Behavior of ethylene and ethane within single-walled carbon nanotubes. 1-Adsorption and equilibrium properties

Abstract

Endohedral adsorption properties of ethylene and ethane onto single-walled carbon nanotubes were investigated using a united atom (2CLJQ) and a fully atomistic (AA-OPLS) force fields, by Grand Canonical Monte Carlo and Molecular Dynamics techniques. Pure fluids were studied at room temperature, T = 300 K, and in the pressure ranges 4 × 10 −4 < p < 47.1 bar (C 2 H 4 ) and 4 × 10 −4 < p < 37.9 bar (C 2 H 6 ). In the low pressure region, isotherms differ quantitatively depending on the intermolecular potential used, but show the same qualitative features. Both potentials predict that ethane is preferentially adsorbed at low pressures, and the opposite behavior was observed at high loadings. Isosteric heats of adsorption and estimates of low pressure Henry's constants, confirmed that ethane adsorption is the thermodynamically favored process at low pressures. Binary mixtures of C 2 H 4 /C 2 H 6 were studied under several (p, T ) conditions and the corresponding selectivities towards ethane, S, were evaluated. Small values of S < 4 were found in all cases studied. Nanotube geometry plays a minor role on the adsorption properties, which seem to be driven at lower pressures primarily by the larger affinity of the alkane towards the carbon surface and at higher pressures by molecular volume and packing effects. The fact that the selectivity towards ethane is similar to that found earlier on carbon slit pores and larger diameter nanotubes points to the fact that the peculiar 1-D geometry of the nanotubes provides no particular incentive for the adsorption of either species.

Behavior of ethylene and ethane within single-walled carbon nanotubes, 2: dynamical properties

“…In the previous paper of this series (Cruz and Müller 2008) we showed that a fine description of the fluid molecules, in terms of a fully atomistic intermolecular potential model, as opposed to a simpler coarse-grained unitedatom model, captured some effects which otherwise would be lost when describing the adsorption on nanotubes. In Fig.…”

“…4). One needs to keep in mind that the highest density systems correspond to a nanopore which is totally filled, as determined in part I (Cruz and Müller 2008). Generally speaking, the MSD curves of Fig.…”

“…The intermolecular potentials used in the MD simulations have already been thoroughly described in part I of this work (Cruz and Müller 2008), and will be summarized here for completeness. Interactions between particles are modeled employing the all-atom OPLS (AA-OPLS) force-field, with the parameterization given by Jorgensen and co-workers (Jorgensen et al 1996;McDonald et al 1997), and considering explicit electrostatic charges.…”

“…To study the effect on dynamics of fluid confinement, a simulation cell was built from the final configuration of a previous conducted adsorption run (Cruz and Müller 2008); the corresponding volume, containing the (16, 0) zig-zag nanotube and the adsorbed molecules, was separated from the bulk fluid phase and replicated four times along the z-axis to produce a 30 × 30 × 206 Å supercell with 3104 graphitic carbon atoms. To monitor the effect of nanotube chirality on the fluids transport properties, the zig-zag SWCNT was replaced either by a chiral (12, 6) tube or an armchair (9, 9) tube, having, respectively, a total number of graphitic carbon atoms of 3066 and 3006.…”

“…In a previous work, henceforth designated as part I (Cruz and Müller 2008), we have presented detailed computer simulation results on the equilibrium and adsorption properties of C 2 H 4 and C 2 H 6 and their corresponding binary mixtures, when bulk fluids were put in contact with SWCNTs. Two different force-fields were employed, namely a coarse grained model and a fully atomistic potential.…”