Nucleation Induction Time Research Articles

Solvent-Modulated Nucleation and Crystallization Kinetics of 12-Hydroxystearic Acid: A Nonisothermal Approach

The solvent type strongly affects the nucleation behavior of 12HSA and therefore strongly influences the peak nucleation rate, fiber length, spatial distribution of mass, and degree of branching. Using nonisothermal kinetic models, a correlation was established among the activation energy of nucleation, fiber length, and induction time of 12HSA nucleation in different solvents. However, there was no correlation between any measurable parameter and solvent polarity. Activation energies varied from 2.2 kJ/mol in methyl oleate to 15.8 kJ/mol in glycerol. Nucleation behavior and structure were strong functions of the cooling rate, with distinct regions observed above and below 5-7 degrees C/min for fiber length, induction time, rate constant, and peak nucleation rate. The abrupt changes in the rate of nucleation, crystal growth rate constant, and degree of branching around this cooling rate are related to whether the nucleation and crystal growth processes are governed by mass transfer or thermodynamics. Furthermore, the Avrami equation accurately predicted several structural features of the fibrillar network such as fiber length and, to a lesser extent, induction time.

Titanium Oxide Nanoparticles Precipitated from Low‐Temperature Aqueous Solutions: I. Nucleation, Growth, and Aggregation

Titanium oxide (TiO2) thin films can be prepared by controlled hydrolysis of a titanium tetrachloride (TiCl4) precursor in an aqueous solution at temperatures below 100°C. As part of an effort to understand the formation of such TiO2 films, the kinetics of the nucleation, growth, and aggregation of TiO2 nanoparticles in aqueous solutions of TiCl4 were studied using dynamic light scattering and a transmission electron microscope (TEM). A higher degree of supersaturation, produced by a higher solution temperature, a higher concentration of TiCl4, or a higher pH, results in a shorter nucleation induction time, a faster initial growth/aggregation rate, and a larger aggregate size. The interfacial energy of TiO2 nanoparticles was found to be 0.072 J/m2 from the homogeneous nucleation theory. Further investigations by a high‐resolution scanning electron microscope (SEM) and TEM for the as‐deposited films showed the structure hierarchy organized from nanocrystalline particles precipitated in the supersaturated solution. As a result, the effect of solution conditions on the nucleation, growth, and aggregation of TiO2 nanoparticles can provide useful guidance for tailoring of microstructures of the TiO2 films.

Evidence of liquid water formation during methane hydrates dissociation below the ice point

Dissociation of small methane hydrate samples formed from water droplets of size 0.25–2.5 mm has been investigated below the ice melting point in the temperature range of 240–273 K, where the self-preservation effect is observed for bulk hydrates. The experiments included optical microscopy observations combined with P– T measurements of the dissociation conditions for the methane hydrates. For the first time, the formation of supercooled liquid water during the hydrate dissociation was reliably detected in the temperature range of 253–273 K. The formation of the liquid phase was visually observed. The induction time of the ice nucleation for the metastable liquid water depended from the dissociation temperature and a size of water droplets formed during the hydrate dissociation. It was found that in the temperature range of 253–273 K values of the dissociation pressure for the small hydrate samples fall on the extension of the water–hydrate–gas equilibrium curve into the metastable region where supercooled water exist. The average molar enthalpy of 51.7 kJ/mol for the dissociation of the small methane hydrate samples in the temperature range of 253–273 K was calculated using Clausius–Clapeyron equation. This value agrees with the enthalpy of dissociation of bulk methane hydrates into water and gas at temperatures above 273 K.

IDENTIFICATION AND CORRELATION OF MIXING TIMES IN OPPOSED-JET MIXERS

When opposed-jet mixers are employed to achieve rapid mixing in processes such as pharmaceutical precipitations, an unambiguous value of the mixing time is needed for comparison to characteristic times such as the induction time for crystal nucleation. We used selectivity in the fourth Bourne reaction to assess mixing quality in two types of jet mixers: Y mixers and a confined impinging jet (CIJ) mixer. We developed an equal-volume mixing model based upon Baldyga and Bourne (1999a) to interpret the reaction selectivity results in terms of a Damkohler number, defined as the ratio of an unambiguously defined physical mixing time to the characteristic reaction time. A value of unity for the Damkohler number corresponded to a reaction selectivity of 0.05, which has often been assumed in the past (Mahajan and Kirwan, 1996; Johnson and Prud'homme, 2003a). The results showed that both mixers provided rapid mixing of the order of 10 ms at jet velocities of 10 m/s. Experimental mixing times were correlated with jet velocity, diameter, and viscosity using simplified turbulent mixing theories, particularly the Corrsin model, which includes both meso- and micro-mixing terms. The relevance of a combined meso- and micro-mixing mechanism in these devices was also supported by recent CFD simulations in a CIJ mixer (Liu and Fox, 2006).

Influence of the Roughness, Topography, and Physicochemical Properties of Chemically Modified Surfaces on the Heterogeneous Nucleation of Protein Crystals

In this study, the influence of some factors on the heterogeneous nucleation of hen egg-white lysozyme (E.C. 3.2.1.17) on a series of chemically modified surfaces has been investigated. Microbatch crystallization experiments were conducted on the microscope glass slides that were treated with poly-L-glutamic acid (PLG), poly(2-hydroxyethyl methacrylate) (P2HEMA), poly(methyl methacrylate) (PMMA), poly(4-vinyl pyridine) (P4VP), and (3-aminopropyl)triethoxysilane (APTES). An optical microscope with a heating/cooling stage was employed to measure the induction time of heterogeneous nucleation. The surface topography and roughness were characterized by atomic force microscopy. Contact angles for crystallization solution on the investigated surfaces were measured by a contact angle meter. From the theoretical analysis, the energetic barrier to heterogeneous nucleation was found to increase at higher contact angles and to decrease at higher roughness. Experimentally, a qualitative increase of the induction time of the heterogeneous nucleation on P2HEMA, APTES, and PMMA surfaces with the contact angle was observed. Such surfaces as P2HEMA, PLG, and APTES, which were of higher roughness, were shown to promote the heterogeneous nucleation. In addition, the surface with specific topography is expected to increase the possibility of the formation of a critical nucleus. Finally, the P4VP surface appeared to suppress the heterogeneous nucleation as a result of the electrostatic interaction between the lysozyme and P4VP molecules.

The nucleation and growth of calcium phosphate by amelogenin

The nucleation processes involved in calcium phosphate formation in tooth enamel are not well understood but are believed to involve proteins in the extracellular matrix. The ability of one enamel protein, amelogenin, to promote the nucleation and growth of calcium phosphate was studied in an in vitro system involving metastable supersaturated solutions. It was found that recombinant amelogenin (rM179 and rp(H)M180) promoted the nucleation of calcium phosphate compared to solutions without protein. The amount of calcium phosphate increased with increasing supersaturation of the solutions and increasing protein concentrations up to 6.5 μg/mL. At higher protein concentrations, the amount of calcium phosphate decreased. The kinetics of nucleation was studied in situ and in real time using a quartz crystal microbalance (QCM) and showed that the protein reduced the induction time for nucleation compared to solutions without protein. This work shows a nucleation role for amelogenin in vitro which may be promoted by the association of amelogenin into nanosphere templates, exposing charged functionality at the surface.

Nucleation and Growth Kinetics of PbS Quantum Dots in Oxide Glass: Spectroscopic and Microscopic Studies in the Dilute Range

The kinetics of nucleation and growth of PbS nanocrystals or quantum dots in a boroaluminosilicate host glass have been studied in the dilute range of the precipitating phase (∼2 wt % PbS) using infrared absorption spectroscopy and transmission electron microscopy. The temperature dependence of the experimental steady-state nucleation rates have been found to be several orders of magnitude lower than the predictions based on the classical nucleation theory. The induction times for nucleation at various temperatures are found to be more than 2 orders of magnitude longer than the shear relaxation time scale of the host glass, although the average activation energies are found to be similar in both cases (∼200 kJ/mol). The growth curves at all temperatures show an extremely sublinear time dependence of particle radius r ∼ t0.15-0.20 indicating an early onset of ripening process which also results in a saturation in the nucleation rate. A strong deviation from the typical scaling relations of r ∼ t1/2 or t1/3 has been hypothesized to result from a growth mechanism based on cluster diffusion and attachment.

Role of epitaxy of nucleating agent (NA) in nucleation mechanism of polymers

The role of “epitaxy” of nucleating agent (NA) in nucleation mechanism of polymers was studied to formulate the nucleation rate (I) as a function of concentration of NA (CNA) in mixture of polymer and NA and lateral size of a NA crystal (aNA), I∝CNA/aNA. It is proved that the epitaxy of NA controls nucleation mechanism by confirming above formula experimentally by observing nucleation by means of optical microscopy. We also clarified that heterogeneous nucleation is a probabilistic phenomenon by confirming that “induction time of nucleation (τi)” did not depend on CNA. We established a method to obtain reliable I and τi by adding NA which has been a difficult problem in polymer science.

Quasi-Emulsion Precipitation of Pharmaceuticals. 2. Application to Control of Polymorphism

Under a number of mixing conditions, a quasi-emulsion is formed during solute precipitation in which the solvent [e.g., 60 mass % polyethylene glycol 300 (PEG300) in water] is more viscous than the antisolvent (e.g., water).8 This microscopically segregated dispersion typically results in an increase in nucleation induction time, and this behavior can be exploited to control the polymorphic form produced during antisolvent precipitation. For three pharmaceutical solutes that form polymorphs (methylparaben, p-aminobenzoic acid, and sulfanilamide), it is demonstrated that quasi-emulsion precipitation results in the formation of the stable polymorphic form. On the other hand, when the fluid is rapidly mixed to homogeneity, a metastable polymorph form is first produced. Thus, polymorph formation can be highly mixing sensitive. When a solute is dissolved in a solvent less viscous than the antisolvent as is the case for the precipitation of glycine from water with aqueous PEG300, quasi-emulsions are not formed, and the metastable polymorph is first formed unless extreme fluid segregation (no agitation) is employed.

A Probabilistic Approach to Model the Nonisothermal Nucleation of Triacylglycerol Melts

Crystallization studies are usually performed under isothermal conditions. Kinetic parameters characterizing the isothermal nucleation and growth processes can be obtained using classical nucleation and growth models. However, crystallization regimes found in nature, as well as those used in food and pharmaceutical processing, are rarely isothermal. Focusing on the nucleation stage, the approach followed in this work was to define a new parameter to characterize the driving force of nucleation, the supercooling-time exposure (β), which not only depends on the difference between the melting temperature (Tm) and the onset temperature of nucleation (Tc) ΔTc but also on the induction time of nucleation, tc, and therefore the cooling rate (φ), namely, β = (1/2)ΔTctc = ΔTc2/2φ. An exponential probability density function of the values of β was utilized to model changes in nucleation rate as a function of β in the form J/Jmax = . From this parametrization procedure, the energy of activation for the nucleation pr...

The Concept of Delayed Nucleation in Nanocrystal Growth Demonstrated for the Case of Iron Oxide Nanodisks

A comprehensive study of iron oxide nanocrystal growth through non-hydrolitic, surfactant-mediated thermal reaction of iron pentacarbonyl and an oxidizer has been conducted, which includes size control, anisotropic shape evolution, and crystallographic phase transition of monodisperse iron oxide colloidal nanocrystals. The reaction was monitored via in situ UV-vis spectroscopy, taking advantage of the color change accompanying the iron oxide colloid formation, allowing measurement of the induction time for nucleation. Features of the synthesis such as the size control and reproducibility are related to the occurrence of the observed delayed nucleation process. As a separate source of iron and oxygen is adopted, phase control could also be achieved by sequential injections of oxidizer.

A Kinetic Study on Tetrahydrofuran Hydrate Crystallization

Various aspects of crystallization of THF hydrate have been studied. It has been found that formation of THF hydrate is possible between solution concentrations of 9 and 46% by weight. Induction time for nucleation in various solutions has been determined. The results indicate that the lowest induction time for nucleation corresponds to solutions with a THF concentration of 19% by weight. This solution concentration matches with the THF composition in the crystalline hydrate. The metastable zone for primary nucleation has been investigated and concluded that for each stirrer speed, THF hydrate forms at a higher temperature when the solution concentration is 19% by weight. The next conclusion is that the higher the stirrer rate, the narrower the metastable zone. Nucleation and growth rates have been evaluated from a series of experiment by s-plane analysis. Compared with the growth rate, nucleation rate has been found to be a stronger function of the stirrer speed. Both rates increase with subcooling but in the case of the growth rate, only a slight increase has been observed at high levels of subcooling and stirrer speeds. A regression analysis resulted in derivation of kinetics correlations between the significant process variables and the consistency between the correlations and the experimental data has been examined.

Influence of Ultrasound on the Nucleation of Polymorphs of p-Aminobenzoic Acid

p-Aminobenzoic acid crystallizes in two different polymorphic forms: the α-form and the β-form. The α-form crystals are needle-shaped, while the β-form crystals have a more favorable prismatic shape. The system is enantiotropic with the transition temperature at approximately 25 °C. Below the transition temperature, the β-form is the thermodynamically stable polymorph but can only be produced at very slow supersaturation generation either in water or in ethyl acetate. In the present work, the influence of ultrasound on the nucleation of p-aminobenzoic acid polymorphs has been investigated by use of several different sonication intensities and schemes. It is shown that sonication significantly reduces the induction time for nucleation. By using controlled sonication, we were able to more reproducibly crystallize the β-form at more reasonable cooling rates. In addition, sonication is found to quite selectively favor the appearance of the β-polymorph. It is even possible to produce the pure β-form above the transition temperature where it is the metastable form and impossible to produce without sonication. The α-form structure is based on centro symmetric dimers formed by the association of carboxylic acid groups, while the β-form contains four-membered hydrogen-bonded rings of alternating amino and carboxylic acid groups. It is suggested that ultrasound disturbs the building up of the dimers in the solution and thus favors the crystallization of the β-polymorph.

Effect of ultrasonic irradiation on nucleation phenomena in a Na 2HPO 4 · 12H 2O melt being used as a heat storage material

The effect of ultrasound on nucleation phenomena in the heat storage material Na 2HPO 4 · 12H 2O was investigated by determining the primary nucleation probability and induction time, and by looking at heat generation phenomena in the initial stage of nucleation. The experimental results show that the primary nucleation probability dramatically increased, and the induction time decreased under the ultrasound irradiation, and in addition, the rate of temperature rise was dependent upon the ultrasonic output. Based on these results and the theoretical relationship between the number of primary nuclei and the heat generation rate, it is proposed that the number of primary nuclei depends upon the ultrasonic output.

Crystallization and melting behavior of homogeneous and heterogeneous linear low‐density polyethylene resins

AbstractThe crystallization behavior of homogeneous and heterogeneous linear low‐density polyethylenes (LLDPE) was investigated by evaluating the characteristics of melting traces obtained by differential scanning calorimetry (DSC). Based on the isothermal experimental results, the concept of the effective nucleation induction time is suggested. In the initial crystallization stage, the Avrami equation in conjunction with the effective induction time can be used to successfully describe the overall crystallization kinetics. Avrami exponents 2, 1.5, and 1 were found to apply in regimes III, II, and IM, respectively, as identified by the modified Hoffman‐Lauritzen (MHL) equation. The kinetic parameters estimated from evaluating the linear crystallization behavior during spherulitic growth experiments using polarized light microscopy (PLM) are in agreement with the overall crystallization kinetic parameters obtained from DSC experiments. POLYM. ENG. SCI., 45:1140–1151, 2005. © 2005 Society of Plastics Engineers

4-Aminophenylacetic acid

This thesis deals with the controlled crystallization of small organic molecules and is focused on solubility and polymorphism. The solubility was determined for phenylacetic acid, p-hydroxyphenylacetic acid, p-aminophenylacetic acid, p-hydroxybenzoic acid and ibuprofen in both water and in a range of organic solvents. Data is discussed from the standpoint of molecular aspects of solute – solvent interactions and by estimated solid phase activity. It was shown that better understanding could be acquired by making a qualitative analysis of the molecular interactions in the solution and the crystal structure of the compounds in question. Solubility predictions that are carried out by the UNIFAC method are not sufficiently accurate to serve as a basis for a reliable design of a crystallization process or selection of a suitable solvent since they deviate more than 15% from experimental values. The reason for the discrepancies are related to uncertainties in the prediction of activity coefficients by UNIFAC, as well as, difficulties in the estimation of the activity of the solid state. p-Aminobenzoic acid (PABA) has been crystallized from thirteen different solvents either by slow cooling, after which the product is allowed to mature in suspension, or by rapid cooling followed by immediate isolation. Two different polymorphs have been crystallized. The system is found to be enantiotropic with the transition temperature of 25 °C, below which the β-form is the stable polymorph. The α-form was obtained from all solvents by both methods. The β-form is obtained only in carefully controlled conditions from water and ethyl acetate, well below the transition temperature. Often the α-form appears concomitantly. It is shown in this work that sonication significantly reduces the induction time for nucleation. The β-form crystallizes more reproducibly and at higher cooling rates when controlled sonication is used. In addition sonication is found to selectively favor the appearance of one of the polymorphs. Producing the pure β-form was possible even above the transition temperature where other crystallization techniques were only capable of producing the stable α-form. The α-form structure is based on centro symmetric dimers formed by association of carboxylic acid groups. It is suggested that the preference for nucleation of the α-polymorph is related to the formation of dimers in the supersaturated solution. Only at the condition where the formation of dimers is reduced sufficiently, (i.e. in the polar solvents or when sonication is applied) the nucleation of the β-form is favored.

Analytical Techniques for Nucleation Studies in Lipids: Advantages and Disadvantages

ABSTRACT: Crystallization is generally considered a 2‐step process. The 1st step, nucleation, involves the formation of molecular aggregates with a critical size great enough to become stable. During the 2nd step, nuclei grow and develop into crystals. Distinguishing between nucleation and growth constitutes a major challenge in lipid crystallization studies. Thus, it is of great importance to discuss the information obtained from the different techniques that are usually used to study nucleation behavior such as nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), rheological techniques, light‐scattering techniques such as turbidimetry and scanning diffusive light scattering (SDLS), polarized light microscopy (PLM), and laser polarized optical sets such as laser polarizedlight turbidimetry (LPLT). Techniques to describe the nucleation process must be very sensitive to disregard growth. When crystallization is followed by methods such as DSC, NMR, and rheological measurements, at times, small amounts of crystals are present in the melt before any solids are detected. Clearly, at this stage, well beyond the induction time for nucleation (τ), these methods are measuring crystal growth. Techniques of low sensitivity for solid fat contents lower than 0.1% must not be used to evaluate nucleation effects. Sensitive turbidimeters with detectors that saturate below 0.3% solid fat content give good results as do scanning diffusive light‐scattering equipment. Although the PLM technique is sensitive enough for these kinds of studies, an understanding of important basic concepts is essential. Laser optical sets are the most appropriated methods to study nucleation in fats systems.

Kinetics of binary nucleation of vapors in size and composition space

We reformulate the kinetic description of binary nucleation in the gas phase using two natural independent variables: the total number of molecules g and the molar composition x of the cluster. The resulting kinetic equation can be viewed as a two-dimensional Fokker-Planck equation describing the simultaneous Brownian motion of the clusters in size and composition space. Explicit expressions for the Brownian diffusion coefficients in cluster size and composition space are obtained. For characterization of binary nucleation in gases three criteria are established. These criteria establish the relative importance of the rate processes in cluster size and composition space for different gas phase conditions and types of liquid mixtures. The equilibrium distribution function of the clusters is determined in terms of the variables g and x. We obtain an approximate analytical solution for the steady-state binary nucleation rate that has the correct limit in the transition to unary nucleation. To further illustrate our description, the nonequilibrium steady-state cluster concentrations are found by numerically solving the reformulated kinetic equation. For the reformulated transient problem, the relaxation or induction time for binary nucleation was calculated using Galerkin's method. This relaxation time is affected by processes in both size and composition space, but the contributions from each process can be separated only approximately.

Nucleation of calcium carbonate in presence of citric acid, DTPA, EDTA and pyromellitic acid

The influence of four calcium complexing additives, i.e., citric acid (CIT), diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) and pyromellitic acid (PMA), and their concentration on the induction time of calcium carbonate nucleation has been studied. The experiments were performed by rapidly mixing a sodium carbonate solution and a calcium chloride solution of various concentrations. The induction time was obtained by recording the white light absorption of the solution. Chemical speciation was used to estimate the initial thermodynamic driving force of each experiment. The induction time was found to increase with additive concentration. The effect varies from one additive to another. CIT causes the greatest increase in induction time and PMA the least. Using classical nucleation theory the experimental data were evaluated in terms of the interfacial energy. In pure water a value of 37.8 mJ m −2 was obtained, showing good agreement with other works. CIT, DTPA and EDTA caused a notable increase of the interfacial energy at a concentration of 0.5 mmol l −1. PMA does not appear to have any effect at all on the interfacial energy. Different mechanisms for the influence of the additives on the measured induction time and on the estimated interfacial energy are discussed.

Influence of Additives on Nucleation of Vanillin: Experiments and Introductory Molecular Simulations

Nucleation of vanillin (VAN) in 2-propanol/water in the presence of additives, viz., acetovanillone (AVA), ethyl vanillin (EVA), guaiacol (GUA), guaethol (GUE), 4-hydroxyacetophenone (HAP), 4-hydroxybenzaldehyde (HBA), and vanillic acid (VAC) is investigated experimentally and by molecular simulations. In the experimental work, the induction time for nucleation is measured at different temperatures and levels of supersaturation using a multicell apparatus. A large variation in the experimental data is observed, and this variation is analyzed by statistical methods. By classical nucleation theory, the induction time data are used to estimate the solid-liquid interfacial energy of vanillin for each VAN-additive system. At 1 mol %, the interfacial energy becomes lower in the presence of AVA, EVA, HAP, and VAC, while it becomes higher in the presence of the other additives. As the additive concentration increases from 1 to 10 mol %, the interfacial energy also increases. The interfacial energy ranges from 6.9 to 10.1 mJ m -2 . Molecular modeling, implemented in the program Cerius2, is used to simulate and examine the surface chemistry of the likely crystal growth faces of VAN (i.e., {0 0 1} and {100}). To evaluate the additive-crystal face interaction energy, two approaches are used: the surface adsorption method and the lattice integration method. Both experimental and molecular simulation results indicate that the additives studied are potential modifiers of the nucleation of VAN. However, a simple and clear relationship between the experimental values of interfacial energy and the calculated interaction energies for the most important crystal faces of VAN cannot be identified. The modeling does not concern the actual nucleation but rather the conditions of a growing surface and are based on several severe simplifications. Obviously, this simplistic approach does not sufficiently capture the influence of additives on the nucleation of vanillin.