Dissolution of the barite (001) surface by the chelating agent DTPA as studied with non-contact atomic force microscopy

DTPA (diethylenetriaminepentaacetic acid) is a chelating agent widely used for removal of barium sulfate (barite) scale in the petroleum industry. In this paper we report ex-situ investigations of barite dissolution in deionized water and in 0.18 M DTPA aqueous solutions. Non-contact atomic force microscopy (NC-AFM) was used to observe dissolution on the BaSO₄ (001) cleavage surface. Dissolution was carried out at room temperature in a 10 ml reactor. Each sample was first etched in solution and dried before examination by NC-AFM. Dissolution on the BaSO₄ (001) surface took place via development of etch pits. In deionized water, triangular etch pits were observed on the (001) terraces at room temperature. And, zigzag shaped etch pits were found at the edges of steps. In DTPA solutions, etch pits on the (001) terraces were observed and these became deeper and longer with increasing time. The geometry of these etch pits was trapezoidal, and/or trapezohedral. To explain this characteristic morphology caused by dissolution we suggest that the active sites of one DTPA molecule bind to two or three Ba²⁺ cations exposed on the (001) surface.     

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.     

Hydrogels coupled with self-assembled monolayers: an in vitro matrix to study calcite biomineralization

This paper describes the control of the nucleation and growth of calcite crystals by a matrix composed of an agarose hydrogel on top of a carboxylate-terminated self-assembled monolayer (SAM). The design of this matrix is based upon examples from biomineralization in which hydrogels are coupled with functionalized, organic surfaces to control, simultaneously, crystal morphology and orientation. In the synthetic system, calcite crystals nucleate from the (012) plane (the same plane that is observed in solution growth). The aspect ratio (length/width) of the crystals decreases from 2.1 +/- 0.22 in solution to 1.2 +/- 0.04 in a 3 w/v % agarose gel. One possible explanation for the change in morphology is the incorporation of gel fibers inside of the crystals during the growth process. Etching of the gel-grown crystals with deionized water reveals an interpenetrating network of gel fibers and crystalline material. This work begins to provide insight into why organisms use hydrogels to control the growth of crystals.     

The Morphology of Etch Pits During Thermal Treatment of Drugs and its Dependence on the Features of Their Crystallochemical Structure

Topochemistry of the initial stages of evaporation and dissolution of monoclinic single crystals of paracetamol and phenacetin was studied. Thermal treatment of these crystals showed that the morphology of etch pits depicts the symmetry of etched planes. The shapes of pits formed during chemical etching of the cleavage plane of a paracetamol crystal by different etchants were not similar to each other. The chemical etching of the cleavage plane of a phenacetin crystal resulted in the formation of pits stretched along the same direction, independently of the chosen solvent. An interpretation of this result is suggested, based on the analysis of the anisotropy of the crystal structure and presence of steric hindrance.This revised version was published online in November 2005 with corrections to the Cover Date.     

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.     

Growth and Dissolution of Organic Crystals with “Tailor-Made” Inhibitors—Implications in Stereochemistry and Materials Science

When crystals of organic compounds are grown in the presence of growth inhibitors, there is a change in crystal morphology. A stereochemical correlation exists between the crystal structure, its modified morphology, and the molecular structure of the inhibitor. This correlation has been successfully exploited for the efficient resolution of conglomerates, the engineering of organic crystals with desired morphologies, the direct and relative assignment of the absolute configurations of chiral molecules and crystals, and for the design of a new model for the spontaneous generation of optical activity. In an analogous way dissolution of organic crystals in the presence of these growth inhibitors induces etch pits at preselected faces. The effect of solvent on crystal growth has been analyzed in some model systems. The experimental results are complemented by atom-atom potential energy calculations.     

Scanning-induced growth on single crystal calcite with an atomic force microscope

We report observations of localized growth on the (1014) surface of single-crystal CaCO3 in supersaturated solutions while scanning with the tip of an atomic force microscope (AFM). At low contact forces, AFM scanning strongly enhances deposition along preexisting steps. This enhancement increases rapidly with increasing solution supersaturation, and is capable of filling in multilayer etch pits to produce defect-free surfaces at the resolution of the AFM. Attempts to achieve similar deposition rates in the absence of scanning require high supersaturations that produce three-dimensional crystal nuclei, which are important defects. Localized deposition produced by drawing the AFM tip back and forth across step edges can produce monolayer deposits extending well over a micron from the scanned area. These tip-induced deposits provide convincing evidence for the importance of ledge diffusion in calcite crystal growth.     

Heterogeneous nucleation and growth of calcium carbonate on calcite and quartz

The precipitation of calcium carbonate as a binding salt for the consolidation of loose sand formations is a promising approach. The heterogeneous nucleation and growth of calcite were investigated in supersaturated solutions. The ionic activities in the solutions tested were selected so that they included both supersaturations in which crystal growth took place only following the introduction of seed particles and supersaturations in which precipitation occurred spontaneously past the lapse of induction times. In the latter case the supersaturation conditions were sufficiently low to allow the measurement of induction times preceding the onset of precipitation. The stability domain of the calcium carbonate system was established at pH 8.50, 25 degrees C, measuring the induction times in the range between 30 min and 2 h. The rates of precipitation following the destabilization of the solutions were measured from the pH and/or concentration-time profiles. The induction times were inversely proportional and rates proportional to the solution supersaturation as expected. The high-order dependence of the rates of precipitation on the solution supersaturation suggested a polynuclear growth mechanism. Fitting of the induction time-supersaturation data according to this model yielded a value of 64 mJ/m2 for the surface energy of the calcite nucleus. In the concentration domain corresponding to stable supersaturated solutions, seeded growth experiments at constant supersaturation showed a second-order dependence on the rates of crystal growth of calcite seed crystals. Inoculation of the stable supersaturated solutions with quartz seed crystals failed to induce nucleation. Raising supersaturation to reach the unstable domain showed interesting features: calcite seed crystals yielded crystal growth kinetics compatible with the polynuclear growth model, without any induction time. The presence of quartz seed crystals reduced the induction times and resulted in nucleation in the bulk solution. The kinetic data in the latter case were consistent with the polynuclear growth model and the surface energy for the newly forming embryo was calculated equal to 31.1 mJ/m2, because of the dominantly heterogeneous nature of the process.     

Scanning tunneling microscopy and spectroscopy of wet-chemically prepared chlorinated Si111 surfaces

Chlorine-terminated Si(111) surfaces prepared through the wet-chemical treatment of H-terminated Si(111) surfaces with PCl5 (in chlorobenzene) were investigated using ultrahigh vacuum scanning tunneling microscopy (UHV cryo-STM) and tunneling spectroscopy. STM images, collected at 77 K, revealed an unreconstructed 1 x 1 structure for the chlorination layer, consistent with what has been observed for the gas phase chlorination of H-terminated Si(111). However, the wet-chemical chlorination is shown to generate etch pits in the Si(111) surface, with an increase in etch pit density correlating with increasing PCl5 exposure temperatures. These etch pits were assumed to stabilize the edge structure through the partial removal of the <112> step edges. Tunneling spectroscopy revealed a nonzero density of states at zero bias. This is in contrast to the cases of H-, methyl-, or ethyl-terminated Si(111), in which similar measurements have revealed the presence of a large conductance gap.     

The Inhibition of Calcite Dissolution/Precipitation: 1,2-Dicarboxylic Acids

The dissolution of calcite under conditions of high pH (8.0-9.0) is shown to be strongly inhibited by ca. 10 mM levels of the fully deprotonated forms of succinic acid, phthalic acid, and maleic acid. Channel flow cell measurements are used in each case to deduce the appropriate rate law for dissolution. For the maleate dianion it is demonstrated that the inhibition is likely due to the blocking of dissolution/growth sites at which CaCO₃ units are incorporated into or removed from the crystal lattice, whereas for the other two ions it arises from competitive Langmuirian adsorption of the dicarboxylate ions and CO^2-₃ on the calcite surface.     

碳酸钡单晶微米锥阵列在方解石(104)面上的外延生长

通过液固界面上的溶解-沉淀耦合反应在Ba(NO₃)₂乙醇-水溶液中实现了毒重石晶型的碳酸钡在方解石(CaCO₃)晶体基底上的外延生长, 得到碳酸钡的单晶微米锥阵列. 碳酸钡微米锥的长轴平行于毒重石晶体的[001]方向,同时也与方解石基底[001]晶向相同, 其俯视图为六边形, 具有近似的六方对称性. 随反应时间的增加, 外延生长形成的碳酸钡微米锥的尺寸增加, 但其轴径比逐渐减小. 通过改变乙醇-水混合溶剂中的乙醇含量或者Ba(NO₃)₂浓度也能调控碳酸钡晶体的尺寸和形貌. 随着混合溶剂中乙醇含量与Ba(NO₃)₂浓度的提高, 溶液中BaCO₃的过饱和度增加, 通过外延生长在方解石的(104)表面形成的BaCO₃阵列结构的密集程度逐渐增加, 尺寸逐渐减小, 形貌从微米锥逐渐转变为微米柱状结构. 经过对晶化过程及毒重石和方解石晶体结构分析,提出了在方解石表面外延生长形成的毒重石微米锥单晶阵列结构的形成过程机理: 该过程为界面溶解-沉淀耦合反应的过程,方解石的溶解和毒重石的外延生长过程同时进行, 由于两种晶体在方解石基底的(104)晶面与(001)晶面上具有中高度错配值, 毒重石晶体在方解石的这两个晶面上发生Volmer-Weber型的外延生长, 逐渐形成在靠近基底处包覆有方解石台阶的毒重石微米锥单晶阵列结构.     

Thermodynamics of epitaxial calcite nucleation on self-assembled monolayers

Classical heterogeneous nucleation theory is used to describe the epitaxial nucleation of calcite on self-assembled monolayers (SAMs). Both spherical and faceted clusters are considered. The use of faceted clusters reveals a useful relation between the shape of very small crystals and the ratio of the heterogeneous and homogeneous nucleation barriers. The experimental approach of this paper concerns the measurement of the threshold driving forces for both homogeneous and heterogeneous nucleation of calcite. This is accomplished by preparing solutions with well-defined driving forces and by measuring the resulting types of nucleation that are observed after a fixed experimental time. The results of the experiments and the theoretical shape analysis are compared, and it is shown that in the experiments no homogeneous nucleation of calcite occurs for driving forces up to at least Deltamu/k(B)T approximately equal to 6.0. A calculation of the critical cluster size for heterogeneous nucleation results in a range of 2-28 growth units and faceted critical clusters from 3-28 growth units, depending on the value of the surface free energy of calcite. These sizes are 50-100 times smaller than the crystalline domain sizes of SAMs and therefore small enough to explain the promoting effect of the substrate.     

Crystalline order of a water/glycine film coadsorbed on the (104) calcite surface

For biomineralization processes, the interaction of the surface of calcite crystals with organic molecules is of particular importance. Especially, biologically controlled biomineralization as in exoskeletons of mollusks and echinoderms, e.g., sea urchin with single-crystal-like spines and shells,1-3 requires molecular control of seed formation and growth process. So far, experiments showing the obvious influence of organic molecules on the morphology and habit of calcite crystals have demonstrated the molecular dimension of the interaction.4-7 Details of the kinetics of growth and dissolution of mineral surfaces influenced by additives are available,8,9 but other experimental data about the structure of the organic/inorganic interface on the atomic scale are rare. On the other hand, complicated organic macromolecules which are involved in biomineralization are numerous, with only a small fraction solved in structure and function so far.10-13 Therefore, model systems have to be designed to provide a basic understanding for the interaction process.14 Using grazing incidence X-ray diffraction combined with molecular modeling techniques, we show that glycine molecules order periodically on the calcite (104) face in competition with the solvent water when exposed to an aqueous solution of the most simple amino acid. In contrast to the general concept of the charge-matching fit of organic molecules on mineral surfaces,4,14 glycine is not attached to the calcite surface directly but substitutes for water molecules in the second hydration layer.     

Thermal conditions of growth and the necking evolution of Si,GaSb AND GaAs

The configuration of thermal gradient is illustrated for various types of crucible rotation, which is important for the creation of dislocations, which decreases along the grown axis of crystal. A new mechanism for dislocation elimination during the growth is proposed to explain this phenomenon, which provides a good agreement with the experimental results. The concentration of etch pits rapidly decreased from the beginning to the end of the crystals and the dislocation densities in the middle portion of all investigated crystals were found less than 102 cm^-2. The shallow vertical temperature gradients and virtually flat solidification interface prevented thermal stress from their building up in the crystals. As a result, the dislocation formation had random distribution. Using good necking procedures and choosing an appropriately oriented starting crystal with the shoulder angle <38.94° (assuming growth in <111> direction) it is possible to produce almost dislocation-free crystals without resorting to additional doping normally employed to reduce dislocation formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogen plasma interaction with (100) diamond surfaces

Polycrystalline diamond films exhibiting (100) oriented surfaces have been subject to a low pressure hydrogen plasma for durations up to 20 h. The topography of spatially defined 20 × 20 μm areas of the samples were imaged by atomic force microscopy at intervals during the plasma exposure. The mean surface roughness of individual (100) crystallites decreased from ca. 2.4 nm to <1 nm over the period and was independent of the twist and tilt angles of the crystallite. Whilst small hillock growth features were etched completely by the plasma treatment, there was no evidence for etch pits evident in similar experiments carried out with (100) natural diamond. Very low lateral etch rates of the (100) crystallites of 28 ± 4 nm/h were measured for crystallites bounded by (111) planes. High resolution XPS analysis of the C(1s) and O(1s) transitions of the same samples showed that the surface graphitic phase, present in the as-prepared samples, was removed to below detectable limits. The surface oxygen content was reduced from around 9-10% to ca. 3% after prolonged plasma exposure. The C(1s) and O(1s) band contours revealed the presence of oxygen in the form of ether and carbonyl functional groups. The ether:carbonyl: areal density ratio on (100) crystallites decreased only slightly from 83:17 to 64:37 after 20 h of plasma treatment based on fitting of the O(1s) band envelope. Etching products arising from the plasma interaction with the diamond surface were not detected by either optical emission spectroscopy or mass spectrometry.     

The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg²⁺ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg²⁺ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.     

The nature of clay volatiles and condensates and the effect on their environment

When clays are heated, a mass loss occurs due to the evolution of volatiles. Most of these are water vapour, but in addition minor amounts of a complex assemblage of other species are liberated. The corresponding condensates are colloidal suspensions. The composition of the volatiles and condensates and the release patterns of the gases are reviewed. The nature of the assemblages varies from one clay to another and depends on the thermal regime. Both volatiles and condensates are very reactive. Their reactivity persists even after prolonged storage. They act as acid catalysts in reactions with organic matter and decompose calcite and other carbonates. Condensates were found to etch the surfaces of quartz crystals and to dealuminate and partly destroy an Al-rich faujasite (zeolite). Possible implications of reactions of clay volatiles and condensates for natural processes are discussed.     

High-magnesian calcite mesocrystals: a coordination chemistry approach

While biogenic calcites frequently contain appreciable levels of magnesium, the pathways leading to such high concentrations remain unclear. The production of high-magnesian calcites in vitro is highly challenging, because Mg-free aragonite, rather than calcite, is the favored product in the presence of strongly hydrated Mg(2+) ions. While nature may overcome this problem by forming a Mg-rich amorphous precursor, which directly transforms to calcite without dissolution, high Mg(2+)/Ca(2+) ratios are required synthetically to precipitate high-magnesian calcite from solution. Indeed, it is difficult to synthesize amorphous calcium carbonate (ACC) containing high levels of Mg, and the Mg is typically not preserved in the calcite product as the transformation occurs via a dissolution-reprecipitation route. We here present a novel synthetic method, which employs a strategy based on biogenic systems, to generate high-magnesian calcite. Mg-containing ACC is produced in a nonaqueous environment by reacting a mixture of Ca and Mg coordination complexes with CO(2). Control over the Mg incorporation is simply obtained by the ratio of the starting materials. Subsequent crystallization at reduced water activities in an organic solvent/water mixture precludes dissolution and reprecipitation and yields high-magnesian calcite mesocrystals with Mg contents as high as 53 mol %. This is in direct contrast with the polycrystalline materials generally observed when magnesian calcite is formed synthetically. Our findings give insight into the possible mechanisms of formation of biogenic high-magnesian calcites and indicate that precise control over the water activity may be a key element.     

铝箔在盐酸中的交流电侵蚀研究——Ⅲ.交流电频率的影响

用10-100Hz频率的交流电研究了铝箔在盐酸溶液中的电解侵蚀,以获得高的表面积扩大率K₀值。K₀与频率的关系曲线上存在一相对最佳频率f_m。f_m随温度升高移向较高频率端。20Hz与35℃条件下得到的K₀值最大。频率增高有利于使侵蚀膜增厚,温度升高使膜溶解。用膜重G击破电位E_p与f之间的关系解释了K₀的变化。讨论了铝箔原始表面状态对侵蚀过程的影响。     

铝箔在盐酸中的交流电侵蚀的研究—Ⅱ.50周交流电正半周侵蚀

五种不同纯度电容器用铝箔(99.97—99.999%Al)在盐酸中用50周交流电正半周进行侵蚀。由于侵蚀膜是在断电半周低pH值下形成的,属透明钝化型薄膜,不堵塞侵蚀孔,因此不出现纯交流电侵蚀时的"掉粉"、"减薄"现象。实验结果表明,侵蚀形态以及表面积扩大率与箔的纯度,也即其自钝化能力有关。用电位波形图解释了得到的现象。