Thermodynamics and Kinetics of Carbon Dioxide Adsorption on HiPco Nanotubes

We present the results of a combined experimental and computational study of CO2 adsorption on purified HiPco nanotubes. Isotherms were measured at six temperatures between 147 and 207 K, below the bulk triple point for CO2. Unlike the case of other adsorbates on HiPco nanotube bundles, adsorption isotherms at corresponding temperatures do not reveal the presence of any resolvable substeps. The isosteric heat values derived from the measured isotherms are lower than the latent heat of sublimation for most of the loadings, with the exception of a narrow range at very low coverage. Results from grand canonical Monte Carlo simulations show that this is due to the much larger contribution of the CO2-CO2 interactions (owing mostly to presence of the electrostatic component) that greatly exceeds the size of the gas-surface interaction as the coverage increases beyond the monolayer. Measurements of the kinetics of adsorption show that the equilibration time increases with sorbent loading, which is typical of systems with relatively larger adsorbate-adsorbate interactions.