Resistance due to structure

It thus appears that in real zeolite crystals diffusion over long distances reflects the influence of surface and internal barriers rather than the pore structure of the idealized framework. As a result the apparent intracrystalline diffusivities often show a strong dependence on the length scale of the measurement. Measurements by QENS and neutron spin-echo methods over distances corresponding to a few unit cells often approach the theoretical values derived by MD calculations for an ideal lattice. Similar values are often obtained by PEG NMR when the measurement is made over short distances. Measurements by most macroscopic methods are on the length scale of the crystals, and these tend to yield lower apparent diffusivities as a consequence of the intrusion of surface barriers and internal resistances due to structural defects. 

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