Dispersion of hydroxyapatite nanocrystals stabilized by polymeric molecules bearing carboxy and sulfo groups

Abstract

Calcium phosphate buildup is important for biological processes (e.g., bones and teeth growth) and negatively affects the efficiency and functioning of water systems (e.g., cooling water systems and boilers) as a result of scale buildup by salts precipitation. Thus, stable aqueous dispersions of calcium phosphate particles can effectively prevent the formation and buildup of scale in water systems. In this study, the stabilization of calcium phosphate nanocrystals (NCs) was studied by forming a supersaturated solution in the presence of poly acrylic acid/2-acrylamide-2-methylpropane sulfonic acid p(AA/AMPS) containing carboxy and sulfo group functionalities. Calcium phosphate NCs were formed and subsequently aggregated in a supersaturated aqueous solution containing 2-phosphonobutane-1,2,4-tricarboxylic acid without any copolymers. Aggregation of calcium phosphate NCs was suppressed in the presence of polymeric molecules containing carboxy groups, whereas molecules bearing sulfo groups were ineffective in dispersing calcium phosphate NCs. However, copolymers bearing both functional groups enhanced the dispersibility of calcium phosphate NCs. Using a combination of light scattering and surface charge measurements along with electron imaging, we found that the inhibition of calcium phosphate precipitation originates from hydroxyapatite nanocrystals (HANCs) with a size smaller than 100 nm in solution. While the carboxy groups in the copolymer adsorbed on the surface of the HANCs, the sulfo groups provided these species with an overall negative surface charge, thereby increasing their colloidal stability via electrostatic repulsion. These results indicated that the aggregation of the HANCs can be effectively hindered using sulfo/carboxy bifunctional copolymers.