Unsupported alkoxide-derived silica membranes

Symmetric silica membranes can be prepared as thin unsupported sheets. The process involves partial hydrolysis of tetraethyl orthosilicate which is then poured into polystyrene dishes. The thickness of the layer in the dish is less than 1 mm and completion of the hydrolysis and gelling take place through incorporation of atmospheric moisture. The layer detaches from the bottom of the dish at an early stage such that the microstructure is symmetric with pores about 10 nm in size. The porosity is continuous through the thickness of the gel. The gels dry to rigid foils which fit into a commercial membrane holder in line with a gas stream. The symmetric membranes show gas permeabilities which are inversely proportional to the square root of the gas species molecular weight.