Applied predictive thermodynamics (ThermAP). Part 2. Apatites containing Ni2+, Co2+, Mn2+, or Fe2+ ions

Apatites are minerals encountered in many fields including geochemistry, nuclear and environmental sciences as well as medicine. This ubiquity is likely related to the diversity of ion substitutions that the apatite structure can accommodate, making of it an excellent “ion reservoir” either in natural settings or for the intentional production of doped systems with tailored properties. Despite this widespread interest for apatite compounds, however, only few studies are dedicated to study their thermodynamic properties. Yet, their knowledge becomes necessary for assessing stability domains and understanding evolutionary trends in solution or upon heating, for example. Recently, the experimental thermodynamics of 33 phosphate apatite compounds (deriving from the composition M10(PO4)6X2) have been reviewed and their comparison allowed the development of the additive predictive model “ThermAP” (Applied Predictive Thermodynamics) capable of adequately predicting properties such as standard enthalpies (ΔHf∘), Gibbs free energies of formation (ΔGf∘), or entropies (S°) at T=298K, for any composition involving ions among M2+=Ca2+, Ba2+, Sr2+, Mg2+, Cd2+, Pb2+, Cu2+, Zn2+ and X−=OH−, F−, Cl− or Br−. Although experimental data for apatites involving other divalent cations such as Ni2+, Co2+, Mn2+ or Fe2+ do not seem to be available, the exploration of apatites doped with these ions is appealing from a practical and fundamental viewpoint, for example for understanding geochemical events, or when using apatite precipitation for the elimination of metal cations from industrial wastewaters, or else for conferring magnetic properties to apatite systems in medicine. Based on multiple physico-chemical correlations, the present contribution extends the additive predictive model ThermAP to Ni-, Co-, Mn(II)- and Fe(II)-doped apatites. It provides for the first time estimations of enthalpies, Gibbs free energies of formation and entropies, unveiling the general stability ranking Mn(II)-apatite>Fe(II)-apatite>Co-apatite⩾Ni-apatite. This additive approach also allows one to estimate these properties for any composition in view of enabling thermodynamic calculations for applicative or fundamental purposes

Thermochemistry of yavapaiite KFe(SO4)2: Formation and decomposition

Yavapaiite, KFe(SO4)2, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO4)2, ΔH°f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol−1.K−1 and ΔG°f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe2O3 + H2O has been calculated, at pH2O = 1 atm, the phase boundary lies at 219 ± 2°C

Purification of biomimetic apatite-based hybrid colloids intended for biomedical applications: a dialysis study

The field of nanobiotechnology has lately attracted much attention both from therapeutic and diagnosis viewpoints. Of particular relevance is the development of colloidal formulations of biocompatible nanoparticles capable of interacting with selected cells or tissues. In this context, the purification of such nanoparticle suspensions appears as a critical step as residues of unreacted species may jeopardize biological and medical outcomes, and sample purity is thus increasingly taken into account by regulatory committees. In the present work, we have investigated from a physico-chemical point of view the purification by dialysis of recently developed hybrid colloids based on biomimetic nanocrystalline apatites intended for interacting with cells. Both Eu-doped (2 mol.% relative to Ca) and Eu-free suspensions were studied. The follow-up of the dialysis process was carried out by way of FTIR, TEM, XRD, pH and conductivity measurements. Mathematical modelling of conductivity data was reported. The effects of a change in temperature (25 and 45 ◦C), dialysis medium, and starting colloid composition were evaluated and discussed. We show that the dialysis method is a well-adapted and cheap technique to purify such mineral–organic hybrid suspensions in view of biomedical applications, and we point out some of the characterization techniques that may prove helpful for following the evolution of the purification process with time

Jarosite stability on Mars

Jarosite, a potassium (sodium) iron sulphate hydrated mineral, has recently been identified on the martian surface by the Opportunity rover. Using recent thermochemical data [Drouet and Navrotsky, 2003, Geochim. Cosmochim. Acta 67, 2063–2076; Forray et al., 2005, Geochim. Cosmochim. Acta, in press], we calculate the equilibrium decomposition curve of jarosite and show that it is thermodynamically stable under most present martian pressures and temperatures. Its stability makes jarosite potentially useful to retain textural, chemical, and isotopic evidence of past history, including possible biological activity, on Mars

Formation and evolution of hydrated surface layers of apatites

Nanocrystalline apatites exhibit a very fragile structured hydrated surface layer which is only observed in aqueous media. This surface layer contains mobile ionic species which can be easily exchanged with ions from the surrounding fluids. Although the precise structure of this surface layer is still unknown, it presents very specific spectroscopic characteristics. The structure of the hydrated surface layer depends on the constitutive mineral ions: ion exchanges of HPO42- ions by CO32- ions or of Ca2+ by Mg2+ ions result in a de-structuration of the hydrated layer and modifies its spectroscopic characteristics. However, the original structure can be retrieved by reverse exchange reaction. These alterations do not seem to affect the apatitic lattice. Stoichiometric apatite also shows HPO42- on their surface due to a surface hydrolysis after contact with aqueous solutions. Ion exchange is also observed and the environments of the surface carbonate ions seem analogous to that observed in nanocrystalline apatites. The formation of a hydrated layer in aqueous media appears to be a property common to apatites which has to be taken into account in their reactivity and biological behavior

Chemical Diversity of Apatites

Apatites can accommodate a large number of vacancies and afford multiple ionic substitutions determining their reactivity and biological properties. Unlike other biominerals they offer a unique adaptability to various biological functions. The diversity of apatites is essentially related to their structure and to their mode of formation. Special charge compensation mechanisms allow molecular insertions and ion substitutions and determine to some extent their solubility behaviour. Apatite formation at physiological pH involves a structured surface hydrated layer nourishing the development of apatite domains. This surface layer contains relatively mobile and exchangeable ions, and is mainly responsible for the surface properties of apatite crystals from a chemical (dissolution properties, ion exchange ability, ion insertions, molecule adsorption and insertions) and a physical (surface charge, interfacial energy) point of view. These characteristics are used by living organisms and can also be exploited in material science

Bone mineral: update on chemical composition and structure

Bone mineral: update on chemical composition and structur

Physico-chemical properties of nanocrystalline apatites: Implications for biominerals and biomaterials

Nanocrystalline apatites play an important role in biomineralisation and they are used as bioactive biominerals for orthopaedic applications. One of the most interesting characteristics of the nanocrystals, evidenced by spectroscopic methods, is the existence of a structured surface hydrated layer, well developed in freshly formed precipitates, which becomes progressively transformed into the more stable apatitic lattice upon ageing in aqueous media. The hydrated layer is very fragile and irreversibly altered upon drying. Several routes leading to different apatite compositions are found in biological systems. The loosely bound ions of the hydrated layer can be easily and reversibly substituted by other ions in fast aqueous ion exchange reactions. These ions can either be included in the growing stable apatite lattice during the ageing process or remain in the hydrated layer. The adsorption properties of nanocrystals appear to be strongly dependent on the composition of the hydrated layer and on ageing. The surface reactivity of the apatite nanocrystals can play a part in different biomaterials and could explain the setting reactions of biomimetic calcium phosphate cements and the possibility of obtaining adherent nanocrystalline coatings on different substrates

Synthesis and characterization of non-stoichiometric nickel-copper manganites

Non-stoichiometric nickel–copper manganites Ni Cu Mn h O were synthesized by thermal decomposition of x y 32x2y 3d / 4 41d mixed Ni Cu Mn C O , nH O oxalates in air at low temperature (623–673 K). X-ray diffraction showed that, x / 3 y / 3 (32x2y) / 3 2 4 2 for a nickel content x $0.1, the oxalates precipitated presented a mixed crystal structure up to a limit value of copper Ni extent, whereas the oxalates obtained with x ,0.1 were not mixed. This could be explained by the intermediate structure Ni of nickel oxalate (b orthorhombic form) between those of copper and manganese (a monoclinic form) oxalates. The structure (a or b) of the mixed oxalates obtained was also investigated and their lattice parameters are given. The Ni Cu Mn h O oxides crystallize in the spinel structure in a wide range of composition and a stabilizing effect x y 32x2y 3d / 4 41d 2 21 of copper was evidenced. They are highly divided (Sw.100 m g ) however Sw tends to decrease with increasing y . Cu The non-stoichiometry d of such nickel–copper manganites was for the first time determined by selective titration (gas chromatography) of the oxygen released during TPR experiments in argon. The technique is presented and the results, along with those obtained with manganese oxide Mn O and nickel manganites synthesized in the same conditions, showed that d 5 8 depended both on the decomposition temperature of the oxalate and on the chemical composition of the oxide. Such results should provide interesting data concerning the cationic distributions of these non-stoichiometric nickel–copper manganites

IR spectroscopic study of NO and CO adsorptions on nonstoichiometric nickel-copper manganites

The adsorptions and co-adsorption of nitric oxide and carbon monoxide on nonstoichiometric nickel and nickel–copper manganites have been investigated in situ by transmission infrared spectroscopy. Time-dependent and temperature-dependent data have been acquired to investigate the nature of the molecule–surface interactions. The surface chemistry of NO was found to be particularly rich, involving numerous surface species (mononitrosyls, dinitrosyls, adsorbed N2O, nitrites/nitrates), whereas that of CO was somewhat simpler. The competitive adsorption of NO and CO was evidenced. However, with time, NO tended in all cases to colonize the surface of the samples, and Cu+ cations were shown to be the only access points of CO to the surface of such oxides in the presence of NO. Finally, a time sequence of reactions was shown in the case of NO adsorption