RESTRICTED DIFFUSION AND MOLECULAR EXCHANGE PROCESSES IN POROUS MEDIA AS STUDIED BY PULSED FIELD GRADIENT NMR

摘要

The spatial and time resolution of the PFG NMR technique is directly proportional to the amplitude of the magnetic field gradient. In standard NMR equipment (for example, NMR scanners) the typical gradient amplitude is limited to a 1 T/m. Using such gradient pulses it is possible to investigate translational displacements of fluid molecules on the distances about a few microns only. Producing higher magnetic field gradients involves serious technical difficulties. Nevertheless, while in the 1980s the maximum amplitude of PFG did not exceed 10 T/m, at present magnetic field gradients of about 100 T/m and higher are used in several laboratories. For a long time we used a gradient with a maximum amplitude up to 50-100 T/m. With a special probehead, we have achieved real gradient values up to 500 T/m. This considerably broadens the scope of the method and its applications. It also opens up a new window to study porous media by PFG NMR in the range beyond the sub-micron level. Some preliminary measurements demonstrate the potential of this NMR technique to characterize the pore space over distances below 1 μm. Such information is essential for the study of relationships between the pore geometry and transport properties of the porous media.