The basis of the transition-site model theory for amorphous solids in general was outlined; the present extension to inorganic glasses has revealed that the transition sites (TS) of this theory, which were the 3-dimensional saddle-points critical in the molecular sieving action, equate to the doorways long recognized in permeation through amorphous silica and other inorganic glasses. The TSM, which views permeation as a primary process, was contrasted with the conventional sorption–diffusion model for permeation. It was pointed out that in the sorption–diffusion model, the widely accepted analysis into two apparently distinct factors – sorption (equilibrium) and diffusion (kinetic) – had the fundamental flaw that these factors were not independent, since both involve the sorbed state. By contrast, the transition-site model focuses on the permeating molecule in only two states: as the free gas, and as inserted in a doorway D; hence the characteristics of these doorways – (unperturbed) diameter σD, spacing λ, and the thermodynamic parameters θ (force constant) and ν (entropy increment) for the insertion process – could be evaluated. The theory was applied to literature data on the permeation through microporous silica hollow-fibre membranes of: Ar, He, H2, N2, O2, CO, CO2, CH4 and C2H4, at 25 to 200C. The derived Arrhenius parameters for the permeation of these gases (excepting He) lead to estimates of the four doorway-parameters: σD, 125pm; λ, ca. 30nm; θ, 0.43nN; ν, 1.7pN/K; these values lie within the ranges of those obtained with the glassy organic polymers. Some “secondary effects”, shown particularly by CO and CO2, were interpreted as host–guest interactions at the doorway. The behavior of He was anomalous, the permeation rising linearly with temperature. This confirmed that the transition-site model could be applied to gas permeation by activated molecular sieving for this type of inorganic membrane.

Permeation of Gases Through Microporous Silica Hollow-Fiber Membranes - Application of the Transition-Site Model. P.Molyneux: Journal of Membrane Science, 2008, 320[1-2], 42-56