Physisorption in confined geometry

Abstract

The physisorption of molecules in confined geometry, i.e., in pores of atomic size such as found in zeolites, has been investigated using a simple pairwise-additive Lennard-Jones potential and an effective-medium model. In a spherical geometry, it is found that the equilibrium distance D corresponding to the lowest equilibrium energy is reduced to about 90% of the pair equilibrium distance σe. This originates from the increased dominance of long-range forces in the condensed state. The enhancement of the physisorption energy due to surface curvature and confinement effects reaches its maximum value of 5.05, relative to the flat surface, when D=0.899σe. This value must be compared to the factor of 8 which was derived previously [D. H. Everett and P. C. Powl, J. Chem. Soc. Faraday Trans. 1 72, 619 (1976); E. G. Derouane, J.-M. André, and A. A. Lucas, Chem. Phys. Lett. 137, 336 (1987)] using a simple van der Waals model neglecting repulsion forces. It is also concluded that molecules can be strongly trapped in pores which are substantially narrower than their free (gaseous phase) sizes, the situation of lowest energy corresponding to R=D=0.899σe and the sorption energy remaining negative down to R=D=0.749σe (R denotes the pore radius).