Concerning the novel silica membrane material, the cobalt impregnated in the silica matrix may exist as covalently bound cobalt, forming Si–O–Co. This perhaps facilitates the surface to substitute the –OH groups, along with the hydrophobic –CH3 groups, on the surface making the silica surface hydrophobic and decreasing the possibilities for the burn-off of the methyl groups upon heat treatment at 400 °C in air.