  Gases adsorbed within bundles of carbon nanotubes (inside of the nanotubes or
in the interstitial channels between the tubes) exhibit a variety of phase
transitions with the help of interactions between molecules in neighboring
channels or tubes. Because the channels/tubes are widely separated, these
transverse interactions are weaker than the (longitudinal) interactions within
the same channel. The transition temperatures that result are therefore lower
than those of typical two- or three-dimensional transitions of the same species
of molecules. We discuss here the condensation transition of such a gas to form
a liquid, expressing the transition behavior in universal form, where the
reduced critical temperature T_c* is a universal function of the reduced
transverse interaction.
