Factors governing the fluorination of hydroxyapatite by an ionic liquid

Fluorapatite exhibits improved bone-bonding and chemical durability under neutral and acidic conditions compared with hydroxyapatite. Typically, an aqueous fluoride solution is used for F− incorporation into the hydroxyapatite. Use of an ionic liquid as F− source provides highly-concentrated ionic environment and can lead to more effective fluorination. In this study, two ionic liquids were added during the synthesis of hydroxyapatite under various conditions, and the differences in the crystalline structure and chemical durability of the resulting fluorapatites were investigated. 1-ethyl-3-methylimidazolium tetrafluoroborate was quite effective for F− incorporation, but not 1-ethyl-3-methylimidazolium hexafluorophosphate. When the temperature was increased to 80 °C, not only the hydroxyapatite or fluorapatite but also CaF2 was generated as a by-product. When the initial pH was decreased to 7, the formed product fully converted to CaF2. The solubility of the product tended to be suppressed in acidic conditions with increase in amount of fluorine substituted in the crystal lattice