Size-selective transport of uncharged solutes through multilayer polyelectrolyte membranes

Several recent studies demonstrated highly selective ion transport through multilayer polyelectrolyte membranes. This paper examines the transport of neutral molecules through multilayer polyelectrolyte films and shows significant size-based discrimination among organic analytes. Simple 7-bilayer poly(styrene sulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) films deposited on porous alumina exhibit a glucose/sucrose selectivity of similar to150 in both diffusion dialysis and nanofiltration. However, selectivity among smaller solutes is fairly low (methanol/glycerol approximate to 2 and glycerol/glucose approximate to 8). Because inorganic ions are generally smaller than glycerol, these results suggest that size-based selectivity in ion transport through PSS/PAH films is minimal. High selectivity in nanofiltration by PSS/PAH membranes is accompanied by relatively high solute rejections. For example, 7-bilayer PSS/PAH membranes exhibit a methanol rejection of 70% and a sucrose rejection of >99.9%. Although such high rejections will preclude the use of these membranes in sugar separations, they will allow removal of organic pollutants from water. The high water flux through PSS/PAH films (0.9 m(3)m(-2)d(-1) at 4.8 bar) would also be important in water purification. Capping PSS/PAH films with a few bilayers of poly(acrylic acid) (PAA)/PAH increases glycerol/glucose diffusion-dialysis selectivity from 8 to 75. Thus, controlling film composition allows tailoring of membrane properties. Simulations of nanofiltration and diffusion dialysis data for 7-bilayer PSS/PAH membranes suggest that these films have pores with radii of 0.4-0.5 nm.