Physisorption kinetics in carbon nanotube bundles

The kinetics of gas uptake on different regions of carbon nanotube bundles is investigated by means of a kinetic Monte Carlo scheme. A lattice-gas description is used to model the adsorption of particles on a one-dimensional chain of sites under two types of dynamics: (a) external kinetics, in which the chain is on the bundle's external surface directly exposed to the gas, and (b) pore-like kinetics, expected to occur inside the tubes and interstitial channels, where adsorption occurs via gas diffusion from the ends. From the time evolution of the coverage at a fixed temperature, equilibration times are obtained as a function of chemical potential (or amount adsorbed). The equilibration time of the external phase decreases linearly as the coverage increases toward monolayer completion; the rate at which this occurs strongly depends on the ratio between the binding energy and the temperature. Because of this dependence, unexpectedly long waiting times can be observed for very low coverages in systems with relatively high binding energies. The adsorption rate in pore-like phases is typically 2 orders of magnitude slower than that of external phases. We show how this large disparity between adsorption rates can hinder the observation of adsorption inside the tubes and in the interstitial channels during measurements of adsorption isotherms.