A CTRW interpretation of simulated single-file diffusion in zeolites

Single-file diffusion of molecules adsorbed in the channels of zeolite is a phenomenon occurring at different time and length scales, which is difficult to simulate by standard methods, because it often occurs for large molecules adsorbed in microporous materials showing parallel non-crossing channels, with large energy barriers to diffusion.. Two interesting examples are given by water adsorbed in two different Li containing zeolites, Li-ABW and bikitaite, which show parallel straight channels where hydrogen-bonded linear chains of water molecules run along the axis of the channels, parallel to regular rows of lithium ions sticking to the channel surface. Extensive Molecular Dynamics (MD) simulations of the dynamical of water in these zeolites at different loading and temperature were performed by this research group. It resulted that the energy barrier for water molecules to cross one another in the same channel is much larger than kT event at any reasonably high temperature, so that the diffusion resulted single-file in all the simulation conditions. The simulation details and results are reported in Refs. [3] and [4], were it was shown that for time scales ranging from a few to some hundreds of picoseconds depending on temperature and loading the mean square displacement (MSD) is proportional to the square root of time as expected for single-file diffusion.