Surprising diffusion behaviour in the restricted regions of silicalite

One of the chief uses of porous materials such as zeolites is in the separation of mixtures of hydrocarbons. Molecular sieve properties of an adsorbate-zeolite system are known to depend on the shape and size of the adsorbate and the dimensions of the channels or cages. Molecular dynamics simulations of simple Lennard-Jones particles in silicalite as a function of the adsorbate diameter show unexpected behaviour; the diffusion coefftcient exhibits a peak when the diameter of the adsorbate approaches the channel diameter. This interesting result implies a breakdown of the geometrical criterion and has far-reaching implications in diverse fields such as diffusion across bio-membranes in cells, separation of mixtures by zeolites and host-guest chemistry