Adsorption of potassium N-phenylglycinate on hydroxyapatite: role of solvents and ionic charge

Abstract

The adsorption of the potassium salt of N -phenylglycine (KNPG) on synthetic hydroxyapatite from aqueous and ethanol solutions was studied at 22°C. The adsorption isotherm of KNPG from aqueous solutions is Langmuirian in shape, and the analysis showed that one glycinate ion was adsorbed per about two (100) faces of the unit cell of hydroxyapatite. For each glycinate ion adsorbed, 3.5 phosphate ions were released to the aqueous solution while the calcium concentration decreased slightly. The adsorption from ethanol (99.8%) was total and irreversible from dilute solutions up to a threshold concentration, and reversible and Langmuirian thereafter from concentrated solutions, while the amounts of phosphate and calcium ions in solution are negligible. At maximum coverage from ethanol solution, the surface is fully occupied by reversibly adsorbed molecules which are perched on a one-to-one basis on the top of irreversibly adsorbed molecules totally covering the substrate. The amount of irreversibly adsorbed solute is thus about equal to the reversibly adsorbed solute and about twice the maximum amount adsorbed from aqueous solutions. That the adsorption involves electrically neutral molecular units and not electrically charged ions from ethanol is demonstrated by the irreversibly and maximally adsorbed molecules which totally occupy the surface, showing no ionic repulsion between them, and by the absence of ionic exchange of the molecules with the substrate. The study may have a bearing on the role of solvents upon the chemistry of N -phenylglycine analogs and their salts, both of which are used as surface-active polymerization initiators in the bonding of prosthetic resins to teeth.