Variation of ion-exchange capacity, zeta potential, and ion-transport selectivities with the number of layers in a multilayer polyelectrolyte film.

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 of (PDADMAC/PSS)n films deposited on a Ge crystal shows that coatings with fewer than 7 PDADMAC/PSS bilayers do not absorb significant amounts of SCN- or Ni(CN)4(2-) but coatings with more than 7 bilayers exhibit an ion-exchange capacity of about 0.5 mol/L of film. Consistent with ion-exchange, Ni(CN)4(2-) is the anion that is predominantly absorbed from equimolar mixtures of SCN- and Ni(CN)4(2-), even though SCN- initially exchanges into the film more rapidly than Ni(CN)4(2-). For silicon-supported PSS/PDADMAC films terminated with PSS, zeta potentials change from negative to positive as the number of adsorbed bilayers increases, presumably because of a high number of anion-exchange sites inside the film. These changes in film properties dramatically affect ion transport through (PSS/PDADMAC)nPSS-coated alumina membranes. The Cl-/SO4(2-) selectivities of these membranes are >30 with (PSS/PDADMAC)4PSS films but only 3 with (PSS/PDADMAC)6PSS films. Trends in zeta potentials and selectivities with increasing numbers of bilayers are consistent with the exponential growth mechanism, where a polycation absorbs 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. Apparently, not all of the charge on the polycation is compensated by the polyanion; therefore, anion-exchange sites remain in the film, and the presence of this positive charge yields decreased Cl-/SO4(2-) selectivity.