An overview is presented of the various phases predicted to occur when gases are absorbed within a bundle of carbon nanotubes. The behavior may be characterized by an effective dimensionality, which depends on the species and the temperature. Small molecules are strongly attracted to the interstitial channels between tubes. There, they undergo transitions between ordered and disordered quasi-one dimensional (1D) phases. Both small and large molecules display 1D and/or 2D phase behavior when adsorbed within the nanotubes, depending on the species and thermodynamic conditions. Finally, molecules adsorbed on the external surface of the bundle exhibit 1D behavior (striped phases), which crosses over to 2D behavior (monolayer film) and eventually 3D behavior (thick film) as the coverage is increased. The various phases exhibit a wide variety of thermal and other properties that we discuss here.

The kinetics of gas uptake on different regions of carbon nanotube bundles is investigated by means of a kinetic Monte Carlo scheme. A lattice-gas description is used to model the adsorption of particles on a onedimensional chain of sites under two types of dynamics: (a) external kinetics, in which the chain is on the bundle's external surface directly exposed to the gas, and (b) pore-like kinetics, expected to occur inside the tubes and interstitial channels, where adsorption occurs via gas diffusion from the ends. From the time evolution of the coverage at a fixed temperature, equilibration times are obtained as a function of chemical potential (or amount adsorbed). The equilibration time of the external phase decreases linearly as the coverage increases toward monolayer completion; the rate at which this occurs strongly depends on the ratio between the binding energy and the temperature. Because of this dependence, unexpectedly long waiting times can be observed for very low coverages in systems with relatively high binding energies. The adsorption rate in pore-like phases is typically 2 orders of magnitude slower than that of external phases. We show how this large disparity between adsorption rates can hinder the observation of adsorption inside the tubes and in the interstitial channels during measurements of adsorption isotherms.

We explore the behavior of neon, xenon, and methane films adsorbed on the external surface of a bundle of carbon nanotubes. The methods used are classical: a ground state calculation, by grand potential energy minimization, and the grand canonical Monte Carlo (GCMC) method of simulation. Our results are similar to those found recently in a GCMC study of Ar and Kr. At low chemical potential (pressure) the particles form a quasi-one dimensional phase within the groove formed by two contiguous tubes. At higher chemical potential, there occurs a "three-stripe" phase aligned parallel to the groove (except for xenon). This is followed by monolayer and bilayer phases. The low temperature monolayer phase is striped; the number of stripes per nanotube is a quantized function of the adatom size. In the neon case, the bilayer regime also includes a second layer groove phase. Our results are compared with recent thermal and diffraction experiments. We find no evidence of a zig-zag phase reported recently.

A study is presented of the effects of gas (especially H2) absorption within the interstitial channels of a bundle of carbon nanotubes. The ground state of the system is determined by minimizing the total energy, which includes the molecules' interaction with the tubes, the intertube interaction, and the molecules' mutual interaction (which is screened by the tubes). The consequences of swelling include a reduced threshold pressure for gas uptake and a 2.7% increase in the tubes' breathing mode frequency.

Adsorption properties of several gases (Ne, CH 4 , Ar, Xe) on the external surface of a carbon nanotube bundle are investigated. Calculations are performed at low coverage and variable temperature, and for some fixed temperatures as a function of coverage. Within a simple model (in the limit of very low coverage) we are able to study the evolution of the film's thermal properties from those of a one dimensional (1D) fluid to those of a 2D film. In addition, grand canonical Monte Carlo simulations are performed in order to identify a second layer groove phase, which occurs once a monolayer of atoms covers the external surface. We derive from the simulations the isosteric heat, compresibility and specific heat as a function of coverage. We evaluate alternative models in order to derive quantum corrections to the classical results. We compare our findings with those of recent adsorption experiments.