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Surface functionalization of mesoporous Membranes: Impact on Pore Structure and Gas Flow Mechanisms.

Membranes showing monomodal pore size distributions with mean pore diameters of 23, 33 and 60 nm are chemically functionalized using silanes with varying chain length and functional groups like amino-, alkyl-, phenyl-, sulfonate- and succinic anhydride. Their influence on morphology, pore structure and gas flow are investigated. For this, single gas permeation measurements at pressures around 0.1 MPa are carried out at temperatures ranging from 273 to 353 K using He, Ne, Ar, N2, CO, CO2, CH4, C2H4, C2H6 and C3H8. Results show pore size and pore volume linearly depending on the length of the functional molecules as expected for monolayer deposition. However, the gas flow through functionalized membranes is disproportionally decreased up to a factor of around 10. Hence, the decreased pore size and pore volume cannot explain the large drop in flow. Furthermore, there is no specific dependency between the decrease in flow and temperature or gas type other than the relation proposed by Knudsen (√RTM)-1. Considering the large variety of functional molecules used, it is very surprising, that no correlations between the type of functional group and the flow has been found. The decrease in flow, however, is strongly dependent on the chain length of the silanes (factor of 10 at ~ 2 nm length). This leads to the conclusion that the observed effect is not caused by sorption driven processes. It is proposed that steric interactions between functional groups and gas molecules lead to increased residence times on the surface and longer molecular trajectories, which, in turn, leads to a decrease in flow. In membrane design, any surface modification should, therefore, make use of functionalizing agents with chain length as short as possible.