Permeation porometry: Effect of probe diffusion in the condensate

Permeation porometry, aka “permporometry” is used as a tool to investigate the connected pore size distribution of asymmetric meso-porous membranes. The method is based on the selective filling of specific sized pores with a condensable vapor, and the analysis of a permeating non-condensable gaseous probe through the remaining open pores. The technique is frequently proposed as a convenient means of detecting pinholes in supported membranes. However, artifacts are found to arise in the desorption isotherm due to a combination of chemical diffusion of the probe gas through the pore-blocking liquid, cyclohexane, and the desorption of cyclohexane from the support. These artifacts can be misinterpreted as pinholes and/or overwhelm the contributions of real pinholes. γ-Alumina membranes of various thickness, supported on smooth homogeneous α-Al2O3 compacts, are used to demonstrate the occurrence of these artifacts. Non-condensable probe gasses O2 and Ne are used to identify the crucial difference between the Knudsen and chemical diffusion of the probe through either pinholes or a condensate filled membrane. In addition it is shown how analysis of chemical diffusion permeance through the membrane can be used to obtain the membrane's tortuosity for chemical diffusion.