Microstructure of porous media and field‐cycling nuclear magnetic relaxation spectroscopy
Microstructure of porous media and field‐cycling nuclear magnetic relaxation spectroscopy
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多孔介质的微观结构和场循环核弛豫磁谱
DOI：
10.1063/1.355834
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发表时间：
1994
影响因子：
3.2
通讯作者：
J. Niess
中科院分区：
文献类型：
--
作者：
S. Stapf;R. Kimmich;J. Niess
作为一类特殊的核磁共振（NMR）弛豫机制，考虑了平移位移介导的再取向。这特别指的是其中分子被限制于无序结构的系统，所述无序结构限定（a）局部优先取向并允许（B）具有一定降维的平移自由度。实例是吸附在大分子、颗粒聚集体和多孔介质表面上的分子。因此，分子沿沿着这种限制结构扩散的偶极相关函数不仅反映了分子动力学，而且反映了限制系统的结构性质。使用场循环和其他NMR弛豫技术，在3×102-3×108 Hz范围内测量了几种多孔材料的自旋晶格弛豫时间的频率依赖性。数据进行了数值分析，使用“取向结构因子”，这是引入特设和呈现的表面形成的分子位移的几何限制的波数分布。这个分布由幂律和单峰项组成。结果进行了讨论，相对于微观结构。特征长度尺度可以通过引入在短时间间隔内有效的平移扩散系数的数据来估计。
As a special class of nuclear‐magnetic‐resonance (NMR) relaxation mechanisms, reorientations mediated by translational displacements, is considered. This particularly refers to systems in which molecules are confined to disordered structures defining (a) local preferential orientations and permitting (b) translational degrees of freedom with a certain reduced dimensionality. Examples are molecules adsorbed on surfaces of macromolecules, particle aggregates, and porous media. The dipolar correlation function of molecules diffusing along such confining structures therefore does not only reflect the molecular dynamics but also the structural properties of the confining system. Using field‐cycling and other NMR relaxation techniques the frequency dependence of the spin‐lattice relaxation times of several porous materials were measured in a range 3×102–3×108 Hz. The data were numerically analyzed using an ‘‘orientational structure factor’’ which was introduced ad hoc and which renders the distribution of wave numbers of the surfaces forming the geometrical restrictions of molecular displacements. This distribution turned out to consist of a power‐law and a single‐peak term. The results are discussed with respect to the microstructure. Characteristic length scales can be estimated by bringing in data of the translational diffusion coefficient effective in short time intervals.