Molecular transport in nanopores: a theoretical perspective
Molecular transport in nanopores: a theoretical perspective
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DOI：
10.1039/c1cp21166h
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发表时间：
2011-01-01
影响因子：
3.3
通讯作者：
Nicholson, David
中科院分区：
文献类型：
--
作者：
Bhatia, Suresh K.;Bonilla, Mauricio Rincon;Nicholson, David
纳米孔中的分子传输在许多新兴的气体分离和储存纳米技术以及纳米流体中起着核心作用。运输理论提供了对影响运输及其相互作用的机制的理解，并可以导致可用于通过过程分析和优化来推进这些纳米技术的易于处理的模型。我们回顾了一些最有影响力的理论在小孔隙和密闭空间的流体输运。从世纪的Knudsen公式出发，讨论了在Maxwell-Stefan扩散方程中具有共同出发点的尘埃气体模型和其他几种相关方法。特别是，模型的概念基础和有效性的假设和必要的简化，以获得其最终结果进行了分析。它示出的吸附的效果经常被忽略，或在一个特设的基础上处理，如通过划分成气相和表面扩散贡献的孔通量。此外，虽然它是司空见惯的假设，横截面的压力是均匀的，它表明，这违反了吉布斯-迪昂关系，它是化学势，基本上保持恒定的横截面，近平衡的密度分布，即使在运输过程中保存。尘埃气体模型和Maxwell-Stefan模型的表面扩散进行了分析，并讨论了它们的优点和缺点，说明了使用冲突的参考框架的选择在前一种情况下，和分配适当的值的二元扩散率在后一种情况下的重要性。振荡器模型，在这个实验室中开发的，这是精确的低密度限制下的漫反射条件下，被证明是代表一个进步的经典努森公式，虽然后者经常出现作为一个基本部分，许多运输模型。分布式摩擦模型，也在本实验室开发的多组分运输在任何努森数的研究进行了讨论，并与以前的方法进行了比较。最后，对本文的理论展望和未来的研究需求进行了讨论。
Molecular transport in nanopores plays a central role in many emerging nanotechnologies for gas separation and storage, as well as in nanofluidics. Theories of the transport provide an understanding of the mechanisms that influence the transport and their interplay, and can lead to tractable models that can be used to advance these nanotechnologies through process analysis and optimisation. We review some of the most influential theories of fluid transport in small pores and confined spaces. Starting from the century old Knudsen formulation, the dusty gas model and several other related approaches that share a common point of departure in the Maxwell-Stefan diffusion equations are discussed. In particular, the conceptual basis of the models and the validity of the assumptions and simplifications necessary to obtain their final results are analysed. It is shown that the effect of adsorption is frequently either neglected, or treated on an ad hoc basis, such as through the division of the pore flux into gas-phase and surface diffusion contributions. Furthermore, while it is commonplace to assume that cross-sectional pressure is uniform, it is demonstrated that this violates the Gibbs-Duhem relation and that it is the chemical potential that essentially remains constant in the cross-section, as near-equilibrium density profiles are preserved even during transport. The Dusty Gas model and Maxwell-Stefan model for surface diffusion are analysed, and their strengths and weaknesses discussed, illustrating the use of conflicting choices of frames of reference in the former case, and the importance of assigning appropriate values for the binary diffusivity in the latter case. The oscillator model, developed in this laboratory, which is exact in the low density limit under diffuse reflection conditions, is shown to represent an advance on the classical Knudsen formula, although the latter frequently appears as a fundamental part of many transport models. The distributed friction model, also developed in this laboratory for the study of multi-component transport at any Knudsen number is discussed and compared with previous approaches. Finally, the outlook for theory and future research needs are discussed.