Structure–Mobility Relations of Molecular Diffusion in Interface Systems

Abstract

The behaviour of molecules in contact with interfaces may significantly deviate from the patterns known from their behaviour in the bulk. This is in particular true with respect to molecular diffusion, i.e. to the random movement of the constituents of the interface systems. As a non-invasive technique and owing to the large spectrum of rather specific information accessible by this technique, pulsed field gradient (PFG) NMR has proved to be the method of choice for such investigations.

After a short introduction into the principles of PFG NMR and its particular strengths and limitations, various examples of the benefit of exploiting this technique in diffusion studies of interface systems are given. They include the investigation of nanoporous materials such as zeolites, where the application of PFG NMR has revolutionized the understanding of intracrystalline diffusion. As typical examples for structure-mobility relations, the propagator representation and the various (structure-dependent) patterns of concentration dependence of self-diffusion are presented. In the investigation of sediments (such as beds of sand grains), the influence of the interface between the fluid and solid phases is shown to affect both molecular propagation and nuclear magnetic relaxation, which is shown to eventually lead to a new route of attaining fractal geometries. Finally, the internal structurization of self-organized multicomponent polymer systems containing diblock copolymers is shown to give rise to peculiarities in the propagation patterns of the individual constituents. The conclusion summarizes the presented principles of structure-mobility correlations and refers to the numerous further examples in nature and technology, some of which – see e.g. the contributions by Fritzsche et al., Bräuer et al., Freude et al., Kremer et al., Michel et al., and Arnold et al. – may be found in further chapters of the book.