Crystallization process of carbonate substituted hydroxyapatite nanoparticles in toothpastes upon physiological conditions: an in situ time-resolved X-ray diffraction study.

The crystallization process in recently developed toothpastes, containing nanoparticles of carbonate substituted hydroxyapatite (nano-CHA), was investigated. For this purpose, the non-conventional Energy Dispersive X-Ray Diffraction technique, that demonstrated to be a powerful tool to follow in situ phase transformations, was applied, for the first time, to products of pharmaceutical-cosmetic interest. Two types of toothpastes, containing 15 and 20 wt% of nano-CHA, respectively, have been studied. It was observed that, after mixing the toothpastes with water and saliva in order to reproduce in vivo conditions, a crystallization of nano-CHA takes place. Such process occurs in a characteristic time of (22 +/- 1) min for the toothpaste containing 15 wt% of nano-CHA and of (3.9 +/- 0.5) min for the one containing 20% of nano-CHA. For both toothpastes, a 10% increase in grain dimensions was observed over an average characteristic time of (55 +/- 5) min.