Structure-related diffusion under molecular confinement

Abstract

Molecular diffusion under confinement by highly ordered nanoporous structures is subjected to a hierarchy of particle-particle correlations. These correlations result in a variety of peculiarities of molecular diffusion. Three examples are discussed in detail. As a consequence of the confinement by the host matrix, molecular displacements in different directions often intimately depend on each other, resulting in an interdependence of the principal elements of the diffusion tensor (correlation rule of diffusion anisotropy). If intracrystalline channel systems confine molecular propagation to one direction, molecules with diameters sufficiently exceeding the channel radii are subjected to single-file diffusion, whose spatial-temporal dependence significantly deviates from normal diffusion. In networks of intersecting single-file systems with different affinities to the reactant and product molecules, the interplay of diffusion and reaction leads to deviations from a homogeneous pore filling over the sample and to an enhanced output of reaction products in comparison with channel networks of equal adsorption affinities (reactivity enhancement by molecular traffic control). The presented theoretical concepts are compared with experimental results, in particular with the findings of pulsed field gradient (PFG) NMR diffusion studies.