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

Abstract

The present study combines experimental and theoretical approaches to investigate the competitive precipitation of calcium phosphates (CaPs) in aqueous solution in order to understand and control both the structural and textural properties of the synthesized hydroxyapatites (HAps). Some of the precipitation reactions were followed by in situ Raman spectroscopy or achieved under kinetically controlled conditions. The CaP precursors of HAps were identified as a function of the precipitation pH of the medium and the order of introduction of the precursor ions in the synthesis reactor. Their formation was rationalized by calculations based on a homogeneous nucleation model. Depending on the synthesis conditions, precipitation reaction pathways of HAps are proposed by bringing together the kinetic model developed in the present study and our previous thermodynamic model. HAps are complex materials due to the ease with which large amounts of crystallographic defects, such as carbonates and hydrogen phosphates, can be incorporated in their structure. As these defects play a key role in material sciences (bone substitute, heterogeneous acid-base catalysis, etc.), the present work also includes the analysis of the formation of these crystallographic defects in the apatitic framework, allowing a better control of their incorporation through careful selection of operating parameters.

Thermodynamics-Oriented Crystal Growth Enables Lignocellulose-Enticed Nanostructured Hydroxyapatite with Boosted Phosphorus Fertilizer Utility

“…Inspired by the lowest solubility product constant (K sp ) of HAP among those reported calcium phosphates, transforming refractory calcium phosphates (RCPs) such as calcium hydrogen phosphate (CHP, CaHPO 4 ), calcium pyrophosphate (CPP, Ca 2 P 2 O 7 ), and tricalcium phosphate (TCP, Ca 3 (PO 4 ) 2 ) to HAP seems achievable in thermodynamics (see the rationale of Text S1). As previously reported, 20,21 these RCPs could appear in the form of intermediates to formulate HAP when soluble Ca and P precursors arising from PRs were used. It is thus anticipated that directly modulating thermodynamic behaviors of the RCP dissolution−recrystallization process could develop a new synthesis route of nanoenabled HAP.…”

“…Consequently, the percent yield (defined as the ratio of actual yield to calculated yield) was 102.3, 58.9, and 103.3% for mCHP-HAP, mCPP-HAP, and mTCP-HAP, respectively. The minor discrepancy for mCHP-HAP and mTCP-HAP could arise from small deposited carbon over HAP nanoparticles, further evidence of which was provided later, while the significant discrepancy for mCPP-HAP, besides carbonaceous material deposition, probably implied that the recrystallization efficiency of dissociated CPP to formulate HAP was unsatisfactory, likely due to the kinetically sluggish cleavage of phosphate group dimers (P 2 O 7 4– ) to monomers. , With the validated simulation, the TGE related to the hydrothermal process for HAP synthesis was derived (the lower panel of Figure a), finding that the TGE concerning molar HAP made from these RCPs followed the order of mCPP-HAP (−6.01 × 10 6 J/mol) < mCHP-HAP (−3.82 × 10 6 J/mol) < mTCP-HAP (−2.90 × 10 6 J/mol), which corresponded to the change of percent yield. It should be noted that HAP phases were never identified for these RCPs during the simulation cases with the absence of lignocellulose while they were generated experimentally.…”

“…The minor discrepancy for mCHP-HAP and mTCP-HAP could arise from small deposited carbon over HAP nanoparticles, further ) to monomers. 15,21 With the validated simulation, the TGE related to the hydrothermal process for HAP synthesis was derived (the lower panel of Figure 1a), finding that the TGE concerning molar HAP made from these RCPs followed the order of mCPP-HAP (−6.01 × 10 6 J/mol) < mCHP-HAP (−3.82 × 10 6 J/mol) < mTCP-HAP (−2.90 × 10 6 J/mol), which corresponded to the change of percent yield. It should be noted that HAP phases were never identified for these RCPs during the simulation cases with the absence of lignocellulose while they were generated experimentally.…”

Design Strategies and Biomimetic Approaches for Calcium Phosphate Scaffolds in Bone Tissue Regeneration

“…However, more systematic approaches are required to elucidate the fine physicochemical mechanisms determining the ion doping extent and particularly to the mutual interaction between different ions competing to enter in the same crystal sites. In this context, an accurate control of synthesis parameters driving kinetic and thermodynamically-driven phenomena relevant for the formation of the apatite crystal, can be a tool to better control the formation of crystallographic defects and the attainment of ion-doped apatites with more precise design of their chemical composition [ 137 ].…”

Counterintuitive Crystallization: Rate Effects in Calcium Phosphate Nucleation at Near-Physiological pH

“…Other works have observed DCPD and PCAA forming through octacalcium phosphate (OCP) and amorphous calcium phosphate (ACP) precursors, both under thermodynamically and kinetically controlled conditions. , These works, however, used much higher concentrations and faster addition rates, compared to those of the titrations above. This likely leads to kinetic influences affecting all experiments, as the nucleation is not completely under thermodynamic control, even under the experimental conditions described as thermodynamic by the authors.…”