MICROSTRUCTURE OF POROUS-MEDIA AND FIELD-CYCLING NUCLEAR MAGNETIC-RELAXATION SPECTROSCOPY

As a special class of nuclear-magnetic-resonance (NMR) relaxation mech anisms, reorientations mediated by translational displacements, is con sidered. This particularly refers to systems in which molecules are co nfined to disordered structures defining (a) local preferential orient ations and permitting (b) translational degrees of freedom with a cert ain reduced dimensionality. Examples are molecules adsorbed on surface s of macromolecules, particle aggregates, and porous media. The dipola r correlation function of molecules diffusing along such confining str uctures therefore does not only reflect the molecular dynamics but als o the structural properties of the confining system. Using field-cycli ng and other NMR relaxation techniques the frequency dependence of the spin-lattice relaxation times of several porous materials were measur ed in a range 3 X 10(2)-3 X 10(8) Hz. The data were numerically analyz ed using an ''orientational structure factor'' which was introduced ad hoc and which renders the distribution of wave numbers of the surface s forming the geometrical restrictions of molecular displacements. Thi s distribution turned out to consist of a power-law and a single-peak term. The results are discussed with respect to the microstructure. Ch aracteristic length scales can be estimated by bringing in data of the translational diffusion coefficient effective in short time intervals .