The equilibrium probability distribution of n methane molecules, adsorbed in the interior of n α-cages of all-silica ZK4 zeolite, was modelled using a modified hypergeometric distribution where the effects of mutual exclusion between particles were extracted from lengthy molecular dynamics simulations. The trajectories were analyzed in terms of time-correlation functions for fluctuations in the occupation number of the α-cages. The analysis revealed the correlations induced by the spatial distribution of the adsorbed molecules coupled with a migration mechanism in which a molecule could pass one-by-one from one α-cage to another. These correlations led to cooperative motion, which manifested itself as a non-exponential decay of the correlators.

Understanding Diffusion in Confined Systems: Methane in a ZK4 Molecular Sieve. a Molecular Dynamics Simulation Study. Dementis, P., Fenu, L.A., Suffritti, G.B.: Journal of Physical Chemistry B, 2005, 109[38], 18081-7