Adsorptive bonding to hydroxyapatite I: adsorption of anthranilic acid - the effect of solvents. Search for surface bonding groups for coupling agents to teeth.

A study was initiated to explore the nature of surface-active groups of model compounds that might cause chemisorption of comonomers used in the polymerization of dental resins on hydroxyapatite (the main constituent of bone) and thus effect a strong and durable bond between the resins and hard tooth tissues. In an effort to accomplish this objective, adsorption of anthranilic acid on hydroxyapatite was studied from three solvents: 95% aq. ethanol, dimethyl sulphoxide and methylene chloride. The adsorption is reversible from the first two solvents and the isotherms follow Langmuir plots. Adsorption is irreversible from methylene chloride; a constant amount of adsorbate is removed from different solutions above a certain threshold concentration but all adsorbate is removed from solutions below this threshold concentration. The saturation amounts obtained from the two Langmuir plots and the maximum irreversibly adsorbed amount in the above three solvents are 11.4, 6.0 and 17.2 mmol/100 g, respectively. These, together with the surface area (41.0 m2/g; BET, N2) of the hydroxyapatite, yield the effective molecular areas of 60, 114, and 40 A2, respectively. These areas can be matched, on geometrical considerations, with areas of 60, 110, and 40 A2 of the model adsorbate molecule rotating about; (i) its centre, lying flat on the surface, (ii) its carboxyl group at 45 degrees to the surface, and (iii) the carboxyl group in an upright position, respectively.