore
The Journal of Chemical Physics, 2009
We consider the influence of the relative size of the gas reservoir on the states visited by a simple fluid adsorbed in a nanopore of ideal geometry (a slit). We focus on the intermediate states that appear in between the main hysteresis branches comprising gaslike and liquidlike states and we study the adsorption and desorption paths actually followed by the system as one changes the reservoir size. We find that these paths may display discontinuous sections associated with transitions between different nonuniform states. We also discuss the stability of the states in such situations.
Langmuir : the ACS journal of surfaces and colloids, 2017
Using molecular simulations, we investigate the relationship between the pore-averaged and position-dependent self-diffusivity of a fluid adsorbed in a strongly attractive pore as a function of loading. Previous work (Krekelberg, W. P.; Siderius, D. W.; Shen, V. K.; Truskett, T. M.; Errington, J. R. Connection between thermodynamics and dynamics of simple fluids in highly attractive pores. Langmuir 2013, 29, 14527-14535, doi: 10.1021/la4037327) established that pore-averaged self-diffusivity in the multilayer adsorption regime, where the fluid exhibits a dense film at the pore surface and a lower density interior pore region, is nearly constant as a function of loading. Here we show that this puzzling behavior can be understood in terms of how loading affects the fraction of particles that reside in the film and interior pore regions as well as their distinct dynamics. Specifically, the insensitivity of pore-averaged diffusivity to loading arises from the approximate cancellation of...