Unsupported SiO2-based organic-inorganic membranes .2. Surface features and gas permeation

Gas permeation of unsupported hybrid membranes, obtained by co-hydrolysis of various Si(OEt)(4)/MeSi(OEt)(3)(TEOS/MTES) mixtures, is studied with Ar, He and N-2. The separation performance of these hybrid membranes is higher than for pure SiO2 sol-gel derived membranes and depends on chemical composition: a considerable increase in separation factor alpha(He/N-2) is found as the amount of MTES increases; moreover, gas permeability decreases with increasing organic modification of the network. Membrane characterization is performed by N-2 adsorption-desorption measurements and low-temperature differential scanning calorimetry (DSC) and dynamic contact angle (DCA) analyses. Data are related to chemical composition, affecting both the chemical nature of the surface and gas permeation. Results indicate that gas permeation through hybrid membranes may favour the Knudsen flow model or surface diffusion mechanism, depending on the TEOS/MTES ratio.