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Abstract: Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at , whereas a sudden inhibition from was activated at . The presence of the -switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of . [ABSTRACT FROM AUTHOR]

Abstract: In situ Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) studies on dolomite (101̄4) were performed during exposure to supersaturated aqueous solutions (supersaturated in dolomite, calcite, aragonite, vaterite, huntite and magnesite) at pH = 9 at various Ca2+/Mg2+ aqueous ion activity ratios. At high saturation ratios, rapid growth of a single layer (∼3 Å thick) of a carbonate followed by much slower growth of a second layer was observed. Growth of the second layer was highly inhibited, suggesting that the first layer was essentially self-limited, and inhibited further layer-by-layer growth. The growth of the first layer was observed over a wide range of Ca2+/Mg2+ ratios, suggesting that the dolomite surface is favorable to formation of a range of Ca-Mg carbonates. LFM data revealed contrast in the tip-surface frictional forces on the first grown layer, but this contrast was only observed in layers grown from middle to high Ca2+/Mg2+ solutions. Thus, LFM may have detected or responded to differences in the structure and/or composition between the first layer relative and the dolomite substrate. Dissolution of the first layer occurred from significantly supersaturated solutions relative to ordered stoichiometric dolomite permitting an estimate of the excess interfacial strain energy of up to 10 mJ/m2. [Copyright &y& Elsevier]

Magnesite (104) dissolution kinetics were studied in acidic aqueous solutions (2.0 < pH < 4.2) at temperatures between 60 and 90°C by atomic force microscopy (AFM). Comparison of dissolution fluxes obtained by AFM and chemical methods revealed six to seven times larger dissolution fluxes obtained by chemical analysis. Corresponding empirical activation energies were found to be 74 ±22 kJ/mol and 41 ± 4 kJ/mol (at pH 4.2) for the AFM and chemical methods, respectively. The empirical reaction order with respect to proton concentration was 0.36 ± 0.13 and 0.47 ± 0.03 for AFM and chemical methods, respectively. These comparisons suggest that the two experimental measurement methods differ as a result of the different sampling length scales associated with the methods. Negligible changes in step dissolution velocity with changes in bulk pH were found, suggesting that the principal source of increasing dissolution flux with decreasing pH is an increase in step density. However, the observed stable step orientation, which is dependent on pH, suggests that more than one proton adsorption equilibrium should be used to describe the surface chemistry of magnesite in acidic solution. [Copyright &y& Elsevier]

The rate of growth of ionic minerals from solutions with varying aqueous cation:anion ratios may result in significant errors in mineralization rates predicted by commonly-used affinity-based rate equations. To assess the potential influence of solute stoichiometry on barite growth, step velocities on the barite (0 0 1) surface have been measured at 108 °C using hydrothermal atomic force microscopy (HAFM) at moderate supersaturation and as a function of the aqueous barium:sulfate ratio ( r ). Barite growth hillocks at r ∼ 1 were bounded by 〈 1 2 0 〉 steps, however at r < 1, kink site densities increased, steps followed a direction vicinal to 〈 1 2 0 〉 , and the [0 1 0] steps developed. At r > 1, steps roughened and rounded as the kink site density increased. Step velocities peaked at r = 1 and decreased roughly symmetrically as a function of r , indicating the attachment rates of barium and sulfate ions are similar under these conditions. We hypothesize that the differences in our observations at high and low r arise from differences in the attachment rate constants for the obtuse and acute 〈 1 2 0 〉 steps. Based on results at low r , the data suggests the attachment rate constant for barium ions is similar for obtuse and acute steps. Based on results at high r , the data suggests the attachment rate constant for sulfate is greater for obtuse steps than acute steps. Utilizing a step growth model developed by Stack and Grantham (2010) the experimental step velocities as a function of r were readily fit while attempts to fit the data using a model developed by Zhang and Nancollas (1998) were less successful. [ABSTRACT FROM AUTHOR]

Step velocities on the celestite (0 0 1) surface have been measured as a function of temperature (23–45 °C), saturation state ( S = 1.1–2.2), ionic strength ( I = 0.01, 0.06, and 0.1 M), and aqueous strontium:sulfate ratio ( r = 0.01–100) using atomic force microscopy (AFM). Celestite growth hillocks were flanked by [0 1 0]-aligned step edges, which are polar, and step edges vicinal to 〈1 2 0〉, which are non-polar. [0 1 0] step velocities increased with temperature and saturation state, however step velocities did not vary significantly with ionic strength. Step velocities were non-linear with saturation state, suggesting a change in mechanism at high S as compared with low S. At constant S , the step velocities were maximized at r = 1 and decreased significantly at extreme r , demonstrating the governing role of solute stoichiometry. We successfully fit the step velocity data as a function of r using the Stack and Grantham (2010) nucleation and propagation model. Based on the results as a function of ionic strength and r , the mechanism at low S is likely ion-by-ion attachment to the step with an activation energy of 75 (±10) kJ mol −1 . At high S the mechanism is a combination of the one at low S and possibly attachment of a neutral species such as an ion pair with an activation energy of 43 (±9) kJ mol −1 . [ABSTRACT FROM AUTHOR]

Abstract: A multiscale carbon substrate is fabricated by growing carbon nanotubes (CNT) on multicellular graphitic foam using an intermediate oxide nanolayer. Abundant CNT growth is seen on the surface throughout, including the walls of inner pores. The Brunauer–Emmett–Teller (BET) technique with nitrogen has been used to measure the specific surface area (SSA) of this multiscale structure. It is seen that attachment of CNT provides over a one hundred fold increase in usable SSA of the material with negligible increase in volume and a weight gain of only 2.5%. These results correlate well with semi-empirical predictions obtained by combining analytical modeling and microstructural data. [Copyright &y& Elsevier]

Abstract: Dissolution of celestite (001) was studied by atomic force microscopy as a function of solution undersaturation. In solutions near saturation with respect to celestite, dissolution of the mineral took place exclusively by removal of ions from existing step edges. The onset of etch pit nucleation was observed at a critical saturation state of . Below this saturation state, dissolution took place both at existing step edges and via the creation of new steps surrounding the etch pits. The dissolution rates of celestite exhibited a non-linear dependence on saturation state. Basic crystal dissolution/growth models were inadequate to describe the non-linearity, but a model that incorporates a critical undersaturation provided an improved fit to data collected at high undersaturation. A simple model for dissolution at low undersaturation also fit the rate data well, but in light of the conditions necessary to produce new step edges, the rate coefficient in this model is poorly constrained due to the effects of sample surface history. Consideration of the process of topographic relaxation, consisting of changes in the surface microtopography (i.e., step density) resulting from changes in solution conditions, led to predicted relaxation times on the order of days for the celestite–water interface. [Copyright &y& Elsevier]

Force spectroscopy is used to investigate crude oil/brine/rock interactions at reservoir temperatures and pressures, up to 120 °C and 10 MPa, for the first time. A newly-developed, high pressure, high temperature atomic force microscope (HP/HT-AFM) is used in combination with newly-designed, crude-oil functionalized AFM probes compatible with these conditions. Our results show that both temperature and pressure have significant impact on adhesion forces, demonstrating the necessity of testing in this environment to obtain results that accurately reflect interactions at reservoir conditions. The HP/HT-AFM design introduced here also allows changing many other variables such as brine composition, dissolved gases, injection fluid additives, the type of crude oil, as well as the type of mineral surface. Consequently, the system introduced here has the capability to investigate interfacial interactions at reservoir conditions with unprecedented specificity, providing the basis for the development of optimized and tailored oil recovery techniques. [ABSTRACT FROM AUTHOR]

Abstract: Direct measurements of calcite faces were performed using in situ atomic force microscopy (AFM) to reveal the dissolution processes as a function of solution saturation state and temperature. Time-sequential AFM images demonstrated that step velocities at constant temperature increased with increasing undersaturation. The anisotropy of obtuse and acute step velocities appeared to become more significant as solutions approached equilibrium and temperature increased. At saturation state Ω >0.02, a curvilinear boundary was formed at the intersection of two acute steps and the initially rhombohedral etch pit exhibited a nearly triangular shape. This suggests that the and steps may not belong to the calcite-aqueous solution equilibrium system. Further increase in the saturation state (Ω ⩾0.3) led to a lack of etch pit formation and dissolution primarily occurred at existing steps, in accordance with . Analysis of step kinetics at different temperatures yielded activation energies of 25±6kJ/mol and 14±13kJ/mol for obtuse and acute steps, respectively. The inconsistencies in etch pit morphology, step anisotropy, and step activation energies from the present study with those of studies far-from-equilibrium can be explained by increased influence of the backward reaction, or growth, near-equilibrium. We propose that the backward reaction occurs preferentially at the acute–acute kink sites. The kinetics and effective activation energies of near-equilibrium calcite dissolution presented in this work provide accurate experimental data under likely CO2 sequestration conditions, and thus are crucial to the development of robust geochemical models that predict the long-term performance of mineral-trapped CO2. [Copyright &y& Elsevier]

Abstract: Cleaved surfaces of dolomite were studied using ex-situ X-ray photoelectron spectroscopy (XPS) following exposure of the surfaces to various experimental conditions. Dolomite samples exposed to air, to a highly undersaturated solution (0.1M NaCl, pH=9), and to solution with a supersaturation (−Δμ/kT) of 5.5 (pH=9) were investigated with semiquantitative methods of analysis to ascertain the degree of non-stoichiometry resulting at the dolomite surface from reactive conditions. It was found that the dolomite cleavage surface in undersaturated solution was not altered significantly from the stoichiometric surface termination. The composition of the cleaved surface after exposure to supersaturated solution, a surface known to have self-limiting growth characteristics under similar conditions, was found to be Ca2+ rich (Ca x Mg2− x (CO3)2, 1.7> x >1.3). The observations, while underscoring differences in hydration/dehydration kinetics of the two alkaline earth cations, suggest that achievement of equilibrium at dolomite–water interfaces may be subject to significant barriers from both undersaturated and supersaturated solutions. [Copyright &y& Elsevier]

The iron oxide–water interface is of interest not only in geochemical and environmental processes, but also in fields ranging from corrosion to photocatalysis. The structure of α-Fe2O3 (001) surfaces is not fully understood, and questions have arisen recently concerning different terminations of (001) terraces; a so-called Fe-termination is expected, but under some conditions an O-termination may also be possible. Ultra-high vacuum (UHV) scanning tunneling microscope (STM) studies report evidence for an O-termination in coexistence with an Fe-termination, but other studies find no evidence for an O-termination. Molecular mechanics studies suggest that an O-termination should be possible in an aqueous environment. An O-termination could result from dissolution; if Fe atoms were to dissolve from an Fe-termination, an O-termination would presumably result (and vice-versa). We imaged hematite (001) surfaces in air and aqueous solution using STM. To aid interpretation of the images, we use a resonant tunneling model (RTM) parameterized using ab initio calculations.Our STM and RTM results are consistent with mixed O- and Fe-terminated (001) surfaces. For acid-etched surfaces we find evidence for a periodic (with wavelength of 2.2 ± 0.2 nm) arrangement of nominal O- and Fe-terminated domains. Two different borders between domains should occur, one in which the Fe-termination is high relative to the O-termination, and the reverse. The different domain borders have significantly different heights, consistent with RTM calculations. This agreement allows us to conclude that the Fe-termination is topographically high on most terraces. Surface domains are observed in aqueous solutions at the atomic scale, and appear to be very unreactive on tens-of-seconds time scales at pH 1. [Copyright &y& Elsevier]