Interaction of Zinc Ions with Hydroxylapatite

Abstract

The interaction between zinc ions in aqueous nitrate solutions and hydroxylapatite, the structural prototype for the principal inorganic constituent of tooth and bone, was studied. The nature of this interaction may be important in elucidating the role of zinc oxide commonly dissolved in the phosphoric acid solutions used to etch hard tooth tissues prior to the application of dental resins. This study has also a direct bearing on the suitability of zinc ions as candidate “mordants” for use with surface-active comonomers that promote adhesion between restorative resins and hard tooth tissues.

The reaction of zinc ions with hydroxylapatite diminishes with time even at dilute concentrations, and is not complete after several days at room temperature. X-ray analysis shows the formation of hopeite [Zn₃(PO₄)₂.4H₂O)] and the zinc analogue [Zn₂ (OH)PO₄] of libethenite. The initial phase of the interaction may be explained by two proposed rate processes occurring sequentially. One involves the exchange of the surface calcium ions with the zinc ions in solution and the other seems to depend inversely on the amount of adsorbed zinc ions. The rate constant of the second kinetic process is a linear function of the ionic strength of solution.