Synthesis of Hydroxyapatite Nanoparticles by Controlled Precipitation in the Presence of Sodium Dodecyl Sulfate

Abstract

Hydroxyapatite nanoparticles (NPs) have been synthesized by controlled precipitation in the presence of a surfactant, sodium dodecyl sulfate. The sizes, shapes, and structures of the NPs depend on the sequence of mixing the reagents . When the addition of sodium phosphate to a surfactant solution is followed by the addition of calcium nitrate, rodlike hydroxyapatite NPs are formed, the length of which decreases with a rise in surfactant concentration. As the sodium dodecyl sulfate concentration increases, the ζ-potential of the NPs decreases to zero; then, its absolute value grows to –55 mV. This suggests that a surfactant bilayer is formed on the NP surface, with the bilayer shielding the surface of the growing crystals. The shielding leads to the formation of smaller NPs at high surfactant concentrations. When the addition of calcium nitrate to the surfactant solution is followed by the addition of sodium phosphate, the synthesis most likely occurs on the surface of micelles, which partially consist of calcium dodecyl sulfate. Therefore, the sulfate phase remains preserved in the synthesized particles, which is confirmed by X-ray diffraction analysis. At higher surfactant concentrations in the reaction medium, the presence of flexible cylindrical micelles is possible. The precipitation of hydroxyapatite on the surface of such micelles results in the formation of wavy threadlike particles.