Search

Dissolution and epitaxial growth on dolomite [CaMg[(C[O.sub.3]).sub.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.

Dissolution of the (104) surface of magnetite (MgCO3) was studied as a function of bulk solution pH over the range 2.0 less than pH less than 5.0 at 60 degree using atomic force microscopy (AFM). The AFM images revealed a dramatic increase in step density associated with a large increase in dissolution flux that was attributed to the protonation of terrace-adsorbed carbonate sites.

Step growth kinetics on barium sulfate (001) is measured as a function of step length, supersaturation, and temperature. The results demonstrate that knowledge of the kink kinetics, as a function of temperature is important in testing the applicability of the classical thermodynamic model to kinetic data.

In contrast to most ionic minerals studied by SFM, the periclase (001) surface dissolves not by retreat of monolayer steps parallel to (001), but by retreat of a rough surface perpendicular to (001). At pH < 2, dissolution has an additional contribution from retreating macro-steps at the edges of nearly square pits. The macro-steps have heights up to 120 nm. In general, step direction is parallel to [110] and equivalent directions. During dissolution at low pH, a soft near-surface region is formed. Other investigators have shown that the near-surface region is protonated. Protonation is supposed to stabilize the (111) surface of periclase. Due to the structural similarities between periclase (111) and brucite (001), and similar dissolution rates of periclase and brucite at pH < 5, we conclude that during dissolution the periclase (001) surface is restructured into '(111) nano-facets' representing brucite (001)-like layers and appearing as a rough and soft surface in SFM images. The most probable reasons that the slopes of these macro-steps (up to 50 [degrees]) are lower than the slopes of perfect (111) facets are the likely poorly ordered structure of these layers, microtopography on these surface facets, and tip-surface convolution in SFM. By measuring the vertical position of the surface vs. time, we calculated the dissolution rate. At pH 1 and pH 2 we found the rates to be 17.1 [+ or -] 5.8 x [10.sup.-6] and 5.7 [+ or -] 3.7 x [10.sup.-6] mol/[m.sup.2].s, respectively. These rates are in reasonable agreement with previously reported rates of periclase and brucite (001) dissolution, and are consistent with the idea that the MgO (001) surface consists of Mg[(OH).sub.2] (001)-like layers.

One of the promising technologies expected to improve environmental chemistry research is the scanning probe microscopy, which enables researchers to gain new insights into the fundamentals of interfacial chemistry. The utility of scanning probe microscopy in environmental chemistry is in its ability to monitor key energetic parameters of reactive sites, especially between fluids and particles. The techniques ability to monitor real-time chemical reactions is also important in gaining insights into combinatorial chemistry that will enhance the effectivity of chemical treatment strategies.

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]

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]

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]

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]

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]