Effect of sodium polyacrylate on the hydrolysis of octacalcium phosphate

Octacalcium phosphate (OCP) hydrolysis into hydroxyapatite (HA) has been in vestigated in aqueous solutions at different concentrations of sodium polya crylate (NaPA). In the absence of the polyelectrolyte, OCP undergoes a comp lete transformation into HA in 48 h. The hydrolysis is inhibited by the pol ymer, which is significantly adsorbed on the crystals, up to about 22 wt.%. A polymer concentration of 10(-2) mM is sufficient to cause a partial inhi bition of OCP to HA transformation, which is completely hindered at higher concentrations. The small platelet-like crystals in the TEM images of parti ally converted OCP can display electron diffraction patterns characteristic either of OCP single crystals or of polycrystalline HA, whereas the much b igger plate-like crystals exhibit diffraction patterns characteristic of OC P single crystals. The polyelectrolyte adsorption on OCP crystals is accomp anied by an increase of their mean length and by a significant reduction of the coherence length of the perfect crystalline domains along the c-axis d irection. It is suggested that the carboxylate-rich polyelectrolyte is adso rbed on the hydrated layer of the OCP (100) face, thus inhibiting its in si tu hydrolysis into HA. (C) 2000 Elsevier Science Inc: All rights reserved.