(604f) Diffusion Mechanisms of Fluids Confined in Carbon Nanotubes, Carbon Nanotube Bundles and Hierarchial Carbons: Single-File, Fickian and Dual-Mode Diffusion

Carbon nanotubes have received extensive interest in materials science and chemical engineering due to their possibilities of being used as host structures for gas separation; they can yield faster diffusion rates of fluid molecules compared to other widely used materials (e.g., zeolites). Several diffusion mechanisms are possible in narrow cylindrical pores. Among the most important are ballistic motion, Fickian diffusion, and single-file diffusion. Ballistic motion occurs for very short times, before the molecule has had a chance to collide with anything. For longer times the motion becomes either Fickian or single-file. If the pore is large enough, the molecules will diffuse in 3-dimensions as they would in a bulk fluid (Fickian diffusion), but if the pore diameter becomes small enough the diffusion will crossover from Fickian to single-file diffusion, where the molecules can no longer pass each other. Clearly single-file motion is much slower than Fickian, which is much slower than ballistic.

We use a combination of Grand Canonical Monte Carlo and Molecular Dynamics simulations to investigate the adsorption and self-diffusion properties of pure component Lennard-Jones Ar and mixtures of Lennard-Jones fluids (e.g., Ar, Kr, Xe, Ne) in isolated carbon nanotubes and carbon nanotube bundles for a range of densities and nanotube diameters.

In the case of the diffusion of fluids in severe confinement, the fluids are inevitably trapped in or close to a state of Knudsen diffusion where the fluid-wall interactions are important. For pure component fluids, the carbon nanotube transition diameter is a function of the fluid-wall interaction diameter. A single transition diameter of the carbon nanotube can be extracted for each fluid. For mixtures, dual diffusion mechanisms can occur in isolated carbon nanotubes, in which one component exhibits Fickian diffusion while the other shows single-file diffusion. We show that for carbon nanotube bundles and hierarchical carbons a dual diffusion mechanism can also occur, with 3-dimensional Fickian diffusion occurring inside the larger pores and single-file diffusion occurring simultaneously in smaller pores or interstices. Both results have implications for separations and for applications of porous materials with bimodal pore size distributions as catalysts and electrodes.