Determination of the activation energies of phase transition for calcium orthophosphates based on powder X-ray diffraction data

Kinetic studies of the transformation of calcium orthophosphates metastable precipitates were performed under different synthesis conditions. Phase composition and degree of crystallinity were investigated by X-ray powder diffraction analysis. In acidic solution, precipitates of CaHPO4⋅2H2O (DCPD) and CaHPO4 (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 is precipitated, which is explained by the high degree of supersaturation (i.e., the high rate of precipitation compared to that in acidic media). The diffraction peaks of DCPD and DCPA are found to be 0.3–0.45 degrees lower relative to their reference data, which is caused by decrease of lattice strain during early stage of crystallization. Furthermore, the initial molar ratio of Ca/P in reagent mixture was found to play subordinary role in determining the composition of final calcium phosphate precipitates. The effect of pH on the composition of precipitates is illustrated by the solubility isotherms of pure calcium orthophosphates. Given that the intensities of diffraction peaks are proportional to planar density of the material in the given plane, we propose, for the first time, to determine activation energy of phase transformation of calcium orthophosphate from X-ray powder diffraction patterns. Based on this relationship developed, the activation energy for the recrystallization DCPD and DCPA are 10.2 and 13.1 kJ/mol, respectively and for the phase transition of DCPD to DCPA – 36.7 kJ/mol. Further recrystallization to most thermodynamically stable Ca10(PO4)6(OН)2 hydroxyapatite (HA) occurs at the activation energy of 5.2 kJ/mol. These findings are critical on phase transition and transformation of calcium phosphate minerals.

Purification of precursors of calcium orthophosphates synthesis by co-precipitation method

The purification of the synthesis precursors of calcium phosphates from the toxic microimpurities Cu(II), Cd(II), As(III) and Pb(II) by co-precipitation with a part of the target product was studied. It was found that a maximum extraction of Cu(II), Cd(II) and As(III) from the CaCl2 solutions was achieved in the acidic and alkaline media. When precipitating calcium phosphates from the H3PO4 solutions, the following patterns regarding the degree of co-precipitation of the microimpurities with increase of pH were observed: the degree of co-precipitation of Cd(II) decreases rapidly, whereas the degree of the co-precipitation of As(III) goes through the maximum and Cu(II) is removed completely. Pb(II) ions are also completely removed in both CaCl2 and H3PO4 solutions. It was shown that in order to purify the CaCl2 solution, it is necessary to add at least 15 g of H3PO4 per 1 dm3 of the solution and then adjust pH to the value of 2.5 to 3.0 by neutralizing the solution with ammonia. In order to remove the impurities from the phosphoric acid solution, it is recommended to carry out the co-precipitation in two following stages: firstly, the ions of Cd(II), Cu(II) and Pb(II) are removed at pH of 6.0–6.5, which requires at least 4 g of CaCl2 per 1 dm3 of the acid, and then the arsenic residues are removed at pH of 8.0–8.25, which requires at least 13 g of CaCl2 per 1 dm3 of the solution.

Synthetic amorphous calcium phosphates (ACPs):preparation, structure, properties and biomedical applications

Amorphous calcium phosphates (ACPs) represent a metastable amorphous state of other calcium orthophosphates (abbreviated as CaPO4) possessing variable compositional but rather identical glass-like physical properties, in which there are neither...

Microwave-Assisted Fabrication of Mesoporous Silica-Calcium Phosphate Composites for Dental Application

Herein, the microwave-assisted wet precipitation method was used to obtain materials consisting of mesoporous silica (SBA-15) and calcium orthophosphates (CaP). Composites were prepared through immersion of mesoporous silica in different calcification coating solutions and then exposed to microwave radiation. The composites were characterized in terms of molecular structure, crystallinity, morphology, chemical composition, and mineralization potential by Fourier-transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), and scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDX). The application of microwave irradiation resulted in the formation of different types of calcium orthophosphates such as calcium deficient hydroxyapatite (CDHA), octacalcium phosphate (OCP), and amorphous calcium phosphate (ACP) on the SBA-15 surface, depending on the type of coating solution. The composites for which the progressive formation of hydroxyapatite during incubation in simulated body fluid was observed were further used in the production of final pharmaceutical forms: membranes, granules, and pellets. All of the obtained pharmaceutical forms preserved mineralization properties.

Synthesis of calcium orthophosphates by chemical precipitation: re-evaluation of the solubility products of hydroxyapatites

For the first time, a linear correlation was found between the specific solubility products of calcium orthophosphates and their molar ratio Ca/P (the correlation coefficient being R2=0.9742). Nevertheless, the values of solubility products of tricalcium phosphate (pKS=26–29), hydroxyapatite (pKS=116.8) and its Ca-deficient forms (pKS~85) cannot be correlated. We proposed to adjust these values of solubility products in accordance with the obtained correlation dependence as follows: pKS is 40, 155 and 114–155 for tricalcium phosphate, hydroxyapatite and Ca-deficient hydroxyapatite, respectively. The calculated solubility diagrams (isotherms) with adjusted solubility products agree reasonably well with the known experimental data, which could not be explained and were not accepted by the scientific community up to the present day. Based on well-known ideas of chemical thermodynamics, we suggested an explanation of the correlation between the specific solubility products of calcium orthophosphates and their molar ratios Ca/P. The developed model of dissolution process is based on a comparison between crystal lattice energy and hydration energy of calcium ions. The experiments on chemical precipitation of orthophosphates were performed at pH 4–10 and at a constant molar ratio Ca/P=1.5; their results showed that only one single metastable form, calcium hydrogen phosphate, is precipitated within the entire pH range at the temperature of 200С. The obtained sediments were isothermally exposed at the temperature of 2500С in a mother solution for 6 hours; such a treatment resulted in a full dehydration of calcium hydrogen phosphate and its partial transformation into a more stable form (hydroxyapatite). The mass fraction of hydroxyapatite increases from 14% to 70% and the degree of crystallinity decreases from 20% to 5% with increasing the value of solution pH. It was concluded that the rate of phase transition from CaHPO4 to Ca10(PO4)6(OH)2 in influenced by the concentration of hydroxide ions that are incorporated into a crystal lattice of hydroxyapatite.

Synthesis of Calcium Orthophosphates by Chemical Precipitation in Aqueous Solutions: The Effect of the Acidity, Ca/P Molar Ratio, and Temperature on the Phase Composition and Solubility of Precipitates

Studies on chemical precipitation of the calcium orthophosphates have shown that their phase compositions do not vary depending on molar ratio Ca/P but are sensitive to solutions acidity and temperature. These are two key factors that determine the phase transformation progress of metastable phases into less soluble precipitates of the phosphates. It was proposed to compare calcium orthophosphates solubility products with calcium cations quantities in their formulas. It was found that there was a linear correlation between calcium orthophosphates specific solubility products and their molar ratios Ca/P if hydroxyapatite and its Ca-deficient forms were excluded from consideration. It was concluded that the relatively large deviations of their solubility products from the found correlation should be thought of as erroneous data. That is why solubility products were changed in accordance with correlation dependence: pKS for hydroxyapatite was 155, pKS for Ca-deficient hydroxyapatites was 114–155. The solubility isotherms, which were calculated on the basis of the corrected pKS values, coincided with the experimental data on solid-phase titration by Pan and Darvell.

Functionalized calcium orthophosphates (CaPO4) and their biomedical applications

Due to the chemical similarity to natural calcified tissues (bones and teeth) of mammals, calcium orthophosphates (abbreviated as CaPO4) appear to be good biomaterials for creation of artificial bone grafts.