PREPARATION AND PROPERTIES OF SURFACE-MODIFIED CERAMIC MEMBRANES .3. GAS PERMEATION OF 5 NM ALUMINA MEMBRANES MODIFIED BY TRICHLORO-OCTADECYLSILANE

Stable trichloro-octadecyl silane (ODS) derivatives of a 5 nm gamma-al umina ceramic membrane were prepared. Gas permeabilities of the untrea ted membrane did not show Knudsen diffusion at 20 degrees C. Gas perme abilities of the ODS membrane were three orders of magnitude lower; He , Ne, Ar, CO2, C3H8 have near constant permeabilities 360 mol s(-1) m( -2) bar(-1), except methane which has the highest permeability of the group, 481 mol s(-1) m(-2) bar(-1). The mechanism of diffusion is solu tion/diffusion. Remarkably, permeabilities of ODS-alumina membrane wer e reduced by 5 x after exposure to a pressure difference of 1 atm (act ive layer side) against vacuum for only 10 min. The effect was metasta ble but could be reversed on standing for several hours, reversal of p ressure difference or after washing with (hydrocarbon solvent) toluene . The mechanism was presumed to be due to movement of the octadecyl-hy drocarbon chains of the silane monolayer causing a partially blocked p ore structure; perhaps a unique example of self-fouling.