Intermediate Water on Calcium Phosphate Minerals: Its Origin and Role in Crystal Growth

Water molecules are known to play crucial roles both in the formation and biological function of materials. Herein, we show the presence of "intermediate water" on an inorganic solid material, hydroxyapatite. In vitro experiments revealed that Mg substitution of apatite significantly enriched the amount of intermediate water, possibly due to the proton transfer to a hydrogen-bonded network of water around HPO₄^2- on divalent-cation-deficient apatite surfaces. The intermediate water formation related to a markedly suppressed protein adsorption on apatite. Analysis of bone apatites suggested that the intermediate water on minerals could play crucial roles in regulating crystal growth.