(157a) Adsorption And Diffusion Of Alkanes And Alkenes In Carbon Nanotubes

Adsorption isotherms, isosteric heats of adsorption of pure substances and binary mixtures were simulated in SWCNTs and bundles of SWCNTs. Hydrocarbons of various chain lengths were investigated, namely, ethane to dodecane. Configurations Bias Monte Carlo Simulations (CBMC) in the grand canonical ensemble were executed. Diffusion calculations were done by Molecular Dynamics for some alkanes employing a novel Lowe-Andersen thermostat [1,2] which was extended to chain-like molecules and mixtures. This technique maintains the advantages of both simplicity and efficiency. The same diffusivities and other properties of the flexible framework system were reproduced by the novel algorithm. The TraPPE force field was used for alkanes and alkenes.

The simulations have shown that there is a direct interrelationship between the Henry coefficients, isosteric heats of adsorption, and the chain lengths of the molecules. The inflection points of the isotherms were investigated. For hexane the change of the positions of the molecules relative to the pore wall during the progress of adsorption was calculated. Rearrangements of the molecules at higher pressures could be observed. Binary mixture adsorption simulations showed that at low pressure the stronger adsorbing molecules (e.g. longer chains) are preferentially adsorbed. At higher pressure, molecules which allow a higher packing density are preferred (entropic effect). Simulations of adsorption on bundles of SWCNTs demonstrated that a considerable amount of molecules are adsorbed in the interstitial space if the molecules can penetrate this space.

The diffusion simulations have shown that at low loadings the wall flexibility cannot be neglected. For high loadings of ethane and butane, the diffusivities of ethane in the mixture are lower than for pure ethane, because the butane molecules are slowing them down.

[1] S. Jakobtorweihen, M.G. Verbeek, C.P. Lowe, F.J. Keil, and B. Smit, Phys. Rev. Lett. 95 (2005), 044501 [2] S. Jakobtorweihen, C.P. Lowe, F.J. Keil, and B. Smit, J. Chem. Phys. 124 (200