Saxs study of silica gel formation from aqueous silicate solutions

In situ SAXS measurements of silica gel formation in the presence of different cations are presented as a function of reaction time. Conditions used are similar to those applied to prepare catalyst supports. The data can be interpreted in terms of aggregation of elementary silica particles with sizes of approximately 1 nm, thus finally forming a continuous silica gel network. The aggregates are fractal with respect to their masses, in the range 2–10 nm. The dimensionality of the fractals can be determined from the SAXS curves; it increases from 1.8 to 2.5 during gelformation, corresponding with a diffusion limited aggregation process (DLA) of gel formation followed by densification of the silica gel network. The finding that the rate of change in fractal dimensionality does not change as a function of cation, indicates strongly that gelformation is a diffusion limited aggregation process.Beatrixlaan 12, 6165 CX Geleen, The Netherlands     

化学气相反应法合成SiC超细粉的成核和生长过程的研究

借助于ＴＥＭ观察，讨论了化学气相反应法合成ＳｉＣ超细粉的成长机理，分析结果显示，由于气体流速不同，合成的ＳｉＣＦ超细粉颗粒尺寸和颗粒形貌有较大变化，颗粒尺寸和颗粒形貌的变化成核和生长机理决定，一般涉及到均匀成术怜惜是相成核，团聚和熔融。     

A Plug Flow Model for Chemical Reactions and Aerosol Nucleation and Growth in an Alkali-Containing Flue Gas

The paper presents a numerical model for the simulation of gas to particle conversion and the chemical changes during cooling of a flue gas from the combustion of fuels rich in volatile alkali species. For the homogeneous nucleation of alkali species the model uses the classical theory modified by the Tolman coefficient for the size dependent surface tension. A special adaptation of the theory to the nucleation of seeds of chloride salt from an equilibrium mixture of monomer and dimer chloride vapor molecules is also made. The growth of particles occurs by condensation and agglomeration. Different multicomponent growth models are treated. The local gas phase composition is determined from a gas phase chemical equilibrium calculation combined with finite reaction rate kinetics for slower reactions. The model is useful in the analysis of boiler operation with respect to the formation of particles, HCl, SO₂, and deposits.     

Particle nucleation and growth mechanisms in miniemulsion polymerization of styrene

Styrene miniemulsion polymerizations stabilized by sodium lauryl sulfate in combination with a reactive costabilizer, lauryl methacrylate (LMA) or stearyl methacrylate (SMA), were studied. A small amount of extremely hydrophobic dye was incorporated into monomer droplets (10² nm in diameter) to investigate particle nucleation and growth mechanisms. In addition to monomer droplet nucleation, particle nuclei generated in the aqueous phase (homogeneous nucleation) also play an important role in both LMA‐ and SMA‐containing polymerization systems. The way that these two nucleation mechanisms compete with each other is closely related to the water solubility of the costabilizer (LMA > SMA). The fraction of latex particles originating from homogeneous nucleation increases with decreasing hydrophobicity of the costabilizer. Zeta potential data of latex particles and the molecular weight and molecular weight distribution of emulsion polymers provide supporting evidence for the proposed competitive particle nucleation and growth mechanisms. © 2002 Society of Chemical Industry     

Winsor I-like (micro)emulsion polymerization of styrene initiated by oil-soluble initiator

Batch emulsion polymerization of styrene initiated by an oil-soluble initiator and stabilized by non-ionic emulsifier (Tween 20) has been investigated. The rate of polymerization vs. conversion curve shows the two non-stationary rate intervals typical for the non-stationary-state polymerization. This behavior is a result of the continuous particle nucleation and the decrease of monomer concentration at the reaction loci with increasing conversion. The initial increase of the polymerization rate is attributed to the increase of particle number and the polymerization proceeding under the monomer-saturated condition—the Winsor I-like (micro)emulsion polymerization. The decrease of the polymerization rate is the result of the depressed transfer of monomer from the monomer reservoir to the reaction loci. Above 50 °C the monomer emulsion separates into two phases: the upper transparent monomer phase and the lower blue colored (microemulsion) phase. The polymerization mixture consists of the microdroplets (act as the reaction loci) and large degradable monomer droplets (act as the reservoir monomer and emulsifier). The continuous release of emulsifier from the monomer phase and the microdroplets induce the continuous particle nucleation up to high conversion. The initial formation of large particles results from the agglomeration of unstable growing particles and monomer droplets. The size of large (highly monomer-swollen) particles decreases with conversion and they merge with the growing particles at ca. 40-50% conversion. The coarse initial emulsion transformed during polymerization to the fine (semitransparent) polymer emulsion as a result of the continuous particle nucleation, the shrinking of highly monomer-swollen polymer particles and the depletion of monomer droplets. The low overall activation energy of polymerization is mainly ascribed to the decreased barrier for entering radicals into the latex particles with increasing temperature.     

Particle growth modeling of gas phase polymerization of butadiene

Butadiene polymerization in the gas phase is modeled by a polymeric multilayer model. Intraparticle mass and heat transfer effects are studied. The effects of catalyst size and diffusivity of butadiene on the radial profile of monomer concentration in polymeric particles and on the rate of particle growth are significant. Intraparticle temperature gradients do appear to be negligible under normal reaction conditions. External boundary layer heat effects are studied for various operation conditions. The model predicts that there is no significant temperature rise of the polymeric particles, even in the case of large catalyst particles. The effect of deactivation of active sites on the rate of particle growth is also studied. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 203–212, 1997     

Mass transfer and thermodynamic effects in emulsion polymerisation

The course of emulsion polymerisation is shown to depend on the water-solubility of the monomers concerned. With insoluble monomers, the mass transfer rate of monomer to polymer particles can sometimes be exceeded by the polymerisation rate, and viscosity within the particles is appreciable throughout the course of the reaction. Thus, surfactant is adsorbed before monomer droplets disappear. With more soluble monomers, mass transfer rates of monomer are high and since the equilibrium concentration of polymer in particles is low the monomer droplets disappear early in the reaction. Consequently, water-insoluble monomers often conform to Smith-Ewart behaviour, whereas water-soluble monomers do not. With water-soluble monomers the final particle number is proportional to the cube of the initial surfactant concentration.     

Population balance modeling of particle size distribution in monomer‐starved semibatch emulsion polymerization

The evolution of particle size distribution (PSD) in the monomer‐starved semibatch emulsion polymerization of styrene with a neat monomer feed is investigated using a population balance model. The system under study ranges from conventional batch emulsion to semicontinuous (micro)emulsion polymerization depending on the rate of monomer addition. It is shown that, contrary to what is often believed, the broadness of PSD is not necessarily associated with the length of nucleation period. The PSDs at the end of nucleation are found to be independent of surfactant concentration. Simulation results indicate that at the completion of nucleation the particle size is reduced and the PSD narrows with decreasing rate of monomer addition despite nucleation time increasing. The broad distribution of particles frequently encountered in semibatch emulsion polymerizations is therefore attributed to stochastic broadening during the growth stage. The zero‐one‐two‐three model developed in this article allows perceiving that the dominant kinetic mechanism may be different for particles with different sizes. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Effect of the carboxylic acid monomer type on the emulsifier‐free emulsion copolymerization of styrene and butadiene

Carboxylated styrene–butadiene rubber latexes were prepared through the emulsifier‐free emulsion copolymerization of styrene and butadiene with various carboxylic acid monomers. The effects of various carboxylic acid monomers on the particle formation process were investigated. The type of carboxylic acid monomer strongly affected the particle nucleation. The number of particles and thus the polymerization rate increased with the increasing hydrophobicity of the carboxylic acid monomers. There was a significant difference in the polymerization rate per particle. The results showed that particle nucleation and growth were dependent on the hydrophilic nature of the carboxylic acid monomers. The average particle diameter of the carboxylated styrene–butadiene rubber latexes in the dry state was obtained through some calculations using direct measurements of the average particle diameter in the monomer‐swollen state by a dynamic light scattering technique. Several parameters, such as the polymerization rate, number of latex particles per unit of volume of the aqueous phase, and polymerization rate per particle, were calculated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Controlled Nucleation in Aerosol Reactors for Suppression of Agglomerate Formation: A Numerical Study

The formation of agglomerated particles can be a detriment to the production of highly sinterable materials. It has been seen experimentally that agglomerates are almost exclusively constructed from 10–30-nm primary particles. This paper develops criteria for the avoidance of these small primary particles through control of the nucleation rate. The relationship between particle size and number density can be used to obtain regions of stable and unstable particle growth. It is seen that the critical source rate for the prevention of runaway nucleation can be scaled without knowledge of the monomer number density.     

Polymerization of olefins through heterogeneous catalysis X: Modeling of particle growth and morphology

The development of a detailed model describing particle growth in olefin copolymerization systems is presented. The Multigrain Model considers in detail monomer sorption, mass transfer, and changing porosity within the growing particle, as well as heat and mass transfer across the external film of the particle. The model predicts catalyst performance, including polymerization rates and particle morphology, in different reactor media without parameter adjustment. Internal void fractions are calculated through an examination of the relative growth rates within the growing particle. The model is used to examine the effects of mass transfer limitations, prepolymerization, and nonuniform metal distribution on the particle growth process. Model predictions of morphology show the same trends as observed experimentally.     

火焰法合成纳米TiO2颗粒生长过程

在新型火焰反应器生产纳米TiO2的过程中使用TEM微栅在不同火焰高度位置处进行原位取样分析,得到生长过程中纳米TiO2颗粒的粒径和形态. TiO2颗粒经历了成核、生长、聚并、烧结的过程. 调节反应物浓度为7.9×10-5~5.7×10-3 mol/L,研究了不同反应物浓度对纳米颗粒生长过程的影响,高前驱体浓度形成较高的单体浓度,使颗粒间碰撞几率增加,从而得到粒径较大的颗粒,产物粒径17~85 nm. 调节CH4和O2流量,改变温度场,研究温度对颗粒生长过程的影响,在相同反应物浓度条件下,较高的温度下形成分散性好、一次粒径为63 nm的颗粒,而在较低的温度下形成的颗粒一次粒径为35 nm,但颈部烧结严重;增加喷嘴气流速度减小了反应停留时间,颗粒粒径从63 nm减小到36 nm.     

Synthesis of Fe3O4/PMMA composite latex particles: Kinetic modeling

In this work, a generalized mathematical model was developed to estimate the variation of particle concentration during the entire course of soapless emulsion polymerization of methyl methacrylate with ferrofluid. Two mechanisms for the nucleation and growth of particles throughout the polymerization reaction were discussed: Mechanism I – seeded polymerization; and Mechanism II – self‐nucleation polymerization. Here, the self‐nucleation included homogeneous nucleation and micelle nucleation. Coagulation between particles, which came from different nucleation mechanisms during the course of polymerization, was considered and included in this model. When appropriate parameters were selected, this model could be successfully used to interpret the variation of particle concentration during the entire reaction. Under different conditions, rate of polymerization, number of radicals in each particle, average molecular weight of polymers, and rate constant of termination were also calculated. All of them explained the experimental results quite well. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4925–4934, 2006     

用阴离子Gemini 9 B-6-9 B乳化剂合成苯乙烯-丙烯酸丁酯共聚物纳米乳液

以壬基酚、1,6-二溴己烷、氯磺酸为主要原料合成的阴离子Gemini磺酸盐表面活性剂9 B-6-9 B为乳化剂,苯乙烯(St)、丙烯酸丁酯(BA)为单体,合成了乳胶粒径小于100 nm且分布均匀的苯乙烯-丙烯酸丁酯共聚物纳米乳液,考察了聚合温度、乳化剂用量、引发剂用量、单体配比、单体含量对乳液的影响,探讨了乳胶粒子成核机理。结果表明,随着温度的升高,乳胶粒子的平均粒径减小,转化率与凝胶率增大;随着乳化剂或引发剂用量的增加,乳胶粒子的平均粒径和凝胶率减小,转化率增大;随着软单体比例的增加,乳胶粒子的平均粒径与转化率增大,粒径分布变宽,凝胶率减小;单体用量增大,乳胶粒子的平均粒径与凝胶率增大,转化率降低;乳胶粒子的粒径呈单峰分布,可能是按胶束成核机理形成的。     

Nanoparticle formation through solid‐fed flame synthesis: Experiment and modeling

The preparation of silica nanoparticles through solid‐fed flame synthesis was investigated experimentally and theoretically. Monodispersed submicrometer‐ and micrometer‐sized silica powders were selected as solid precursors for feeding into a flame reactor. The effects of flame temperature, residence time, and precursor particle size were investigated systematically. Silica nanoparticles were formed by the nucleation, coagulation, and surface growth of the generated silica vapors due to the solid precursor evaporation. Numerical modeling was conducted to describe the mechanism of nanoparticle formation. Evaporation of the initial silica particles was considered in the modeling, accounting for its size evolution. Simultaneous mass transfer modeling due to the silica evaporation was solved in combination with a general dynamics equation solution. The modeling and experimental results were in agreement. Both results showed that the methane flow rate, carrier gas flow rate, and initial particle size influenced the effectiveness of nanoparticle formation in solid‐fed flame synthesis. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

乳液聚合成核阶段的模拟与分析

建立了乳液聚合成核阶段的Monte Carlo模型，并用计算机对一个体积为10^-17m^3的微型反应器中苯乙烯的乳液聚合进行了模拟。以计算机生成随机数作为自由基被胶束和乳胶粒捕获的几率，模拟了在微型反应器中每一个自由基的生成、被胶束或乳胶粒捕获的过程以及每一个乳胶粒的生成及增长过程。通过对每一个乳胶粒在增长过程中各参数的统计计算，研究了乳液聚合成核阶段诸参数(乳胶粒数目、乳胶粒直径与粒径分布、单体转化率、聚合反应速率等)与乳化剂浓度[S]及引发剂浓度[I]的关系。结果表明，苯乙烯的乳液聚合体系中乳胶粒数目与[S]^0.5996[I]^0.4016成正比：在成核阶段乳胶粒直径分布先变宽后变窄，乳液聚合过程中乳胶粒直径分布有自动变窄的趋势；成核阶段持续时间t12与[S]^0.60[I]^0.60成正比，成核阶段结束时的单体转化率X12与[S]^1.20[I]^0.20成正比。     

The critical conditions for secondary nucleation of silica colloids in a batch Stöber growth process

In this work, we determined experimentally the critical total surface area of silica seeds in the solution to avoid the formation of new particles (i.e. secondary nucleation) in a batch Stöber growth process. This critical surface area corresponded to the balance between the generation rate of reaction intermediate from the hydrolysis of TEOS and its consumption rate via the growth of existing seeds. Further calculation showed that the secondary nucleation was probably related to the average distance between seed particles. When the distance between seeds exceeded a critical value for each growth condition, i.e. for each generation rate, one would then observe secondary nucleation. Otherwise, simple growth was observed. Higher generation rate corresponded to shorter distance.     

Generation of nano-sized Ar–N2 compound particles by homogeneous nucleation and heterogeneous growth in a supersonic expansion

The generation of nano-sized particles was numerically investigated for the heterogeneous growth of nitrogen on homogeneously nucleated argon seed particles in a supersonic expansion. Gas dynamic equations for 1D supersonic flow were solved together with the classical nucleation theory and Hill's growth model.Heterogeneous growth on homogeneously nucleated seeds effectively increased the final particle size to the range of 10–200 nm. The new process could increase the particle mass by a factor of 10 relative to the case of pure gas expansion under comparable pressure and temperature conditions. Increase in particle size resulted predominantly from the enhanced heterogeneous condensation of nitrogen, and the growth by argon condensation could even be reduced relative to the case of pure gas expansion. Final particle size easily reached 200 nm, which has been very difficult to achieve by any of the existing homogeneous growth processes.     

Synthesis of poly(butyl methacrylate) in three-component cationic microemulsions

The polymerization of butyl methacrylate in three-component microemulsions prepared with the cationic surfactant dodecyltrimethlammonium bromide is reported here as a function of monomer and surfactant content in parent microemulsions, type and concentration of initiator, and temperature. Fast reaction rates and high conversions are achieved in all cases. Final latexes are bluish-opaque and stable, and contain spherical particles with diameters in the range of 20 to 30 nm, depending on composition of the parent microemulsions and reaction conditions. Each of these particles is composed of a few macromolecules of high molecular weight (2 to 4 × 10⁶ Dalton). Both particle size and average molecular weight remain constant throughout the reaction, suggesting a continuous nucleation mechanism. Analysis of the molecular weight distribution indicates that the controlling termination mechanism is chain-transfer to monomer. © 1996 John Wiley & Sons, Inc.     

Modeling of the monomer role and the coalescence limitation in primary particle growth

The general dynamic equation (GDE) has been numerically solved to simulate the growth of ultrafine particles (UFPs) in a tubular aerosol reactor, approximating the particle size distribution by a lognormal function. The GDE includes all the terms describing diffusion, thermophoresis, nucleation, condensation and coagulation. We have also considered the efficiency of liquid-like coagulation to primary particles. The data calculated from our model were compared with those from the previous model and also with some experimental results from a TiO₂ UFP generator. The condensation term, which we split from a single coagulation term in the previous model, well described the monomer contribution to the particle growth. Introduction of one adjustable parameter, the efficiency of coagulation, was successful in limiting the growth of primary particles and fit the experimental data.