Applying Alkyl-Chain Surface Functionalizations in Mesoporous Inorganic Structures: Their Impact on Gas Flow and Selectivity Depending on Temperature

Porous inorganic capillary membranes are prepared to serve as model structures for the experimental investigation of the gas transport in functionalized mesopores. The porous structures possess a mean pore diameter of 23 nm which is slightly reduced to 20 nm after immobilizing C-16-alkyl chains on the surface. Gas permeation measurements are performed at temperatures ranging from 0 to 80 degrees C using Ar, N-2, and CO2. Nonfunctionalized structures feature a gas transport according to Knudsen diffusion with regard to gas flow and selectivity. After C-16-functionalization, the gas flow is reduced by a factor of 10, and the ideal selectivities deviate from the Knudsen theory. CO2 adsorption measurements show a decrease in total amount of adsorbed gas and isosteric heat of adsorption. It is hypothesized that the immobilized C-16-chains sterically influence the gas transport behavior without a contribution from adsorption effects. The reduced gas flow derives from an additional surface resistance caused by the C-16-chains spacially limiting the adsorption and desorption directions for gas molecules propagating through the structure, resulting in longer diffusion paths. In agreement, the gas flow is found to correlate with the molecular diameter of the gas species (CO2 < Ar < N-2) increasing the resistance for larger molecules. This affects the ideal selectivities with the relation N-2/Ar < Ar/CO2 < N-2/CO2. The influence on selectivity increases with increasing temperature which leads to the conclusion that the temperature induced movement of the C-16-chains is responsible for the stronger interaction between gas molecules and surface functional groups.