Defect and Nondefect Interstitial Channel Availability in Carbon Nanotube Bundles: Comparison of Modeling with Experiments

There is a controversy in the literature over whether small gases, such as Ne, can adsorb in the interstitial channels (ICs) of carbon nanotube bundles. We distinguish between two types of ICs: nondefect, defined as ICs in perfectly packed arrays of nanotubes having all of the same diameter (homogeneous) defined where three tubes meet, and defect ICs, present in bundles composed of nanotubes having different diameters (heterogeneous) where four or more tubes meet. We have performed grand-canonical Monte Carlo simulations of Ne on various model carbon nanotube bundles in order to explore the role of nondefect and defect ICs in adsorption. We have performed simulations on closed and partially opened homogeneous and heterogeneous bundles. We have computed the specific surface area, the isosteric heats of adsorption, and adsorption isotherms from our simulations, and we have compared these values to experimentally measured quantities for Ne adsorption on carbon nanotubes produced by the HiPco process. Analysis of our results indicates that gases do not adsorb in nondefect ICs. When considering Ne adsorption alone, there is ambiguity about whether homogeneous or heterogeneous bundles are better models. However, taken together with previous work on Ar, CH 4, and Xe, we conclude that a model consisting of heterogeneous bundles with ∼11% open nanotubes, having nondefect ICs blocked, but allowing adsorption in the defect interstitials best describes the reported experimental data for these gases on HiPco nanotubes. Furthermore, we find that quantum mechanical diffraction effects must be taken into account for modeling Ne adsorption at the experimental conditions.