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Abstract: Diffusion invariably involves motion within a medium. An universal behaviorobserved is that self diffusivity exhibits a maximum as a function of the sizeof the diffusant when the diffusant is confined to a medium, as a result ofwhat is known as the Levitation Effect. Such a maximum in self diffusivity hasbeen seen in widely differing medium : microporous solids, dense liquids andclose-packed solids, ions in polar solvents, etc. The effect arises because theforces exerted on the diffusant by the medium in which it is confined is aminimum 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 ofzeolite Y. Several different simulations in which the two atoms of the modeldiatomic species interact with equal strengthexample, $O 2$, the symmetriccase and with unequal interaction strengths example, $CO$, asymmetric caseare 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 alarge enough bond length which fits snugly into the 12-ring window of zeoliteY. For weakly asymmetric case, a weak maximum is seen as a function of the bondlength of the diatomic species. However, for strongly asymmetric case, nomaximum in self diffusivity is seen for all the bond lengths studied. Thisdemonstrates close relation between symmetry and the diffusivity maximum andprovides a direct evidence for the need of force cancellation to observe theLevitation Effect.



Autor: Manju Sharma, S. Yashonath

Fuente: https://arxiv.org/







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