α-Tricalcium phosphate hydrolysis to octacalcium phosphate:: effect of sodium polyacrylate

alpha-Tricalcium phosphate (alpha-TCP) hydrolysis into octacalcium phosphate (OCP) has been investigated in phosphoric acid solution at different concentrations of sodium polyacrylate (NaPA). The hydrolysis process has been followed by powder X-ray diffraction, infrared absorption and scanning electron microscopy analyses. In the absence of the polyelectrolyte, alpha-TCP undergoes a complete transformation into OCP in 24 h. The presence of polyacrylate in solution inhibits the hydrolysis so that a NaPA concentration of 0.5 mum is sufficient to lengthen the time required to complete the hydrolysis to 4 days. The variation of Ca2+ concentration in the soaking solution suggests that the transformation occurs through alpha-TCP dissolution followed by OCP precipitation. The delayed OCP nucleation and growth in the presence of polyacrylate implies a preferential adsorption of the polyelectrolyte on the growing, OCP crystals, which induces an anisotropic reduction of the coherence lengths of the perfect crystalline domains. (C) 2002 Elsevier Science Ltd. All rights reserved.