For many years the loading dependence of the diffusion coefficient of molecules adsorbed in nano-porous materials has been considered as a solved problem; determine the diffusion coefficient at low loading and assume that Darken assumption to hold [1], i.e, the transport diffusion coefficient is independent of the loading. The molecular simulations of Skoulidas and Sholl, however, showed that in zeolites for many cases this Darken assumption did not hold [2] and showed examples in which the diffusion coefficients decreased, increased, or remained constant as a function of loading. Whereas one can understand that with increasing loading the mobility of the molecules decreases resulting in a decrease of the diffusion coefficient, finding a molecular explanation for an increasing diffusion coefficient as a function of loading is less obvious.

In this work we show how changes in diffusion coefficients can be related to changes in the free energies barriers underlying the diffusion behavior[3]. These changes in the free energy barriers can be directly related to differences in adsorption behavior [4,5].

1. A. I. Skoulidas and D. S. Sholl, J. Phys. Chem. B 105 (16), 3151 (2001).
2. A. I. Skoulidas and D. S. Sholl, J. Phys. Chem. A 107 (47), 10132 (2003).
3. E. Beerdsen, B. Smit, and D. Dubbeldam, Phys. Rev. Lett. 93, art. no 248301 (2004).
4. E. Beerdsen, D. Dubbeldam, and B. Smit, Phys. Rev. Lett. 95, 164505 (2005).
5. E. Beerdsen, D. Dubbeldam, and B. Smit, Phys. Rev. Lett. 96, 044501 (2006).