The surface structure and properties of synthetic colloidal calcium hydroxyapatite, Ca
10(PO
4)
6(OH)
2 (abbreviated as CaHap), were investigated. The CaHap surface possesses three kinds of P-OH groups acting as adsorption sites for H
2O, CO
2, CH
3I, CH
3OH and so on. The number of surface P-OH groups was ca. 2.6 groups nm
−2. When the CaHap was outgassed above 400
oC, the surface P-OH groups were dehydroxylated to form surface P-O-P groups. Based on the results obtained, the surface design of CaHap by modifying it with various inorganic and organic molecules was made. The hydrophilic CaHap surface turned into a hydrophobic surface by modification with monoalkyl phosphates and hexamethyldisilazane. The surface P-O-Si(CH
3)
3 groups of CaHap modified with hexamethyldisilazane were combusted at 500
oC in air to form surface P-O-Si(OH)
3 groups. The number and types of surface P-OH groups of the CaHap could be controlled by treating with pyrophosphoric acid.
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