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 examined with respect to ordinary and Levy walk diffusion mechanisms. The orientational structure factor formalism of the "reorientation mediated by translational displacements" (RMTD) mechanism originally derived for ordinary diffusion is generalized to Levy walks. The two cases can be distinguished experimentally using field-cycling NMR relaxometry. The low-frequency spin-lattice relaxation dispersion is influenced by the dynamics on the surfaces as well as by the surface geometry. The experiments were carried out with polar and nonpolar liquids filled into porous glasses and fine particle agglomerates (ZnO, TiO2). The spin-lattice relaxation dispersion of polar and nonpolar adsorbate species shows dramatic differences, and reflects the limits of "strong" and "weak" adsorption, respectively. The low-frequency behavior is explained by RMTD along the surfaces. At temperatures below the freezing point of the confined liquids, one or two molecular diameter thick surface layers remain unfrozen. Molecular dynamics in the interfacial liquid in these nonfreezing surface layers (NFLs) were also studied. The propagators relevant for RMTD are shown to depend on whether the sample is frozen or not. In the NFL case, an ordinary Gaussian displacement distribution function applies, whereas a Levy walk surface diffusion process with a Cauchy distribution tends to dominate in the strong-adsorption limit. On a much longer length scale beyond the so-called retention time when diffusion becomes normal, field gradient NMR diffusometry was applied. Confinement of the liquid adsorbate to the pore space or, in frozen samples, to the NFL reduces the diffusion coefficient mainly due to the geometrical restriction. In the case of NFLs, the reduction amounts to one order of magnitude relative to the bulk values. (C) 1998 American institute of Physics. [S0021-9606(98)70340-X].