Microstructure of porous media probed by NMR techniques in sub-micrometer length scales.

It is shown that field-cycling NMR relaxation spectroscopy in combination with pulsed-gradient spin-echo diffusion studies especially in the supercon fringe field version are suitable techniques for the investigation of length scales of porous media in the range 10 A to 10 microns. Data for water adsorbed in fine particle agglomerates, porous glass and ceramics are reported. An orientational structure factor is introduced permitting the characterization of hydrated surfaces on the basis of reorientations mediated by translational displacements of the adsorbed molecules. Known lengths such as the mean pore or particle size have been reproduced in this way. In length scales beyond these structural elements, the geometry of the internal surfaces can be discussed in terms of wavenumber-space fractals.