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Transition from single-file to Fickian diffusion for binary mixtures in single-walled carbon nanotubes.

The transition from single-file diffusion to Fickian diffusion in narrow cylindrical pores is investigated for systems of rigid single-walled armchair carbon nanotubes, solvated with binary mixtures of Lennard-Jones fluids (Ar/Ne, Ar/Kr, and Ar/Xe). A range of effects is examined including the mixture concentration, the size ratio of the two components, and the nanotube diameter. The transition from single-file to Fickian diffusion in varying carbon nanotube diameters is analyzed in terms of the Fickian self-diffusivity and the single-file mobility of the mixture components. It is found that the single-file to Fickian carbon nanotube transition diameter is a unique property of the individual molecule's diameter and remains unchanged regardless of the mixture composition. In applications of binary mixtures, each component may crossover from single-file to Fickian diffusion in a different carbon nanotube diameter, giving rise to bimodal diffusion in some nanotubes. This transition allows for one species to diffuse in single-file while the other diffuses by a Fickian mechanism, yielding orders of magnitude difference between the self-diffusional rates of the two molecules. This phenomenon might be further extended to alter the diffusional motion of molecules in nanoporous materials.