Kinetics of crystal growth of mirabilite in aqueous supersaturated solutions

The crystallization of sodium sulfate decahydrate (Na₂SO₄·10H₂O, mirabilite) from supersaturated solutions was investigated using stable supersaturated solutions seeded with mirabilite seed crystals. The experiments were done in batch, stirred reactors in which the supersaturated solutions were prepared either by dissolution of sodium sulfate anhydrous at 32 °C followed by cooling to 18 or 20 °C or by mixing equal volumes of equimolar ammonium sulfate and sodium hydroxide solutions at 20 °C. Inoculation of the solutions supersaturated only with respect to mirabilite with seed crystals was accompanied with temperature increase of the thermostated solution. Despite the fact that crystal growth was initiated with seed crystals, the process started past the lapse of induction times inversely proportional to the solution supersaturation. The rates of crystal growth were measured both from the temperature rise and from the concentration–time profiles, which were linearly correlated. The measured crystal growth rates showed a parabolic dependence on supersaturation at low supersaturations. For higher values this dependence changed to linear, a behavior consistent with the BCF spiral crystal growth model. The morphology of the crystals growing at 20 °C showed typical prismatic habit, while at 18 °C when crystallized from cooled sodium sulfate solutions changes in the crystal habit to a leaf like morphology were observed.     

Direct AFM observations of impurity effects on a lysozyme crystal

Impurity effects on the growth of tetragonal lysozyme crystals have been studied using in situ atomic force microscopy. Commercially available hen egg white lysozyme was characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis with silver staining, and purified by re-crystallization and successive high pressure liquid chromatography. On the (1 1 0) crystal surface, there was no significant difference in morphology between crystals grown in commercial and in purified solutions. On the (1 0 1) surface, however, a large number of small particles were found when the crystal was grown in the commercial solution, while the surface grown in the purified solution was quite smooth. Among the typical residual impurities contained in commercial lysozyme, only covalently bound lysozyme dimer yielded such particles. From measurements of particle separation and an estimate of the critical nucleation size, we infer that the particles reduced the step velocity according to the mechanism described by Cabrera et al. [N. Cabrera, D.A. Vermilyea, in: R.H. Doremus et al. (Eds.), Growth and Perfection of Crystals, 1958, P. 393].     

Crystallization-in-emulsion process of a melted organic compound: In situ optical monitoring and simultaneous droplet and particle size measurements

The crystallization-in-emulsion process allows the production of solid particles exhibiting specific features. Here, the batch crystallization process carried out by cooling a melted oil dispersed as an oil-in-water emulsion was studied. Two experimental set-ups allowing the in situ visualization of the nucleation and growth phenomena occurring in the dispersed liquid phase were developed. Observations in quiescent medium of motionless droplets having a diameter of few tens of micrometers showed that primary nucleation started on the inner surface of the droplets. The fast growth of the crystals consumed all the liquid contained within each droplet and was confined within each droplet by the oil–water interface. Solid polycrystalline particles similar in size to the parent droplets were produced. Dynamic tracking of the transient evolution of the size distributions of the two populations of droplets and solid particles during the cooling process in a stirred vessel was carried out using an in situ optical probe. It was shown that the droplets crystallized very progressively during cooling, starting with the largest droplets and ending with the smaller size droplets since the induction time of primary nucleation was dependent on droplet volume. In dilute conditions (1% wt% of dispersed phase) each droplet was converted into a single solid particle. Secondary nucleation based on inter-droplet collisions was not observed in these conditions.     

Batch Precipitation of Potash Alum Crystals by Mixing Aqueous Solutions of the Component Salts

The effects of temperature, supersaturation, excess of reacting component salts and method of precipitation on the batchwise precipitation of potash alum crystals, by mixing aqueous solutions of potassium sulphate and aluminium sulphate, have been investigated. From the primary characteristics of the process (crystal size, size distribution and yield) relative crystal growth rates and relative nucleation rates are calculated and interrelation of these rates with the corresponding supersaturation leads to a value of the nucleation order. It is concluded that secondary nucleation plays an important role in the batch precipitation of potash alum crystals.     

Influence of UV-exposure on the crystallization and optical properties of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass. Photoinduced crystalline phase precipitation results in refractive index variations in UV exposed areas of PTR glass. The precipitation of silver containing particles which occurs during photo-thermo-refractive process increases the optical absorption of the samples in the range 350 nm to NIR wavelengths and the growth of sodium fluoride crystals and their aggregation increases light scattering in visible and NIR regions. We show that one effect of the UV-exposure is a decrease in the crystallization temperature by ～50 °C compared to the unexposed areas as measured by differential scanning calorimetry, which we attribute to an increase in nucleation rate. Using spectro-photometric measurements, a linear function is fitted to the changes in the amplitude of the absorption band of the silver containing particles versus the UV-dosage. The root mean square scatter of the data from the linear function is better than 0.99 and the slope of the function is 0.32 ± 0.01 cm/J. The IR absorption of PTR sample, measured by laser calorimetry shows that the increase of the absorption in infrared region at 1.1 μm, is due to the tail of the absorption band of silver containing particles having maximum at 465 nm. We finally show that after hyper-development, one effect of UV-exposure at 325 nm on the crystallization kinetics of PTR glasses is a decrease in particle sizes from micron size to nanometers size. But additional investigations demonstrate that smaller dosage UV-exposures (a few tens of milliwatts) increase the optical scattering by one order of magnitude. Optical micrographs taken after UV-exposure and hyper-development reveal the use of smaller dosages enhances nucleation rate without preventing the growth of large crystals and therefore induces higher scattering.     

In situ atomic force microscopy studies of surface morphology,growth kinetics,defect structure and dissolution in macromolecular crystallization

Surface morphologies of thaumatin, catalase, lysozyme and xylanase crystals were investigated using in situ atomic force microscopy. For thaumatin, lysozyme and xylanase crystals, growth steps having a height equal to the unit cell parameter were produced both by screw dislocations and two-dimensional nuclei. Growth of catalase crystals proceeded in alternating patterns exclusively by two-dimensional nucleation and the successive deposition of distinctive growth layers, each having a step height equal to half the unit cell parameter. The shapes of islands on successive layers were related by 2-fold rotation axes along the 〈0 0 1〉 direction. Experiments revealed that step bunching on crystalline surfaces occurred either due to two- or three-dimensional nucleation on the terraces of vicinal slopes or as a result of uneven step generation by complex dislocation sources. Growth kinetics for thaumatin and catalase crystals were investigated over wide supersaturation ranges. Strong directional kinetic anisotropy in the tangential step growth rates in different directions was seen. From the supersaturation dependencies of tangential step rates and the rates of two-dimensional nucleation, the kinetic coefficients of the steps and the surface free energy of the step edge were calculated. Adsorption of impurities which formed filaments on the surfaces of catalase and thaumatin crystals was recorded. Cessation of growth of xylanase and lysozyme crystals was also observed and appeared to be a consequence of the formation of dense impurity adsorption layers. Crystal growth resumed upon scarring of the impurity adsorption layer and clearing of the crystal surface with the AFM tip. Adsorption of three-dimensional clusters, which consequently developed into either properly aligned multilayer stacks or misaligned microcrystals was recorded. For catalase crystals, incorporation of misoriented microcrystals as large as 50×3×0.1 μm³ produced elastic deformations in growth layers of ≈0.6%, but did not result in the defect formation. Etching experiments on catalase crystals revealed high defect densities.     

Seeded growth of AlN on N- and Al-polar 〈0001〉 AlN seeds by physical vapor transport

We demonstrated seeded growth of AlN on large-area Al- and N-polar <0 0 0 1>-oriented AlN seeds using the physical vapor transport method (PVT). In both cases, crystals having a diameter of 15 mm were obtained from 5 mm seeds. Based on growth step and terrace width analyses, it was found that the N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.     

Determining the primary nucleation and growth mechanism of cloxacillin sodium in methanol–butyl acetate system

The primary nucleation and growth mechanism of cloxacillin sodium in methanol–butyl acetate system are determined on the basis of induction time measurements. The induction time of cloxacillin sodium is experimentally determined by the laser scattering method at different supersaturations and temperatures. The measured induction times are then treated using the models of mononuclear and polynuclear mechanisms. It is discovered that the primary nucleation mechanism of cloxacillin sodium is identified as polynuclear mechanism, which relates the induction time and the supersaturation for various growth mechanisms. On the basis of these analyses, the growth mechanism of cloxacillin sodium is two-dimensional nucleation-mediated growth at 285 K, and changes into spiral growth with increasing temperature (at 291 and 297 K).     

Growth kinetics of sodium perborate from batch crystallization

The size distribution of sodium perborate crystals was continually monitored using a Malvern sizer during batch crystallization from aqueous solutions carried out under falling supersaturation established at the experiment onset. The growth rate was determined from the time shift of the crystal size distribution expressed in cumulative oversize numbers. The size independent overall growth rate was first order with respect to supersaturation for crystals larger than 150 μm. Crystals between 20 and 150 μm exhibited a significant size-dependent growth rate. Furthermore, the fraction of crystals smaller than 20 μm, formed by primary nucleation, grew extremely slowly or did not grow at all.     

Quartz crystal microbalance as a tool for scale deposition monitoring in Bayer process conditions

This paper describes the study of bayerite or gibbsite scale formation and growth from supersaturated sodium aluminate solutions using a quartz crystal microbalance. Analysis of frequency vs time curves and scanning electron microscopy images allowed us to propose a surface nucleation mechanism that leads to cementation of particles produced by catastrophic secondary nucleation. We also highlighted the influence of the filtration of the supersaturated sodium aluminate solution step on the scale nucleation kinetics. It was observed that use of filter with porosity below 2.5 μm delays the formation of scale.     

Crystallization kinetics of 1Na2O · 2CaO · 3SiO2 · glass monitored by electrical conductivity measurements

We investigated the kinetics of crystal nucleation, growth, and overall crystallization of a glass with composition close to the stoichiometric 1Na₂O · 2CaO · 3SiO₂. The nucleation and subsequent growth of sodium-rich crystals in this glass decreases the sodium content in the glassy matrix, drastically hindering further nucleation and growth. Compositional changes of the crystals and glassy matrix at different stages of the crystallization process were determined by EDS. These compositional variations were also monitored by electrical conductivity measurements, carried out by impedance spectroscopy, in glassy, partially, and fully crystallized samples. The electrical conductivity of both crystalline and glassy phases decreases with the increase of the crystallized volume fraction. Starting at a crystallized volume fraction of about 0.5, the crystalline phase dominates the electrical conductivity of the sample. This behavior was corroborated by an analysis of the activation energy for conduction. We show that electrical conductivity is highly sensitive and can indicate compositional shifts, changes in the spatial distribution of mobile ions in the glassy matrix. Conductivity measurements are thus a powerful tool for the investigation of complex heterogeneous systems, such as partially crystallized glasses and glass-ceramics.     

Investigation of the growth rate of β-cyclodextrin in water during both flow-cell and batch experiments

Growth rate measurements of β-cyclodextrin in water were performed both ways. Firstly, experiments were conducted with single monocrystals located in a supersaturation-controlled flow cell. Diffusional limitations and perturbations due to a competition between surface secondary nucleation and growth at high level of supersaturation have been put in evidence. The evolution of the growth rate with supersaturation has been modelled with a BCF law, assuming a screw dislocation mechanism. Secondly seeded cooling batch trials have been carried on in a well-mixed suspension crystallizer in order to assess the growth rate of the seeds. Refractometry was used as an in situ sensor for measuring the evolution of the concentration of the solute. Measurements of the crystals size distribution of the seeds and of the final crystals are performed off line with laser diffraction technique. A kinetic law with three parameters allows a consistent assessment of the growth of the seeds. The comparison of the two sets of data shows that overall growth rate of the seed is partially limited by diffusion. Nevertheless, the estimation of a surface integration growth kinetic coefficient from batch trials is rather difficult. The theoretical framework of nucleation models developed by Mersmann et al. (Crystallization Technology Handbook, second ed., Marcel Dekker, New York, 2001, pp. 45–80 and 81–144) coupled with the estimated growth kinetics can therefore be used to better monitor the seeding process during batch crystallization operations so as to favor the growth of the seed crystals.     

Heterogeneous nucleation mechanisms and formation of metastable phase assemblages induced by different crystalline seeds in a rapidly cooled andesitic melt

Pt-capsules loaded with a Pt-coil and two crystalline seeds immersed into an andesitic melt were rapidly cooled from 1300 to 1100 °C with a rate of 3 °C/min at atmospheric pressure and air oxygen fugacity. Results show that the Pt-coil does not induce any heterogeneous crystallization process as well as iron diffusion process from the melt into the platinum substrate. In contrast, the presence of crystalline seeds in a solidifying andesitic melt significantly alters the phase assemblage, composition and texture of the new-forming crystals in response to a heterogeneous nucleation mechanism and the formation of metastable phases.     

Influence of emulsion types on nucleation kinetics and growth habit in the cooling crystallization

Experiments on unseeded batch cooling crystallization were made to investigate the influence of emulsion [solution in non‐solvent (S/NS) emulsion and non‐solvent in solution (NS/S) emulsion] on crystal size and growth habit of various materials such as hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine, ammonium sulfate, potassium dihydrogen phosphate (KDP), and γ‐glycine. Size of crystals obtained from the S/NS emulsions was found to be significantly larger than that obtained from the NS/S emulsions. Those results were explained by broad induction time distribution of nucleation in the isolated solution droplets and subsequent seeding effect by free movement of early‐induced crystals by vigorous agitation. As a result, the population density of crystals was shown to be smaller in the S/NS emulsions than that in the NS/S emulsions. In the S/NS emulsions, desupersaturation rate should be slow and high supersaturation is subsequently generated during crystal growth stage. Therefore, it may be concluded that crystal morphology of the materials with supersaturation dependent growth habit, such as ammonium sulfate, KDP and γ‐glycine, can be controlled by selection of emulsion type.     

Semi-Batch Precipitation of Calcium Oxalate Monohydrate

Calcium oxalate monohydrate (COM) formation on the semi-batch precipitation of CaCl₂ and Na₂C₂O₄ solutions in the stoichiometric ratio carried out at pH = 6 and 37 or 70°C, respectively, was studied. When a certain level of supersaturation in the system is reached, individual COM crystals of a rather uniform size are formed. These crystals then grown and form ‘loose’ agglomerates that later develop into compact and spherical particles. The particle size distribution (PSD) rapidly shifts during early stages of precipitation towards larger sizes as a result of crystal growth and agglomeration. Later the PSD reaches a shape and position on the size axis that remain virtually constant with progressing precipitation. COM agglomerates consisting of mainly intergrown crystals are formed by mechanism of primary and secondary agglomeration. The primary agglomeration can constitute an important factor in urolithiasis.     

Incidence of crystal growth conditions on the formation of macroscopic liquid inclusions in ciclopirox crystals

Using an accurate and rigorous protocol, the crystal growth behavior in solution of the antifungal drug ciclopirox was investigated with the aim to identify the experimental factors responsible for the appearance of macroscopic inclusions of saturated solution. Counterintuitively, these inclusions are produced only when the relative supersaturation is below a critical threshold; simultaneously the crystals exhibit a hexagonal morphology. An increase in the driving force leads to a rod-shaped morphology without inclusion and without any change of the crystal phase, as confirmed by X-ray diffraction. The correlation between this morphological transition and the occurrence of liquid inclusions could be rationalized by considering the associated change of growth rates of {(1 1 1)} and {(1 1 -1)} faces. However, the nature of the solvent, the presence of impurities and diffusion in solution appeared to have no detectable incidence on the formation of liquid inclusions, inducing that the presumed contribution of a chemical adsorption phenomenon—side products or solvents—could not be established.     

The induction time,interfacial energy and growth mechanism of maltitol in batch cooling crystallization

Maltitol is crystallized with seeds by cooling mode in industry, often with large amount of fine crystals and wide crystal size distribution. To eliminate the fine nucleation, it's necessary to understand the nucleation kinetics. In this work, the solubility of maltitol in water was measured by the gravimetric method, the nucleation kinetics of maltitol in batch cooling crystallization was investigated using focus beam reflectance measurement (FBRM), and a novel method was proposed to determine the induction time from the trend of the crystal median chord given by FBRM. Based on the relationship between the induction time and the supersaturation, the nucleation mechanism was obtained, including homogenous nucleation at high supersaturation and heterogenous nucleation at low supersaturation. Additionally, combining the classical nucleation theory (CNT) and Arrhenius’ principle, the crystal‐solution interfacial energy and the energy barrier of homogenous nucleation were calculated. From the scanning electron microscope (SEM) images, the growth mechanism of maltitol was identified as surface nucleation growth and the surface entropy factor calculated from the kinetic analyses of t_ind data corroborated this growth mechanism.     

A novel process to prepare a hollow silica sphere via chitosan-polyacrylic acid (CS-PAA) template

Hollow silica spheres were synthesized by using a chitosan-polyacrylic acid (CS-PAA) template. Polyacrylic acid (PAA) was titrated into the dissolved chitosan solution to form CS-PAA particles. For forming the core-shell particle, colloidal silica which was prepared from homogeneous nucleation was mixed with a solution of CS-PAA particles. Colloidal silica was adsorbed on to the surface of the CS-PAA particles to form a shell structure. CS-PAA could be removed from the core of the core-shell particles by adjusting with HCl to pH < 1 for forming the hollow silica structure. The outside particle size diameter, shell thickness, specific surface, pore diameter and pore volume of the final hollow silica sphere are about 200 nm, 20 nm, 350.95 m²/g, 5.4078 nm and 0.516768 cm³/g, respectively.     

Crystal growth of ZrW2O8 and its optical and mechanical characterization

ZrW₂O₈ is known for its isotropic negative thermal expansion over a wide of range of temperature from ?272 to 777 °C. However, ZrW₂O₈ melts incongruently at 1257 °C and is stable only over a short temperature interval between 1105 and 1257 °C. This makes the growth of single crystals a formidable challenge. In order to study the intrinsic properties of this compound, a repeatable, viable single crystal growth strategy is required. Here we report a simple, self-seeding, self-fluxing single crystal growth process which resulted in single crystals of ZrW₂O₈ up to about 4 mm in size. Grown crystals were characterized by X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Mechanical properties of the crystals were studied using nanoindentation.     

Impurity effect of iron(III) on the growth of potassium sulfate crystal in aqueous solution

Growth rates of the {1 1 0} faces of a potassium sulfate crystal were measured in a flow cell in the presence of traces of impurity Fe(III) (up to 2 ppm) over the range of pH=2.5–6.0. The growth rate was significantly suppressed by the impurity. The effect became stronger as the impurity concentration was increased and at pH<5. It became weaker with increasing supersaturation. It also became weaker as the pH was increased and at pH>5 it finally disappeared completely. The concentration and supersaturation effects on the impurity action were reasonably explained with a model proposed by Kubota and Mullin [J. Crystal Growth, 152 (1995) 203]. The surface coverage of the active sites by Fe(III) is estimated to increase linearly on increasing its concentration in solution in the range examined by growth experiments. The impurity effectiveness factor is confirmed to increase inversely proportional to the supersaturation as predicted by the model. Apart from the discussion based on the model, the pH effect on the impurity action is qualitatively explained by assuming that the first hydrolysis product of aqua Fe(III) complex compound, [Fe(H₂O)₅(OH)]²⁺, is both growth suppression and adsorption active, but the second hydrolysis product, [Fe(H₂O)₄(OH)₂]⁺, is only adsorption active.