  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.
