Van't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
Chem Soc Rev. 2015 May 21;44(10):2812-36. doi: 10.1039/c4cs00440j. Epub 2015 Mar 12.
Molecular diffusion is an omnipresent phenomena that is important in a wide variety of contexts in chemical, physical, and biological processes. In the majority of cases, the diffusion process can be adequately described by Fick's law that postulates a linear relationship between the flux of any species and its own concentration gradient. Most commonly, a component diffuses down the concentration gradient. The major objective of this review is to highlight a very wide variety of situations that cause the uphill transport of one constituent in the mixture. Uphill diffusion may occur in multicomponent mixtures in which the diffusion flux of any species is strongly coupled to that of its partner species. Such coupling effects often arise from strong thermodynamic non-idealities. For a quantitative description we need to use chemical potential gradients as driving forces. The transport of ionic species in aqueous solutions is coupled with its partner ions because of the electro-neutrality constraints; such constraints may accelerate or decelerate a specific ion. When uphill diffusion occurs, we observe transient overshoots during equilibration; the equilibration process follows serpentine trajectories in composition space. For mixtures of liquids, alloys, ceramics and glasses the serpentine trajectories could cause entry into meta-stable composition zones; such entry could result in phenomena such as spinodal decomposition, spontaneous emulsification, and the Ouzo effect. For distillation of multicomponent mixtures that form azeotropes, uphill diffusion may allow crossing of distillation boundaries that are normally forbidden. For mixture separations with microporous adsorbents, uphill diffusion can cause supra-equilibrium loadings to be achieved during transient uptake within crystals; this allows the possibility of over-riding adsorption equilibrium for achieving difficult separations.
分子扩散是一种普遍存在的现象，在化学、物理和生物过程的广泛领域中都很重要。在大多数情况下，扩散过程可以用菲克定律来充分描述，该定律假定任何物质的通量与其自身浓度梯度之间存在线性关系。最常见的情况是，一种物质沿着浓度梯度扩散。本篇综述的主要目的是强调许多导致混合物中一种成分向上游运输的情况。在多组分混合物中，可能会发生向上的扩散，其中任何物质的扩散通量与其伴侣物质强烈耦合。这种耦合效应通常源于强烈的热力学非理想性。为了定量描述，我们需要使用化学势梯度作为驱动力。由于电中性约束，离子物质在水溶液中的传输与其伙伴离子耦合；这种约束可能会加速或减速特定离子。当发生向上扩散时，我们在平衡过程中观察到瞬态过冲；平衡过程在组成空间中遵循蜿蜒的轨迹。对于液体、合金、陶瓷和玻璃的混合物，蜿蜒的轨迹可能会导致进入亚稳态组成区域；这种进入可能导致旋节分解、自发乳化和 Ouzo 效应等现象。对于形成共沸物的多组分混合物的蒸馏，向上扩散可能允许跨越通常禁止的蒸馏边界。对于使用微孔吸附剂的混合物分离，向上扩散可以在晶体内部的瞬态吸收过程中导致超过平衡的负载；这为克服吸附平衡以实现困难的分离提供了可能性。
