Size-selective transport of uncharged solutes through multilayer polyelectrolyte membranes

Several recent studies demonstrated highly selective ion transport through multilayer polyelectrolyte membranes. This thesis 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 ∼150 in both diffusion dialysis and nanofiltration. However, selectivity among smaller solutes is fairly low (methanol/glycerol ≈2 and glycerol/glucose ≈8). High selectivity in nanofiltration by PSS/PAH membranes is accompanied by relatively high solute rejections. Although such high rejections will preclude the use of these membranes in sugar separations, they may prove useful in applications such as removal of organic pollutants from water. The high water flux through PSS/PAH films (0.9 m3m-2d-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 may allow tailoring of membrane properties for specific separations. Simulations of nanofiltration and diffusion dialysis data for 7-bilayer PSS/PAH membranes suggest that these films have a porosity of 2 to 3% and pores with radii of 0.4 to 0.5 nm.