Carbon nanotubes have been under intense study since their discovery nearly a decade ago. Two of their unique properties that have generated much interest and excitement is the possibility of exploiting their hollow structure for use as membranes for separations and taking advantage of their unusually high Young's modulus in novel composite materials. This talk will discuss computational investigations of two aspects of these properties of carbon nanotubes.
The first part of the talk will use classical molecular dynamics simulations to study the flow of molecular species, such as methane, ethane, ethylene, and butane, through carbon nanotubes at room temperature. Both uniform molecule cases and binary mixtures were considered. The interatomic forces in the simulations are calculated using a classical reactive empirical bond-order hydrocarbon potential coupled to long-range Lennard-Jones potentials. The simulations show that the intermolecular and molecule-nanotube interactions strongly affect molecular diffusion and separations in molecular mixtures. For example, normal-mode molecular thermal diffusion is predicted for methane for nearly all the nanotube diameters considered. In contrast, ethane and ethylene are predicted to diffuse by normal-mode, single-file mode, or at a rate that is transitional between normal-mode and single-file diffusion over the time scales considered in the simulations depending on the diameter of the nanotube. These mechanisms stay relatively unchanged in the case of mixtures, although the diffusion coefficients do change compared to the pure molecule type case. Significant separations are predicted for methane/butane mixtures but little separation is seen for methane/ethane mixtures. The effects of atomic termination at the nanotube opening and pore-pore interactions within a nanotube bundle on the diffusion results are also considered.
The second part of the talk will examine the effect of filling the hollow nanotubes on the mechanical properties of carbon nanotubes is also considered in a separation series of simulations. Single-walled and multi-walled nanotubes that are capped at both ends and filled with various gases, such as methane and xenon, and clusters, such as C60, are subjected to compressive, tensile and shear forces. The effect of the filling on the responses of the nanotubes is quantitatively assessed and compared to the responses of empty nanotubes. The influence of density will also be discussed.