Theory of sorption hysteresis in nanoporous solids: Part II Molecular condensation (original) (raw)
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A Poromechanical Model for Sorption Hysteresis in Nanoporous Polymers
Sorption hysteresis in nanoporous polymer is an intriguing phenomenon that involves coupling between sorption and deformation. Based on the mechanism revealed at the microscopic level using molecular simulation, a poromechanical model is developed capturing all relevant physics and yielding a quantitative description. In this model, the coupling between sorption and deformation is described by a poromechanics framework. More in detail, an upscaling process from the molecular mechanism is implemented to model the hysteresis through the state change of each element upon deformation. We provide two solutions of the model, a numerical one based on the finite element method and an analytical one based on uniform strain assumption. The results from both solutions agree well with the molecular simulation and experimental results, therefore capturing and describing adequately sorption hysteresis. The developed model illustrates that water forms different structural distributions upon adsorption and desorption. A parametric study shows that sorption hysteresis is influenced by material properties. We find that a softer material with stronger adsorbent-adsorbate interaction tends to exhibit more profound sorption hysteresis. The developed model, which relies on the concepts of sorption-deformation coupling and multi-scale modeling from atomistic simulations to domain dependent theory, paves the way for a new direction of modeling sorption hysteresis.
Langmuir, 1995
Capillary hysteresis in cylindrical nanopores has been studied using MCM-41 as the prime example of a mesoporous material. These materials, due to their regular pore structure, can be considered to be candidates for reference adsorbents for standardizing adsorption measurements and methods for characterization of porous solids. They provide a unique opportunity for verification of theoretical models employed for predicting phase equilibrium in confined geometry. Three samples with monodisperse pore channels have been synthesized and examined using X-ray diffraction (XRD). Nitrogen adsorption isotherms were modeled using nonlocal density functional theory (NLDFT) in a wide range of pore sizes (18-80 Á). Theoretical isotherms for pore channels with sizes corresponding to those identified by XRD were compared with experimental isotherms at different temperatures between 70 and 82 K. The latter have been measured independently on two different adsorption setups. The theoretical thermal dependence of the thermodynamic adsorption-desorption hysteresis predicted by NLDFT is confirmed by the experimental measurements. It is shown that at 77.4 K NLDFT quantitatively predicts equilibrium phase transitions in cylindrical channels of MCM-41. Theoretical and experimental results prove that the nitrogen hysteresis observed at temperatures below 77.4 K is associated with metastability of the adsorption branch of the isotherm. The absence of experimental hysteresis on samples with pore size of about 40 Á at temperatures above 77.4 K cannot be explained by the capillary critical temperature for a given pore size being achieved as was assumed previously.
Capillary-condensation hysteresis in naturally-occurring nanoporous media
A B S T R A C T Persistent uncertainties in understanding fluid phase behavior in natural nanoporous media, including shale rock, remain a significant challenge to fully utilizing tight geological formations as both globally significant sources of hydrocarbon fuels and repositories for greenhouse gas sequestration. By measuring isotherms of nbutane and n-pentane in kerogen-rich shale cores at temperatures from 4.9 to 65.6°C, we show that shale nanopores can induce a phase transition known as capillary condensation upon adsorption or capillary evaporation upon desorption. For both adsorbates, capillary condensation and capillary evaporation took different paths, thus forming hysteresis loops that increased in size with increasing temperature. While isotherms of nbutane were expectedly reproducible, surprisingly those for n-pentane were not. This was due to irreversible kerogen swelling induced by the n-pentane. To further investigate this phenomenon, we measured scanning isotherms of n-pentane at 4.9 and 65.6°C. Similar to the primary hysteresis loops, successive scanning measurements during adsorption resulted in different isotherm shapes, while those for desorption remained consistent. This implies differences in the physics governing adsorption and desorption, which may rely on the pore structure and fluid elasticity, respectively. These results comprise the first observations of hysteresis loop broadening at high temperatures, irreproducible hysteresis, and scanning isotherms during capillary condensation measurements in a natural nanoporous medium. By viewing these results in the context of the current hypotheses on capillary condensation derived from previous studies using synthetic nanopores, we conclude that https://doi.
The Journal of chemical physics, 2014
Hysteresis and discontinuities in the isotherms of a fluid adsorbed in a nanopore in general hamper the determination of equilibrium thermodynamic properties, even in computer simulations. A way around this has been to consider both a reservoir of small size and a pore of small extent in order to restrict the fluctuations of density and approach a classical van der Waals loop. We assess this suggestion by thoroughly studying through Monte Carlo simulations and density functional theory the influence of system size on the equilibrium configurations of the adsorbed fluid and on the resulting isotherms. We stress the importance of pore-symmetry-breaking states that even for modest pore sizes lead to discontinuous isotherms and we discuss the physical relevance of these states and the methodological consequences for computing thermodynamic quantities.
Molecular simulation of adsorption and intrusion in nanopores
Adsorption, 2008
This paper reports Monte Carlo simulations of the adsorption or intrusion in cylindrical silica nanopores. All the pores are opened at both ends towards an external bulk reservoir, so that they mimic real materials for which the confined fluid is always in contact with the external phase. This realistic model allows us to discuss the nature of the filling and emptying mechanisms. The adsorption corresponds to the metastable nucleation of the liquid phase, starting from a partially filled pore (a molecular thick film adsorbed at the pore surface). On the other hand, the desorption occurs through the displacement at equilibrium of a gas/liquid hemispherical interface (concave meniscus) along the pore axis. The intrusion of the nonwetting fluid proceeds through the invasion in the pore of the liquid/gas interface (convex meniscus), while the extrusion consists of the nucleation of the gas phase within the pore. In the case of adsorption, our simulation data are used to discuss the validity of the modified Kelvin equation (which is corrected for both the film adsorbed at the pore surface and the curvature effect on the gas/liquid surface tension).
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 Sorption-Induced Deformation in Atomistic Nanoporous Materials
Langmuir, 2019
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Characterization of nanoporous materials from adsorption and desorption isotherms
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001
We present a consistent method for calculation of pore size distributions in nanoporous materials from adsorption and desorption isotherms, which form the hysteresis loop H1 by the IUPAC classification. The method is based on the nonlocal density functional theory (NLDFT) of capillary condensation hysteresis in cylindrical pores. It is implemented for the nitrogen and argon sorption at their boiling temperatures. Using examples of MCM-41 type and SBA-15 siliceous materials, it is shown that the method gives the consonant pore size distributions calculated independently from the adsorption and desorption branches of the sorption isotherm. The pore size distributions, pore volumes and specific surface areas calculated from nitrogen and argon data are consistent. In the case of SBA-15 materials, the method evaluates also the amount of microporosity. The results of the NLDFT method are in agreement with independent estimates of pore sizes in regular nanoporous materials.
Adsorption Hysteresis in Porous Solids
Journal of Colloid and Interface Science, 1998
Hysteresis has been observed in adsorption isotherms for a number of gas-solid systems and, generally, is attributed to adsorption in mesoporous materials with capillary condensation. This behavior is classified as Type IV or Type V in the IUPAC classification scheme. Here, lattice theory is used to predict adsorption behavior in pores. The Ono-Kondo theory is used with appropriate boundary conditions for fluid adsorption in infinite and semi-finite slit-like pores. It is shown that there can be phase transitions in the adsorbed phase which lead to hysteresis in kinetically controlled experiments. However, hysteresis in equilibrium behavior is exhibited only in pores of finite length. For finite-length pores, the interface geometry is predicted to be different during the processes of adsorption and desorption and this difference in interface shape leads to hysteresis. This simple molecular model is able to predict the change in the interface geometry without invoking the Kelvin equation or the macroscopic concept of surface tension.
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.