Texture and Formation Mechanism of Fibrous Calcium Hydroxyapatite Particles Prepared by Decomposition of Calcium–EDTA Chelates

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Abstract
The texture of fibrous calcium hydroxyapatite (Ca10‐(PO4)6(OH)2, CaHAP) particles that were prepared by the decomposition of calcium–ethylenediaminetetraacetic acid (calcium–EDTA) chelates at 100°C under various pH conditions (pH values of 5–10) was investigated by various means. Well‐crystallized fibrous CaHAPs were produced at pH .6. The stoichiometry of the CaHAPs with a chemical formula of Ca10−x(HPO4)x(PO4)6−x(OH)2−x(H2O)x was improved by increasing the decomposition pH. All the CaHAPs had unit‐cell dimensions of a= 0.9436 ± 0.0003 nm and c= 0.6881 ± 0.0006 nm, exhibiting an enlarged a value. The finding of mesoporosity of CaHAPs by nitrogen gas (N2) adsorption measurement indicated that the CaHAPs were produced by an agglomeration of primary particles. Furthermore, the nonstoichiometric CaHAPs that formed at pH 6 developed ultramicropores, which were accessible to water (H2O) molecules but not to N2 molecules, by the elimination of H2O molecules that were adsorbed in interstices of primary particles in less‐orderly crystallized CaHAPs and/or by dehydration of HPO42− groups. These findings by gas adsorption techniques could give evidence for the agglomeration mechanism to attain a polycrystalline CaHAP, although they exhibited good crystallinity with large size.

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