Structure-related diffusion under molecular confinement

Molecular diffusion under confinement by highly ordered nanoporous structur es is subjected to a hierarchy of particle-particle correlations. These cor relations result in a variety of peculiarities of molecular diffusion. Thre e examples are discussed in detail. As a consequence of the confinement by the host matrix, molecular displacements in different directions often inti mately depend on each other, resulting in an interdependence of the princip al elements of the diffusion tensor (correlation rule of diffusion anisotro py). If intracrystalline channel systems confine molecular propagation to o ne direction, molecules with diameters sufficiently exceeding the channel r adii are subjected to single-file diffusion, whose spatial-temporal depende nce significantly deviates from normal diffusion. In networks of intersecti ng single-file systems with different affinities to the reactant and produc t molecules, the interplay of diffusion and reaction leads to deviations fr om a homogeneous pore filling over the sample and to an enhanced output of reaction products in comparison with channel networks of equal adsorption a ffinities (reactivity enhancement by molecular traffic control). The presen ted theoretical concepts are compared with experimental results, in particu lar with the findings of pulsed field gradient (PFG) NMR diffusion studies.