Adsorption-induced shape transitions in bistable nanopores with atomically thin walls (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.
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).
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.
Molecules, 2021
The finite pore volume Guggenheim–Anderson–de Boer (fpv-GAB) adsorption isotherm model has been considered as a simple tool which not only enables us to analyze the shape of isotherms theoretically, but also provides information about pore diameter. The proposed methodology is based on the geometrical considerations and the division of the adsorption space into two parts: the monolayer and the multilayer space. The ratio of the volumes of these two spaces is unambiguously related to the pore diameter. This ratio can be simply determined from the N2 adsorption isotherm by its fitting with the use of fpv-GAB model. The volume ratio is equal to the ratio of the adsorption capacities in the monolayer and the multilayer—two of the best-fit parameters. The suggested approach has been verified using a series of isotherms simulated inside ideal carbon nanotubes. The adsorption data for some real adsorbents has also been used during tests. The studies performed have proven that diameters est...
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]
Positional ordering of hard adsorbate particles in tubular nanopores
The phase behavior and structural properties of a monolayer of hard particles is examined in such a confinement where the adsorbed particles are constrained to the surface of a narrow hard cylindrical pore. The diameter of the pore is chosen such that only first-and second-neighbor interactions occur between the hard particles. The transfer operator method of [Percus and Zhang, Mol. Phys. 69, 347 (1990)] is reformulated to obtain information about the structure of the monolayer. We have found that a true phase transition is not possible in the examined range of pore diameters. The monolayer of hard spheres undergoes a structural change from fluidlike order to a zigzaglike solid one with increasing surface density. The case of hard cylinders is different in the sense that a layering takes place continuously between a low-density one-row and a high-density two-row monolayer. Our results reveal a clear discrepancy with classical density functional theories, which do not distinguish smecticlike ordering in bulk from that in narrow periodic pores.
Effect of Morphological Defects on Gas Adsorption in Nanoporous Silicas
Journal of Physical Chemistry C, 2007
This paper reports a molecular simulation study on the adsorption of a simple fluid in nanoporous silicas with or without morphological defects (constrictions). All of the pores considered in this work are of a finite length and are connected to a bulk reservoir so that they mimic real materials for which the confined fluid is always in contact with the external gas phase. The adsorption isotherms for the regular cylindrical pores conform to the typical experimental behavior for MCM-41 as the adsorbed amount increases continuously in the multilayer adsorption regime until a jump occurs due to capillary condensation of the fluid within the pore. The evaporation pressures are lower than the condensation pressures, so that hysteresis loops are observed. The condensation and evaporation mechanisms for the pores with constrictions depart significantly from what is observed for regular nondefective nanopores. Depending on the size, length, and number of constrictions in the pores, the filling and emptying processes are found to be of different nature. In every case, these mechanisms involve the coexistence between the confined liquid and some gas nanobubbles that are trapped within the main cavities of the pore. It is also found that the finite length of the pore introduces some heterogeneity in the adsorption and desorption processes; the filling and emptying of the regions (constrictions and cavities) near the pore surface differs from those of the regions in the pore center. The desorption process can occur through different mechanisms such as cavitation, pore blocking effects with at equilibrium evaporation, or several combined pore blocking effects. Adsorption isotherms can be used to assess and characterize morphological defects in nanopores. In contrast, our results suggest that microcalorimetry experiments such as measurements of the isosteric heat of adsorption cannot be used to gain information regarding such defects as all of the data for the regular and constricted pores fall on the same curve.
Low coverage adsorption in cylindrical pores
Surface Science, 1998
We present a theoretical exploration of the adsorption of rare gases in carbon nanotubes. In both the classical and the quantum cases, nanotube adsorption provides a nearly ideal realization of quasi-one-dimensional ( 1 D) matter. We have studied the adsorption potentials, the gas surface virial coefficient and the isosteric heat of adsorption. Comparison shows a much stronger binding of the adsorbate in the tubes than at the planar surface of graphite. As a consequence, one can easily adsorb sufficiently many atoms to be measurable in a thermodynamic or scattering experiment. In studying the low coverage adsorption we find great sensitivity to the species, the assumed potential model, and the radius of the tubes. The effect of interactions between the adsorbed particles is evaluated in the ID classical case. ~' 1998 Elsevier Science B.V.
Characterization of nanopores by standard enthalpy and entropy of adsorption of probe molecules
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004
Numerous theories have been developed for the determination of pore size distribution in adsorbent materials. The calculated pore size distribution is sensitive to the model selected for adsorption in slits, spheres, or cylinders and cannot be independently verified by experimental methods. Physical constants which are independent of theory can be determined from equilibrium thermodynamic experiments. Given an adsorbent material and a probe molecule, these constants are: (1) pore volume; (2) standard molar enthalpy of adsorption at the limit of zero pressure; (3) standard molar entropy of adsorption at finite pressure. The standard enthalpy and entropy changes are from the initial state of a perfect gas to the final state of a hypothetical adsorbed gas that obeys Henry's law. These standard thermodynamic properties characterize the interaction of a single probe molecule with the surface of the nanopores.