The adsorption of two polymeric phosphonates on hydroxyapatite (HA), a simple model of biological hard tissue, was investigated. At low polymer concentrations and a given amount of HA it was found that solution viscosity, the rate of adsorption, the total amount adsorbed, and the resistance to desorption all depend on the ionic strength of the solution and the presence of calcium and phosphate ions. Further, it was found that during adsorption phosphate is liberated into solution in molar amounts exceeding the molar amounts of phosphonate groups present on the adsorbed polymer chains. These results are consistent with an adsorption mechanism in which a phosphonate-containing macromolecule at low concentration approaches an HA surface, causes phosphate to be released from the Stern layer, becomes attached at a few adsorption sites through phosphate/phosphonate exchange, and then rearranges by spreading over the surface and becoming attached at more sites. Higher ionic strengths and calcium binding compress the polyelectrolyte coil. The compressed coil occupies less space and, hence, both the amount and rate of adsorption is enhanced. Phosphate competes for adsorption sites and, hence, inhibits adsorption and promotes desorption. These various parameters are expected to affect the protection which adsorbed polyphosphonates afford HA mineral against acid dissolution, and, hence, are of potential importance in dental and medical applications.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Surfaces, Coatings and Films
- Colloid and Surface Chemistry