Abstract
AbstractKinetic studies of the transformation of calcium orthophosphates metastable precipitates are performed under different synthesis conditions. The phase composition and degree of crystallinity are investigated by X‐ray powder diffraction analysis. In acidic solution, precipitates of CaHPO 4 . 2H 2 O (DCPD) and CaHPO 4 (DCPA) are formed at the early stage of precipitation, with the degree of crystallinity at the range of 17–35%. Specifically, DCPD precipitates at 30 °C and anhydrous DCPA at 50 °C. In alkaline solution (pH 8–10), only amorphous forms of calcium orthophosphate are precipitated, which is explained by the high degree of supersaturation. The effect of pH on the composition of precipitates is illustrated by the solubility isotherms of pure calcium orthophosphates. It is determined the activation energy of phase transformation of calcium orthophosphate from X‐ray powder diffraction patterns. Based on this relationship developed, the activation energies for the recrystallization DCPD and DCPA are 10.2 and 13.1 kJ mol−1, respectively, and for the phase transition of DCPD to DCPA – 36.7 kJ mol−1. Further recrystallization to most thermodynamically stable hydroxyapatite occurs at the activation energy of 5.2 kJ mol−1. These findings are critical to the phase transition and transformation of calcium phosphate minerals.
Highlights
Among inorganic compounds of calcium and phosphorus, calcium orthophosphates are of particular interest
A detailed understanding of the mechanism of phase transformation during synthesis as well as during dissolution is needed, In the Ca(OH)2–H3PO4–H2O system, the formation of eight individual calcium orthophosphates with a molar ratio of Ca/P in the range from 0.5 to 1.67 are well known: Ca(H2PO4)[2].H2O, Ca(H2PO4)[2], CaHPO4 2H2O (DCPD), CaHPO4 (DCPA), -Ca3(PO4)[2], Ca3(PO4)[2], Ca8(HPO4)2(PO4)4.5H2O and HA
We show that the kinetics of the reaction is controlled by distinct differences in activation energies, which form different metastable phases and limit phase transformations
Summary
Among inorganic compounds of calcium and phosphorus, calcium orthophosphates are of particular interest They are the mineral basis of bone tissue and are considered as promising biomaterials. The changes in physical and chemical properties, including crystallography and thermodynamic parameters in calcium phosphates such as due to ion substitution and phase transformations are well studied in the 20th century 1 It is unclear so far why certain forms of calcium phosphates are precipitated under specific pH of the solution, Ca/P molar ratio, or precipitation method. Tricalcium phosphate is formed by heat treatment above 800°C from calcium and phosphorus precursors at a Ca/P ratio of 1.5 This phosphate is stable at room temperature but hydrolyzed in an aqueous solution, which indicates its metastable nature
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