The transition from single-file diffusion to Fickian diffusion in narrow cylindrical pores was 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 was 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 various carbon nanotube diameters was analyzed in terms of the Fickian self-diffusivity and the single-file mobility of the mixture components. It was found that the single-file to Fickian carbon nanotube transition diameter was a unique property of the diameter of the individual molecule and remained the same, regardless of the mixture composition. In binary mixtures, each component could cross over from single-file to Fickian diffusion at a different nanotube diameter, thus giving rise to bimodal diffusion in some nanotubes. This transition permitted one species to diffuse in single-file mode, while the other diffused via a Fickian mechanism. This led to orders of magnitude differences between the self-diffusion rates of the two molecules. This phenomenon was also expected to alter the diffusional motion of molecules in nanoporous materials.

Transition from Single-File to Fickian Diffusion for Binary Mixtures in Single-Walled Carbon Nanotubes. Q.Chen, J.D.Moore, Y.C.Liu, T.J.Roussel, Q.Wang, T.Wu, K.E.Gubbins: Journal of Chemical Physics, 2010, 133[9], 094501