Alternating deposition of oppositely charged polyelectrolytes on porous supports is an attractive way to synthesize selective, ultrathin ion-separation membranes. This study reports the use of Cu2+ as a template in poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) membranes to control fixed charge density and hence enhance anion-transport selectivity. Alternating deposition of PAA partially complexed with Cu2+ and PAH on porous alumina supports followed by removal of Cu2+ and deprotonation results in fixed −COO- sites in these films. Cu2+-templated PAA/PAH membranes show a 4-fold increase in Cl-/SO4 2- transport selectivity compared to pure PAA/PAH membranes deposited under similar conditions. Cross-linking of these templated membranes further increases Cl-/SO4 2- selectivity by an order of magnitude to a value of 610. Changing the top layer of cross-linked, Cu2+-templated membranes from negatively charged PAA to positively charged PAH decreases Cl-/SO4 2- selectivity 10-fold, indicating that Donnan repulsion from a negatively charged surface is a major factor behind selectivity. Modeling studies suggest that selectivity is due to both Donnan exclusion and diffusivity differences among ions.

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-/SO4 2- 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+/Mg2+) selectivity. Transport simulations suggest that both Donnan exclusion and selective diffusion contribute to selectivity.

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