Single-File Diffusion Observation

Molecular diffusion in zeolites of unidimensional channel structure is studied by pulsed field gradient NMR. In contrast to ordinary diffusion, the mean square displacement increases in proportion with the square root of the observation time. This behavior is explained by assuming that the molecules are unable to pass each other (``single-file'' diffusion). The diffusivity of an isolated molecule estimated from the mobility in the single-file system is 2 orders of magnitude larger than the largest intracrystalline diffusivities in zeolites so far observed. The finding is supported by both the observed concentration dependence and molecular dynamics simulations.