Bound Water in Hydroxyapatite Prepared by Wet Process

Abstract

The relationship between heating weight loss and structures of hydroxyapatite(HAp*)prepared by wet process was studied by thermal analysis(TG-DTA), X-ray diffractometry and scanning electron microscopic observation(SEM).
The reaction products, precip itated from aqueous solutions (H₃PO₄ or Na₂HPO₄+CaCl₂) of various pH's(3∼11) at 25, 50 and 100°C, contained about 3∼25 wt% of H₂O. From analysis of heating weight loss and X-ray diffraction patterns of the products, it is presumed that the amounts of H₂O in HAp* expressed as the value of n (HAp(H₂O)) depend on the reaction temperature, pH and concentration of the reagent in the reaction system. The approximate n values were 2 at 100°C, 4 and 6 at 50°C and 6, 8, 12 and 16(amorphous) at 25°C. The precipitates(n≅8, 16) formed at 25°C changed to the materials with n≅6 by the 1 day digestion in solution. From TG-DTA, H₂O of HAp* were mainly dehydrated step by step with unit of 1/2, 1, 2 and 4×N(N=1, 2, 3) mol H₂O.
Kissinger plots analysis revealed that the activation energy (ΔE¹; 1 st step) of dehydration process were 10, 13, 17, 20 and 25∼28 kcal/mol⋅H₂O, depending on the value of n. The lattice constant(c₀=6.84∼6.90Å) of HAp* changed with the value of n. SEM observation indicated that HAp* has a layer structure.
Experimental results s uggest that HAp*, prepared by wet process, has the lameller structure and has stoichiometric bound water in the form of interlameller water.