Inter- and Intratube Self-Diffusion in n-Heptane Adsorbed on Carbon Nanotubes

We have experimentally measured long-range self-diffusivity of normal heptane adsorbed on single-walled carbon nanotubes at 275 K. This was done by observing the displacement kinetics of heptane by a deuterium-labeled molecule, 1-deuteroheptane. We also performed MD simulations of heptane self-diffusion inside an individual nanotube under full loading. The experimentally measured diffusion coefficient ( D = 7.7 × 10 -12 cm 2 s -1 ) was 8 orders of magnitude lower than the diffusion coefficient inside an individual nanotube from the simulation ( D = 8.2 × 10 -4 cm 2 s -1 ). Because heptane only adsorbs in the nanotube interior at 275 K, we conclude that the experimentally observed long-range transport is rate-limited by a very slow exchange of heptane between different nanotubes in the bulk SWNT sample. At the same time, the mixing of heptane inside individual nanotubes is fast. The reasons for slow diffusion between different tubes could be the potential energy barriers at the entry points into the nanotubes, as well as desorption−adsorption barriers for molecules migrating through the bulk.