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Deviations from the ideal single-file behavior in actual systems, such as zeolitic host-guest systems with a one-dimensional channel structure, may occur if (i) there is a possibility of mutual passages of the diffusants and (ii) the influence of the file boundary on the movement of the diffusants cannot be neglected. The conditions under which such deviations may become relevant are investigated both by analytical methods and molecular dynamics simulations. The consequences of these deviations for experimental studies of singlefile diffusion are discussed.

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Translational diffusion of molecules in one-dimensional channel systems has been measured by quasi-elastic neutron scattering. The scattering function for a single-file diffusion model has been derived in order to differentiate this transport model from normal 1D diffusion. In the AlPO 4 -5 structure, ordinary 1D diffusion is observed for methane and ethane, whatever the loading, which indicates that the molecules are able to pass each other. The diffusion coefficients for both molecules are on the order of 10 -9 m 2 s -1 . On the other hand, a quite different regime is obtained by varying cyclopropane concentration in the same structure. At low loading, normal 1D diffusion is observed because the molecules can be considered as isolated, but at higher concentration cyclopropane is found to follow single-file diffusion. However, the mobility is too small to be determined. In the less open channel system of ZSM-48, ordinary 1D diffusion is found for small methane concentration, the diffusion coefficient being 2.5 × 10 -9 m 2 s -1 , at 155 K. At medium loading, single-file diffusion is observed with a mobility factor of 2 × 10 -12 m 2 s -1/2 . This is the first time that this technique has provided experimental evidence of single-file diffusion in zeolites.