对称性、悬浮效应和尺寸相关扩散系数极大值
Symmetry, Levitation Effect and Size Dependent Diffusivity Maximum
摘要：
扩散总是涉及介质内的运动。观察到的普遍行为是，当扩散剂被限制在介质中时，作为所谓的悬浮效应的结果，自扩散率表现出最大值作为扩散剂的尺寸的函数。这种最大的自扩散率已被认为是在广泛不同的介质：微孔固体，致密液体和紧密堆积的固体，离子在极性溶剂等的影响，因为施加在扩散剂的介质中，它被限制的力量是最小的扩散剂的大小，其中自扩散率是最大的。我们在这里报告的模拟双原子物种被限制在笼的沸石Y。几种不同的模拟，其中两个原子的模型双原子物种相互作用的强度相等（例如，$O_2$，对称的情况下）和不等的相互作用强度（例如，$CO$，不对称的情况下）在这里建模。此外，双原子物种的键长是变化的。对称情况下的计算结果表明，当键长足够大时，自扩散系数最大，键长正好与Y型沸石的12环窗口相吻合。对于弱不对称情况，弱最大值被看作是双原子物种键长的函数。然而，对于强不对称的情况下，没有最大值的自扩散被视为所有的键长研究。这表明了对称性和扩散率最大值之间的密切关系，并为观察悬浮效应需要力抵消提供了直接证据。
英文摘要：
Diffusion invariably involves motion within a medium. An universal behavior observed is that self diffusivity exhibits a maximum as a function of the size of the diffusant when the diffusant is confined to a medium, as a result of what is known as the Levitation Effect. Such a maximum in self diffusivity has been seen in widely differing medium : microporous solids, dense liquids and close-packed solids, ions in polar solvents, etc. The effect arises because the forces exerted on the diffusant by the medium in which it is confined is a minimum for the size of the diffusant for which self diffusivity is a maximum. We report here simulations on a diatomic species confined to the cages of zeolite Y. Several different simulations in which the two atoms of the model diatomic species interact with equal strength(example, $O_2$, the symmetric case) and with unequal interaction strengths (example, $CO$, asymmetric case) are modeled here. Further, the bond length of the diatomic species is varied. Our results for the symmetric case shows that self diffusivity is maximum for a large enough bond length which fits snugly into the 12-ring window of zeolite Y. For weakly asymmetric case, a weak maximum is seen as a function of the bond length of the diatomic species. However, for strongly asymmetric case, no maximum in self diffusivity is seen for all the bond lengths studied. This demonstrates close relation between symmetry and the diffusivity maximum and provides a direct evidence for the need of force cancellation to observe the Levitation Effect.
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