A novel algorithm for modeling the influence of the host lattice flexibility in molecular dynamics simulations was extended to chain-like molecules and mixtures. This technique, based upon a Lowe-Andersen thermostat, preserved the advantages of simplicity and efficiency. The same diffusivities and other properties of the flexible framework system were reproduced. The computationally demanding flexible host lattice simulations could be advantageously avoided. Using this methodology, a study was made of the influence of flexibility upon the diffusion of n-alkanes inside single-walled carbon nanotubes. Results were presented for the diffusion of two mixtures (methane-helium, ethane-butane). Using these results, an investigation was made of the accuracy of theories which described diffusion in the Knudsen regime. For the dynamics in carbon nanotubes, the Knudsen diffusivities were much too low. The Smoluchowski model gave better results. The extended Smoluchowski model could reproduce the present simulation results obtained for a rigid host lattice. This model was modified so as to treat collisions with a flexible interface correctly, as well. As the tangential momentum accommodation coefficient was required for the theoretical models, a simple concept was introduced in order to calculate it.

Diffusion of Chain Molecules and Mixtures in Carbon Nanotubes: the Effect of Host Lattice Flexibility and Theory of Diffusion in the Knudsen Regime. S.Jakobtorweihen, C.P.Lowe, F.J.Keil, B.Smit: Journal of Chemical Physics, 2007, 127[2], 024904