Effect of magnetic field on sodium arsenate metastable zone width and crystal nucleation kinetics for crystallization

Sodium arsenate, the main component of arsenic-containing solid waste pollutants, causes serious environmental health threats. Crystallization is one of the effective methods for separating and purifying sodium arsenate from arsenic-alkali residue lixivium. However, the crystallization process is limited for its low observability and the lack of separation and purification data. In this work, a laser detection system with a magnetic field generator was designed, and the solubility, metastable zone width, interfacial tension, interfacial entropy factor, crystal nucleation, and growth rate of sodium arsenate were investigated in a constant composition environment. The results showed that the solubility, metastable zone width, interfacial tension, and interfacial entropy factor decreases with the presence of a magnetic field. The magnetic field shortened the crystallization induction time and changed the nucleation and growth rate of sodium arsenate. Under the magnetic field, the nucleation rate increased from 2.43 × 10¹⁶ to 8.98 × 10¹⁷ (s m³)⁻¹, and the growth rate decreased from 4.94 × 10⁻⁸ to 2.73 × 10⁻⁸ (s m³)⁻¹, the growth mechanism of sodium arsenate as a continuous growth mode was unchanged. In addition, the X-ray diffraction and infrared showed that the crystal structure of sodium arsenate is unaffected by the magnetic field, indicating that the enhancement of the crystallization process of sodium arsenate with the magnetic field could be a feasible method in engineering application.     

The life of an anise-flavored alcoholic beverage: does its stability cloud or confirm theory?

The well-known alcoholic beverage Pastis becomes turbid when mixed with water due to the poor solubility of trans-anethol, the anise-flavored component of Pastis in the water solution formed. This destabilization appears as the formation of micrometer-sized droplets that only very slowly grow in size, thus expanding the life of the anise-flavored beverage. The slow growth has been attributed to an extremely low interfacial tension of the droplets. Fitting experimental droplet growth rates to an Ostwald ripening model, interfacial tensions were deduced in the past. Direct determination of the interfacial tensions was not yet reported on these systems. We have measured the interfacial tensions and used these data to predict droplet growth rates using an Ostwald ripening model and a model for creaming of the droplets. The interfacial tension was measured to be about 11 mN/m for a 30/70 w/w % ethanol/water mixture, and it decreases slightly to a value of 1.4 mN/m in the case of a 70/30 w/w % ethanol/water mixture. These values are not as low as those deduced in the past. The theoretical predictions for both the Ostwald ripening rates and the creaming rates, using the directly measured interfacial tensions, are found to contradict with the experimental results on Ostwald ripening and creaming. While the experiments on Ostwald ripening show an increase in stability with increasing ethanol concentration, the results based on our interfacial tension measurements in combination with the same Ostwald ripening model show a decrease in stability with an increase in ethanol concentration. Further research is needed to understand fully which parameters play a role in both droplet growth and the stability of these three-component emulsions to elucidate the current discrepancy between model and experiment. This could be useful for a better control of "spontaneous emulsification" processes.     

Crystallization kinetics of stretched natural rubber

Studies of the crystallization kinetics of natural rubber networks held in simple extension are reported. In these experiments the length of the specimen was held constant, and the crystallization process was followed by the decay in stress that occurred. A wide range of extension ratios and crystallization temperatures was encompassed by these experiments. From an analysis of the shapes of the crystallization isotherms, it can be concluded that major changes take place in the nucleation and growth processes as the extension ratio is increased. This conclusion is in accord with reports in the literature of changes in the crystallite morphology with extension ratio. Analysis of the temperature coefficient of crystallization, by means of nucleation theory, indicates a substantial increase in apparent interfacial free energies with increased deformation. This latter observation is interpreted to indicate a departure from a correlated crystallization process to one where isolated crystallites are formed.     

Transcrystallization of PTFE fiber/PP composites—III. Effect of fiber pulling on the crystallization kinetics

The effect of shear rates on the transcrystallization of polypropylene (PP) on the polytetrafluoroethylene (PTFE) fibers has been quantitatively investigated using a polarized optical microscope equipped with a hot stage and a tensile testing machine. The PTFE fibers were pulled at different rates, from 0.17 to 8.33 μm/s, to induce a range of shear rates, about 0.02 to 1.16 1/s, in the PP melt adjacent to the fiber. The induction time, nucleation rate, and saturated nucleation density at the fiber surface were determined at various crystallization temperatures. It was found that both the nucleation rate and the saturated nucleation density increase with increasing shear rates. However, the induction time is significantly reduced. Based on the theory of heterogeneous nucleation, the interfacial free energy difference functions Δσ;_TCL of PP on PTFE fibers at different levels of shear rates were determined and compared with that obtained from crystallization under quiescent conditions. Results showed that the magnitude of Δσ_TCL decreased to be about one-third of that for the quiescent crystallization, when a shear rate of 1.16 1/s was applied. The application of a shear stress to the supercooled PP melt by fiber pulling leads to enhance the development of transcrystallinity. Moreover, both the thickness and the crystal growth rate of transcrystalline layers were found to increase with the increasing rate of fiber pulling, especially at low crystallization temperatures where regime III prevails (see text). Surface morphology of PTFE fibers was revealed using a scanning electron microscope and an atomic force microscope. It is argued that the presence of fibrillar-type features at the fiber surface is the main factor responsible for the development of transcrystallinity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1361–1370, 1998     

溶液间歇结晶过程成核与生长阶段的定量识别

为定量识别溶液间歇结晶过程中的成核和生长阶段,基于晶粒数目和粒度的变化对粒度分布(CSD)的二阶和三阶矩量影响程度的不同,定义并关联了无因次变量K和K^*.添加晶种KNO₃-H₂O溶液结晶过程模拟计算的结果表明,K和K^*值均呈先降后升的变化趋势,成核时单调下降,生长过程中单调上升;且K与K^*值较接近.测定了KNO₃-H₂O溶液自发成核结晶过程中溶液浓度和透光率的变化,用K^*判据定量识别出成核阶段和生长阶段,并与晶体线性生长速率模型检验的结果相吻合.K值的计算依赖于CSD和结晶动力学参数,而K^*作为成核和生长阶段的模型判据,由实验测定的溶液浓度和透光率计算得到.     

Particle nucleation and growth models

This review concentrates on the progress of modeling the nucleation process of particles by the balanced nucleation-growth (BNG) process. The BNG model will be compared with other models that try to predict material nucleation. Compared to other models, the BNG model allows quantifying the nucleation rate, maximum growth rate, and supersaturation during the nucleation period as a function of nucleation efficiency and maximum growth rate of the crystals. From this model, equations are derived that correlate the number of stable crystals formed with molar addition rate of reactants, solubility of the crystals, and temperature. The BNG model predicts the experimental result that many crystallization processes result in a limited number of crystals followed by growth. The model also predicts that factors like diffusion and kinetically controlled growth process, Ostwald ripening agents and growth restrainers control the crystal number. Equations are given for each of the variables that agree with experiments. The BNG model predicts the conditions for renucleation (formation of new crystals during precipitation). It leads to new equations for the prediction of crystal number and crystal size during controlled continuous precipitation in the continuous stirred tank reactor (CSTR) as a function of precipitation conditions.     

Non-isothermal crystalliztion kinetics of poly(phenylene sulfide) with low crosslinking levels

Poly(phenylene sulfide) (PPS) with different crosslinking levels was successfully fabricated by means of high- temperature isothermal treatment (IT). The crosslinking degree of PPS was increased with IT time as revealed by Fourier-transform infrared spectroscopy and dynamic viscosity measurements. Its influence on the non-isothermal crystallization behaviors of PPS was studied by differential scanning calorimeter (DSC). The crystallization peak temperature of PPS with 6 h IT was 15 K higher than that of the one with 2 h IT at 30 K/min cooling rate. The non-isothermal crystallization data were also analyzed based on the Ozawa model. The Ozawa exponent m decreased from 3.5 to 2.2 at 232℃ with the increase of the IT time, suggestive of intensive thermal oxidative crosslinking reducing the crystallite dimension as PPS crystal grew. The reduced cooling crystallization function K(T) was indicative of the larger activation energy of crosslinked PPS chain diffusion into crystal lattice, resulting in a slow crystal growth rate. Additionally, the overall crystallization rate of PPS was also accelerated with the increase of crosslinking degree from the observation of polarized optical micrograph. These results indicated that the chemical crosslinked points and network structures formed during the high-temperature isothermal treatment acted as the effective nucleating sites, which greatly promoted the crystallization process of PPS and changed the type of nucleation and the geometry of crystal growth accordingly.     

Effects of organic additives on crystallization process and the adsorption performances of zeolite A

The crystallization of zeolite A was monitored by measuring the adsorption capacities of synthetic products. The influences of organic additives on the crystallization process and adsorption performances of zeolite were investigated. SDS (sodium dodecyl sulphonate), TWEEN (Tween-80), and PEG (poly(ethylene glycol)) shorten the induction period by reducing the interfacial energy while SCMC (sodium carboxymethylcellulose) can prolong the induction period by increasing the interfacial energy. TEA (triethanolamine) can also suppress the nucleation through reducing the effective supply of aluminum. All the organic additives but SCMC diminish the rate of crystal growth. CTAB (cetyltrimethylammonium bromide) causes the destruction of crystal structure and reduce the concentration of OH^? ions. As a result, the rate of crystal growth is significantly reduced. Meanwhile, PAM (poly(acrylamide)), SDS, TWEEN, HMTA (hexamethylenetetramine), and PEG increase the viscosities of synthesis systems, thus, diminish the growth rate. PAM restrains the transformation of zeolite A crystal into hydroxysodalite one, therefore, tremendously improves the stability of crystals of zeolite A. In addition, PAM can promote the rates of n-hexane adsorption on zeolite 5A because of the impact of PAM on the crystal-size distributions of zeolite 5A.     

Temperature effects for isothermal polymer crystallization kinetics

We adopt the cluster size distribution model to investigate the effect of temperature on homogeneous nucleation and crystal growth for isothermal polymer crystallization. The model includes the temperature effects of interfacial energy, nucleation rate, growth and dissociation rate coefficients, and equilibrium solubility. The time dependencies of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots) are presented for different temperatures. The denucleation (Ostwald ripening effect) is also investigated by comparing moment and numerical solutions of the population balance equations. Agreement between the model results and temperature-sensitive experimental measurements for different polymer systems required strong temperature dependence for the crystal-melt interfacial energy.     

Crystallization and Transformation Mechanism of Calcium Carbonate Polymorphs and the Effect of Magnesium Ion

The crystallization of calcium carbonate was carried out by mixing CaCl(2) and Na(2)CO(3) solutions. The morphology of precursor formed prior to the nucleation of the polymorphous crystals (calcite and vaterite) varies depending on the feed concentration. The faster nucleation rate of polymorphous crystals in 0.2 mol/L than in 0.05 mol/L solution results in the prompt disappearance of the precursor at 0.2 mol/L. In 0.05 mol/L solutions the lifetime of the precursor is relatively long. The crystallization fraction of vaterite increases with the feed concentration and decreases with the addition rate of Na(2)CO(2) solution. Vaterite takes on the various morphologies of the aggregates of the primary flocculation body (spherulite) depending on the crystallization conditions. Vaterite transforms to calcite by a direct solution-mediated mechanism. During crystallization the concentration attains a stationary value, which increases with the feed concentration and decreases with the addition rate of Na(2)CO(2) solution. This may be due to the crystal size decrease expected from the Gibbs-Kelvin equation. Magnesium ion suppresses the transformation of vaterite by inhibiting the growth of the calcite. Magnesium ion is selectively included in calcite and causes the increase of the attained concentration and the remarkable change in the morphology of calcite especially in 0.05 mol/L solution. Copyright 2001 Academic Press.     

Polylactide stereocomplex crystallites as nucleating agents for isotactic polylactide

A nucleation efficiency scale for isotactic poly(L ‐lactide) (PLLA) was obtained with self‐nucleation and nonisothermal differential scanning calorimetry experiments. The maximum nucleation efficiency occurred at the highest concentration of self‐nucleating sites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D ‐lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher temperatures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heterogeneous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation efficiency scale, we evaluated a series of PLLA/PDLA blends (0.25–15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation efficiency was largely dependent on the thermal treatment of the sample. The nucleating ability of the stereocomplex was most efficient when it was formed well before PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 300–313, 2001     

Localized Crystallization of Enantiomeric Organic Compounds on Chiral Micro‐patterns from Various Organic Solutions

The controlled crystallization of enantiomers of an organic compound (a cyclic phosphoric acid derivative) on templated micro‐patterned functionalised surfaces is demonstrated. Areas where a complementary chiral thiol has been located were effective heterogeneous nucleation centres when a solution of the compound is evaporated slowly. Various organic solvents were employed, which present a challenge with respect to other examples when water is used. The solvent and the crystallization method have an important influence on the crystal growth of these compounds. When chloroform was employed, well‐defined crystals grow away from the surface, whereas crystals grow in the plane from solutions in isopropanol. In both cases, nucleation is confined to the polar patterned regions of the surface, and for isopropanol growth is largely limited within the pattern, which shows the importance of surface chemistry for nucleation and growth. The apparent dependence on the enantiomer used in the latter case could imply stereo‐differentiation as a result of short‐range interactions (the templating monolayer is disordered, even at the nanometre scale). The size of the pattern of chiral monolayer also determines the outcome of the crystallization; 5 μm dots are most effective. Despite the low surface tension of the samples (relative to the high surface tension of water), differential solvation of the polar and hydrophobic layers of the solvents allows crystallization in the polar regions of the monolayer, therefore the polarity of the regions in which heterogeneous nucleation takes place is indeed very important. Despite the complex nature of the crystallization process, these results are an important step towards to the use of patterned surfaces for heterogeneous selective nucleation of enantiomers.     

Interfacial Engineering for Oriented Crystal Growth toward Dendrite-Free Zn Anode for Aqueous Zinc Metal Battery

Zn deposition with a surface-preferred (002) crystal plane has attracted extensive attention due to its inhibited dendrite growth and side reactions. However, the nucleation and growth of the Zn(002) crystal plane are closely related to the interfacial properties. Herein, oriented growth of Zn(002) crystal plane is realized on Ag-modified surface that is directly visualized by in situ atomic force microscopy. A solid solution HCP-Zn (~1.10 at. % solubility of Ag, 30 °C) is formed on the Ag coated Zn foil (Zn@Ag) and possesses the same crystal structure as Zn to reduce its nucleation barrier caused by their lattice mismatch. It merits oriented Zn deposition and corrosion-resistant surface, and presents long cycling stability in symmetric cells and full cells coupled with V₂O₅ cathode. This work provides insights into interfacial regulation of Zn anodes for high-performance aqueous zinc metal batteries.     

Formation of stable vaterite with poly(acrylic acid) by the delayed addition method

The crystallization of CaCO3 was examined by changing the addition time of poly(acrylic acid) (PAA) to an aqueous solution of calcium carbonate by selectively interacting with the crystal at different stages during the crystal-forming process. The precipitation of CaCO3 was carried out by a double jet method to prevent heterogeneous nucleation on glass walls, and the sodium salt of PAA was added by a delayed addition method. In the initial presence of PAA in an aqueous solution of calcium carbonate, PAA acted as an inhibitor for the nucleation and growth of crystallization. However, it was found that stable vaterite particles were successfully obtained by delaying the addition of PAA from 1 to 60 min. The vaterite particles were stable in the aqueous solution for more than 30 days, and the CaCO3 particles were formed by a spherulitic growth mechanism. It is suggested that PAA strongly binds with the Ca2+ ion on the surface of CaCO3 particles to stabilize the unstable vaterite form effectively. Upon changing the addition time of PAA, we found that CaCO3 particles were formed through different formation mechanisms in selectively controlled crystallization at different stages during the crystallization process.     

Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene

In this study, memory effect of mesomorphic isotactic polypropylene (iPP) was investigated using polarized optical microscope and small‐angle X‐ray scattering. Differing from classical memory effect, mesomorphic iPP melt had a higher growth rate and a higher memory temperature. The relative growth rate increased with increasing crystallization temperature. Lauritzen–Hoffman plots indicated that the increased growth rate arose from reduced surface nucleation barrier. The highest memory temperature was estimated to be 185 °C, which was close to the equilibrium melting point of iPP crystal. Additionally, Small‐angle X‐ray scattering measurements showed that a liquid crystal layer might exist between lamellar and amorphous layers. Based on above results, a crystallization model was proposed. In the mesomorphic iPP melt, there exist aggregates structurally similar to β phase except α‐phase crystal residuals, which cannot act as nucleation sites or transform to β crystal through surface nucleation. The only way for the aggregate is to transform to α crystal during crystal growth. The aggregate decreases the surface nucleation barrier and promotes the helical growth, leading to higher growth rate. Only when the aggregate relaxes to polymer coils through thickening at a higher temperature, can the memory effect be erased. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1573–1580     

The use of phase-inverting emulsions to show the phenomenon of interfacial crystallization on both heating and cooling

Highly anomalous crystallization behavior has been achieved in phase-inverting emulsion systems by using nonionic surfactants that induce nucleation. In particular, nucleation can be inhibited at the phase inversion, allowing systems held at, or near, this temperature to undergo crystallization either on heating or cooling. This new phenomenon is demonstrated for 27.4 wt % aqueous glycine solutions emulsified in decane using Span 20 Tween 20 blends. The inhibitory effect on interfacial nucleation at/near the phase inversion is readily shown by the maximum in the induction time for crystallization found in systems at/near the phase-inversion temperature. These findings are unprecedented. An extremely rapid rise in nucleation rate is expected on cooling glycine solutions, owing to the associated increase in supersaturation, the driving force for crystallization. The origin of this highly anomalous behavior is thought to be the low droplet interfacial tension, gammaow, that occurs at the phase-inversion temperature, which results primarily in a substantially increased contact angle between the glycine critical nucleus and the droplet interface. This may present a paradigm shift in crystallization strategies through the use of tunable contact-angle nucleators.     

Study of clay nanocomposites of the biodegradable polyhexamethylene succinate. Application of isoconversional analysis to nonisothermal crystallization

Nanocomposites of organomodified montmorillonites and the biodegradable polyester derived from hexanediol and succinic acid were prepared by the solution‐casting method using chloroform as solvent. Samples were studied by means of X‐ray diffraction and transmission electron microscopy. Intercalated structures differentiated by the stacking mode between silicate layers were observed. The highest variability in interlayer spacing was found when C30B organoclay was added. In this case, hydroxyl groups of the modifier could interact with polar carbonyl groups of the polyester. Thermal stability and crystallization behavior under both isothermal and nonisothermal conditions were evaluated. The overall crystallization rate of the intercalated nanocomposites was higher than that of the neat polyester due to a significant increase in their nucleation density, which compensated for their lower crystal growth rate. Isoconversional analysis was used to determine effective activation energies and to estimate nucleation and transport energy parameters from nonisothermal hot crystallization experiments. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2234–2248, 2008     

聚苯硫醚/尼龙6共混物界面对结晶行为的影响

高分子作为材料时 ,其力学性能受其结晶形态的影响 ,而其结晶形态与其结晶行为有关 .结晶性聚合物共混物中结晶组分由于第二组分存在 ,改变了结晶组分在熔体时的化学与物理环境 .因此 ,其结晶组分的结晶行为不仅取决于两组分在熔体时的相容性 ,而且与第二组分是否起到异相晶核作用和 /或两组分间界面是否诱导成核作用有关 ,从而影响共混物中结晶组分的结晶行为 ,导致共混物力学性能的改变[1～ 4] .在ＰＰＳ/ＰＡ6共混物中 ,由于ＰＰＳ的熔点和熔体结晶温度都比ＰＡ6的高 ,共混物熔体降温结晶ＰＰＳ是在ＰＡ6熔体存在下发生结晶 ,而ＰＡ6是在…     

Monte Carlo simulations of single crystals from polymer solutions

A novel "anisotropic aggregation" model is proposed to simulate nucleation and growth of polymer single crystals as functions of temperature and polymer concentration in dilute solutions. Prefolded chains in a dilute solution are assumed to aggregate at a seed nucleus with an anisotropic interaction by a reversible adsorption/desorption mechanism, with temperature, concentration, and seed size being the control variables. The Monte Carlo results of this model resolve the long-standing dilemma regarding the kinetic and thermal roughenings, by producing a rough-flat-rough transition in the crystal morphology with increasing temperature. It is found that the crystal growth rate varies nonlinearly with temperature and concentration without any marked transitions among any regimes of polymer crystallization kinetics. The induction time increases with decreasing the seed nucleus size, increasing temperature, or decreasing concentration. The apparent critical nucleus size is found to increase exponentially with increasing temperature or decreasing concentration, leading to a critical nucleus diagram composed in the temperature-concentration plane with three regions of different nucleation barriers: no growth, nucleation and growth, and spontaneous growth. Melting temperatures as functions of the crystal size, heating rate, and concentration are also reported. The present model, falling in the same category of small molecular crystallization with anisotropic interactions, captures most of the phenomenology of polymer crystallization in dilute solutions.     

Mechanistic study of the synthesis of Au nanotadpoles, nanokites, and microplates by reducing aqueous HAuCl4 with poly(vinyl pyrrolidone)

This article describes a simple approach to anisotropic Au nanostructures with various shapes by reducing HAuCl 4 with poly(vinyl pyrrolidone) (PVP) in aqueous solutions without the use of any additional capping agent or reductant. In this approach, the commercially available PVP servers as a mild reducing agent thanks to its hydroxyl (-OH) end groups, enabling kinetic control over both nucleation and growth. As the volume of HAuCl 4 solution added to the reaction was increased, the morphology of Au nanostructures evolved from nanotadpoles to nanokites and then triangular and hexagonal microplates. The slow reduction rate associated with the mild reducing power of PVP plays a critical role in forming nanoplates during nucleation as well as their growth into highly anisotropic nanostructures. Electron microscopy studies reveal that the nanotadpoles and nanokites are formed through the linear fusion of small Au particles (<10 nm) to the initially formed nanoplates, whereas the microplates result from the continuous addition of Au atoms to the side faces of nanoplates. Through this morphological control, the localized surface plasmon resonance peaks of these Au nanostructures can be tuned in the visible and near-IR regions.