Size-effects in the dissolution of hydroxyapatite: an understanding of biological demineralization

Abstract

A comprehensive dissolution theory that integrates size effects and surface energy control into the overall reaction rate has been developed. This model is successfully applied to the dissolution of hydroxyapatite (HAP) at different undersaturations, using the highly reproducible Constant Composition (CC) method. It is found that the commonly used rate law is not suitable for the dissolution of nanoscale particles where the sizes of the crystallites must be taken into account. Dissolution is induced by the formation of pits and continues with the spreading of their stepwaves. However, only the larger pits (of size greater than a critical value, r^*) are active, with stepwaves contributing to dissolution and the spreading velocities are also dependent on the pit sizes, decreasing with decreasing pit size. Size effects dominate the reaction when the crystallite sizes are of the same order as r^*, resulting in self-inhibition of dissolution and even reaction suppression. A metastable zone of undersaturation manifests itself when particle and pit critical sizes approach each other. These findings are also applicable for in vivo demineralization of biological materials such as tooth enamel. As the biomaterials select nanosized particles as their basic building blocks, the size-effects are magnified and the materials are insensitive to demineralization. These effects confer on biominerals the ability to be stabilized against dissolution in the undersaturated biological milieux.