Positional ordering of hard adsorbate particles in tubular nanopores (original) (raw)
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Adsorption‐Induced Structural Phase Transformation in Nanopores
Angewandte Chemie International Edition, 2017
We report a new type of structural transformation occurring in methane adsorbed in micropores. The observed methane structures are defined by probability distributions of molecular positions. The mechanism of the transformation has been modeled using Monte Carlo method. The transformation is totally determined by a reconstruction of the probability distribution functions of adsorbed molecules. The methane molecules have some freedom to move in the pore but most of the time they are confined to the positions around the high probability adsorption sites. The observed high‐probability structures evolve as a function of temperature and pressure. The transformation is strongly discontinuous at low temperature and becomes continuous at high temperature. The mechanism of the transformation is influenced by a competition between different components of the interaction and the thermal energy. The methane structure represents a new state of matter, intermediate between solid and liquid.
Quasi-One-Dimensional Phase Transitions in Nanopores: Pore-Pore Correlation Effects
Physical Review Letters, 1997
For adsorbates confined within a single, sufficiently narrow cylindrical pore, no phase transitions occur because the system is too close to the one-dimensional limit. We study the influence of intermolecular correlations between adsorbed molecules in neighboring cylindrical pores, using molecular simulation. For a simple model of methane in the molecular sieve ALPO 4-5, we find that a phase transition between two fluid states ("gas" and "liquid") occurs below a critical temperature that is depressed relative to the bulk value. [S0031-9007(97)04210-5]
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.
Adsorption-induced shape transitions in bistable nanopores with atomically thin walls
Physical Review E
Atomically thin cylindrical nanopores can change shape in response to physically adsorbed gas inside. Coupled to a gas reservoir, an initially collapsed pore can expand to allow the adsorbed gas to form concentric shells on the inner part of the pore, driven by adsorption energetics, not gas pressure. A lattice gas model describes the evolution of the pore/nanotube shape and absorbed gas as a function of gas chemical potential at zero temperature. We found that narrow-enough tubes are always expanded and gas inside adsorbs in sequences of concentric shells as the gas chemical potential increases. Wider tubes, which are collapsed without gas, can expand with one or more concentric shells adsorbed on the inner surface of the expanded region.
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).
Orientational ordering and layering of hard plates in narrow slitlike pores
Physical Review E, 2019
We examine the ordering behavior of hard plate-like particle in a very narrow slit-like pore using the Parsons-Lee density functional theory and the restricted orientation approximation. We observe that the plates are orientationally ordered and align perpendicularly (face-on) to the walls at low densities, a first order layering transition occurs between uniaxial nematic structures having n and n+1 layers at intermediate densities and even a phase transition between a monolayer with parallel (edge-on) orientational order and n layers with perpendicular one can be detected at high densities. In addition to this, the edge-on monolayer is usually biaxial nematic and a uniaxial-biaxial nematic phase transition can be also seen at very high densities.
Phase equilibria of water in cylindrical nanopores
Physical Chemistry Chemical Physics, 2001
Phase equilibria of water in cylindrical nanopores were simulated in the Gibbs ensemble. The decrease of the critical temperature in the conÐnement compared to the bulk value attains 35% in the pores with radius
The structure of frozen phases in slit nanopores: A grand canonical Monte Carlo study
The Journal of Chemical Physics, 2002
Freezing of soft spheres in slit nanopores is investigated using Grand canonical Monte Carlo simulations. The pores are in equilibrium with a liquid located close to the liquid-solid coexistence region in the bulk Lennard-Jones phase diagram. In addition to layering, the confined fluid is found to possess in-plane order, leading to the formation of frozen phases which give rise to a sequence of solid-solid transformations as the pore width is varied. Transformations between n layered triangular to nϩ1 layered square lattices and between n layered square to triangular lattices, are observed for nϭ1, 2, 3, and 4. The transition from triangular to square lattices occurs via an intermediate buckled phase which is characterized by increased out-of-plane motion, while maintaining in-plane triangular order. Buckling was found to decrease with increasing number of layers. The transition between square to triangular lattices at a fixed number of layers is accompanied by a lowering of the solvation force, resulting in a doublet in the solvation force maxima. Influence of fluid-wall interactions on the nature of the frozen phases are studied by comparing the structures formed with a 10-4-3 and 10-4 fluid-wall potential. The solid structures are classified based on their closest 3D counterparts.
Adsorption of flexible oligomers into narrow cylindrical pores has been studied by means of several versions of classical density functional theory (CDFT) and Monte Carlo simulation. The adsorption process is interesting to study due to the competition between the entropic depletion of oligomers from the pores and the wall-oligomer attraction. It is also challenging to describe using current CDFTs, which tend to overestimate the amount of the adsorbed fluid. From a comparison of several different CDFT approaches, we find that this is due to the assumption of ideal or freely jointed chain conformations. Moreover, it is demonstrated that it is impossible to obtain a reasonable description of the adsorption isotherms without taking into account accurate contact values in the distribution functions describing the structure of the reference monomer fluid. At low densities, more accurate result are obtained in comparison with Monte Carlo simulation data when accurate contact values are incorporated into the theory rather than the more commonly used hard-sphere contact value. However, even the CDFT with accurate contact values still overestimates the amount of the adsorbed fluid due to the ideal or freely jointed chain approximation, used for the description of chain conformations in most CDFT approaches. We find that significant improvement can achieved by employing selfconsistent field theory, which samples self-avoiding chain conformations and decreases the number of possible chain conformations, and, consequently, the amount of the adsorbed fluid. © 2013 AIP Publishing LLC. [http://dx.
Adsorption and Phase Transitions in Slit-like Pores with Differently Adsorbing Walls
Journal of Colloid and Interface Science, 2001
We study the adsorption of a lattice gas in a slit-like pore with different walls. The density profiles are evaluated in the framework of the mean-field approximation. We show that changes in the potential field exerted by one wall can lead to substantial modifications of the phase behavior of the fluids.