Kimmich R, Stapf S, Maklakov A I, Skirda V D, Khozina E V
Universität Ulm, Sektion Kernresonanzspektroskopie, Germany.
Magn Reson Imaging. 1996;14(7-8):793-7. doi: 10.1016/s0730-725x(96)00165-8.
Diffusion coefficients of 10 different polar and nonpolar liquids filled in porous glasses with mean pore diameters of 4 or 30 nm were determined with the aid of the NMR field-gradient technique. In the time scale of these experiments (0.3 to 500 ms) diffusion coefficients were found to be time independent. Within the experimental error, no influence of the polarity of the adsorbate can be stated. The diffusion coefficients of all investigated fluids in glass with 4 and 30 nm pores were reduced by factors of 0.17 and 0.63, respectively, relative to the bulk values. This relatively weak reduction can be explained by considering the known porosities of the adsorbents. The second objective of this study was to examine the diffusion behaviour below the melting point of adsorbates in porous glass. Fluids confined in pores do not freeze at the bulk freezing temperatures. In this respect, two phases must be distinguished. A maximal two monolayer thick film adsorbed on the inner surfaces does not crystallize at all, whereas the "free" fraction of the fluid in the pores freezes at reduced temperatures according to the Gibbs-Thompson relation. The nonfrozen surface layers form a network in which self-diffusion can be investigated. Experiments have been carried out with cyclohexane. A reduction factor of 0.06 was found relative to the extrapolated values of the entirely unfrozen fluid in porous glass with a mean pore diameter of 30 nm. It is, thus, demonstrated that molecules in adsorption layers virtually retain their translational degrees of freedom along the surfaces. The lowering of the diffusivity is mainly due to the geometric restriction rather than to the interaction with the surface.
借助核磁共振场梯度技术测定了填充在平均孔径为4纳米或30纳米的多孔玻璃中的10种不同极性和非极性液体的扩散系数。在这些实验的时间尺度（0.3至500毫秒）内，发现扩散系数与时间无关。在实验误差范围内，无法说明被吸附物极性的影响。相对于本体值，所有研究流体在孔径为4纳米和30纳米的玻璃中的扩散系数分别降低了0.17倍和0.63倍。这种相对较弱的降低可以通过考虑吸附剂的已知孔隙率来解释。本研究的第二个目标是研究被吸附物在多孔玻璃中熔点以下的扩散行为。限制在孔隙中的流体在本体凝固温度下不会凝固。在这方面，必须区分两个相。吸附在内表面上的最大两层单分子层厚的薄膜根本不会结晶，而孔隙中流体的“自由”部分会根据吉布斯-汤姆逊关系在降低的温度下凝固。未冻结的表面层形成一个网络，在其中可以研究自扩散。已经用环己烷进行了实验。相对于平均孔径为30纳米的多孔玻璃中完全未冻结流体的外推值，发现降低因子为0.06。因此，证明了吸附层中的分子实际上沿表面保留了它们的平动自由度。扩散率的降低主要是由于几何限制，而不是与表面的相互作用。
