Development of a thermodynamic approach to assist the control of the precipitation of hydroxyapatites and associated calcium phosphates in open systems

Abstract

The understanding of the precipitation mechanisms of calcium phosphates (CaPs) remains extremely challenging despite its fundamental interest for material sciences and its practical implications in medicine, paleosciences, and chemistry, especially...

On the Comprehensive Precipitation of Hydroxyapatites Unraveled by a Combined Kinetic–Thermodynamic Approach

“…The OCP → HAp transformation will be discussed in detail in section III. The evolution of the Raman contributions shown in Figure 3a for the synthesis carried out at pH 6.5 via the Ca → P route is therefore fully consistent with the earlier thermodynamic 28 (Figure 1Bc) and present kinetic predictions (Figure 1Aa).…”

“…However, despite careful selection of the operating parameters likely to favor a thermodynamic control (a low addition rate of the precursor solution possibly associated with a subsequent aging step and moderate reaction temperatures), in some cases, this thermodynamic modelling of the precipitation of CaPs appeared to be insufficient to account for the observed differences in HAps morphologies, the amounts of hydrogen phosphate defects present in the apatitic crystallographic structure, or the presence of OCP in some samples. 28 The present study provides further insights into the understanding of the precipitation reaction pathways of CaPs with the help of complementary kinetic data. In order to identify the nature of the reaction intermediates leading to the thermodynamically most stable products (DCPA or HAp), two sets of experiments have been carried out: (i) in situ Raman spectroscopy monitoring of the initial steps of syntheses likely to favor a thermodynamic control and (ii) additional syntheses performed under kinetically controlled conditions (a high addition rate of the precursor solution and absence of the aging step).…”

“…CaPs were obtained via a co-precipitation procedure using an automated reactor (Optimax 1001 synthesis workstation from Mettler Toledo, Figure S1). 28 (1.67), an appropriate volume of the calcium solution (200 mL), previously degassed by bubbling N 2 for 15 min, was added from a sample vial into the reactor at controlled speed (50 mL/min).…”

“…The Ca/P bulk ratio of the prepared CaPs was determined following the protocol described in our previous study. 28 Diffuse Reflectance Infrared Fourier Transform Spectroscopy. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded using a TENSOR II spectrometer (Bruker) equipped with a MCT detector.…”

“…In a recent study, we have developed a thermodynamic model adapted to continuous dropwise addition syntheses that enables to distinguish the thermodynamic products (DCPA and HAp) as a function of the pH of the co-precipitation medium. 28 The importance of the synthesis route (Ca → P vs P → Ca) has also been highlighted with the identification of a thermodynamic differentiation region in the 3.8-5.3 pH range at the beginning of the addition step. However, despite careful selection of the operating parameters likely to favor a thermodynamic control (a low addition rate of the precursor solution possibly associated with a subsequent aging step and moderate reaction temperatures), in some cases, this thermodynamic modelling of the precipitation of CaPs appeared to be insufficient to account for the observed differences in HAps morphologies, the amounts of hydrogen phosphate defects present in the apatitic crystallographic structure, or the presence of OCP in some samples.…”