Nitrogen adsorption on carbon nanotube bundles: Role of the external surface

Nitrogen adsorption on two types of single-walled carbon nanotube bundles at both subcritical and supercritical temperatures is studied using Gibbs ensemble Monte Carlo simulation to understand the role of the external surface in the type of isotherm. On an infinite periodic hexagonal bundle without an external surface, which was mostly used in previous theoretical studies, subcritical adsorption is of type I with two steps in the adsorption isotherm and two maxima in the isosteric heat corresponding to adsorption, first forming annuli inside the nanotubes, and then with increased coverage at the centers of the nanotubes. Supercritical adsorption is also of type I, but with one step in the adsorption isotherm and a single maximum in the isosteric heat. Also, at high pressures the interstitial channels between the nanotubes become accessible to nitrogen molecules. On a small isolated hexagonal bundle with an external surface, subcritical adsorption is of type II, as has been observed in experiments, with two steps in the adsorption isotherm and two maxima in the isosteric heat. The first step corresponds to adsorption at the internal annuli and the grooves between the nanotubes, and the second step results from adsorption at the ridges on the external surface of the bundle and at the centers of the nanotubes. At higher coverages multilayer adsorption and wetting occur on the external surface as the bulk phase approaches saturation. Supercritical adsorption is of type I, without a step in the adsorption isotherm or a maximum in the isosteric heat, and there is no nitrogen adsorption in the interstitial channels even at high pressures. These results demonstrate the important role of the external surface of the nanotube bundle in the character of adsorption isotherm, and provide a substantial physical explanation for the difference between experimental observation and previous theoretical prediction.