Phases of ethane adsorbed on purified HiPco single-walled carbon nanotubes

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We have used adsorption isotherms to study ethane films on bundles of purified HiPco single-walled carbon nanotubes (SWNTs). We infer the existence of a possible phase transition in this system from a sudden change in the temperature dependence of the coverage interval corresponding to a feature present in the isotherms. The possible phase transition occurs near monolayer completion, at approximately $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We also measured isotherms at low fractional coverages and relatively high temperatures $(220--240\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ in order to determine the binding energy of ethane on the SWNT bundles. We obtained a value of $308\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for this quantity; this is 1.76 times larger than the binding energy of ethane on graphite. We used isotherm results obtained at all measured temperatures and coverages to determine the coverage dependence of the isosteric heat of adsorption for this system.