Molecular simulation of the kinetic selectivity of a model silica system

Equilibrium and non-equilibrium molecular dynamics (MD) simulation results are presented for the transport of low pressure gases composed of both structureless (spherical) and linear molecules within and through membranes generated by randomly 'etching' silicon atoms and concomitant oxygens from an atomistic model of dense amorphous silica. The MD simulation results demonstrate that while an infinite medium possessing the morphological properties of the moderately dense etched glass is non-percolating, thin layers of this material are kinetically selective. In agreement with earlier results for carbon molecular sieves, it is shown that the permeabilities of individual gases within a layer of the etched glass depend exponentially on the layer (or membrane) thickness. The relationship between the finite length scale of the microvoids of the medium and the size and shape of the diffusing gas molecules is explored.