Role of tail chemistry on liquid and gas transport through organosilane-modified mesoporous ceramic membranes

To develop hydrophobic ceramic membranes, the permeability of liquids (water, ethanol, hexane, and CO_2(l)) and gases (CO₂ and N₂) were examined in unmodified and surface-modified 1 kDa titania and 5 nm γ-alumina membranes. Octyltrichlorosilane (Cl₃SiC₈H₁₇) and its fluorinated analog trichloroperfluorooctylsilane (Cl₃SiC₂H₄C₆F₁₃) were chosen as modifying agents. SEM/EDS analysis revealed tri-bonding of the silanes to both materials with no evidence of a thick polymerized surface layer. Characterization by CO₂ and N₂ gas permeability indicated Knudsen diffusion in all membranes with minimal CO₂ selectivity, except for 5 nm C8F (CO₂/N₂ = 2.6) where a surface-flow mechanism was apparent. In contrast, surface modification yielded significant solvent- and modifier-dependent differences in liquid permeability. For example, hexane exhibited the greatest permeability in the C8H mesoporous titania and γ-alumina membranes, while liquid CO₂ (fluorophilic) permeability was the greatest with the C8F membranes. As expected, the hydrophobic C8F membranes were impermeable to water. Low gas and liquid permeabilities in the C8F-modified membranes were consistent with large transport resistances due to the bulky fluorinated tails. These results demonstrate that silane surface modification can be used to tailor liquid transport behavior and improve apolar solvent flux in ceramic membranes relative to polar solvents. In addition, these membranes have proven amenable to liquid CO₂, a green solvent alternative.