Binding and ion-exchange analysis in the process of adsorption of anionic polyelectrolytes on barium sulfate

The adsorption of poly(acrylic acid) and poly(styrenesulfonic acid) (PSS) is studied on a barium sulfate powder. Comparison of the polyelectrolytes shows that the difference in binding strength corroborates the difference between the ligand strengths with barium ions. The surface excess dependence on pH correlates with the density of the ionized groups. The electrokinetic potential follows the surface coverage and the ionization ratio of the polymer up to the onset of the adsorption plateau, but continues to increase above that point. This peculiarity is explained by the release of barium ions from the adsorption layer into the solution. The phenomenon is attributed to the complexing power of unadsorbed molecules. Analysis of the displacement of small ions (Na+, SO42-) shows that adsorbed PSS releases sulfate ions from the surface and sodium counterions from the polymer. The displacement ratio for sulfate ions (SO42-/PSS monomer units) remains constant over the adsorption isotherm, but that for sodium ions is constant only up to about two-thirds of the maximum coverage. From the data we deduced that about half of the monomer units of the adsorbed PSS molecules bind with surface barium ions. The other half form barium and sodium sulfonates whose ratio varies with the concentration of unadsorbed molecules.