Diffusion Path Reversibility Confirms Symmetry of Surface Barriers

The mass transfer resistance encountered by guest molecules entering or leaving a microporous host material (the surface barrier) is often comparable with or even greater than the internal diffusional resistance and therefore has a major effect on the overall rate of molecular uptake and release. However, direct measurement of surface resistance is difficult and this has impeded our understanding of this phenomenon. It has recently been suggested that surface resistance may be asymmetric greater for a molecule leaving the pore than for the one that is entering. To investigate this hypothesis, we have carried out molecular dynamics simulations with a model system, methane in silicalite with partial pore blocking at the external surface. The results confirm complete symmetry of the diffusion paths for adsorption and desorption and therefore of the surface resistance, in accordance with the principle of microscopic reversibility.