Holistic approach to dissolution kinetics : linking direction-specific microscopic fluxes, local mass transport effects and global macroscopic rates from gypsum etch pit analysis

Dissolution processes at single crystal surfaces often involve the initial formation and expansion of localized, characteristic (faceted) etch-pits at defects, in an otherwise comparatively unreactive surface. Using natural gypsum single crystal as an example, a simple but powerful morphological analysis of these characteristic etch pit features is proposed that allows important questions concerning dissolution kinetics to be addressed. Significantly, quantitative mass transport associated with reactive microscale interfaces in quiescent solution (well known in the field of electrochemistry at ultramicroelectrodes) allows the relative importance of diffusion compared to surface kinetics to be assessed. Furthermore, because such mass transport rates are high, much faster surface kinetics can be determined than with existing dissolution methods. For the case of gypsum, surface processes are found to dominate the kinetics at early stages of the dissolution process (small etch pits) on the cleaved (010) surface. However, the contribution from mass transport becomes more important with time due to the increased area of the reactive zones and associated decrease in mass transport rate. Significantly, spatial heterogeneities in both surface kinetics and mass transport effects are identified, and the morphology of the characteristic etch features reveal direction-dependent dissolution kinetics that can be quantified. Effective dissolution velocities normal to the main basal (010) face are determined, along with velocities for the movement of [001] and [100] oriented steps. Inert electrolyte enhances dissolution velocities in all directions (salting in), but a striking new observation is that the effect is direction-dependent. Studies of common ion effects reveal that Ca2+ has a much greater impact in reducing dissolution rates compared to SO42−. With this approach, the new microscopic observations can be further analysed to obtain macroscopic dissolution rates, which are found to be wholly consistent with previous bulk measurements. The studies are thus important in bridging the gap between microscopic phenomena and macroscopic measurements

Factors Affecting Bubble Size in Ionic Liquids

This study reports on understanding the formation of bubbles in ionic liquids (ILs), with a view to utilising ILs more efficiently in gas capture processes. In particular, the impact of the IL structure on the bubble sizes obtained has been determined in order to obtain design principles for the ionic liquids utilised. 11 ILs were used in this study with a range of physico-chemical properties in order to determine parametrically the impact on bubble size due to the liquid properties and chemical moieties present. The results suggest the bubble size observed is dictated by the strength of interaction between the cation and anion of the IL and, therefore, the mass transport within the system. This bubble size – ILs structure–physical property relationship has been illustrated using a series of QSPR correlations. A predictive model based only on the sigma profiles of the anions and cations has been developed which shows the best correlation without the need to incorporate the physico-chemical properties of the liquids. Depending on the IL, selected mean bubble sizes observed were between 56.1 and 766.9 μm demonstrating that microbubbles can be produced in the IL allowing the potential for enhanced mass transport and absorption kinetics in these systems

The Effect of the CO32- to Ca2+ Ion activity ratio on calcite precipitation kinetics and Sr2+ partitioning

<p>Abstract</p> <p>Background</p> <p>A proposed strategy for immobilizing trace metals in the subsurface is to stimulate calcium carbonate precipitation and incorporate contaminants by co-precipitation. Such an approach will require injecting chemical amendments into the subsurface to generate supersaturated conditions that promote mineral precipitation. However, the formation of reactant mixing zones will create gradients in both the saturation state and ion activity ratios (i.e., <inline-formula><m:math name="1467-4866-13-1-i1" xmlns:m="http://www.w3.org/1998/Math/MathML"><m:msub><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:msup><m:mrow><m:msub><m:mrow><m:mi>O</m:mi></m:mrow><m:mrow><m:mn>3</m:mn></m:mrow></m:msub></m:mrow><m:mrow><m:mn>2</m:mn><m:mo class="MathClass-bin">-</m:mo></m:mrow></m:msup></m:mrow></m:msub><m:mo class="MathClass-bin">/</m:mo><m:msub><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:msup><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mn>2</m:mn><m:mo class="MathClass-bin">+</m:mo></m:mrow></m:msup></m:mrow></m:msub></m:math></inline-formula>). To better understand the effect of ion activity ratios on CaCO₃precipitation kinetics and Sr²⁺co-precipitation, experiments were conducted under constant composition conditions where the supersaturation state (Ω) for calcite was held constant at 9.4, but the ion activity ratio <inline-formula><m:math name="1467-4866-13-1-i2" xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:mo class="MathClass-open">(</m:mo><m:mrow><m:mi>r</m:mi><m:mo class="MathClass-rel">=</m:mo><m:msub><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:msup><m:mrow><m:msub><m:mrow><m:mi>O</m:mi></m:mrow><m:mrow><m:mn>3</m:mn></m:mrow></m:msub></m:mrow><m:mrow><m:mn>2</m:mn><m:mo class="MathClass-bin">-</m:mo></m:mrow></m:msup></m:mrow></m:msub><m:mo class="MathClass-bin">/</m:mo><m:msub><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:msup><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mn>2</m:mn><m:mo class="MathClass-bin">+</m:mo></m:mrow></m:msup></m:mrow></m:msub></m:mrow><m:mo class="MathClass-close">)</m:mo></m:mrow></m:math></inline-formula> was varied between 0.0032 and 4.15.</p> <p>Results</p> <p>Calcite was the only phase observed, by XRD, at the end of the experiments. Precipitation rates increased from 41.3 ± 3.4 μmol m^-2min^-1at <it>r = </it>0.0315 to a maximum rate of 74.5 ± 4.8 μmol m^-2min^-1at <it>r = </it>0.306 followed by a decrease to 46.3 ± 9.6 μmol m^-2min^-1at <it>r </it>= 1.822. The trend was simulated using a simple mass transfer model for solute uptake at the calcite surface. However, precipitation rates at fixed saturation states also evolved with time. Precipitation rates accelerated for low <it>r </it>values but slowed for high <it>r </it>values. These trends may be related to changes in effective reactive surface area. The <inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" name="1467-4866-13-1-i1"><m:msub><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:msup><m:mrow><m:msub><m:mrow><m:mi>O</m:mi></m:mrow><m:mrow><m:mn>3</m:mn></m:mrow></m:msub></m:mrow><m:mrow><m:mn>2</m:mn><m:mo class="MathClass-bin">-</m:mo></m:mrow></m:msup></m:mrow></m:msub><m:mo class="MathClass-bin">/</m:mo><m:msub><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mi>C</m:mi><m:msup><m:mrow><m:mi>a</m:mi></m:mrow><m:mrow><m:mn>2</m:mn><m:mo class="MathClass-bin">+</m:mo></m:mrow></m:msup></m:mrow></m:msub></m:math></inline-formula> ratios did not affect the distribution coefficient for Sr in calcite (D^P_Sr²⁺), apart from the indirect effect associated with the established positive correlation between D^P_Sr²⁺and calcite precipitation rate.</p> <p>Conclusion</p> <p>At a constant supersaturation state (Ω = 9.4), varying the ion activity ratio affects the calcite precipitation rate. This behavior is not predicted by affinity-based rate models. Furthermore, at the highest ion ratio tested, no precipitation was observed, while at the lowest ion ratio precipitation occurred immediately and valid rate measurements could not be made. The maximum measured precipitation rate was 2-fold greater than the minima, and occurred at a carbonate to calcium ion activity ratio of 0.306. These findings have implications for predicting the progress and cost of remediation operations involving enhanced calcite precipitation where mineral precipitation rates, and the spatial/temporal distribution of those rates, can have significant impacts on the mobility of contaminants.</p

Investigation of spin wave dynamics in Au/CoFeB/Au multilayers with perpendicular magnetic anisotropy

Abstract We have carried out an experimental investigation of the spin-wave dynamics in the Au/CoFeB/Au multilayer consisting of a ferromagnetic film with thicknesses of 0.8, 0.9 and 1.0 nm. We employed the Brillouin light scattering spectroscopy to measure the frequency of the spin waves in dependence on the wave vector. Additionally, we characterized the samples by ferromagnetic resonance measurements. We found that the considered samples exhibit perpendicular magnetic anisotropy with low damping, indicating small pumping effects. Furthermore, we found a nonreciprocal dispersion relation pointing at a non-negligible Dzyaloshinskii–Moriya interaction. These results make the Au/CoFeB/Au multilayer a compelling subject for further analysis and as a potential material for future applications within magnonics

Hydration effects on gypsum dissolution revealed by in situ nanoscale atomic force microscopy observations

© 2016 Elsevier Ltd. Recent work has suggested that the rates of mineral dissolution in aqueous solutions are dependent on the kinetics of dehydration of the ions building the crystal. Dehydration kinetics will be ultimately determined by the competition between ion-water and water-water interactions, which can be significantly modified by the presence of background ions in solution. At low ionic strength, the effect of electrolytes on ion-water (electrostatic) interactions will dominate (Kowacz et al., 2007). By performing macroscopic and in situ, microscopic (atomic force microscopy) dissolution experiments, the effect of background electrolytes on the dissolution kinetics of gypsum (CaSO4·2H2O) cleavage surfaces is tested at constant, low ionic strength (IS = 0.05) and undersaturation (saturation index, SI = -0.045). Dissolution rates are systematically lower in the presence of 1:1 background electrolytes than in an electrolyte-free solution, regardless of the nature of the electrolyte tested. We hypothesize that stabilization of the hydration shell of calcium by the presence of background ions can explain this result, based on the observed correlations in dissolution rates with the ionic surface tension increment of the background ion in solution. Stabilization of the cation hydration shell should favor dissolution. However, in the case of strongly hydrated ions such as Ca2+, this has a direct entropic effect that reduces the overall dG of the system, so that dissolution is energetically less favorable. Overall, these results provide new evidence that supports cation dehydration being the rate-controlling step for gypsum dissolution, as proposed for other minerals such as barite, dolomite and calcite

An atomic force microscopy study of the growth of a calcite surface as a function of calcium/total carbonate concentration ratio in solution at constant supersaturation

Calcite growth experiments using atomic force microscopy (AFM) were conducted at two constant values of supersaturation (Qi = 5.248 and £22 = 6.457) while varying the Ca2+to CO32-concentration ratio. The calcite growth rate and the morphology of growth depend on the solution stoichiometry. At a constant degree of supersaturation, the growth rate was highest when the cation/total carbonate anion ratio, r*, was equal to 1 but decreased nonsymmetrically for higher or lower values of r*. The observed dependence of growth, rates on solution stoichiometry can be explained by nonequivalent attachment frequencies of cation and anion at ratios that differ from 1. At the same time, the morphology of the closing etch pits and of the forming nuclei was different when the rate changed, suggesting a change in the crystal growth mechanism. © 2009 American Chemical Society

Separation of mandelic acid enantiomers using solid-liquid biphasic systems with chiral ionic liquids

This work aims to take full advantage of ionic liquids’ (ILs) “designer solvent” nature in order to expand the applicability of solid-liquid biphasic systems (SLBS) as alternative chiral resolution techniques. To this purpose, twelve chiral ILs, bearing chirality on the cation or the anion, were used as chiral selectors in SLBS to selectively precipitate mandelic acid enantiomers. The precipitation studies were performed using aqueous solutions of the chiral ILs or their chiral precursors, where the impact of the chemical structure and chirality was investigated. The most efficient chiral ILs were employed to evaluate the influence of different operational conditions on the formed precipitate, namely resolution time, resolution speed, temperature, initial concentration of chiral IL, initial concentration of racemic mandelic acid and water content. When compared to their precursors, chiral ILs showed an enhanced ability to induce enantioselective precipitation. The composition of the precipitate formed, and the molecular-level mechanisms leading to enantioselective precipitation, were also uncovered. Due to cooperative interactions between the chiral IL and the target mandelic acid enantiomer, the [N4444][D-Phe]-based SLBS proposed allowed to obtain maximum enantiomeric excess of 51% in a single-step, without any further additives.publishe