Hydrothermal synthesis of hydroxyapatite whisker from amorphous calcium phosphate and the effect of carboxylic acid

Hydroxyapatite (Ca-10(PO4)(6)(OH)(2), HAp) whisker was synthesized by hydro thermal crystallization of amorphous calcium phosphate (Ca-3(PO4)(2). nH(2) O, ACP) with 0-5.0 mol.dm(-3) of different carboxylic acid solutions such a s acetic acid, malic acid, and citric acid at 140-220 degreesC, in autoclav e. The products were characterized by X-ray diffractometry, Fourier-transfo rm infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). Th e shape of the products changed by kinds of carboxylic acid used, and the l argest HAp whisker was obtained under conditions such as acetic acid of con centration 1.0 mol.dm(-3), hydrothermal temperature of 200 degreesC, and ho lding time of 3 h; the resulting whisker was hexagonal prismatic with 100 m um in length and 4 pm in diameter, and had Ca/P atomic ratio of 1.56 +/-0.0 2. The formation of HAp whisker is ascribed to adsorption of dimer-carboxyl ic acid on the a-plane of HAp. Accordingly, carboxylic acid promoted growth toward the c-axis. The existence of dimer-carboxylic acid was also influen ced by the acid dissociation constant of carboxylic acid. Synthesis conditi ons for HAp whisker was calculated from the dissociation constant.