Molecular transport in nanopores: a theoretical perspective

Bhatia, Suresh K.; Bonilla, Mauricio R.; Nicholson, D.

doi:10.1039/c1cp21166h

Cited by 154 publications

(161 citation statements)

References 192 publications

(422 reference statements)

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“…It was investigated theoretically employing various methods [10][11][12][13][14][15][16][17][18]. In continuum models [11,12,17], the molecular translocation through the channel is viewed as a one-dimensional diffusion in the effective potential created by a complex network of intermolecular and molecular/pore interactions.…”

Section: Introductionmentioning

confidence: 99%

Theoretical analysis of selectivity mechanisms in molecular transport through channels and nanopores

Agah

Pasquali

Kolomeisky

2015

The Journal of Chemical Physics

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Selectivity is one of the most fundamental concepts in natural sciences, and it is also critically important in various technological, industrial and medical applications. Although there are many experimental methods that allow to separate molecules, frequently they are expensive and not efficient. Recently, a new method of separation of chemical mixtures based on utilization of channels and nanopores has been proposed and successfully tested in several systems. However, mechanisms of selectivity in the molecular transport during the translocation are still not well understood. Here we develop a simple theoretical approach to explain the origin of selectivity in molecular fluxes through channels. Our method utilizes discrete-state stochastic models that take into account all relevant chemical transitions and can be solved analytically. More specifically, we analyze channels with one and two binding sites employed for separating mixtures of two types of molecules. The effects of the symmetry and the strength of the molecular-pore interactions are examined. It is found that for one-site binding channels the differences in the strength of interactions for two species drive the separation. At the same time, in more realistic two-site systems the symmetry of interaction potential becomes also important. The most efficient separation is predicted when the specific binding site is located near the entrance to the nanopore. In addition, the selectivity is higher for large entrance rates into the the channel. It is also found that the molecular transport is more selective for repulsive interactions than for attractive interactions. The physical-chemical origin of the observed phenomena are discussed.

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Section: Introductionmentioning

confidence: 99%

Theoretical analysis of selectivity mechanisms in molecular transport through channels and nanopores

Agah

Pasquali

Kolomeisky

2015

The Journal of Chemical Physics

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“…Nevertheless, adsorption of gases in mesoporous materials deviates from the Knudsen model due to the highly complex disordered structure of solid network [106], which do not meet requirement of the Knudsen model [104].…”

Section: Classical Theories Of Transport Phenomenamentioning

confidence: 99%

“…Here, the Maxwell-Stefan (MS) description of mass transfer appears to provide the most promising approach [104,107]. Chapman and Enskog [108], as well as Hirschfelder, Curtiss and Bird [109] have derived the generalized Maxwell-Stefan diffusion equations [104].…”

Section: Classical Theories Of Transport Phenomenamentioning

confidence: 99%

“…The Knudsen model [20] was originally developed by Martin Knudsen for low density non-interacting hard sphere gases diffusing in a tube surface. At such low densities, the mean free path of the diffusing particle λ is assumed to be large in comparison with the tube diameter d (Knudsen limit = ≫ 1), thus the frequency of molecular collision would be negligible compared to the wall-molecule contacts [104]. Consequently, wall-molecule collisions, as well as the pressure gradient are the only driving forces of molecular diffusion in this model [104].ThecorrectedversionofKnudsen'stheoryfollows[105]…”

Section: Classical Theories Of Transport Phenomenamentioning

confidence: 99%

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Structural modelling of silicon carbide-derived microporous carbon and its application in CO2 capture and separation of volatile gases from moist streams

Farmahini¹

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The adsorption of volatile gases in microporous materials has wide applications in gas separation, including CO 2 capture and natural gas dehydration. It has also attracted a large amount of attention for energy storage applications such as electrochemical energy storage in supercapacitors and adsorptive methane/hydrogen storage. Nevertheless, the development of efficient gas storage/separation technologies requires fundamental knowledge of fluid transport in narrow pore spaces, since the behavior of fluids under tight confinement differs markedly from that in the bulk.Along with the rapid development of a broad range of new microporous materials such as carbide derived carbons (CDCs), carbon nanotubes and metal-organic frameworks, there has been wide growth in their potential applications, especially in emerging nanotechnologies, thus the development of this understanding has become even more crucial.This study contributes to such understanding by investigating adsorption and transport of industrially important gases in the microporous structure of silicon carbide-derived carbon (SiC-DC). Initially, a realistic model of silicon carbide-derived carbon is developed using the hybrid reverse Monte Carlo (HRMC) method, which reliably represents the atomistic structure of the actual carbon sample and successfully predicts its gas adsorption and fluid transport properties.Following this part, structural characteristics of the constructed model are investigated using different computational methods to reveal the underlying correlations between such characteristics and the behavior of fluid transport in the highly disordered structure of SiC-DC. This study also explores adsorption and self-diffusion of fluid molecules in the amorphous SiC-CD model, providing more insight into the internal resistances and energy barriers to gas diffusion. The strong influence of structural heterogeneity arising from the disordered nature of the microporous carbon on molecular diffusion is also examined here using a range of computational techniques. The computational methods employed include molecular dynamics (MD) simulation, nudged elastic band (NEB) method and analysis of the free energy map of the system. It is shown that disordered structure of SiC-DC has larger energy barriers to diffusion of carbon dioxide, rather than methane despite smaller molecular size of CO 2 . It is also demonstrated that activation energy barriers for methane obtained from MD simulation are in remarkable agreement with that of macroscopic kinetic uptake at low loading, which is an indication of accuracy of the HRMC constructed model in capturing internal resistances and constrictions of the actual sample. However, quasi elastic neutron scattering (QENS) measurements suggests a smaller activation energy barrier for methane compared to the results obtained from MD simulations and macroscopic kinetic uptake experiments, which is due to the much smaller length scale in QENS compared to the macroscopic length scale. IIIDue to the significance of water a...

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“…In that case, the validity of the Knudsen model becomes very questionable. In order to measure the role of the adsorption strength in cylindrical pores at low pressure, the Henry's law equilibrium constant has been examined by Bhatia et al [79]. Following the well-known hypergeometric potential [79,80], the equilibrium constant is written as ), the equilibrium constant approaches to unity, otherwise the adsorption effect becomes significant.…”

Section: Knudsen Modelmentioning

confidence: 99%

Molecular simulation of CO2 capture from flue gas and natural gas using carbon nanotubes

Liu

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Nanoporous carbons including the well-shaped carbon nanotubes (CNTs) and the amorphous carbons, activated carbon fiber-15 (ACF-15) and silicon carbide derived carbon (SiC-DC) have been investigated regarding CO2 separation in this study, in order to reveal the performance of CNTs in separating CO2 from flue/natural gas relative to conventional amorphous carbons. A full understanding of the performance of CNTs in separation of CO2 requests fundamental knowledge of the adsorption and diffusion of sorbates in the CNTs. Since both flue and natural gases are saturated with water, and the carbons will be wetted in the humid separation environment, a thorough investigation of the effect of water vapour and pre-adsorbed water on the adsorption of CO2 and CO2/CH4 and CO2/N2 mixtures in the CNTs and amorphous carbons is vital to determine the potential of CNTs on CO2 separation. On the other hand, the diffusion of sorbates through the CNT has to overcome the interfacial barriers located at the entrance and exit of the CNT, which drag down the transport diffusion of sorbates dramatically, 2-3 orders of magnitude observed in this study, compared to the diffusion in the infinitely long CNT. As there lacks a feasible and effective method to quantitatively measure the interfacial barriers and its relative importance to the overall diffusion resistance, a novel equilibrium molecular dynamics (EMD) simulation method is proposed in this study and employed to measure the interfacial barriers for methane at the entrance and exit of the CNTs.Initially, the adsorption of CO2 in different sizes of CNTs with and without pre-adsorbed water is studied using grand canonical Monte Carlo simulations (GCMC). It is found while the adsorption capacity shows strong dependency on the diameter of the CNT, the chirality of the CNT has negligible impact on the adsorption of CO2. Meanwhile, due to the strong hydrogen-bonding, preadsorbed water molecules form clusters during the adsorption of CO2. It is intriguing to observe that the water clusters are split into smaller ones by the enhanced adsorption of CO2 at high pressures, and this splitting effect in turn enhances the adsorption of CO2 in the CNT, which is because those split smaller water clusters distribute in the CNT, further facilitating the water-CO2 interactions.Further, the adsorption of CO2/CH4 and CO2/N2 mixtures in the CNTs, CNT bundles and the amorphous carbons, in the presence of water vapour and pre-adsorbed water clusters is studied. At first, negligible adsorption of water vapour was observed in the carbons considered, both for the CO2/CH4/H2O and CO2/N2/H2O ternary mixtures saturated water. For the CO2/CH4/H2O ternary mixture, pre-adsorbed water clusters in the carbons attract the water vapour molecules to adsorb on them, leading to noticeable adsorption of water in the CNTs and amorphous carbons, which reduces the adsorption of CO2 and CH4. Nevertheless, the enhanced additional water-adsorbate (CO2+CH4) interactions prompt the adsorption of CO2 over CH4 and hence facili...

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Molecular transport in nanopores: a theoretical perspective

Cited by 154 publications

References 192 publications

Theoretical analysis of selectivity mechanisms in molecular transport through channels and nanopores

Theoretical analysis of selectivity mechanisms in molecular transport through channels and nanopores

Structural modelling of silicon carbide-derived microporous carbon and its application in CO2 capture and separation of volatile gases from moist streams

Molecular simulation of CO2 capture from flue gas and natural gas using carbon nanotubes

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