Energetics of Calcium Phosphate Nanoparticle Formation by the Reaction of Ca(NO₃)₂with (NH₄)₂HPO₄

Abstract

The energetics of the reaction between Ca(NO3)2(aq) and (NH4)2HPO4(aq) leading to the formation of calcium phosphate nanoparticles was investigated by flow calorimetry. The relationship between the observed enthalpy change, the pH of the (NH4)2HPO4(aq) solution, and the elemental composition and morphology of the obtained compounds was studied. Results of elemental analysis, combined thermogravimetry−infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy showed that the change of the pH during the precipitation reaction through the addition of controlled amounts of NaOH(s) to the (NH4)2HPO4(aq) solution leads to significant and reproducible changes of the chemical composition, morphology, and amorphous character of the obtained materials. These changes are reflected by the corresponding enthalpy of reaction, which seems to be predominantly determined by the differences in chemical composition.

Biomimetic Route to Calcium Phosphate Coated Polymeric Nanoparticles: Influence of Different Functional Groups and pH

“…To determine the calcium phosphate phase present in the different samples, X‐ray diffraction (XRD) was performed. Theoretically, different calcium phosphate phases like amorphous calcium phosphate, brushite, octacalcium phosphate, or hydroxyapatite, named in the sequence of increasing thermodynamic stability, are possible depending on the loading conditions 39, 40. Furthermore, the progress of the crystallization has an influence on the resulting crystals, since a thermodynamically more stable form can emerge from a kinetically stable phase that was build in an earlier stage of crystallization 41.…”

Nanodimensional and Nanocrystalline Calcium Orthophosphates

“…Furthermore, the initial pH values and reaction temperatures both play important roles in the morphology of the precipitated apatites, as well as on the phase formation and degree of crystallinity [326]. For example, significant differences in the chemical composition, morphology and amorphous character of nano-sized CDHA produced through the reaction between aqueous solutions of Ca(NO3)2 and (NH4)2HPO4 can be induced, simply by changing the pH of the reactant hydrogen phosphate solution [327]. Thus, the solvent systems, dispersant species and drying methods appear to have effects on the particle size and dispersibility.…”

Nanodimensional and Nanocrystalline Apatites and Other Calcium Orthophosphates in Biomedical Engineering, Biology and Medicine

“…In general, the shape, stoichiometry, dimensions and specific surface area of the apatite nanoparticles appeared to be very sensitive to both the reaction temperature (Figure 5) and the reactant addition rate [270,279,285]. Furthermore, significant differences in the chemical composition, morphology and amorphous character of the CDHA nanoparticles produced through the reaction between aqueous solutions of Ca(NO 3 ) 2 and (NH 4 ) 2 HPO 4 can be induced, simply by changing the pH of the reactant hydrogen phosphate solution [374]. Among the methods described, the thinnest crystals of apatite (60 nm × 15 nm × 0.69 or 0.84 nm) have been prepared by Melikhov et al ; they have been called as “two dimensional crystalline HA” [266], while the smallest ones (size between 2.1 and 2.3 nm, i.e.…”