Diffusive and Superdiffusive Motion of sorbates in Carbon nanotubes

Molecular dynamics simulations of sorbates of different sizes confined to the interior of carbon nanotubes are reported. The mean squared displacement shows gradual change from diffusive for small sorbates to superdiffusive for intermediate sized-sorbates to ballistic for sizes comparable to the channel diameter. We show that this crossover behaviour can be understood on the basis of a gradual decrease of the x-y component of the force with the levitation parameter. The analysis can also help to rationalize some recently published results.Comment: 3 pages, 3 figure

Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO2 and CH4 to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH2, H2PO3, NO2, and CH3. Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO2 to exceed that of CH4 by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH2, H2PO3, NO2, and COOH functional groups can significantly enhance gas binding with respect to a hydrogen-passivated edge, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO2/CH4 selectivity are discussed.Comment: 12 pages, 7 figure

Types of Dependence of Self-Diffusivity on Sorbate Concentration in Parameter Space: A Two-Dimensional Lattice Gas Study

The various factors that influence the dependence of self-diffusivity D on sorbate concentration c has been investigated by means of lattice gas model. Among the parameters varied are the activation energy, the temperature, the number of adsorption sites, and the pattern of adsorption sites. It is shown that the five types of D vs c dependence observed by Karger and Pfeifer based on the PFG-NMR experiments on a variety of guest-zeolite systems can be understood in terms of the variation of just a few parameters. A self-diffusivity type diagram showing under what conditions a given type is found has been constructed. It is shown that even the approximate lattice gas treatment can reasonably predict the D vs c behavior of a number of guest-zeolite systems besides giving insight into why a particular behavior is observed

Study of translational and rotational mobility and orientational preference of ethane in one-dimensional channels

Structural and dynamical properties of ethane in one-dimensional channels of AlPO4-5 and carbon nanotube have been investigated at dilute concentration with the help of molecular dynamics simulation. Density distributions and orientational structure of ethane have been analyzed. Repulsive interactions seem to play an important role when ethane is located in the narrow part of the AlPO4-5 channel. In AlPO4-5, parallel orientation is predominant over perpendicular orientation except when ethane is located in the broader part of the channel. Unlike in the case of single-file diffusion, our results in carbon nanotube show that at dilute concentrations the mean squared displacement, mu(2)(t) approximate to t(alpha), alpha = 1.8. The autocorrelation function for the z-component of angular velocity of ethane in space-fixed frame of reference shows a pronounced negative correlation. This is attributed to the restriction in the movement of ethane along the x- and y- directions. It is seen that the ratio of reorientational correlation times does not follow the Debye model for confined ethane but it is closer to the predictions of the Debye model for bulk ethane

Anomalous diffusion of linear and branched pentanes within zeolite NaY

Molecular dynamics studies of n-pentane and iso-pentane in zeolite NaY are reported. Radial distribution of the pentane isomers within the supercage, angle distribution of the end-to-end vector with the radial vector, distribution of end-to-end distance in the two isomers, average guest-zeolite interaction energy, properties during intercage migration event, diffusivity, autocorrelation function for the centre of mass and angular velocities are reported. Results suggest that iso-pentane has a higher diffusivity and lower activation energy than n-pentane. This surprising result is attributed to the need for change from trans to gauche during intercage migration in the case of n-pentane. This is suggested by the increase in gauche population in n-pentane near the window while no such increase is observed in iso-pentane. Another reason for the higher diffusivity of iso-pentane appears to be due to the similarity of the cross-sectional diameter of iso-pentane with the diameter of the 12-ring windo

Orientational preference and influence of rotation on methane mobility in one-dimensional channels

Molecular dynamics investigations of methane in one-dimensional channel systems of AlPO4−5 and the carbon nanotube are reported. Methane shows contrasting orientational preference over 1+3 and 2+2 orientations inside AlPO4−5 and the carbon nanotube. In AlPO4−5, 2+2 orientation is preferred in narrow parts of the channel while 1+3 orientation is predominant in the wider part of the channel. In the carbon nanotube 2+2 is preferred over 1+3 orientation throughout the channel. Thus, the orientation of methane can yield insight into methane-host interactions. Simulations with and without allowing for rotation of methane suggest that rotation aids translational mobility of methane in both the channel systems studied. It is shown that molecules with certain orientations are associated with lower mobility, both in AlPO4−5 and carbon nanotube. Further, translational motion exhibits diffusive behavior for methane in AlPO4−5 and superdiffusive behavior in the carbon nanotube. These results suggest that particular orientation or rotation in general can strongly influence translational mobility of methane in one-dimensional channels. It is seen that levitation effects influence rotational motion in addition to the translational motion

Dependence of the self-diffusion coefficient on the sorbate concentration: a two-dimensional lattice gas model with and without confinement

Previous pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) measurements on different sorbate-zeolite systems suggest that there exist at least five different types of dependence of self-diffusivity of the sorbate, D, on the concentration, c, of the sorbate. Sorbate-zeolite systems have been modeled as a two-dimensional lattice gas and studied by carrying out Monte Carlo simulations under different conditions. Among the different factors that have been varied are the arrangement of adsorption sites, hop length, nature and strength of the sorbate-sorbate interaction, and the degree of confinement. Surprisingly, even the simple 2-D lattice gas model could yield more than one type of D vs c dependence. The present study provides insight into the possible reasons for a given type of D vs c dependence

Structure and dynamics of benzene in one-dimensional channels

Molecular dynamics investigation of benzene in one-dimensional channel systems A1PO(4)-5, VPI-5, and carbon nanotube is reported. The results suggest that, in all the three host systems, the plane of benzene is almost perpendicular to the channel axis when the molecule is near the center of the channel and the plane of benzene is parallel to the channel axis when the molecule is near the wall of the channel. The density distribution of benzene as a function of channel length, z and the radial distance, r, from the channel axis is also different in the three host structures. Anisotropy in translational diffusion coefficient, calculated in body-fixed frame of benzene, suggests that benzene prefers to move with its plane parallel to the direction of motion in A1PO(4)-5 and VPI-5 whereas in carbon nanotube the motion occurs predominantly with the plane of the benzene perpendicular to the direction of motion.;Anisotropy associated with the rotational motion is seen to alter significantly in confinement as compared to liquid benzene. In A1PO(4)-5, the rotational anisotropy is reversed as compared to liquid benzene thereby suggesting that anisotropy arising out of molecular geometry can be reduced. Reorientational correlation times for C-6 and C-2 axes Of benzene are reported. Apart from the inertial decay of reorientational correlation function due to free, rotation, two other distinct regimes of decay are observed in narrower channels (AIPO(4)-5 and carbon nanotube): (i) an initial fast decay (0.5-2 ps) and (ii) a slower decay (>2 ps) of reorientational correlation function where C-6 decays slower than C-2 Similar to what is observed in liquid benzene. In the initial fast decay, it is seen that the decay for C-6 is faster than C-2 which is in contrast to what is observed in liquid benzene or for benzene confined in VPI-5