THE ANOMALOUS ADSORBATE DYNAMICS AT SURFACES IN POROUS-MEDIA STUDIED BY NUCLEAR-MAGNETIC-RESONANCE METHODS - THE ORIENTATIONAL STRUCTURE FACTOR AND LEVY WALKS

Diffusion of adsorbate molecules along surfaces of porous media was ex amined with respect to ordinary and Levy walk diffusion mechanisms. Th e orientational structure factor formalism of the ''reorientation medi ated by translational displacements'' (RMTD) mechanism originally deri ved for ordinary diffusion is generalized to Levy walks. The two cases can be distinguished experimentally using field-cycling NMR relaxomet ry. The low-frequency spin-lattice relaxation dispersion is influenced by the dynamics on the surfaces as well as by the surface geometry. T he experiments were carried out with polar and nonpolar liquids filled into porous glasses and fine particle agglomerates (ZnO, TiO2). The s pin-lattice relaxation dispersion of polar and nonpolar adsorbate spec ies shows dramatic differences, and reflects the limits of ''strong'' and ''weak'' adsorption, respectively. The low-frequency behavior is e xplained by RMTD along the surfaces. At temperatures below the freezin g point of the confined liquids, one or two molecular diameter thick s urface layers remain unfrozen. Molecular dynamics in the interfacial l iquid in these nonfreezing surface layers (NFLs) were also studied. Th e propagators relevant for RMTD are shown to depend on whether the sam ple is frozen or not. In the NFL case, an ordinary Gaussian displaceme nt distribution function applies, whereas a Levy walk surface diffusio n process with a Cauchy distribution tends to dominate in the strong-a dsorption limit. On a much longer length scale beyond the so-called re tention time when diffusion becomes normal, field gradient NMR diffuso metry was applied. Confinement of the liquid adsorbate to the pore spa ce or, in frozen samples, to the NFL reduces the diffusion coefficient mainly due to the geometrical restriction. In the case of NFLs, the r eduction amounts to one order of magnitude relative to the bulk values . (C) 1998 American institute of Physics. [S0021-9606(98)70340-X].