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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 this paper, a detailed study of the nucleation and growth of plasma assisted oxide coatings on model graphitic substrates has been performed. The surface chemistry and morphology of these films (as they grow on graphite) have been monitored using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). It is shown that oxide coatings start as islands having average height of 3–5 nm. The density and lateral size of these islands grow to form complete layers. Therefore complete and smooth coverage can be obtained with a film thickness in the range of a few nanometers (3–5 nm). The coating chemical composition is identical to SiO2. Detailed XPS analysis during overlayer growth reveals the presence of strong –Si–C– bonds at the graphite–coating interface. This is an important finding that can help to predict the mechanical, thermal and other functional properties of the interface. The durability of this coating at high operating temperatures has been investigated, and it is found to withstand up to 1000 °C heat treatments in argon environment. [Copyright &y& Elsevier]

Abstract: Experimental observation of step (i.e., a linear discontinuity in a surface) growth on calcium carbonate and barium sulfate surfaces was performed using in situ scanning probe microscopy (SPM) in aqueous solutions. Step speeds at very short step lengths (<300nm) were found to increase with step length, but scatter in the data precluded comparison of different theoretical models with regard to the experimental data. Analysis of experimental errors revealed that the data scatter was not unusual and further suggested that significant experimental bias in the determination of step properties such as the step free energy and kinetics of kink propagation and nucleation are introduced if the instrument bandwidth is not appropriate for the specific dynamics of the step. In light of step growth models based on either equilibrium assumptions or non-equilibrium assumptions, these biases tend to favor one model over the other. [Copyright &y& Elsevier]

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]

Looks at the need for solid-fluid interfaces in various types of environmental applications, with reference to scanning probe microscopy (SPM). Validity of models used in environmental remediation processes; Reference to the development of two power surface analytical techniques; Methods used in the treatment of water.

Provides information on a study which investigated the rate and mechanism of periclase (001) surface dissolution. Methodology; Results; Discussion of the findings.

Presents a design of a contact atomic force microscope (AFM) for aqueous solution imaging. Features of the AFM; Advantages of the design; Details on the unit-cell scale vertical resolution of the AFM.

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]

Sr partitioning on calcite crystals growing from Ca 2 + –Sr 2 + –CO 3 2 − solutions was studied by means of friction force microscopy (FFM). Experiments were performed with various Sr 2 + (aq) /Ca 2 + (aq) concentration ratios and total Sr 2 + (aq) concentration in order to examine conditions under which Sr-calcite growth is self-limiting (e.g., the so-called “template effect”) and also to investigate continuous Sr-calcite growth, where spiral growth predominates and Sr incorporation is sector-dependent. In these latter experiments, the goal was to evaluate the utility of friction force microscopy to discriminate sector zoning. Results from the experiments show that friction increases with the incorporation of Sr into the growing calcite layers. The maximum increase in friction was measured at high Sr 2 + (aq) /Ca 2 + (aq) , although a quantitative link between a specific amount of increase in friction to a specific amount of Sr incorporation was not possible to determine due to experimental uncertainties. Nevertheless, it was possible to establish that no change in friction is detectable when Sr incorporation yields a solid composition of Sr 0.05 Ca 0.95 CO 3 . Friction was found to increase during growth of several layers in an incremental fashion. The increase can be linked either to an incremental increase in Sr content in the newly formed calcite, controlled by the thermodynamics of the strained layers necessitated by the substitution of larger Sr cations into the calcite, or to the incremental increase in layer thickness which in turn leads to increases in the probe-surface contact area. No difference in friction could be observed between acute and obtuse sectors under any of the experimental conditions, which was primarily due to the limits of the friction measurement sensitivity. [ABSTRACT FROM AUTHOR]

The adsorption of dissolved organic matter (DOM) to metal (oxy)hydroxide mineral surfaces is a critical step for C sequestration in soils. Although equilibrium studies have described some of the factors controlling this process, the molecular-scale description of the adsorption process has been more limited. Chemical force spectroscopy revealed differing adhesion strengths of DOM extracted from three soils and a reference peat soil material to an iron (oxy)hydroxide mineral surface. The DOM was characterized using ultrahigh-resolution negative ion mode electrospray ionization Fourier Transform ion cyclotron resonance mass spectrometry. The results indicate that carboxyl-rich aromatic and N-containing aliphatic molecules of DOM are correlated with high adhesion forces. Increasing molecular mass was shown to decrease the adhesion force between the mineral surface and the DOM. Kendrick mass defect analysis suggests that mechanisms involving two carboxyl groups result in the most stable bond to the mineral surface. We conceptualize these results using a layer-by-layer "onion" model of organic matter stabilization on soil mineral surfaces. [ABSTRACT FROM AUTHOR]

Stepvelocities of monolayer-height steps on the (101̅4)magnesite surface have been measured as functions of the aqueous magnesium:carbonateratio and saturation index (SI) using a hydrothermal atomic forcemicroscope. At SI ≤ 1.9 and 80-90 °C, step velocitieswere found to be invariant with changes in the magnesium:carbonateratio, an observation in contrast with standard models for growthand dissolution of ionically bonded, multicomponent crystals. However, at high saturation indices (SI = 2.15), step velocities displayeda ratio dependence, maximized at magnesium:carbonate ratios slightlygreater than 1:1. Traditional affinity-based models could not describegrowth rates at the higher saturation index. Step velocities alsocould not be modeled solely through nucleation of kink sites, in contrastto other minerals whose bonding between constituent ions is also dominantlyionic in nature, such as calcite and barite. Instead, they could bedescribed only by a model that incorporates both kink nucleation andpropagation. On the basis of observed step morphological changes atthese higher saturation indices, the step velocity maximum at SI =2.15 is likely due to the rate of attachment to propagating kink sitesovercoming the rate of detachment from kink sites as the latter becomesless significant under far from equilibrium conditions. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this work was to examine the effects of polished crystal-surface orientation and degree of solution undersaturation on the dissolution kinetics of calcite. Crystallographic surface orientations utilized in this study included natural calcite specimens polished approximately parallel to the (104) plane, giving rise to surfaces with flat terraces with few steps, as well as fully kinked surfaces created by sectioning approximately parallel to the (001) plane. Results from inductively coupled plasma optical emission spectroscopy (ICP-OES) and vertical scanning interferometry (VSI) investigations revealed how crystallographic orientations of calcite with initially higher energy surface morphologies were associated with greater extent of reaction, greater surface retreat, and therefore, greater initial transient dissolution rates than those with lower energy initial surface morphologies. However, both the ICP-OES and atomic force microscopy (AFM) results confirm that the effects of crystal orientation become minimal under long-term conditions because (1) variously oriented calcite surfaces exhibited similar “long-term” rates and (2) orientations with high initial reactive site densities developed lower energy morphologies. Estimating a step rate coefficient from the experimental data from a miscut sample suggest that these coefficients may be highly morphology dependent. Results from this study are significant for predicting long term calcite dissolution rates because they suggest that the “long-term” dissolution rate of calcite will be governed by the rates of the rhombohedral faces and microfacets and the time-scale for approaching such conditions is of the order of days at room temperature and slightly alkaline solution conditions. [Copyright &y& Elsevier]

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: A computer algorithm was developed to automatically track the displacement of straight step edges between sequential scanning probe microscopy images of single-crystal surfaces. The program utilizes the Canny edge detection algorithm followed by the Hough Transform of the edge map to identify step edges according to their direction, relative to the image axes, and according to their displacement, relative to the image origin. The tracking of individual steps is facilitated by the fact that straight edges in general maintain their direction and therefore, steps of similar displacement but different direction can be sorted. The algorithm is based on the assumption that the rate of image acquisition is much greater than the rate of (mono)layer growth/dissolution, requiring that changes in step displacement are small in successive images. The change in step displacement in sequential images leads directly to the calculation of the step speed. By tabulating all changes in step displacement through a sequence of images, a statistical representation of the step edge data is produced. The program was evaluated using a sequence of 20 atomic force microscopy images from a calcite (104) surface growing from a supersaturated aqueous solution. The program required, in total, 5 CPU-minutes running on a Pentium 4 processor to compute the mean step speed with 60% precision whereas the equivalent number of measurements performed “by hand” required 6 person-hours at 70% precision. For comparable output, the computer program therefore represents a factor of about 100 decrease in required effort. [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]

Abstract: Plasma-assisted functional films have significant potential in various engineering applications. They can be tailored to impart desired properties by bonding specific molecular groups to the substrate surface. The aim of this investigation was to develop a fundamental understanding of the atomic level growth, coverage and functional effectiveness of plasma nano-films on flat surfaces and to explore their application-potential for complex and uneven shaped nano-materials. In this paper, results on plasma-assisted nano-scale fluorocarbon films, which are known for imparting inertness or hydrophobicity to the surface, will be discussed. The film deposition was studied as a function of time on flat single crystal surfaces of silicon, sapphire and graphite, using microwave plasma. X-ray photoelectron spectroscopy (XPS) was used for detailed study of composition and chemistry of the substrate and coating atoms, at all stages of deposition. Atomic force microscopy (AFM) was performed in parallel to study the coverage and growth morphology of these films at each stage. Combined XPS and AFM results indicated complete coverage of all the substrates at the nanometer scale. It was also shown that these films grew in a layer-by-layer fashion. The nano-films were also applied to complex and uneven shaped nano-structured and porous materials, such as microcellular porous foam and nano fibers. It was seen that these nano-films can be a viable approach for effective surface modification of complex or uneven shaped nano-materials. [Copyright &y& Elsevier]

Dissolution and epitaxial growth on dolomite [CaMg(CO3)2] cleavage surfaces under well-defined aqueous solution conditions were studied in real-time by atomic force microscopy (AFM). Step-retreat speeds in undersaturated solutions were used to predict the growth speed for ordered dolomite utilizing a theoretical step-growth model. Monolayer growth on dolomite under highly supersaturated conditions was observed, but multilayer growth was inhibited due to film disorder enhanced solubility. Ordered dolomite film growth is predicted in supersaturated solutions close to equilibrium, but observed step speeds were immeasurably low under the current laboratory low-temperature conditions. [ABSTRACT FROM AUTHOR]

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]

Although minerals are known to affect the environmental fate and transformation of heavy-metal ions, little is known about their interaction with the heavily exploited silver nanoparticles (AgNPs). Proposed here is a combination of hitherto under-utilized micro-Raman-based mapping and chemometric methods for imaging the distribution of AgNPs on various mineral surfaces and their molecular interaction mechanisms. The feasibility of the Raman-based imaging method was tested on two macro- and microsized mineral models, muscovite [KAl2(AlSi3O10)(OH)2] and corundum (α-Al2O3), under key environmental conditions (ionic strength and pH). Both AgNPs- and AgNPs+ were found to covalently attach to corundum (pHpzc = 9.1) through the formation of Ag-O-Al- bonds and thereby to potentially experience reduced environmental mobility. Because label-free Raman imaging showed no molecular interactions between AgNPs- and muscovite (pHpzc = 7.5), a label-enhanced Raman imaging approach was developed for mapping the scarce spatial distribution of AgNPs- on such mineral surfaces. Raman maps comprising of n = 625-961 spectra for each sample/control were rapidly analyzed in Vespucci, a free open-source software, and the results were confirmed via ICP-OES, AFM, and SEM-EDX. The proposed Raman-based imaging requires minimum to no sample preparation; is sensitive, noninvasive, cost-effective; and might be extended to other environmentally relevant systems. [ABSTRACT FROM AUTHOR]

To better understand the role of spectator ions in barite growth, the kinetics of step edge growth on barite (001) surfaces were studied under various salt solutions. Hydrothermal atomic force microscopy (HAFM) was used to investigate the effect of background electrolytes (NaCl, NaBr, and NaNO3) as a function of saturation index and ionic strength (I) on barite growth sourced at dislocations at 108 °C. Results demonstrate that hillock morphology is affected by I, as well as type of anion, where the prevalence of steps aligned on the [010] direction is highest under Cl-. There is a modest increase in kinetic coefficient of 55-130% with a 10-fold increase in I for each salt. In comparing the kinetic coefficients of the salts at low ionic strength (0.01 M), there is a moderate difference, suggesting that the anion may play a role in barium attachment. [ABSTRACT FROM AUTHOR]

The increased worldwide exploitation of nanomaterials has reinforced the importance of introducing nanoscale aspects into the undergraduate and graduate curriculum. To meet this need, a novel nano-laboratory module was developed and successfully performed by science and engineering students. The main goal of the experiment was to accurately quantify the total silver composition of a nanocolloid with modern inductively coupled plasma-optical emission spectroscopy (ICP-OES) instrumentation in conjunction with two well-established methods that are heavily employed in both research and industrial settings. Specifically, undergraduate and graduate students estimated the total silver composition of Creighton colloidal nanoparticles via the external calibration method (16.3 ± 4.7 mg L-1) and the standard addition method (14.9 ± 4.2 mg L-1) at two emission wavelengths (328.068 and 338.898 nm). The assessment of basic laboratory skills and the class assignments showed that the students successfully mastered the various aspects of sample/standard preparation, the operation of the ICP-OES instrument, and the data analysis. Students' interest and experience in this laboratory were highly rated in the anonymous student evaluations. [ABSTRACT FROM AUTHOR]

Elucidating dissolution kinetics and mechanisms at carbonate mineral–water interfaces is essential to many environmental and geochemical processes, including geologic CO2 sequestration in deep aquifers. In the present work, effects of background electrolytes on dolomite (CaMg(CO3)2) reactivity were investigated by measuring step dissolution rates using in situ hydrothermal atomic force microscopy (HAFM) at 90 °C. Cleaved surfaces of dolomite were exposed to sodium chloride and tetramethylammonium chloride (TMACl) aqueous solutions with ionic strengths (I) ranging from 0 to 0.77 m at pH 4 and pH 9. HAFM results demonstrated that dolomite step retreat rates increased with increasing solution ionic strength and decreasing pH. Comparison of [481̅] and [4̅41] steps revealed that the anisotropy of [481̅] and [4̅41] step speeds became significant as solution ionic strength increased, with NaCl exerting more pronounced effects than TMACl for the same I. To interpret the different trends observed for NaCl and TMACl, a dissolution mechanism involving orientation-dependent ion adsorption and consequent edge free energy changes is proposed. [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]