Sorbates in carbon nanotubes: Transition from diffusive to superdiffusive motion

Molecular dynamics simulations of sorbates of different sizes confined to the interior of carbon nanotubes are reported. The mean squared displacement shows gradual change from diffusive for small sorbates to superdiffusive for intermediate-sized sorbates to ballistic for sizes comparable to the channel diameter. We show that this crossover behaviour can be understood on the basis of a gradual decrease of the x–y component of the force with the levitation parameter. We propose that the relevant quantity to understand the recently reported results by others and ourselves, of diffusivities in carbon nanotube is the levitation parameter and not the smoothness of the carbon nanotube. This explanation helps rationalize in a coherent way all of the recently published results.