INHIBITION OF HYDROXYAPATITE CRYSTAL GROWTH BY PHOSPHONATED HYDROXY CARBOXYLIC ACIDS, NON-PHOSPHONATED HYDROXY CARBOXYLIC ACIDS, AND POLY (ACRYLIC ACID)

Abstract

The influence of inhibitors containing various functional groups (i. e., phosphonate, hydroxyl, and carboxyl) on the kinetics of crystal growth of hydroxyapatite (HAP) has been studied at pH=7.40, 37°C and at low supersaturations. It has been found that the crystal growth of HAP is significantly retarded in the presence of micromolar concentrations of hydroxy phosphonoacetic acid (HPA) and 2-phosphono butane 1, 2, 4- tricarboxylic acid (PBTC), with PBTC being a more potent inhibitor than HPA. Three other inhibitors, α-hydroxy carboxylic acids (i. e., glycolic acid, GLA; malic acid, MLA; mandelic acid, MDA), dicarboxylic acid (malonic acid, MNA), and poly (acrylic acid), PAA, all exhibit inhibitory activity but to a varying degree. Among these inhibitors the overall effectiveness, in terms of decreasing power, is PAA>MLA>MNA>GLA>MDA. The retarding effect of these inhibitors is discussed in relation to the calcium-inhibitor complex formation on the HAP seed crystals and the structural features of the inhibitor molecules. The fit of the experimental data to the Langmuir adsorption isotherm supports a mechanism of inhibition through molecular adsorption of the inhibitor ions on the surface of the growing crystals.