Molecular dynamics simulations on single-file diffusions: Effects of channel potential periods and particle-particle interactions

Abstract

This paper reports molecular dynamics simulations on the diffusion of sulfur hexafluoride SF6 molecules in one-dimensional zeolite ZSM-22 pores. In particular, the simulations explored the effects of the periodic boundary conditions of the ZSM-22 pores and the SF6–SF6 molecular interactions on the time (t) dependence of the mean square displacement (d) of the SF6 molecules. The simulation results clearly indicate that, with time, the molecules undergo three types of diffusions in sequence: a projectile diffusion regime with d∼t2, a single-file diffusion regime with d∼t0.5, and a normal diffusion regime with d∼t1. The time for the transition from the single-file diffusion to the normal diffusion increases with the length of the pores. When the interaction between the SF6 molecules is sufficiently strong, there exists also a suppressed single-file (SSF) diffusion regime in between the single-file and normal diffusion regimes that is characterized by d∼tα with α<0.5. The intermolecule interaction also substantially affects the durations of the single-file diffusion and the SSF diffusion, as well as the time for the transition to the normal diffusion state. A detailed discussion is provided that compares the results from this work with those from previous simulation and experimental works.