Synthesis and hydrolysis of octacalcium phosphate: effect of sodium polyacrylate

Abstract

A structural and morphological investigation has been carried out on octacalcium phosphate (OCP) synthesized in aqueous solution in the presence of different concentrations of sodium polyacrylate (NaPA) (from 0.375 to 5 μM). The presence of the polyelectrolyte inhibits the crystallization of OCP and reduces significantly the coherence length of the perfect crystalline domains. The inhibition of the crystal growth along the directions corresponding to the crystallographic axes is almost isotropic and indicates no preferential regular interaction of polyacrylate with the structure of OCP. In agreement, the lattice constants of OCP do not change as a function of the concentration of the polyelectrolyte, which is not significantly adsorbed on the crystals. Scanning electron microscopy (SEM) images of OCP display the presence of spherules and double spheres. The elongated blades which constitute the spherules and the walls of the hollow spheres become shorter with increase in NaPA concentration. OCP has been submitted to hydrolysis at pH 7.4 and 60°C. The hydrolysis is inhibited by NaPA, which prevents the transformation into hydroxyapatite. The results of IR absorption and thermogravimetric analyses (TG-DTG) indicate that the polyelectrolyte is significantly adsorbed on the products of hydrolysis. Adsorption increases on increasing NaPA concentration in solution up to about 22 wt.%, and it probably takes place through interaction between carboxylate ions and the hydrated layers of the (100) face of the OCP structure.