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An increasing number of studies in the field of optical chemosensors, organic/inorganic composites and membrane technology invoked the interest in porous glasses. Further requirements are high thermal, mechanical and chemical stability, high optical transparency, and good accessibility to the active sites inside the porous structure.
In presented work, we are focused on quantitative identification of gas permeation in porous glass membrane with the range of pore size diameter between 1.5 and 5nm. The complex mechanism of gas transport in these membranes is still not well understood, however, Knudsen diffusion, surface diffusion and configurational diffusion can be considered as rate controlling mechanisms.
Two different versions of experiments were used for measuring gas permeabilities and equilibrium kinetics through the membranes: steady state and dynamic techniques. Permeabilities of He, Ar, N2, Xe, CO2 and C3H8 have been measured. Both dynamic and steady state experiments gave similar values of the permeabilities. In order to describe the observations, anticipating ideal Knudsen diffusion was a first attempt. For pore diameter around 5nm Knudsen diffusion was confirmed as a dominant mechanism. Surprisingly, in a case of smaller pore size diameter, it turned out that the separation factors between the various gases were much bigger than expected Knudsen ratio. Consequently, activated diffusion [1,2] had also to be considered. For membranes of small pore sizes, so-called surface flow due to activated diffusion can be another additional mechanism, because of strong interactions between gas molecules and the pore walls.
To describe these observations the concept of potential barrier between the gas molecules and the solid surface was applied. That means if the molecule after collision has a kinetic energy bigger than the surface potential energy, it passes the potential surface field, and such a flow is called gas-phase flow. When molecules can not pass through the force field they again collide with the surface [3].
Both versions of experiments gave us a lot of information aiming at to quantify mass transfer processes in porous glass media, to better understand the surface flow in nano-separation facilities and to predict the separation factor in micro and mesoporous range. Further work is currently focused on studing modified surface area of the membrane in order to distinguish various adsorption effects. The micro-porous membranes might have a greater potential for gas separation compared to membranes with larger pores.
[1] Shelekin A. B., Dixon A. G., Ma Y. H., AIChE J., 41, 58 (1995)
[2] Bhandarkar M, Shelekin A. B., Dixon A. G., Ma Y. H., J. Mem. Sci., 75, 221 (1992)
[3] Hwang, S. T. and Kammermeyer K., Ind. Eng. Chem., Fundam., 7, 671 (1968)
[4] Do D. D., Park I. S., Rodrigues A., Catal .rev.- Sci. Eng., 38(2), 189 (1996)
