Adsorption of Phosphonylated Polyelectrolytes on Hydroxyapatite

The adsorption of phosphonylated polyphenylene oxide onto hydroxyapatite was investigated as a basis for biomedical applications involving surface modification of teeth and bone. The half-acid, half-ester form of the polymer was found to adsorb rapidly and the rate and amount increased with concentration. Phosphate, but not Ca++, is released into solution during adsorption. The ratio of released phosphate to polymer-bound phosphonate absorbed from solution, is about 2.5 at very low initial polymer concentrations but decreases rapidly to about 0.5 with increased concentration. For equivalent ionic strengths, added salts increase the rate and amount of polymer adsorbed in the order: CaC12 > KC1 = KNO3 > K2HPO4 > no salt. Desorption is enhanced by K2HPO4. These results can be explained by an adsorption mechanism that is controlled by the degree of charge neutralization along the polymer chain. Intrachain charge repulsion results in an extended random-coil conformation and adsorption in a thin layer with binding at many polymer sites. Charge screening, neutralization and chelation cause the random-coil to collapse. A collapsed conformation occupies only a relatively few binding sites and thus adsorption results in the formation of a thick layer. The conformation and binding of adsorbed layers are expected to be of major importance in surface-controlled phenomena such as adhesion, drug delivery, and flocculation.