The results of an adsorption isotherm investigation of methane on closed-ended single-wall nanotube bundles are presented. The isosteric heat of adsorption was determined as a function of the amount of methane adsorbed on the SWNT substrate for coverages in the first layer. The isosteric heat of adsorption is found to decrease with increasing coverage. This behavior provides an explanation for differences in previously reported values for this quantity. Substantial agreement was found between this study and previous reports on the temperature dependence of the pressures at which the two first-layer substeps occur for methane adsorbed on SWNT bundles. There was, however, one exception: we found no evidence suggesting the existence of a phase transition around 90 K in the high pressure substep. We put forth a possible explanation for this difference.

• Received 2 July 2001

DOI:https://doi.org/10.1103/PhysRevB.65.045416