The electrostatic binding of two anionic probe molecules, pyrenetetrasulfonic acid (4-PSA)
and 6-carboxyfluorescein (6-CF), to poly(allylamine hydrochloride) (PAH)/poly(sodium 4-styrenesulfonate)
(PSS) and poly(diallyldimethylammonium chloride) (PDADMAC)/PSS ultrathin multilayer films assembled
onto polystyrene (PS) latex particles has been examined using fluorescence spectroscopy. At the probe
concentrations studied, binding was observed only when the outermost layer was oppositely (positively)
charged to the probe. The amount of probe bound was found to increase linearly with polyelectrolyte
layer number up to about 11 layers (15 nm in thickness for the PAH/PSS films), with saturation binding
occurring beyond 11 layers. The binding data reveal that a minimum of about 10−30% of the cationic
charges of the polycations in the upper region of the multilayer films (where the probe binds) are not
directly electrostatically utilized in the multilayer assembly process through ion-pair binding to oppositely
charged sites on the polyanions. The removal of probe bound to multilayers of different thicknesses upon
subsequent exposure to PSS has also been examined. Evidence that polyelectrolyte multilayer films are
long-range electrostatically coupled systems is presented. The results obtained provide some insight into
the role and extent of electrostatic interactions in polyelectrolyte multilayers.
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