Proton and deuteron field-cycling NMR relaxometry of liquids confined in porous glasses
Proton and deuteron field-cycling NMR relaxometry of liquids confined in porous glasses
复制标题
多孔玻璃中限制液体的质子和氘核场循环核磁共振弛豫测量
DOI：
10.1016/0927-7757(96)03610-2
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
V. Skirda
中科院分区：
文献类型：
--
作者：
S. Stapf;R. Kimmich;R. Seitter;A. Maklakov;V. Skirda
多孔玻璃中的极性和非极性液体已通过质子和氘核场循环核磁共振弛豫测定法进行了研究。平均孔径为4nm、30nm和208nm。自旋晶格弛豫时间 T1 的频率依赖性很大程度上受到吸附质极性的影响。与极性物质相比，非极性液体表现出平坦的 T1 色散。我们将这两种情况分别指定为“弱”和“强”吸附。众所周知，表面的液体分子会采取优先取向，但它们仍保持高扩散率。因此，由平移位移（RMTD）介导的重新定向必须对相关函数有所贡献，该相关函数在比整体大八个数量级的时间尺度上衰减。 Bychuk 和 O'Shaughnessy 最近提出的一种称为体介导表面扩散的机制使得表面的高扩散率变得合理。 Bychuk 和 B. O'Shaughnessy，J. Chem。物理学，101（1994）772]。这些作者发现，表面上有效的位移可以描述为 Lévy 行走的结果。因此它们服从柯西分布。这项工作采用柯西分布对表面相关函数 g(rc) 进行数值推导，它正确地再现了基底的典型相关长度，从而使液体分子执行 Lévy 行走的概念变得合理。当自由液体被冷冻时，极性和非极性吸附物的 T1 分散行为的差异消失，而大约两个单层厚的表面膜仍然是液体。无论极性如何，T1 色散都同样陡峭。这表明，限制在极性表面上的拓扑二维薄层中的非极性液体与强吸附的极性液体经历相同的弛豫机制，而强吸附极性液体则以 RMTD 过程为主。
Polar and non-polar liquids in porous glasses have been studied by proton and deuteron field-cycling NMR relaxometry. The mean pore diameters were 4 nm, 30 nm and 208 nm. The frequency dependence of the spin-lattice relaxation time T1is strongly influenced by the polarity of the adsorbate. Non-polar liquids show a flat T1dispersion compared with polar species. We designate the two cases as “weak” and “strong” adsorption respectively. Molecules of liquids at surfaces are known to adopt a preferential orientation, whereas they retain a high diffusivity. Therefore, reorientations mediated by translational displacements (RMTD) must contribute to the correlation function which decays on a timescale of up to eight orders of magnitude greater than in the bulk. The high diffusivity at the surface is made plausible by a mechanism called bulk-mediated surface diffusion recently proposed by Bychuk and O'Shaughnessy [O.V. Bychuk and B. O'Shaughnessy, J. Chem. Phys., 101 (1994) 772]. These authors found that the displacements effective on the surface can be described as the result of Lévy walks. They therefore obey a Cauchy distribution. This work employs the Cauchy distribution for a numerical derivation of a surface correlation function, g( r c ) , which correctly reproduces the typical correlation lengths of the substrate and thus renders the notion of the liquid molecules performing Lévy walks reasonable. The difference in T1dispersion behaviours of polar and non-polar adsorbates disappears when the free liquid is frozen while the approximately two monolayers thick surface film remains liquid. The T1dispersions are then equally steep irrespective of the polarity. This indicates that a non-polar liquid confined to a thin, topologically two-dimensional layer on a polar surface undergoes the same relaxation mechanism as a strongly adsorbed polar liquid which is dominated by RMTD processes.