Demontis, Pierfranco and Fenu, Luca A. and Suffritti, Giuseppe Baldovino (2005) Understanding diffusion in confined systems: methane in a ZK4 molecular sieve. A molecular dynamics simulation study. The Journal of Physical Chemistry. B, Vol. 109 (38), p. 18081-18087. eISSN 1520-5207. Article.
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The equilibrium probability distribution of N methane molecules adsorbed in the interior of n α cages of the ZK4 zeolite, the all-silica analogue of zeolite A, is modeled by a modified hypergeometric distribution where the effects of mutual exclusion between particles are extracted from long molecular dynamics simulations. The trajectories are then analyzed in terms of time-correlation functions for the fluctuations in the occupation number of the α cages. The analysis digs out the correlations induced by the spatial distribution of the adsorbed molecules coupled with a migration mechanism where a molecule can pass from one α cage to another, one-by-one. These correlations lead to cooperative motion, which manifests itself as a nonexponential decay of the correlators. Our results suggest ways of developing improved lattice approaches that may be useful for studying diffusion in much larger systems and for a much longer observation time.
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