Polymer adsorption on a patchwise heterogeneous surface

The adsorption of the uncharged polymer poly(vinyl-pyrrolidone) (PVP) on a homoionic Na-kaolinite has been studied. Potentiometric acid-base titrations of kaolinite were performed on samples at different concentrations of sodium chloride. An interpretation in terms of the contributions of the individual surface types has been given. Protons are strongly favored over sodium ions at the basal planes. Some striking similarities were observed between the results of the acid-base titrations and the PVP adsorption experiments. PVP adsorbs readily on at least part of the kaolinite surface showing a high affinity character and an adsorbed amount at the plateau of about 1 mg m(-2) total area. The influence of the pH, electrolyte concentration and multivalent ions on the amount adsorbed at the plateau has been investigated. Increasing the pH or the electrolyte concentration leads to a decrease in adsorption. A model is proposed in which PVP adsorbs on edges and basal planes by different mechanisms. The adsorption of PVP on the edges is strongly pH dependent, that of the plates only weakly. Specifically adsorbed protons at the plates act as anchor sites for PVP segments. Multivalent ions do not influence the proposed adsorption mechanism directly but primarily change the surface area accessible for PVP.