Unifying criteria for dissolution kinetics of various hydroxyapatite preparations

A powder suspension method has been used to study the dissolution rate behavior of various hydroxyapatite (HAP) preparations in acidic media containing different levels of calcium and phosphate. Despite the reported variabilities in the properties among differently prepared samples, the present study indicates that, with regard to the dissolution rate behavior, there is an important common characteristic dissolution rate component. There appears to be a dissolution site common to most HAP preparations that may be associated with the predominant surface of the HAP crystal. Also, an attempt is made to correlate the dissolution rate data with the study of Young and Holcomb on the properties of different HAP preparations.     

Adsorption mechanism of polylysine on hydroxyapatite and its effect on dissolution properties of hydroxyapatite

Adsorbed amounts of poly-l-lysine (pLys) and bromide ion on hydroxyapatite (HAp) from aqueous solutions of poly-l-lysine hydrobromide, and amounts of calcium and phosphate ions liberated concurrently from HAp during the adsorption of pLys were determined at 25 degrees . The pLys was adsorbed on HAp by the mechanism of ion-exchange between its amino groups and calcium ions of HAp. The released amount of calcium ion increased, therefore, with the adsorbed amount of pLys. On the other hand, the released amount of phosphate ion first decreased and then increased after attaining a minimum with the equilibrium concentration of pLys. The analysis using an equilibrium dialysis method revealed that the released phosphate ions were mainly in the bound state to the amino groups of pLys remaining in the solution, and that the concentrations of calcium and phosphate ions free from both HAp and pLys were restricted by each other under the law of the solubility product of HAp. The first decrease in the released amount of phosphate ion was concluded to be attributed primarily to the increase in the released amount of calcium ion because pLys remaining in the solution was little in this region. When sodium hydroxide was added to the solution, the adsorbed amount of pLys increased and then slightly decreased with the equilibrium pH of the solution due to the increase or decrease of the electrostatic attractive force between the adsorbate and the adsorbent. However, conformational change in pLys around pH 10 seemed to have little effect on the adsorption.     

On the dissolution and interfacial properties of hydroxyapatite

An acid—base cyclic titration procedure has been developed to elucidate the dissolution behavior of hydroxyapatite. The effects of various inorganic acids, stannous fluoride and citric acid on the solubility of hydroxyapatite are interpreted in terms of the cyclic titration results and electrophoretic mobility measurements. This study confirms the previous results that fluoride ions are essential to achieve any significant reduction in acid dissolution. Although both the fluoride and citrate ions make the zeta potential of hydroxyapatite highly negative, the fluoride reduces the solubility whereas the citrate enhances the dissolution. The differences are explained in terms of the manner in which the complex ions adsorb. The influence of other reagents on dissolution is less significant.     

Incongruent dissolution of hydroxyapatite in the presence of phosphoserine

Concentrations of phosphate and calcium ions, liberated from the surface of hydroxyapatite (HAP) during the adsorption of phosphoserine (PSer), were determined at 30°C. HAP showed a marked incongruent dissolution behavior in the presence of PSer. That is, the concentration of phosphate ion in solution increases with the addition of PSer due to the ion-exchange between PSer and phosphate ion on HAP (molar ratio of the former to the latter=32), whereas the concentration of calcium ion decreases with this release of phosphate ion, because the solubility product of HAP restricts the concentrations of both ions in solution (calculated values of — log (Ca²⁺)¹⁰ (PO ₄ ^3– )⁶ (OH^–)² were 115.8±1.0). The affinity of PSer to HAP was highest at pH 5.8 where the PSer and the HAP surface had the opposite charges. This electrostatic attraction force between PSer and HAP was shielded to some extent by the addition of KCl.     

The kinetics of zinc dissolution in nitric acid

The dissolution of zinc in 0.48–1.49M HNO₃ was studied at 15–25°C, by following simultaneously the concentration changes of the reactants (Zn and HNO₃), intermediate (HNO₂) and product (Zn²⁺) with time. Explicit mechanisms were suggested for the dissolution of zinc in nitric acid. The kinetics of the dissolution process show that it is of the first-order with respect to [Zn] and [HNO₂]. The data obtained show that the dissolution process is diffusion-controlled. The mechanism of zinc dissolution is compared with the mechanism of copper dissolution.

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Die Kinetik der Auflösung von Zink in Salpetersäure

Zusammenfassung Die Auflösung von Zink in 0.48–1.49M HNO₃ wurde bei 15–25°C mittels gleichzeitiger Verfolgung der Konzentrationsänderungen der Reaktanden (Zn und HNO₃), des intermediären HNO₂ und des Produkts Zn²⁺ untersucht. Es wird ein Mechanismus vorgeschlagen. Die Kinetik der Auflösung ist erster Ordnung bezüglich [Zn] und [HNO₂]. Die Daten zeigen, daß der Auflösungsvorgang diffusionskontrolliert ist. Der Mechanismus der Auflösung von Zink wird mit dem der Kupferauflösung verglichen.

     

Self-assembly and influence of the organic counterion in the ternary systems dodecylamine/acrylic acid/water and dodecylamine/methacrylic acid/water

Due to complete proton transfer from the acid to the amine, a reaction between an equimolar mixture of dodecylamine and (meth)acrylic acid leads to the formation of dodecylammonium (meth)acrylate. The latter can be considered as a surfactant with a polymerizable organic counterion. The ternary phase diagrams of the two systems dodecylamine/acrylic acid/water and dodecylamine/methacrylic acid/water are described. Both systems can form isotropic solutions and lyotropic liquid crystalline lamellar phases. Moreover, the system with the methacrylate counterion can also form a cubic phase in the water-rich part of the phase diagram. The difference in the self-organization observed for the two systems is explained by the greater bulkiness and hydrophobicity of the methacrylate. Whereas the acrylate counterion behaves rather like a classic inorganic counterion, the methacrylate counterion resides in the outermost part of the aggregates, giving rise to a change in the surface curvature.     

The influence of hydroxyapatite modification on the cross-linking of polydimethylsiloxane/HAp composites

Hydroxyapatite (HAp) was modified by the action of various hydrophobic agents based on silicon-containing compounds. The influence of the type of applied agent on the thermodynamic and kinetic parameters of the cross-linking of poly(dimethyl siloxane)/HAp composites was investigated. All the modified HAp particles became hydrophobic and these samples were used to synthesize the polysiloxane/hydroxyapatite composites (PDMS/HAp). The possible modes of interaction between the hydroxyapatite and hydrophobing agents were discussed. The most probable interaction between hydroxyapatite and the applied hydrophobing agents is hydrogen bonding. PDMS/HAp composites were formed directly in the cell of the DSC and cross-linking was investigated in situ. It was determined that the introduction of hydroxyapatite into polysiloxane matrices changed the enthalpy of cross-linking, as well as the activation energy of cross-linking and reaction order, while the introduction of modified HAp led to thermodynamic and kinetic parameters more similar to those of the cross-linking of unfilled elastomer.     

The kinetics of the dissolution of monoclinic pyrrhotite in aqueous acidic solutions

Using the rotating disc method, the rates of dissolution of natural monoclinic pyrrhotite, FeS_1.14, in oxygen-free aqueous solutionsS([H⁺]=0.1, [Na⁺]=0.9, [ClO ₄ ^– ]=1.0 mol kg^–1) were determined. In the temperature range 40–90 °C the dissolution reaction occurs under kinetic control; the activation energy being 14±1 kcal mol^–1 (50±5 kJ mol^–1).

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Die Kinetik der Auflösung von monoklinem Pyrrhotin in sauren wäßrigen Lösungen

Zusammenfassung Die Auflösungsgeschwindigkeit von natürlichem monoklinen Pyrrhotin, FeS_1.14, wurde in sauerstofffreien LösungenS([H⁺]=0.1, [Na⁺]=0.9, [ClO ₄ ^– ]=1.0 mol kg^–1) mit Hilfe der Methode der rotierenden Scheibe bestimmt. Im Temperaturbereich von 40–90° erfolgt die Auflösungsreaktion kinetisch kontrolliert, wobei eine Aktivierungsenergie von 14±1 kcal mol^–1 (59±5 kJ mol^–1) gefunden wurde.

     

Contribution to the kinetics of the process of dissolution

Some characteristics of the kinetic equation of dissolution are discussed, bearing in mind that in the course of the process, along with the concentration of the solution, the surface of the soluble substance is also changed. The opportunity for the appication of an exponential kinetic equation is pointed out. Methods are proposed for calculating the solubility and also for using the integral forms of the exponential equation. The meaning of the coefficient of proportionality is explained.

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Modeling the environment influence on the anodic metal dissolution

The influence of the electrolyte composition on the anodic indium dissolution is studied by the RRDE method. A model is designed for the active dissolution of indium according to the hydroxide and anion mechanisms. The model is used for calculating polarization curves and the limiting current of the oxidation of intermediate ions of univalent indium on the ring electrode at different solution compositions and electrode rotation rates. The calculated curves agree well with the experimental ones, which confirms validity of the model. The paper illustrates the way one can analyze the active metal dissolution kinetics in the case of the hydroxide and anion mechanisms. Dedicated to the ninetieth anniversary of Ya.M. Kolotyrkin’s birth.     

The effect of sulfide ion chemisorption on the kinetics of gold dissolution in cyanide solutions

The effect of sodium sulfide additions (from 5 × 10^?6 to 2 × 10^?5 M) on the kinetics of gold dissolution in cyanide solutions of the following composition, M: 0.1 KCN, 0.02 KAu(CN)₂, 0.5 K₂SO₄, pH 10–13 is studied. Hydrosulfide ions are shown to exert a strong catalytic effect on the dissolution kinetics of this metal in a potential range where their adsorption is accompanied by the formation of polysulfides (?0.2 < E < 0.4 V). The reaction acceleration depend on the potential and is 100-fold for E ? 0.1 V. The effect becomes more pronounced as the concentration of hydrosulfide ions increases to 10^?4 M and is almost pH-independent in the pH range from 10 to 13. An attempt to explain the found relationships is undertaken.     

The influence of a radical structure on the kinetics of photopolymerization

A radical initiation ability of new initiating systems in photopolymerization of 2‐ethyl‐2‐(hydroxymethyl)‐1,3‐propanediol triacrylate has been investigated and presented. The evaluation of alkyltriphenyl‐ and tetraalkylborates, iodonium salts, N‐alkoxypyridinium salts, maleimides, phthalimides, 1,3,5‐triazine derivatives and others as a free radical source in combination with suitable photosensitizer for radical polymerization of triacrylate is described. It is assumed that the photochemical decomposition of a coinitiator molecule results in formation of free radicals, which further initiate polymerization. The order of activity of free radical sources on kinetic of photopolymerization was also presented. Different initiator activity can be explained by the difference in the decomposition rate constant and the reactivity of radicals formed toward the double bond of monomer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1575–1589     

The influence of hydrogen on the kinetics of plasmapyrolytic methane conversion

The plasma pyrolysis of methane in the presence of excess hydrogen was kinetically modeled by means of a single pulse shock tube (SPST) technique. Pyrolytic decomposition of methane and the product formation including quenching was investigated as a function of temperature and dwell time in order to elucidate the kinetic role of hydrogen. Methane pyrolysis with an excess of hydrogen leads to lower conversion degrees and lower product yields as compared to pure methane. The decay curves may be described kinetically by first- and second-order ate equations, depending on reaction conditions. An excess of hydrogen causes a decrease in the overall rate constants for the methane decomposition and a straightening of the Arrhenius plots. According to the reaction mechanisms the recombination of primary radicals, especially CH₃, with H₂ molecules and H atoms is significant.     

Microwave assisted dissolution of aluminium oxide samples

The dependence on time of the efficiency of microwave assisted digestion of Al₂O₃ samples with various physico-chemical properties in H₂SO₄ solution (1 + 1) at a temperature of 170 °C and a pressure of 207 kPa has been investigated. Therefore, the concentrations of Al and that of the impurities in the solutions obtained, as well as the masses of residues after different times have been measured. The efficiency of the dissolution was shown to depend on the specific surface area and on the phase-state of the Al₂O₃. The time demand for total dissolution of the samples ranged from 0.1 to 7 hours.     

The kinetics and mechanism of mechanochemical dissolution of chromium in nickel

The regularities of the formation of a solid solution in a Ni-Cr(20 at %) system are studied using X-ray diffraction, optical microscopy, and particle-size distribution analysis within the framework of an energetical approach to the analysis of the kinetics of mechanochemical synthesis. It is established that the curves of the consumption of chromium atoms and the formation of the reaction product (a solid solution of chromium in nickel) coincide with each other. The rate-limiting step of the reaction is the formation of a contact surface between chromium and nickel, while the “stirring” of chromium atoms in nickel matrix has a very high rate. The rate of the formation of the contact surface in the mixture of brittle chromium and plastic nickel is determined by the rate of chromium particle disintegration. To a conversion of about 60%, the reaction kinetics is described by a quadratic dependence on the dose (D) of the mechanical treatment (N ～ D ₂).     

The kinetics of crystal growth and dissolution of strontium oxalate monohydrate

The crystal growth and dissolution of strontium oxalate monohydrate have been studied at 25°C by following the changes in electrical conductivity of supersaturated and subsaturated aqueous solutions seeded with aged crystals of the monohydrate salt. Both processes follow a kinetic equation in which the rate of reaction is proportional to the concentration of the lattice ions. The critical supersaturation or threshold level for homogeneous nucleation and growth of strontium oxalate is considerably lower than that for many other sparingly soluble salts. Sodium pyrophosphate shows a strong inhibiting effect on the rate of crystal growth whereas its influence on the dissolution process is negligible. The thermodynamic association constant for the formation of strontium monooxalate complexes in solution has been determined conductimetrically; the value is 510 l mole⁻¹.     

Microscopic study of hydroxyapatite dissolution as affected by fluoride ions

Fluoride ions play a critical role in preventing tooth decay. We investigated the microscopic effects of fluoride ions on hydroxyapatite (100) surface dissolution using in situ atomic force microscopy. In the presence of 10 mM NaF, individual surface step retraction velocities decreased by about a factor of 5 as compared to NaF-free conditions. Importantly, elongated hexagonal etch pits, which are characteristic of (100) surface dissolution, were no longer observed when NaF was present. The alteration of pit shape is more distinct at a higher NaF concentration (50 mM) where triangular etch pits evolved during dissolution. Furthermore, in a fluoride concentration typical for tap water (10 μM), we observed roughening of individual step lines, resulting in the formation of scalloped morphologies. Morphological changes to individual steps across a wide range of fluoride concentrations suggest that the cariostatic capabilities of fluoride ions originate from their strong interactions with molecular steps.     

The influence of the supramolecular structure of water on the kinetics of vaporization

The kinetics of water vaporization was studied gravimetrically using a Q-1500 D derivatograph with an accuracy of ±5 × 10^?5 g under atmospheric conditions. Various supramolecular structures were created in liquid water using solutions of K, Na, Ba, and Zn chlorides with various concentrations. The kinetic dependences of weight P loss caused by the vaporization of solutions were compared with the data on pure water used to prepare the solutions. The dependence of the rate of vaporization V on the concentration of hydrated ions in solutions was used to show that the rate V is the sum of the rates of vaporization of particles of two types, (a) H₂O molecules and (b) supramolecular formations (H₂O clusters) with H-bonds. As a consequence, a nonlinear (piecewise linear) dependence of the kinetics of vaporization P = f(τ) of water and solutions is observed. The rate of vaporization (V ₁) along the initial P = f(τ) curve portions is substantially (by ～30%) higher than its stationary value (V).