[M-IS13] Interface- and nano-phenomena on crystal growth and dissolution

[MIS13-P08] Visualization of the ionic behavior in the solvent mediated transformation in calcium phosphate

Shin-ichi Hasegawa², *Jun Kawano¹, Yuki Sugiura³, Takashi Toyofuku⁴, Takaya Nagai¹ (1.Faculty of Science, Hokkaido University, 2.Graduate School of Science, Hokkaido University, 3.AIST, 4.JAMSTEC)

Keywords:calcite phosphate, pH, visualization, solvent mediated transformation

Calcium phosphate is the main component of bones and teeth, and therefore it has been the subject of numerous studies. The most stable phase in this system is hydroxyapatite (HAP); recently octacalcium phosphate (OCP) has attracted attention as a precursor of HAP, but its reactivity in solutions were not fully evaluated. When HAP and metastable phase like OCP coexist in an aqueous solution, the solvent-mediated transformation would proceed. In the present study, the ionic behavior in the solvent mediated transformation of OCP to HAP was analyzed by visualizing pH and Ca²⁺ concentration with fluorescent probes.
When HAP and OCP were placed in an aqueous solution with a distance of about 1 mm between them, each crystal began to dissolve, and after a certain amount of time had passed, a concentration gradient was formed between them. When the distance between two crystals were sufficiently large, Ca²⁺ concentration became higher on the OCP side and lower on the HAP side. On the contrary, when the distance between them were smaller, it was observed that Ca²⁺ concentration on the HAP side became higher than on the OCP side. This means that Ca ions flow from the side of stable phase to that of metastable phase, which is the opposite direction to that expected in a normal solvent-mediated phase transformation. This suggests that, in order to understand such phenomena, it is important to clarify the behavior of individual ions, rather than just the solubility product.