Linear free energy relationships between dissolution rates and molecular modeling energies of rhombohedral carbonates

Bulk and surface energies are calculated for endmembers of the isostructural rhombohedral carbonate mineral family, including Ca, Cd, Co, Fe, Mg, Mn, Ni, and Zn compositions. The calculations for the bulk agree with the densities, bond distances, bond angles, and lattice enthalpies reported in the literature. The calculated energies also correlate with measured dissolution rates: the lattice energies show a log-linear relationship to the macroscopic dissolution rates at circumneutral pH. Moreover, the energies of ion pairs translated along surface steps are calculated and found to predict experimentally observed microscopic step retreat velocities. Finally, pit formation excess energies decrease with increasing pit size, which is consistent with the nonlinear dissolution kinetics hypothesized for the initial stages of pit formation.     

Defect induced asymmetric pit formation on hydroxyapatite

Defect sites on bone minerals play a critical role in bone remodeling processes. We investigated single crystal hydroxyapatite (100) surfaces bearing crystal defects under acidic dissolution conditions using real-time in situ atomic force microscopy. At defect sites, surface structure-dependent asymmetric hexagonal etch pits were formed, which dominated the overall dissolution rate. Meanwhile, dissolution from the flat terraces proceeded by stochastic formation of flat bottom etch pits. The resulting pit shapes were intrinsically dictated by the HAP crystal structure. Computational modeling also predicted different step energies associated with different facets of the asymmetric etch pits. Our microscopic observations of HAP dissolution are significant for understanding the effects of local surface structure on the bone mineral remodeling process and provide useful insights for the design of novel therapies for treating osteoporosis and dental caries.     

Correlation of the Pit Depth in Crystal Etching by Dissolution

A theoretically rigorous formulation relating the pit depth to dissolution time for crystal etching is presented and verified. The data of K. Dunn, E. Daniel, P. J. Shuler, H. J. Chen, Y. Tang, and T. F. Yen [J. Colloid Interface Sci. 214, 427 (1999)] for surface dissolution of barite are analyzed and correlated accurately. It is shown that the empirically determined power law pit growth function of F. Hunkeler and H. Bohni [Corrosion, 37(11), 645 (1981)] conforms to a special solution of the present formulation and their empirically determined power law exponent of 1/2 is theoretically justified. In addition, the present study provides some insight into the mechanism of the crystal dissolution rate process and the variation of pit depth during dissolution-induced etching. Copyright 2000 Academic Press.     

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.     

Dissolution of crystallites: surface energetic control and size effects.

Traditional understanding of dissolution assumes that the reaction is spontaneous and continues until equilibrium is reached. This paper presents theoretical and experimental data to support a dissolution mechanism that involves the existence of critical conditions for dissolution, in which the reaction is accompanied by the formation of pits and the subsequent displacement of pit steps. The accompanying increase in surface roughness leads to changes in surface energy with losses of crystal mass that are positive rather than negative and the existence of critical dissolution conditions. Critical pits and dissolution steps are verified experimentally and a relationship between the size and rate of displacement of steps is also demonstrated, in which the rate decreases with size and approaches zero at a critical size, r*. These microscopic step dynamics are consistent with the observed size-effects in bulk dissolution, which cannot be explained using traditional dissolution theories. The observed size effects include self-inhibition, in which the dissolution rate decreases with extent of reaction, dissolution suppression, and periodic resumption. These interesting dissolution phenomena are only readily displayed when the sizes of dissolving crystallites fall in the same range as the critical size (i.e., within 50r*). It is interesting to note that natural biominerals and many nanoparticles fall into this category, so that their suspensions can be dynamically stabilized without dissolution in undersaturated supporting media. The current research implies that dissolution kinetics cannot be understood well without appealing to fundamental physical concepts about the energetic control of dissolution steps on a molecular level. A new dissolution model for crystallites is introduced systemically.     

Formation of artificial micro‐pits on Al alloy with the photon rupture method and the localized corrosion behavior of the formed pits

An in situ artificial micro‐pit fabrication method with an area selective electrochemical measurement technique was applied to investigate the effect of the geometry of artificially formed pits on their localized corrosion behavior in anodized 1000 series aluminum. This technique enables the fabrication of artificial micro‐pits with different aspect ratios (pit depth/pit diameter) in solutions. The aspect ratios of the fabricated artificial micro‐pits in this experiment could be varied from 0.13 to 1.83 by controlling the irradiation time of the focused pulsed YAG laser beam. By applying a constant potential to the final laser‐beam‐irradiated spot in chloride environments, localized dissolution occurred only at the laser beam irradiated area, because the anodic oxide film acted as an insulator. The corrosion current and charge increase with increasing aspect ratio at any applied potential. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparison of secondary-ion mass spectrometry and compleximetric titration for the determination of leaching of magnesium from chrysotile asbestos

The dissolution of chrysotile asbestos in 0.05 M oxalic acid is evaluated by secondary-ion mass spectrometry and by titration of magnesium with EDTA. The results of the two methods agree closely; thus matrix effects in the utilization of sputtering depth profiles of the mass spectrometric determination do not influence the final results.     

Statistical atomic-topographic model of the active dissolution of metals with point lattice defects. The effect of point defects (vacancies and impurity atoms) on the metal dissolution rate

A physicochemical model is put forward for the dissolution (corrosion) of a metal with point lattice defects, i.e., vacancies and impurity atoms with corrosion stability radically different from that of the base metal. The model takes into account the substantial difference in the dissolution rates of the base metal atoms from positions with different numbers of neighboring atoms, which leads to the formation of characteristic nanofragments of the atomic surface relief. These fragments determine the dissolution rate. The point defects substantially affect the dissolution rate of the base metal exclusively due to their active involvement in the formation of the atomic relief. The equations describing this model allow the polarization curves of active dissolution of the base metal to be calculated as a function of the defect concentration.     

Direct evidence of initial pitting corrosion

The chloride ion (Cl⁻) is known as an aggressive species in the aqueous solutions that adsorbs to the imperfect sites on the metal surfaces, such as defects, impurities and second-phase particles. This adsorption process could change the chemical composition and properties, such as ion conductivity, of the passive film. As a result, the passive film becomes less protective and breaks down at some places where the underlying metals are exposed to the electrolyte and dissolved through anodic reaction forming a pit. In the meantime, the hydrolysis (cathodic reaction) occurs inside the pit giving rise to a lower pH and leading to an increased dissolution rate of the metals. Subsequently, more chloride ions migrate into the pit in order to maintain electrical neutrality and adsorb onto the pit surface. The repeating of such process causes the pitting propagation. However, anodic polarization allows the chloride ions to migrate into the passive film more easily. With growth of the pit, more corrosion products make both in plane diffusion across the sample surface and out-of-plane diffusion to bulk solution from the pit mouth [M.G. Fontana, N.D. Greene, Corrosion Engineering, second ed., McGraw-Hill, New York, 1978].     

Electrochemical Corrosion Behavior of Monel Alloy in Carbonate Melts

The interaction of the monel alloy and its corrosion resistance in a melt of alkali metal carbonates in an oxidizing atmosphere was studied. The selectivity of alloy dissolution and modification of the electrode surface after storage at a constant anode potential were analyzed. The generation and development of local corrosion defects (pit corrosion, intercrystallite corrosion, corrosion cracking) on monel alloy (70% nickel, 28% copper), copper, and nickel electrodes in the molten eutectic of lithium, sodium, and potassium carbonates at a working temperature of 773 K were studied. The anode polarization was accompanied by a change in the state of the electrode surface.     

Impact of dissolution on the uncertainty of spent fuel analysis

One of the objectives of the French Alternative Energies and Atomic Energy Commission in the Marcoule Centre is to accurately quantify the composition of nuclear spent fuel, i.e. to determine the concentration of each isotope with suitable measurement uncertainty. These analysis results are essential for the validation of calculation codes used for the simulation of fuel behaviour in nuclear reactors and for nuclear matter accountancy. The different experimental steps are first the reception of a piece of spent fuel rod at the laboratory of dissolution studies, and then dissolution in a hot cell of a sample of the spent fuel rod received. Several steps are necessary to obtain a complete dissolution. Taking into account these process steps, and not only those of analysis for the evaluation of measurement uncertainties, is new, and is described in this paper. The uncertainty estimation incorporating the process has been developed following the approach proposed by the Guide to the Expression of Uncertainty in Measurement (GUM). The mathematical model of measurement was established by examining the dissolution process step by step. The law of propagation of uncertainty was applied to this model. A point by point examination of each step of the process permitted the identification of all sources of uncertainties considered in this propagation for each input variable. The measurement process presented involves the process and the analysis. The contents of this document show the importance of taking the process into account in order to give a more reliable uncertainty assessment to the result of a concentration or isotope ratio of two isotopes in spent fuel.     

Stabilization and acidic dissolution mechanism of single-crystalline ZnO(0001) surfaces in electrolytes studied by in-situ AFM imaging and ex-situ LEED

A combined approach of pH-dependent in-situ AFM topography and ex-situ LEED studies of the stability and dissolution of single-crystalline ZnO(0001)-Zn surfaces in aqueous media is presented. Hydroxide-stabilized and single-crystalline ZnO(0001)-Zn surfaces turned out to be stable within a wide pH range between 11 and 4 around the point of zero charge of pH PZC = 8.7 +/- 0.2. Hydroxide stabilization turned out to be a very effective stabilization mechanism for polar oxide surfaces in electrolyte solutions. The dissolution of the oxide surface started at an acidic pH level of 5.5 and occurred selectively at the pre-existing step edges, which consist of nonpolar surfaces. In comparison, the oxide dissolution along the ZnO(0001) direction proved to be effectively inhibited above a pH value of 3.8. On the basis of these microscopic observations, the mechanistic understanding of the acidic dissolution process of ZnO could be supported. Moreover, both the in-situ AFM and the ex-situ LEED studies showed that the stabilization mechanism of the ZnO(0001) surfaces changes in acidic electrolytes. At pH values below 3.8, the hydroxide-stabilized surface is destabilized by dissolution of the well-ordered radical3. radical3. R30 hydroxide ad-layer as proven by LEED. Restabilization occurs and leads to the formation of triangular nanoterraces with a specific edge termination. However, below pH 4 the surface structure of the crystal itself is ill-defined on the macroscopic scale because preferable etching along crystal defects as dislocations into the bulk oxide results in very deep hexagonal etching pits.     

Understanding the barriers to crystal growth: dynamical simulation of the dissolution and growth of urea from aqueous solution

Both the dissolution and growth of a molecular crystalline material, urea, has been studied using dynamical atomistic simulation. The kinetic steps of dissolution and growth are clearly identified, and the activation energies for each possible step are calculated. Our molecular dynamics simulations indicate that crystal growth on the [001] face is characterized by a nucleation and growth mechanism. Nucleation on the [001] urea crystal face is predicted to occur at a very high rate, followed by rapid propagation of the steps. The rate-limiting step for crystallization is actually found to be the removal of surface defects, rather than the initial formation of the next surface layer. Through kinetic Monte Carlo modeling of the surface growth, it is found that this crystal face evolves via a rough surface topography, rather than a clean layer-by-layer mechanism.     

Determination of Aluminium by Electrothermal Atomization Atomic Absorption Spectrometry in Serum to Characterize Hemodialysis Toxicity

A simple and cost-effective method is described for the determination of aluminum by electrothermal atomic absorption spectrometry (ET-AAS) in serum of hemodialysis patients and healthy subjects. The only preparative step required is the dissolution of the serum sample in 0.2% magnesium nitrate matrix modifier and separate diluents 0.01 M EDTA and 0.1% Triton X-100. The calibration curve was linear from 20 to 100 µg/L with correlation coefficients of 0.9993 and 0.9998 for EDTA and Triton X-100, respectively. The sensitivity of the method for aluminum at the 309.3 nm line was 74 pg. The instrumental and method limits of detection were 2.2 µg/L and 4.4 µg/L, respectively. The aluminum concentrations of forty serum samples from hemodialysis patients and healthy subjects were determined and the mean values were 170.9 ± 6.8 µg/L and 47.3 ± 9.3 µg/L, respectively, whereas the permissible limit for aluminum in blood serum is 10 µg/L. The high level of Al in serum was related to oral phosphate binding agents and dialysis treatment.     

The impact of mineralogy in the U(VI)—Ca—PO4 system on the environmental availability of uranium

Kinetic dissolution studies were conducted on four prominent U-Ca-PO₄ minerals (metaschoepite, becquerelite, chernikovite and metaautunite). Synthetic samples were contacted with four extractants (acetic acid, deionized water, EDTA and sodium bicarbonate) at room temperature at two concentrations, 100 mM and 1 mM. Dissolution progress was monitored by periodic sampling for dissolved U, and dissolution rates were obtained from fits to a three term exponential model. Significant variations were observed in the rate and extent of dissolution among the mineralsexamined. The uranyl phosphates chernikovite and metaautunite proved resistant to dissolution in non-carbonate systems, with dissolution half-times of days to weeks in 100 mM systems and weeks to years in 1 mM systems. In contrast, the uranyl oxide hydrates schoepite and becquerelite were solubilized over much shorter time scales. While 100 mM bicarbonate was successful in dissolving U in all forms, dissolution rates varied among the four minerals. Overall, EDTA was the least sensitive to a 100 to 1 mM drop in its concentration in its solubilization of all four mineral phases, underscoring the importance of organic complexation for the environmental mobility of uranium.     

Dissolution of calcium bilirubinate disk as a model of a gallstone by mixed solutions of 1,3-dimethyl-2-imidazolidinone and ethylenediaminetetraacetic acid

Direct dissolution agents of calcium bilirubinate gallstones were prepared by dissolving chelating agents in aqueous solutions of sodium carbonate containing polar solvent. Dissolution experiments on a calcium bilirubinate disk as a model of a gallstone were performed using a static disk method.

From the amounts dissolved in an hour using dissolution agents with various composition, the most effective conditions for dissolving calcium bilirubinate were determined as follows: (1) the higher the pH of the dissolution agent, the more calcium bilirubinate was dissolved. Thus, pH 8.4 was chosen as the best pH condition within the range of harmless usage to the human body in vivo in limited amount; (2) a combination of ethylenediaminetetraacetic acid (EDTA) as chelating agent for calcium and 1,3-dimethyl-2-imidazolidinone (DMI) as polar solvent; (3) in the mixed system of EDTA and DMI, the most effective concentrations were about 4 wt.% EDTA and about 30 wt.% DMI.

The dissolution agent satisfying the above conditions gave a dissolved amount of about 70 mg dl⁻¹ in 1 h. This amount was considerably higher than those using DMI and EDTA independently. This suggests that dissolution is due to cooperation between EDTA and DMI.     

Topokinetic model of active dissolution of a solid electrode with point lattice defects. analysis and comparison of the results obtained in solving equations with the experimental data

A model that takes into account the consistence between the atomic relief with the metal dissolution (corrosion) rate and also considers the direct effect of point lattice defects (vacancies or atoms of impurities with the corrosion resistance far deviating from that of the base metal) on this consistence is compared with experimental data. Despite the substantial simplifications taken in this model, the latter makes it possible to explain the main manifestations of effect of defects on the metal dissolution rate.     

On the dissolution behaviour of extended chain polyethylene fibres

The influence of stress on the dissolution behaviour of extended-chain high molecular weight polyethylene fibres in p-xylene was investigated. Freely suspended in the solvent, the fibres dissolved at 119.5°, a temperature close to the equilibrium solubility temperature of 118.6° for perfect polyethylene crystals. However, when a stress of 0.4 GPa was exerted by straining the fibre 0.7%, it could withstand a temperature as high as 130° for at least three days. At still higher temperatures the induced stress relaxed completely, and dissolution immediately followed. These phenomena indicate that the fibre has a network structure. The cross-links are of a physical nature. Molecules are connected by topological defects such as entanglements, intertwinings and twist disclinations. These defects are trapped in crystallites; therefore the theory of Gee and Flory is applicable predicting that in such a system dissolution temperature of extended chain crystallites increases with stress. The required stress is transduced by tie molecules bridging the amorphous regions between crystallites. A study of dissolution under stress seems to be a direct method for the detection of topological defects such as entanglements.     

Direct complexometric determination of aluminium in "alzinoy"

A simple and precise method for the complexometric determination of aluminium in "Alzinoy" (a binary alloy of aluminium and zinc) is described. After dissolution of the sample in hydrochloric acid, aluminium, zinc and any lead and iron are complexed with excess of EDTA. The excess of EDTA is titrated with lead solution, with Xylenol Orange as indicator. Ammonium fluoride is then added to decompose the Al-EDTA complex, and the EDTA liberated is titrated with lead solution. Four samples can be analysed in about 45 min.     

Mechanism of anodic oxidation of tungsten in neutral sulphate-fluoride solutions

The anodic oxidation of tungsten has been studied in 1 M Na₂SO₄ solutions containing 0–0.25 M NaF. Steady-state currents measured in the passivation and passivity ranges increase significantly with increasing fluoride concentration, indicating enhanced dissolution of the oxide film. The electrochemical impedance response is dominated by the processes in the barrier layer and at its interface with the electrolyte. The presence of a pseudo-inductive loop in the impedance spectra at intermediate frequencies indicates point defect interaction during film growth and dissolution processes. A kinetic model including the recombination reaction between oppositely charged point defects at the film/solution interface as well as a kinetic scheme for tungsten dissolution through the film mediated by cation vacancies is proposed. It is found to reproduce satisfactorily the steady-state currents and the impedance spectra in the potential range 0.2–2 V. Such a model for the conduction mechanism in the barrier layer is believed to be an essential part of a modelling approach to the formation of a nanoporous overlayer on tungsten in fluoride-containing solutions.