Surface adsorption of fluoride and ionic exchange reactions on hydroxyapatite

Area of Science:

• Dental science
• Biomineralization
• Materials science

Background:

• Fluoride uptake by hydroxyapatite is crucial for dental health.
• Understanding the chemical state of fluoride in teeth is essential for effective treatments.

Purpose of the Study:

• To investigate the chemical form and binding mechanism of fluoride adsorbed by hydroxyapatite and teeth at low concentrations and neutral pH.
• To differentiate between fluoroapatite formation and other fluoride binding states.

Main Methods:

• Experimental analysis of fluoride uptake by hydroxyapatite and teeth.
• Alkali solubility tests (1N KOH, 24 h) to determine fluoride binding.
• Assessment of calcium fluoride formation conditions.

Main Results:

• A significant portion of fluoride uptake was alkali-soluble, indicating it was not fluoroapatite.
• Calcium fluoride did not form due to solubility limits.
• Alkali-soluble fluoride is proposed to be adsorbed onto calcium ions in the hydroxyapatite/enamel hydration layer.

Conclusions:

• Fluoride interaction with hydroxyapatite involves adsorption to calcium ions, not solely fluoroapatite formation.
• This adsorption mechanism is analogous to how other anions and charged polymers interact with hydroxyapatite surfaces.
• A revised model for fluoride interaction with hydroxyapatite is proposed.