Field-cycling NMR relaxometry of water in synthetic saponites: Levy walks on finite planar surfaces

Aqueous suspensions of synthetic saponite clays have been studied using field-cycling NMR relaxometry. The saponite platelets form planar surfaces on which water molecules are adsorbed. As the strong-adsorption limit applies, "bulk mediated surface diffusion'' as originally suggested by Bychuk and O'Shaughnessy is expected to occur. That is, within the so-called retention time, adsorbate molecules tend to perform Levy walks along the surfaces. A corresponding formalism (the "pancake model'') is derived and compared with the experimental data. The low-frequency spin-lattice relaxation dispersion can perfectly be described on this basis. The platelet diameter can be estimated from the peculiar adsorbate dynamics on the surfaces in accordance with literature data. (C) 1999 American Institute of Physics.