Polyelectrolytes. 2. Intrinsic or extrinsic charge compensation? Quantitative charge analysis of PAH/PSS multilayers

Floating multilayers of polyallylamine/polystyrenesulfonate (PAH/PSS) adsorbed to a dimethyldioctadecylammonium. bromide (DODAB) monolayer at the air/water interface (at pH 5.5) are analyzed quantitatively with respect to the internal charge stoichiometry of the polyelectrolytes. To this end the adsorbed amount of polymer is translated into charge per area. It is postulated that the strong acid PSS is fully charged (fully dissociated). The weak polyelectrolyte PAH is also assumed to be fully dissociated. This is vindicated by theoretical considerations and deduced from pH-dependent studies of the isotherms of Langmuir monolayers of negatively charged dimyristoylphosphatidic acid (DMPA) with PAH adsorbed from the subphase. With both polyelectrolytes fully charged the PAH charges significantly overcompensate the PSS charges. The polyelectrolyte charge stoichiometry is not 1:1, and the multilayer must contain some small counterions (Cl-) for charge compensation. It is suggested that this "extrinsic compensation" occurs when the charge density of the polyelectrolytes exceeds a certain threshold. If the distance between the charges is less than the Bjerrum length, the strong binding of the counterions ("Manning condensation") prevents their release upon multilayer formation. This explains why multilayers with PAH with its unusually high charge density contain counterions whereas other polyelectrolyte films are ion-free. Literature data support this hypothesis.