Adsorption of polyacrylic acids and their sodium salts on hydroxyapatite: Effect of relative molar mass

The effect of molar mass on the adsorptive properties of polyacrylic acids and their sodium salts (polymers) has an important bearing on the adhesive mechanisms of restorative resins based on these or similar polymers, The adsorption isotherms of the aqueous solution of polymers were determined on synthetic hydroxyapatite at room temperature to elucidate the role of physico-chemical factors involved in the process. The adsorption of lower molar mass (<90,000) polyacrylic acids shows that the isotherms first rise, reach a maximum, then decrease. In contrast, a higher molar mass (250,000) polyacrylic acids shows no maximum and follows a regular isotherm. The adsorption isotherms of sodium polyacrylates are regular, do not exhibit any maxima, and merge into each other for higher molar mass salts (>60,000), The adsorption is irreversible for the acids as well as for the salts. The concentrations of calcium and phosphate ions were also determined in the filtrates in order to establish any possible correlation with the adsorbed amounts of polymers. The self-association of the polyacrylic molecules by intermolecular hydrogen bonding that may be responsible for the decrease in adsorption at higher concentrations may not play a significant role for higher molar mass acids where individual molecules can fold on themselves as intramolecular hydrogen bonding becomes more dominant. Inter- or intramolecular hydrogen bonding may not play any role in the case of salts because of Coulombic repulsion between the highly charged carboxylate groups.