Preparation and assessment of fluorous supported liquid membranes based on porous alumina

Fluorous media have gained a foothold in the synthetic and analytical communities. There is a need for membranes or films that have good selectivity for transport of molecules with fluorous tags. We show here that supported liquid membranes (SLMs) based on modified porous alumina have the desired properties. Hydrophobic alumina membranes were prepared by surface modification with perfluoroalkanoic acids. The initial contact angles of modified alumina membranes exceed 130°. FTIR shows the loss of –OH stretching modes and the gain of carboxylate carbonyl stretch as well as C–F stretching modes indicative of the surface modification. SEM shows no gross changes in the membrane morphology resulting from the modification. The resulting modified membranes readily imbibe and hold fluorous solvents forming SLMs. Cinnamyl alcohol (1) and its ester with 2H,2H,3H,3H-perfluorononanoic acid (HOOC–(CH2)2–(CF2)5CF3) (2c) were employed as solutes in transport experiments with the membranes. Transport selectivity (ratio of permeabilities of 2c to 1) was used as a measure of effectiveness in the optimization of modification conditions. Conditions for the surface modification leading to maximum selectivity for transport of 2c over 1 by the resulting SLM were to reflux membranes in 3 mM Krytox 157FSH/HFE 7100 solution for 3 h. Finally, the influence of membrane pore size on transport rate and selectivity was investigated for cinnamyl alcohol and its esters with fluorous carboxylic acids HOOC–(CH2)2–(CF2)n−1CF3, n = 2, 4, 6, 8. The transport rate for the smallest pore size (20 nm) and largest solute (n = 8) is anomalously high. This may be due to solute being transported in the modification layer as well as solvent.