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首页> 外文期刊>The Journal of Chemical Physics >The anomalous adsorbate dynamics at surfaces in porous media studied by nuclear magnetic resonance methods. The orientational structure factor and Levy walks
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The anomalous adsorbate dynamics at surfaces in porous media studied by nuclear magnetic resonance methods. The orientational structure factor and Levy walks

机译：通过核磁共振方法研究的多孔介质表面的异常吸附动力学。定向结构因素与征程

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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]. [References: 31]
机译：关于普通和利维步行扩散机制，研究了吸附剂分子沿多孔介质表面的扩散。最初为普通扩散而衍生的“由平移位移介导的重新定向”（RMTD）机制的定向结构因子形式主义被概括为Levy walk。可以使用场循环NMR弛豫法通过实验区分这两种情况。低频自旋晶格弛豫色散受表面动力学以及表面几何形状的影响。实验是通过将极性和非极性液体填充到多孔玻璃和细颗粒附聚物（ZnO，TiO2）中进行的。极性和非极性吸附物种类的自旋晶格弛豫分散度显示出巨大的差异，并分别反映了“强”和“弱”吸附的极限。低频行为由沿表面的RMTD解释。在低于密闭液体凝固点的温度下，一层或两层分子直径厚的表面层保持不冻结。还研究了这些非冻结表面层（NFL）中界面液体的分子动力学。显示与RMTD相关的繁殖器取决于样本是否被冷冻。在NFL情况下，使用普通的高斯位移分布函数，而具有柯西分布的Levy行走表面扩散过程往往在强吸附极限中占主导地位。当扩散变得正常时，在比所谓的保留时间更长的长度尺度上，应用了场梯度NMR扩散法。主要由于几何限制，将液体被吸附物限制在孔隙空间或冷冻样品中限制在NFL会降低扩散系数。在NFL的情况下，相对于体积值，减少量达到一个数量级。 （C）1998美国物理研究所。 [S0021-9606（98）70340-X]。 [参考：31]

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