Evidence for single file diffusion of ethane in the molecular sieve AlPO4-5

We measure the diffusion of ethane in very large crystals of the molecular sieve AlPO4-5 (provided by M. Davis) using pulsed field gradient (PFG) NMR. Thanks to the large crystal size and the long longitudinal relaxation time, the mean square displacement could be monitored over times that arranged over an order of magnitude. The mean square displacement was proportional to the square root of time, providing strong evidence that the ethane molecules are moving in a single file, i.e. they are unable to pass each other. This is the first direct experimental evidence for single-file diffusive motion — a phenomenon that is probably widely prevalent in both industrial catalytic and biological systems.