Polyelectrolyte multilayer (PEM) films formed by layer by layer (LbL) adsorption of polycations and polyanions are attractive as the skins of separation membranes because of their simple deposition, minimal thickness, and wide range of transport properties. Variation of the polyelectrolyte type as well as deposition parameters such as supporting electrolyte concentration, polyelectrolyte concentration, duration and temperature of adsorption allow tailoring of films for specific separations. This dissertation examines the effect of ion-exchange sites in PEM films on ion separations as well as removal of model organic pollutants by PEM membranes. The properties of poly(styrene sulfonate) (PSS)/ poly(diallyldimethylammonium chloride) (PDADMAC) films vary dramatically with the number of polyelectrolyte layers deposited. Attenuated total reflectance infrared spectroscopy shows that coatings with fewer than 7 PDADMAC/PSS bilayers do not absorb significant amounts of SCN- or Ni(CN)42-, but films with >7 bilayers exhibit an ion-exchange capacity of about 0.5 moles per L of film. For silicon-supported PSS/PDADMAC films terminated with PSS, zeta (ζ) potentials change from negative to positive as the number of adsorbed bilayers increases. These changes in film properties dramatically affect ion transport through (PSS/PDADMAC)nPSS-coated alumina membranes. The Cl-/SO42- selectivities of these membranes are >30 with (PSS/PDADMAC)4PSS films but only 3 with (PSS/PDADMAC)6PSS films. Trends in ζ potentials and selectivities with increasing numbers of bilayers are consistent with the exponential growth mechanism where a polycation adsorbs throughout the film to create large numbers of anion-exchange sites, and during polyanion deposition some of the polycation diffuses to the surface of the film to complex with polyanions from solution. Polycation that is not electrically compensated by the polyanion affords anion exchange sites, and the presence of this fixed positive charge yields decreased Cl-/SO42- selectivity. Deliberate introduction of anion-exchange sites in PEMs can greatly enhance selectivities in cation separations. Immersion of PSS/poly(allylamine hydrochloride) (PAH) membranes in aqueous CuCl2 or FeCl3 solutions yields adsorbed Cu2+ or Fe3+ ions that serve as anion-exchange sites. Remarkably, the Na+/Mg2+ selectivity of (PSS/PAH)4 films on 50 kDa polyethersulfone membranes increases from 17.3 to 109.4 after immersion in 5 mM FeCl3 for 10 min. Moreover, this modification is effective even after 18 h of cross-flow nanofiltration. Reflectance FTIR spectroscopy confirms the formation of anion-exchanges sites after Fe3+ or Cu2+ adsorption.Due to its low pressure requirements, nanofiltration is a promising technique for removal of endocrine disrupting and pharmaceutically active compounds from waste water. However, the rejection of small organic compounds by commercial membranes may be too low for effective water treatment. This work shows that coating of commercial NF-90 membrane with a bilayer of poly(acrylic acid)/PAH increases acetaminophen rejection from 65% to 85%. Moreover, (PSS/PAH)7 on porous alumina shows a caffeine rejection of 93% along with a solution flux of 1.33 m3/(m2day) at 4.8 bar.
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