Dissolution of crystallites: surface energetic control and size effects.

Traditional understanding of dissolution assumes that the reaction is spontaneous and continues until equilibrium is reached. This paper presents theoretical and experimental data to support a dissolution mechanism that involves the existence of critical conditions for dissolution, in which the reaction is accompanied by the formation of pits and the subsequent displacement of pit steps. The accompanying increase in surface roughness leads to changes in surface energy with losses of crystal mass that are positive rather than negative and the existence of critical dissolution conditions. Critical pits and dissolution steps are verified experimentally and a relationship between the size and rate of displacement of steps is also demonstrated, in which the rate decreases with size and approaches zero at a critical size, r*. These microscopic step dynamics are consistent with the observed size-effects in bulk dissolution, which cannot be explained using traditional dissolution theories. The observed size effects include self-inhibition, in which the dissolution rate decreases with extent of reaction, dissolution suppression, and periodic resumption. These interesting dissolution phenomena are only readily displayed when the sizes of dissolving crystallites fall in the same range as the critical size (i.e., within 50r*). It is interesting to note that natural biominerals and many nanoparticles fall into this category, so that their suspensions can be dynamically stabilized without dissolution in undersaturated supporting media. The current research implies that dissolution kinetics cannot be understood well without appealing to fundamental physical concepts about the energetic control of dissolution steps on a molecular level. A new dissolution model for crystallites is introduced systemically.     

表面能与晶体生长/溶解动力学研究的新动向

界面现象使物质在结晶过程中出现了临界现象.但最近的研究指出在物质溶解过程中,在表面能量的控制下也存在着临界现象以及尺寸效应.实验发现,当晶体自身小到一 定的程度时(通常在纳米尺度上并和临界蚀坑的大小相近),在溶解过程中其速度会自发降 低,反应被抑制乃至停止.尽管在热力学上表面能的因素可以赋予小颗粒晶体较大的溶解度 ,但表面能却也能通过对临界条件的控制而使这些微粒在动力学上不被溶解.这个发现不仅 解决了纳米颗粒在水溶液中稳定性的问题,而且还从动力学的角度解释了生物矿物选择纳米 尺度作为其基本构成单元的原因.由于表面能和晶体生长/溶解的动力学有着密切的关系, 我们可以通过对表面能的调节来修改它们的动力学速度和晶体的形貌.反过来,也可以用动力学的方法来测定表面能及表面吸附/脱附常数等.相对于常规的界面研究手段,通过生 长和溶解动力学途径所得的数据有着很好的可靠性及重复性.我们认为,晶体生长和溶解的 动力学和表面能的研究相结合,不仅为界面研究提供了新的思路和方法,而且也会推动晶体生长和材料科学的发展.     

Supersaturation control in aragonite synthesis using sparingly soluble calcium sulfate as reactants

Sparingly soluble calcium salts were studied as reactants in the synthesis of needle-like precipitated calcium carbonate (PCC). The morphology and aspect ratio of the PCC particles were characterized with SEM. Polymorphs and crystal size were characterized using X-ray diffraction. The counterions of the sparingly soluble salts influenced the growth kinetics of PCC as well as the polymorphism and morphology of product particles. Either chrysanthemum-like or needle-like aragonite can be synthesized from calcium sulfate and sodium carbonate depending on the supersaturation and synthesis conditions. Low concentration and slow addition rate of sodium carbonate solution were favorable to the formation of aragonite. Addition of sodium sulfate to the reaction system (calcium chloride and sodium carbonate) promoted the formation of aragonite and decreased the crystal size of aragonite due to the decrease of supersaturation and adsorption of sulfate ion. Too much added sodium sulfate, however, did not further increase the aragonite fraction. An optimal temperature for the formation of aragonite was found to be ca. 60 degrees C. Slow dissolution kinetics of sparingly dissoluble salt also is very important for controlling PCC polymorphism and morphology.     

Study of Solubilizing Capacity of Anionic Surfactants (AOS and SLS) in Spray-Dried Pesticide Formulation

The slow dissolution rate exhibited by sparingly water‐soluble pesticide is a major challenge in the pesticide development process. The aim of this study was to improve the dissolution rate of a sparingly water soluble model pesticide, Atrazine. Microparticles containing the model pesticide (atrazine) were produced by spray‐drying technique in the absence and presence of an anionic surfactant. Alpha olefin sulfonate (AOS) and Sodium lauryl sulfate (SLS) were chosen as the surfactants to improve the particle wetting and the dissolution rate. The spray dried particles were characterized by SEM and particle size analyzer and dissolution studies were carried out by USP XXIII paddle method. Also the effect of spray drying on atrazine was studied by DSC. The results obtained showed that the dissolution rate of the spray dried atrazine‐surfactant particles were significantly increased compared to the active. This proves that the better wetting characteristics conferred by the hydrophilic surfactant were responsible for the enhanced dissolution rate of the model pesticide. 50% water dispersible granules (WDG) of atrazine were also prepared by spray‐drying technique. The resulting granules had a unique combination of good flow ability and a good degree of dispersion.     

Determination of interfacial tension from crystallization and dissolution data: a comparison with other methods

Methods for the determination of interfacial tension between a solid and a liquid are reviewed including solubility/particle size, crystallization and dissolution kinetics. The use of solubility as a function of particle size, originally put forward by Ostwald and later corrected by Freundlich, may be unjustified for determining interfacial tension at solid-liquid interfaces. The interfacial tension values between solutions and sparingly soluble minerals such as hydroxyapatite, fluorapatite, brushite, octacalcium phosphate, calcium oxalate monohydrate, barium sulfate, calcium sulfate, calcite, and divalent metal fluorides are discussed. A comparison of these results is made with contact angle or wetting measurements. The interfacial tension values obtained from constant composition reaction kinetics are of the same order of magnitude as those determined using a contact angle method involving thin layer wicking techniques.     

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.     

Condensation of supersaturated water vapor on charged/neutral nanoparticles of glucose and monosodium glutamate

The effects of size, charge, dissolution, and dissociation on the condensation of supersaturated water vapor on monodisperse nanoparticles of glucose and monosodium glutamate (MSG) were investigated in a flow cloud chamber (FCC). The dependence of the critical supersaturation, S(cr), on particle size in the range of 30 to 90 nm and on temperature in the range of 10 to 50 degrees C were determined experimentally. The results show that the experimental S(cr) decreases with increasing particle size at a rate in reasonable agreement with the predictions of the Kohler and Volmer theories of nucleation for soluble particles, but decreases with increasing temperature at a rate higher than the prediction of the Volmer theory. The dissociation of MSG into ions lowers the experimental S(cr) to a value smaller than that for the more soluble glucose, agreeing with predictions. The experimental S(cr) is smaller than the predictions of both theories, and the discrepancy cannot be fully explained by the reductions in surface tension due to the dissolution of particles and curvature dependence. The condensation of supersaturated vapor on singly positively charged particles with diameters of 30, 60, and 90 nm was also examined, and no obvious charge effect on S(cr) was observed.     

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.     

Influence of Operating Conditions on BaSO4 Crystal Size and Morphology in a Continuous Couette Precipitator

Precipitation of a sparingly soluble salt in the annular gap of a continuous Couette reactor with two unpremixed feeds has been experimentally investigated. Barium chloride and sodium sulphate in stoichiometric ratio are fed at different flow rate in the lower part of the reactor; different feeding modes have been considered. The dependence of precipitation yield, mean crystal size and particle size distribution on rotation speed, axial flow rate and initial supersaturation ratio has been studied. Depending on the operating conditions crystals with different morphologies have been obtained, varying from dentritic tabular crystals to complex pyramidal ones and from single crystals to aggregates. This revised version was published online in August 2006 with corrections to the Cover Date.     

Détermination des cinétiques de croissance et de dissolution du perborate de sodium tetrahydrate en lit fluidisé

The authors present the results of an experimental study on the crystallization of tetrahydrate sodium perborate in a fluidized bed. The influences of supersaturation, seed crystal size, temperature and solution velocity on the overall growth rate were examined. The overall dissolution rate determined under comparable hydrodynamical conditions for various seed crystal sizes and temperatures is also reported.The growth rate is not very sensitive to the variations in crystal size, the temperature and the hydrodynamical conditions, but it depends on the supersaturation and the presence of some soluble impurities.The comparison between growth and dissolution rates shows that under actual working conditions the diffusional mass transfer step is not the limiting step of the growth mechanism.     

Green and controllable fabrication of nanocrystals from ionic liquids

Nanocrystals are of great value in delivering poorly soluble drugs as a technique enables enhanced dissolution and bioavailability. The bottom-up technique allows better control of particle properties. However, the commonly used organic solvents are hazardous to environment and operators, and always lead to large particle size and wide size distribution due to failure on controlling the nucleation and crystal growth. The situation is exacerbated in scale-up production. Therefore, in the proof-of-concept study, we evaluated the feasibility of green and controllable fabrication of drug nanocrystals by using biocompatible ionic liquids (ILs) as solvents. Choline based ILs (Ch-ILs) were synthesized via metathesis reactions. Pure paclitaxel nanocrystals of high quality were obtained from Ch-ILs with surface tension higher than 42 mN/m. The sizes were below 250 nm, while the polydispersity indexes were lower than 0.25. Compared with ethanol, choline lactate is superior in controlling the size of the nanocrystals in scale-up production, where the drug concentration was increased by 6 times. The underlying mechanism may be due to the high viscosity and low surface tension of the ILs, which are supposed to benefit homogeneous and burst nucleation. Ch-ILs can be recycled from the process and recovery rate reached 91.1%. Moreover, the applicability of the green technique was validated in a wider range of model drugs and Ch-ILs. In conclusion, ILs are potent solvents in bottom-up technique for green and controllable fabrication of nanocrystals.     

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.     

LiCoO(2) Concaved Cuboctahedrons from Symmetry-Controlled Topological Reactions

Morphology control of functional materials is generally performed by controlling the growth rates on selected orientations or faces. Here, we control particle morphology by "crystal templating": by choosing appropriate precursor crystals and reaction conditions, we demonstrate that a material with rhombohedral symmetry-namely the layered, positive electrode material, LiCoO(2)-can grow to form a quadruple-twinned crystal with overall cubic symmetry. The twinned crystals show an unusual, concaved-cuboctahedron morphology, with uniform particle sizes of 0.5-2 μm. On the basis of a range of synthetic and analytical experiments, including solid-state NMR, X-ray powder diffraction analysis and HRTEM, we propose that these twinned crystals form via selective dissolution and an ion-exchange reaction accompanied by oxidation of a parent crystal of CoO, a material with cubic symmetry. This template crystal serves to nucleate the growth of four LiCoO(2) twin crystals and to convert a highly anisotropic, layered material into a pseudo-3-dimensional, isotropic material.     

Dissolution of ThO2: study of dissolution process with initial 229Th spike

Journal of Radioanalytical and Nuclear Chemistry - The sparingly soluble ThO2 dissolution behaviour was studied under conditions that are expected to prevail in bedrock after the closure of the...     

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.     

On the Stability of Liquid Nanodroplets in Polymerizable Miniemulsions

Liquid nanodroplets within a size range of 50 to 500 nm can easily be prepared by shearing a system containing oil, water and a surfactant. The growth of the nanodroplets can effectively be suppressed by using a strong hydrophobe as an additivie to the oil and an effective surfactant. The hydrophobe acts as an osmotic agent which stabilizes the system against Ostwald ripening. The growth of the droplets by collision is controlled by the density of the surfactant layer. Freshly prepared miniemulsions are “critically stabilized” and show a slow, but pronounced growth, whereas a miniemulsion in “equilibrium” exhibits constant droplet size on longer time scales. Polymerization of the oil droplets of such miniemulsions turns out to be very promising and extends the possibilities of classical emulsion polymerization. Since each droplet can be considered a small reactor in which polymerization reactions take place, the process allows one to create new particle structures, e.g. polyaddition reactions can take advantage of unusual monomers, the incorporation of materials which are not soluble in the continuous phase, and the formation of nanocapsules.     

镁铁双羟基复合金属氧化物的可控合成及晶面生长特征研究

采用成核/晶化隔离法合成镁铁双羟基复合金属氧化物MgFe-LDH,考察了Mg ∶ Fe摩尔比对MgFe-LDH晶形的影响,探讨了晶化温度及晶化时间对晶面生长选择性及晶粒尺寸的影响规律.结果表明,随Mg ∶ Fe摩尔比增大,层板阳离子排列更为规整.晶化温度对晶粒尺寸的影响显著大于晶化时间的影响.晶化温度相同,随晶化时间延长, MgFe-LDH的晶体结构趋于完整,晶粒尺寸变化不大;晶化时间相同,随晶化温度升高,晶体结构趋于完整,晶粒尺寸明显增大.所得到的MgFe-LDH沿a轴方向的晶粒尺寸对晶化温度变化的敏感程度远大于对晶化时间变化的敏感程度,但总是沿a轴方向的晶粒尺寸大于沿c轴方向的尺寸,即[110]晶面的生长速率比[002]晶面的生长速率快.     

The growth of crystals in solution

The crystallization of sparingly soluble salts from their aqueous supersaturated solutions is discussed from the standpoint of two important applications; scale formation and biological mineralization. Theories of crystal growth are outlined and the importance of kinetic factors in determining the nature of the growing phases is discussed. The kinetic factors can be studied by using a highly reproducible seeded growth technique and under certain conditions secondary nucleation can also be induced on the surface of the inoculating seed crystals. The kinetics of crystallization of the alkaline earth surfaces and the calcium phosphates is discussed. In the latter systems, temperature, supersaturation, surface concentration, pH, ionic strength and the presence of foreign ions are important in determining the nature of the phase which grows on the added seed crystals. The mechanism of the retardation of crystal growth by added crystallization inhibitors is illustrated by the influence of organic phosphonate molecules upon the precipitation of calcium carbonate.     

Understanding the barriers to crystal growth: dynamical simulation of the dissolution and growth of urea from aqueous solution

Both the dissolution and growth of a molecular crystalline material, urea, has been studied using dynamical atomistic simulation. The kinetic steps of dissolution and growth are clearly identified, and the activation energies for each possible step are calculated. Our molecular dynamics simulations indicate that crystal growth on the [001] face is characterized by a nucleation and growth mechanism. Nucleation on the [001] urea crystal face is predicted to occur at a very high rate, followed by rapid propagation of the steps. The rate-limiting step for crystallization is actually found to be the removal of surface defects, rather than the initial formation of the next surface layer. Through kinetic Monte Carlo modeling of the surface growth, it is found that this crystal face evolves via a rough surface topography, rather than a clean layer-by-layer mechanism.     

Tracking surface evolution using ligand-assisted dissolution of cobalt oxyhydroxide

The use of surface-specific reactions to probe reactive surface area is a promising direction in materials research. The work presented herein examines how the kinetics of dissolution can be used to quantify particle growth as well as the evolution of site-specific reactive surface area. The dissolution of heterogenite (β-CoOOH) by IDA results in two geometric isomers of Co(IDA)(2)(-): the s-fac and u-fac isomers. The heterogenite particles studied here can generally be described as cylindrical plates, and the relative amount of s-fac isomer produced is found to increase as the height of the plates increases. The quantity of each isomer produced is shown to correlate with the relative number of two different types of surface sites, designated as edge and corner sites, while basal sites are seemingly unreactive. It is hypothesized that u-fac isomer results from the more accessible Co centers at the corner sites, while the s-fac isomer results from the less accessible edge sites. An empirical relationship is developed between the fraction of s-fac isomer produced and the height of the β-CoOOH particles, and this relationship is used to quantify particle growth by analysis of kinetic data. Finally, this new information is used to modify a previously proposed pH-dependent growth model, resulting in a significant improvement in the fit and physical relevance of the model.