It is well known that chlorhexidine digluconate provides an effective microbicidal activity during oral rinsing, and therefore, it was considered worthwhile to investigate its interaction with hydroxyapatite on a fundamental level. The kinetics of uptake (or reaction) of the compound from aqueous solutions by synthetic hydroxyapatite was studied at 23ยฐC for four time periods by monitoring its concentration. There was no uptake at low concentrations for any time period. The uptake curves for higher concentrations shifted towards the lower concentrations as the period increased and became more and more vertically oriented to the concentration axis. The concentrations of calcium ions increased, phosphate ions decreased and hydrogen ions decreased a little for a given period as the concentration of the compound was increased. All of these experimental facts can be qualitatively explained on the basis of the solubility considerations of hydroxyapatite and of chlorhexidine phosphate, the reaction product that slowly precipitates out of the solution. The needle-shaped birefringent crystals of the phosphate salt are clearly visible in the apatite matrix under a microscope, and its refractive index and differential Fourier transform infrared spectra match almost exactly with those of a well-characterized, synthesized phosphate salt.
To explore the nature of interaction, the uptake of chlorhexidine base was studied from p-dioxane and it is irreversible. The uptake is total below a threshold equilibrium concentration and constant above it. 0 1994 John Wiley & Sans, Inc.
Recently, Sodhi, Grad, and Smith" found, by x-ray photoelectron spectroscopy, the retention of CIHx moieties through electrostatic bonding with phosphate groups of hydroxyapatite, but they found no adsorption of gluconate ions. Unfortunately, these authors failed to notice or propose the precipitation of the phosphate salt as a separate phase.
In general, any ionic adsorption on an ionic surface will be conditioned by the charge-determining and specifically the hydrogen ion concentration of the if the solvent is water. Therefore, the constituent ions, calcium, phosphate, and hydroxyl or hydrogen, should play a very important role in any mechanistic evaluation of the uptake from the aqueous solution of chlorhexidine ion on hydroxyapatite. The impact of the interplay of hydrogen bonding between solute, solvent, and substrate on the uptake process should also be carefully ex-pl~red.''-'~ In this report the interaction of ClHxDG with hydroxyapatite, the structural prototype for the principal inorganic crystalline constituent of tooth and bone, is presented to elucidate the role of the constituent ions on the process. The nature of this interaction was further explored by studying the up-