Controlling ion transport through multilayer polyelectrolyte membranes by derivatization with photolabile functional groups

Increasing the net fixed-charge density in multilayer polyelectrolyte membranes using postdeposition reactions results in large enhancements of ion-transport selectivity. To control the fixed-charge density in poly(acrylic acid)/poly(allylamine hydrochloride) (PAA/PAH) films, we partially derivatized PAA using 2-nitrobenzyl bromide. The underivatized -COO- groups still allow adsorption of PAA/PAH membranes on a permeable support, while postdeposition UV irradiation of these films cleaves the 2-nitrobenzyl esters to form fixed-charge sites. Diffusion dialysis experiments show that PAA/PAH membranes prepared with 23%, 50%, and 65% 2-nitrobenzyl esterified PAA exhibit Cl-/SO42- selectivities of 100, 150, and 170, respectively (after photolysis). Underivatized PAA/PAH membranes show a selectivity of only 10. The order of magnitude increase in selectivity resulting from derivatization occurs with a minimal decrease in Cl- flux. By modifying PAH with photolabile 2-nitrobenzyloxycarbonyl groups, we also introduced net, fixed positive charge into PAA/PAH films and improved cation (Na+/mg(2+)) selectivity. Transport simulations suggest that both Donnan exclusion and selective diffusion contribute to selectivity.