Faculty of Physics and Earth Sciences, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany.
Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3109, USA.
Molecules. 2018 Mar 15;23(3):668. doi: 10.3390/molecules23030668.
Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from this pattern, but robust experimental data are scarcely available. Here, we present such data, obtained by monitoring the chain length dependence of the uptake of alkanes in the zeolitic imidazolate framework ZIF-4. A monotonic decrease in diffusivity from ethane to butane was observed, followed by an increase for pentane, and another decrease for hexane. This observation was confirmed by uptake measurements with butane/-pentane mixtures, which yield faster uptake of pentane. Further evidence is provided by the observation of overshooting effects, i.e., by transient pentane concentrations exceeding the (eventually attained) equilibrium value. Accompanying grand canonical Monte Carlo simulations reveal, for the larger alkanes, significant differences between the adsorbed and gas phase molecular configurations, indicating strong confinement effects within ZIF-4, which, with increasing chain length, may be expected to give rise to configurational shifts facilitating molecular propagation at particular chain lengths.
分子扩散通常随着分子尺寸的增加而减缓。在孔径接近客体分子直径的一些主体材料中，会出现偏离这种模式的情况。已经提出了各种理论模型来解释这种偏离模式的现象，但可靠的实验数据却很少。在这里，我们提供了通过监测烷烃在沸石咪唑酯骨架 ZIF-4 中的吸收的链长依赖性而获得的此类数据。观察到从乙烷到丁烷的扩散系数单调下降，随后戊烷增加，而己烷再次下降。这一观察结果通过丁烷/戊烷混合物的吸收测量得到了证实，该测量显示戊烷的吸收更快。通过观察过冲效应（即瞬态戊烷浓度超过（最终达到的）平衡值）提供了进一步的证据。伴随的正则系综蒙特卡罗模拟表明，对于较大的烷烃，吸附相和气相分子构型之间存在显著差异，这表明 ZIF-4 内存在强烈的限制效应，随着链长的增加，可能会导致构象转变，从而在特定链长下促进分子传播。
