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Diffusion of chain molecules and mixtures in carbon nanotubes: The effect of host lattice flexibility and theory of diffusion in the Knudsen regime
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10.1063/1.2753477
/content/aip/journal/jcp/127/2/10.1063/1.2753477
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/2/10.1063/1.2753477

Figures

Image of FIG. 1.
FIG. 1.

(Color online) Schematic illustration of the Lowe-Andersen radius (collision radius) , in the plane. The distance of the butane center of mass to the pore wall is smaller than . Therefore, this molecule will be considered for thermalization. But only the interaction center with the shortest distance to the pore wall will be thermalized (with a certain probability). The component has a probability to be updated of , and the and components have a probability to be updated of . The hexane molecule is nonthermalized, as the center of mass is too far away from the pore wall, although one interaction center of hexane is very close to the pore wall.

Image of FIG. 2.
FIG. 2.

Self-diffusion coefficients of -alkanes inside a (20,0) CNT for the zero loading limit at , calculated with a flexible CNT, with a rigid nanotube without taking the flexibility effect into account, and with the LA-IFC thermostat to take the flexibility influence into account.

Image of FIG. 3.
FIG. 3.

End-to-end vector autocorrelation functions for -butane and -hexane, calculated with a flexible CNT, with a rigid nanotube without taking the flexibility effect into account, and with the LA-IFC thermostat to take the flexibility influence into account. For a better comparison of the butane and the hexane results we normalized the time. For the three butane results we used the time corresponding to a vanishing ACF of the LA-IFC butane result. For the three hexane results we used the time corresponding to a vanishing ACF of the LA-IFC hexane result.

Image of FIG. 4.
FIG. 4.

Self-diffusion coefficients of pure ethane and pure butane inside a (20,0) CNT at , obtained with the LA-IFC thermostat to take the flexibility influence into account, compared to results obtained with a flexible CNT. The lines are added to guide the eye.

Image of FIG. 5.
FIG. 5.

(Color online) Self-diffusion coefficients of a methane-helium mixture inside a (20,0) CNT at , obtained with a rigid nanotube where the flexibility influence is modeled with the LA-IFC thermostat, compared to results obtained with a flexible CNT. The corresponding bulk composition is constant ( and ). The inset shows the mole fractions inside the nanotube as a function of pressure. Lines are added to guide the eye.

Image of FIG. 6.
FIG. 6.

(Color online) Self-diffusion coefficients of a ethane-butane mixture inside a (20,0) CNT at , obtained with a rigid nanotube where the flexibility influence is modeled with the LA-IFC thermostat, compared to results obtained with a flexible CNT. The corresponding bulk composition is constant ( and ). The inset shows the mole fractions inside the nanotube as a function of pressure. Lines are added to guide the eye.

Image of FIG. 7.
FIG. 7.

(Color online) Normalized velocity autocorrelation functions of the direction for different systems. For butane the VACFs for a center and for the center of mass (c.m.) are shown. The arrows and the filled symbols mark the collision times for the different systems. Lines are added to guide the eye.

Image of FIG. 8.
FIG. 8.

(Color online) Comparison of self-diffusion coefficients at the zero loading limit obtained in molecular dynamics simulations with different theoretical approaches. If not otherwise mentioned the flexibility influence is taken into account. The simulation results for methane and helium are taken from Ref. 25. The numbers next to the symbols are the carbon number of the -alkane. (a) -alkanes inside a (20,0) CNT at . The inset shows the results for the rigid case. (b) Methane at . Note that the abscissa is not uniform over the whole range. (c) Deviation between the original Smoluchowski model and simulation results. (d) Deviation between the modified extended Smoluchowski model and simulation results. Note that the relative deviations for the extended Smoluchowski model (nonmodified) is constant for each species, for all methane results and for all helium results (not shown here).

Tables

Generic image for table
Table I.

Parameter sets for the LA-IFC thermostat . The (20,0) and the (13,0) CNTs have a radius of 0.78 and , respectively.

Generic image for table
Table II.

Lennard-Jones parameters and their source.

Generic image for table
Table III.

Self-diffusion coefficients for the zero loading limit at . Errors are given in the subscripts. The (20,0) and the (13,0) CNTs have a radius of 0.78 and , respectively. Note that the results for methane are taken from Ref. 17.

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2007-07-13
2014-04-20
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Diffusion of chain molecules and mixtures in carbon nanotubes: The effect of host lattice flexibility and theory of diffusion in the Knudsen regime
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/2/10.1063/1.2753477
10.1063/1.2753477
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