Acid−Base Equilibria of Weak Polyelectrolytes in Multilayer Thin Films

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Abstract

In this paper, we report on the local apparent dissociation constants of poly(acrylic acid) and poly(allylamine hydrochloride) incorporated in polyelectrolyte multilayer thin films. We assembled 10 polyelectrolyte layers on colloidal silica by the sequential electrostatic adsorption of the polyacid and polybase from aqueous solutions at different pH values and then measured the zeta potential as a function of the solution pH to determine the pKa(app) of each surface layer. The results suggest that the dissociation constant decreases upon adsorption for poly(acrylic acid) and increases in the case of poly(allylamine hydrochloride). These deviations from ideal behavior can be substantial, changing by as much as 4 pH units, and the shifts become more pronounced as the number of adsorbed layers increases. In addition, we found that these pKa(app) shifts are influenced by the pH of the solution used to assemble the thin films but show little dependence on the salt concentration used in the assembly baths.

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