Surface free energies and steam stability of methyl-modified silica membranes

Abstract

Methyl-modified silica membranes have been prepared by acid-catalyzed co-hydrolysis and condensation reactions of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES). The surface wettability, micro-structure and gas permeance of the methyl-modified silica membranes were investigated. The values of dispersion force γ S d , dipole force γ S P and hydrogen bonding force γ S h to the surface tensions for the silica membranes were evaluated by the extended Fowkes equation. The surface free energy and surface wettability of the silica membranes decrease greatly with the increasing of MTES/TEOS ratio mainly because of the contribution of hydrogen bonding force. FTIR analysis shows that the reason is the increase of Si–CH3 group amount and the decrease of O–H group amount on the surfaces of silica membranes. After aging in steam circumstances, the increase of surface free energies results from the increase of O–H amount present on the silica membrane surfaces. The methyl ligands can make the mean pore size and total pore volume of silica membrane larger. Compared with the unmodified silica membrane, the gas permselectivities of the MTES-modified silica membranes have no obvious decrease in despite of the greatly increase of gas permeation rates. As the silica membranes are aged in steam circumstances, the decrease of gas permeation rates in the silica membrane with MTES/TEOS = 0 is far more than that in the silica membrane with MTES/TEOS = 0.8 while their H2/CO2 selectivities have no notable change.