Abstract
Amorphous calcium phosphate (ACP) was synthesized by dropping rapidly aqueous ammonia into calcium dihydrogenphosphate monohydrate (Ca(H2PO4)2·H2O, MCP) solution. The composition of ACP was given within the wide range of Ca/P atomic ratio 1.25~1.55 and was distinguished to three kinds (H-ACP, M-ACP and L-ACP) by different Ca/P atomic ratios. Thermal behavior of water molecule in three types of ACP was investigated by X-ray diffraction, thermal analysis (TG-DTA) and Fourier transformation infrared spectroscopy (FT-IR). Two kinds, the water molecule in ACP existed adsorbed water and bound water. Distinction of these water molecules was possible from Arrehenius plot by relating between heating rates (β) and upper temperature of endothermic peaks (Tm) with thermal analysis (TG-DTA). The activation energy for dehydration of adsorbed water and bound water until about 125°C was able to determine from the gradient of Arrehenius plot. For example, the activation energy for dehydration of bound water in H-ACP of high calcium type (Ca/P atomic ratio 1.55) was 26.4 kcal/mol. It was considered that bound water was stabilized by increasing Ca/P atomic ratio in ACP.
