The reason for the difference at the varied pH values could besought from solubility isotherms. According to the literature data[24–26], the solubility isotherms were calculated and shown inFig. 7. It is clear that CaF2 is the most stable phase at pH<4.2,which explained the occurrence of CaF2 in Fig. 6a. In the pH rangeof 4.2¨10, apatite is the stable phase and have a minimumsolubility around pH=9. Although, when pH values were recycledto 9, CaF2 was transformed into the stable apatite phase as shownby Fig 6b, no rod-like morphologies were developed, whichindicated that the pH recycling method was not suitable to thepreparation of FHAp nanorods. In contrast, surfactant-mediatedcrystal growth provided a convenient means for the preparation ofFHAp nanorods. Tween80 altered the growth rates of crystalsurfaces through adsorbing on them and thus developed the FHApnanorods
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