The effect of citric acid addition on chitosan/hydroxyapatite composites

Chitosan/hydroxyapatite (HAp) composites have been prepared using a co-precipitation method. According to TEM observations, chitosan molecules formed nano-particles of 250×50 nm in size with small HAp crystals. The particles aggregated and the precipitates were reached a maximal size (17 μm) at a chitosan content of 25 wt.%, where the ζ-potential was around zero. This means that the precipitate size increased with a decrease in ζ-potential, i.e. electrostatic repulsion between the nano-particles. The precipitate size decreased at chitosan contents higher than 25 wt.% with excess chitosan molecules covering the nano-particles, resulting in a repulsion force between positively charged particles. When citric acid was added to a suspension of chitosan/HAp composite, the precipitates increased in size. Although, when citric acid was added to a pure HAp suspension, the citric acid showed no effect on the size. This finding suggests that citric acid affects chitosan rather than the HAp, through the formation of an ionic complex. This size effect has been found at citric acid contents lower than 3 wt.%. The precipitates have been compressed into a cylindrical shape under a uni-axial pressure, the compressive strength increased with increasing citric acid content. The composition of the maximal compressive strength agreed well with that of the maximal precipitate size.