  The thermodynamics of adsorption of light alkanes and alkenes (CH4, C2H6,
C2H4, C3H8, and C3H6) in single-walled carbon nanotube bundles is studied by
configurational-bias grand canonical Monte Carlo simulation. The bundles
consist of uniform nanotubes with diameters in the range 11.0 < D (A) < 18.1,
arranged in the usual close-packed hexagonal lattice. The phase space is
systematically analyzed with calculations for adsorption at room temperature
and reduced pressure range of 8.7 x 10-9 < (p/p0) < 0.9. The simulation results
are interpreted in terms of the molecular nature of the adsorbate and the
corresponding solid-fluid interactions. It is shown that confinement in the
internal volume of the bundle (interstitial and intratubular) is energetically
more favorable than physisorption on the external surface (grooves and exposed
surfaces of peripheral tubes), as indicated by the curves of isosteric heat as
a function of reduced pressure. However, the zero-loading properties suggest a
crossover point to this behavior for D = 18 - 19 A. When interstitial
confinement is not inhibited by geometrical considerations, it is possible to
establish the following ordering of the zero-loading isosteric heat by type of
adsorption site.
