Single-file Motion of Polyatomic Molecules in One-dimensional Nanoporous Materials

In a continuation of the study of the mobility of fluids adsorbed in nanoporous materials, molecular dynamics simulations are used to investigate the behaviour of polyatomic ethane molecules adsorbed in AlPO4-5. The current work is based on the use of the united atom approach as a better model than the single-centre ethane used to date. Ethane molecules are modelled as rigid diatoms, and as a result the molecules have more degrees of freedom in the form of the rotational components that are absent in the single-centre ethane model. This represents a more sophisticated model for ethane and is used in the simulations to test earlier findings. Simulations with binary mixtures of methane and ethane also have been conducted with three mixture compositions. The transition from ordinary diffusion to single-file motion for a finite residence time is found to occur at a methyl group diameter of 4.75 Aring. This is identical to the ethane diameter in the earlier study. Thus, only the minimum dimension determines the transition size. Also it is shown that the diatomic molecules undergo free rotation within the channel even when they are in the single-file mode of motion. In the case of binary mixtures, the methane molecules still undergo ordinary diffusion. Ethane molecules exhibit single-file motion at a methyl group diameter of 4.75 Aring. The diffusion coefficient of methane decreases with increasing ethane size, while the trends in the single-file mobility of ethane as a function of methyl group diameter are nonlinear.