Coagulation of nanoparticles in reverse micellar systems: a Monte Carlo model

The process of formation of nanoparticles obtained by mixing two micellized, aqueous solutions has been simulated using the Monte Carlo technique. The model includes the phenomena of finite nucleation, growth via intermicellar exchange, and coagulation of nanoparticles after their formation. Using the model, an exploratory study has been conducted to analyze whether the coagulation of nanoparticles is the reason for the formation of nanoparticles whose sizes are comparable to the size of the reverse micelles. The model explains the possible mechanism of coagulation of semiconductor nanoparticles formed within reverse micelles and its effect on the evolution of their size with time. The model is predictive in nature, and the simulation results compare well with those observed experimentally.     

Encapsulation of silica nanoparticles by miniemulsion polymerization

Hybrid silica/polystyrene nanoparticles were synthesized by miniemulsion polymerization. With the objective to prepare core‐shell hybrid nanoparticles having narrow particle size distributions (PSDs) as well as a high degree of silica encapsulation, the effect of adding surface modifiers, the size of silica nanoparticles, the ratio styrene/silica, the surfactant concentration, and the presence of ethanol in the reaction mixture were studied. A synergistic effect was observed using oleic acid (OA) together with 3‐(trimethoxysilyl)propyl methacrylate (TPM) in the compatibilization step between the organic phase (monomer) and inorganic nanoparticles (silica). Mono and multinuclear eccentric core‐shell hybrid nanoparticles were obtained. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 935–948, 2009     

Size-dependent catalytic behavior of platinum nanoparticles on the hexacyanoferrate(III)/thiosulfate redox reaction

Platinum nanoparticles prepared in reverse micelles have been used as catalysts for the electron transfer reaction between hexacyanoferrate(III) and thiosulfate ions. Nanoparticles of average diameter ranging between 10 and 80 nm have been used as catalysts. The kinetic study of the catalytic reaction showed that for a fixed mass of catalyst the catalytic rate did not increase proportionately to the decrease in particle size over the whole range from 10 to 80 nm. The maximum reaction rate has been observed for average particle diameter of about 38 nm. Particles below diameter 38 nm exhibit a trend of decreasing reaction rate with the decrease in particle size, while those above diameter 38 nm show a steady decline of reaction rate with increasing size. It has been postulated that in the case of particles of average size less than 38 nm diameter, a downward shift of Fermi level with a consequent increase of band gap energy takes place. As a result, the particles require more energy to pump electrons to the adsorbed ions for the electron transfer reaction. This leads to a reduced reaction rate catalyzed by smaller particles. On the other hand, for nanoparticles above diameter 38 nm, the change of Fermi level is not appreciable. These particles exhibit less surface area for adsorption as the particle size is increased. As a result, the catalytic efficiency of the particles is also decreased with increased particle size. The activation energies for the reaction catalyzed by platinum nanoparticles of diameters 12 and 30 nm are about 18 and 4.8 kJ/mol, respectively, indicating that the catalytic efficiency of 12-nm-diameter platinum particles is less than that of particles of diameter 30 nm. Extremely slow reaction rate of uncatalyzed reaction has been manifested through a larger activation energy of about 40 kJ/mol for the reaction.     

Formation of silver bromide precipitate of nanoparticles in a single microemulsion utilizing the surfactant counterion

Silver bromide precipitate of nanoparticles was prepared by addition of silver nitrate aqueous solution to a single microemulsion system consisting of dioctyldimethylammonium bromide, n-decanol, and water in isooctane. The silver ion reacted readily with the surfactant counterion, bromide, to form the precipitate of nanoparticles, which was stabilized in the water pools. The use of the surfactant counterion as a reactant is a new approach to nanoparticle preparation in microemulsions. It is characterized by high reactivity and less dependency on the intermicellar exchange of solubilizate. The effects of the surfactant and the cosurfactant concentrations, the amount of silver nitrate, and the water to surfactant mole ratio, R, were evaluated. Increasing the surfactant concentration at fixed R and amount of silver nitrate enhanced the role of intermicellar nucleation and resulted in the formation of larger particles, while increasing the amount of silver nitrate at fixed values of all the other variables enhanced the direct nucleation and resulted in the formation of smaller particles. Particle aggregation and flocculation took place when the concentration of n-decanol or the value of R was increased. Particle aggregation and flocculation were attributed to the decrease in the interaction between the surfactant protective layer and the nanoparticles in the water pools.     

Modeling Particle Size Distribution (PSD) in Emulsion Copolymerization Reactions in a Continuous Loop Reactor

A detailed dynamic mathematical model that describes the evolution of particle size distributions (PSDs) during emulsion copolymerization reactions in a continuous loop reactor was developed and compared with experimental data. The model is based on the assumption that two distinct particle populations exist: precursor particles and stable latex particles. Precursor particles are colloidally unstable and therefore may undergo coagulation with other precursors and be absorbed by stable latex particles. It is shown that the kinetic model is able to reproduce the rather complex dynamic behavior of the vinyl acetate/Veova10 emulsion copolymerization in a continuous loop reactor, including the development of oscillatory responses of PSDs during reaction start‐up. It is also shown that, for the studied polymerization system, oscillatory responses are obtained only when both particle populations are assumed to exist and when both coagulative and micellar particle nucleations are simultaneously considered.     

Growth of mesoporous silica nanoparticles monitored by time-resolved small-angle neutron scattering

Since the first development of surfactant-templated mesoporous silicas, the underlying mechanisms behind the formation of their structures have been under debate. Here, for the first time, time-resolved small-angle neutron scattering (tr-SANS) is applied to study the complete formation of mesoporous silica nanoparticles. A distinct advantage of this technique is the ability to detect contributions from the whole system, enabling the visualization not only of particle genesis and growth but also the concurrent changes to the coexistent micelle population. In addition, using contrast-matching tr-SANS, it is possible to highlight the individual contributions from the silica and surfactant. An analysis of the data agrees well with the previously proposed "current bun" model describing particle growth: Condensing silica oligomers adsorb to micelles, reducing intermicellar repulsion and resulting in aggregation to form initial particle nuclei. From this point, the growth occurs in a cooperative manner, with condensing silica filling the gaps between further aggregating micelles. The mechanistic results are discussed with respect to different reaction conditions by changing either the concentration of the silica precursor or the temperature. In doing so the importance of in situ techniques is highlighted, in particular, tr-SANS, for mechanism elucidation in the broad field of materials science.     

In Situ Monitoring of Pt Nanoparticle Formation in Ethylene Glycol Solution by SAXS-Influence of the NaOH to Pt Ratio

The temporal evolution of Pt nanoparticle formation in ethylene glycol solution from H(2)PtCl(6)·6H(2)O at 90 °C for different molar ratios of NaOH to Pt (84, 6.5, and 2) in the presence or absence of poly(N-vinyl-2-pyrrolidone) (PVP) as protecting agent was followed in situ by small-angle X-ray scattering (SAXS). The SAXS profiles were analyzed regarding particle size and size distribution using the Guinier approximation and the indirect Fourier transform technique (IFT). The NaOH to Pt ratio has an influence on the integral nanoparticle formation rate as well as on the metal reduction rate and the ratio of nucleation to growth reactions. The fastest nanoparticle formation rate was observed for the NaOH/Pt ratio of 6.5. The obtained results indicate that the differences in the particle formation rate might be due to differences in the reduction rate of the formed Pt complexes. In alkaline reaction media (NaOH/Pt = 84 or 6.5), small nanoparticles with a relatively narrow size distribution were formed. Therefore, it is assumed that for these NaOH/Pt ratios the particle formation is dominated by nucleation reactions. Additionally, the in situ studies point out that nanoparticles prepared at the NaOH/Pt ratio of 84 do not grow further after attaining a certain particle size. For a NaOH to Pt ratio of 2, that means in acidic medium, particle formation should be dominated by growing processes and, therefore, larger particles are formed accompanied by a broader particle size distribution. The influence of PVP on the nanoparticle formation rate is relatively low. However, in acidic medium, the presence of PVP is necessary in order to protect the formed nanoparticles from irreversible aggregation reactions.     

Metalated diblock and triblock poly(ethylene oxide)-block-poly(4-vinylpyridine) copolymers: understanding of micelle and bulk structure

The paper provides new insights into the structure of Pt-containing diblock and triblock copolymers based on poly(ethylene oxide) (PEO) and poly(4-vinylpyridine) (P4VP), using a combination of atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and anomalous small-angle X-ray scattering (ASAXS). Parallel studies using methods contributing supplemental structural information allowed us to comprehensively characterize sophisticated polymer systems during metalation and to exclude possible ambiguity of the data interpretation of each of the methods. AFM and TEM make available the determination of sizes of the micelles and of the Pt-containing micelle cores, respectively, while a combination of XRD, TEM, and ASAXS reveals Pt-nanoparticle size distributions and locations along with the structural information about the polymer matrix. In addition, for the first time, ASAXS revealed the organization of Pt-nanoparticle-filled diblock and triblock copolymers in the bulk. The nanoparticle characteristics are mainly determined by the type of block copolymer system in which they are found: larger particles (2.0-3.0 nm) are formed in triblock copolymer micelles, while smaller ones (1.5-2.5 nm) are found in diblock copolymer micelles. This can be explained by facilitated intermicellar exchange in triblock copolymer systems. For both systems, Pt nanoparticles have narrow particle size distributions as a result of a strong interaction between the nanoparticle surface and the P4VP units inside the micelle cores. The pH of the medium mainly influences the particle location rather than the particle size. A structural model of Pt-nanoparticle clustering in the diblock PEO-b-P4VP and triblock P4VP-b-PEO-b-P4VP copolymers in the bulk was constructed ab initio from the ASAXS data. This model reveals that nearly spherical micellar cores of about 10 nm in diameter (filled with Pt nanoparticles) aggregate forming slightly oblate hollow bodies with an outer diameter of about 40 nm.     

The mechanisms of latex particle formation and growth in the emulsion polymerization of styrene using the surfactant sodium dodecyl sulfate

A new method is presented that provides experimental information which is qualitatively and quantitatively sensitive to assumptions made as to the mechanisms of free radical entry and of latex particle formation in emulsion polymerization systems. The method consists of (1) obtaining (by electron microscopy) the full particle-size distributions (PSDs) at several different times soon after the cessation of latex particle nucleation, (2) using these PSDs to determine the volume dependences of the various rate coefficients governing particle growth by fitting the data to the appropriate evolution equations, and (3) employing these empirical rate coefficients to find that time dependence of the nucleation rate which fits the early-time PSD (again using the evolution equations). This method is quite sensitive to mechanistic assumptions: for example, one is able to determine whether or not the nucleation rate is an increasing or decreasing function of time. The technique is applied to a styrene nucleation system employing sodium dodecyl sulfate as surfactant at well above the critical micelle conventration. The data cannot be fitted even qualitatively by a simple one-step nucleation mechanis, whether it involes micellar entry or homogeneous nucleation. It is found, on the other hand, that the results can be accurately fitted by assuming that coagulation events between primary colloidal particles, perhaps formed by homogeneous nucleation, dominate both the nucleation process and the entry of free radicals into mature latex particles. In addition, the data indicate that the rate of free radical entry into the latex particles decreases with increasing particle size, at least for particles of unswollen radius less than ca. 40 nm.     

Size-controlled synthesis of colloidal platinum nanoparticles and their activity for the electrocatalytic oxidation of carbon monoxide

Using polyvinylpyrrolidone (PVP) as a stabilizing agent, stable colloidal solutions of platinum nanoparticles of different size distributions have been prepared by reducing H2PtCl6 with hydrogen. The UV-vis adsorption peaks at 258 nm due to the adsorption of Pt(IV) species disappear completely, indicating that the Pt(IV) species has been used up and colloidal Pt has been formed. The electrodes have been prepared from aqueous Pt colloids and glassy carbon (GC). The effect of platinum particle size of Pt/GC catalyst electrode on the electrocatalytic oxidation of carbon monoxide has been investigated. The voltammetry shows that a higher potential is needed for the oxidation of absorbed carbon monoxide with a decrease of the platinum particle size for particle sizes larger than 1 nm. But for particle sizes smaller than 1 nm, the potential remains constant while the activity decreases with decreasing the size. The snowlike, well-dispersed, and highly ordered platinum nanoparticles demonstrate high activity in the oxidation reaction of carbon monoxide. The reason may be due to the geometric structure of platinum nanoparticles.     

Formation of Pegylated polyurethane and Lysine-coated polyurea nanoparticles obtained from O/W nano-emulsions

The present work describes the formation of Pegylated polyurethane and Lysine-coated polyurea nanoparticles obtained from O/W nano-emulsions via an interfacial polycondensation process in the aqueous solution/polysorbate 80/diisocyanate/medium chain triglyceride systems. The initial nano-emulsions were prepared using the phase inversion composition (PIC) method. Dynamic light scattering studies revealed the changes in the particle size occurring during the process of nanoparticle formation. Well-defined polymeric nanoparticles with a small particle diameter (below 80 nm) and low polydispersity index were obtained using a highly hydrophilic component (polyethylene glycol or lysine) and an aliphatic diisocyante monomer. FT-IR and AFM studies showed that the polymeric matrix of nanoparticles was built by copolymers derived from reaction between the diisocyanate and the hydroxyl groups of both nonionic surfactant and the highly hydrophilic component. Pegylated-polyurethane and lysine-coated polyurea nanoparticles designed in this study are promising tools for future applications in biomedical sciences.     

Fe3O4 and gamma-Fe2O3 nanoparticles for the adsorption of Co2+ from aqueous solution

The adsorption of Co2+ ions from nitrate solutions using iron oxide nanoparticles of magnetite (Fe3O4) and maghemite (gamma-Fe2O3) has been studied. The adsorption of Co2+ ions on the surface of the particles was investigated under different conditions of oxide content, contact time, solution pH, and initial Co2+ ion concentration. It has been found that the equilibrium can be attained in less than 5 min. The maximum loading capacity of Fe3O4 and gamma-Fe2O3 nanoparticles is 5.8 x 10(-5) and 3.7 x 10(-5) mol m(-2), respectively, which are much higher than the previously studied, iron oxides and conventional ion exchange resins. Co2+ ions were also recovered by dilute nitric acid from the loaded gamma-Fe2O3 and Fe3O4 with an efficiency of 86 and 30%, respectively. That has been explained by the different mechanisms by including both the surface and structural loadings of Co2+ ions. The surface adsorption of Co2+ on Fe3O4 and gamma-Fe2O3 nanoparticles has been found to have the same mechanism of ion exchange reaction between Co2+ in the solution and proton bonded on the particle surface. The conditional equilibrium constants of surface adsorption of Co2+ on Fe3O4 and gamma-Fe2O3 nanoparticles have been determined to be log K=-3.3+/-0.3 and -3.1+/-0.2, respectively. The structural loading of Co2+ ions into Fe3O4 lattice has been found to be the ion exchange reaction between Co2+ and Fe2+ while that into gamma-Fe2O3 lattice to fill its vacancy. The effect of temperature on the adsorption of Co2+ was also investigated, and the value of enthalpy change was determined to be 19 kJ mol(-1).     

Modelling of nano-alloying and structural evolution of bimetallic core-shell nanoparticles obtained via the microemulsion route

A Monte Carlo model has been developed to describe the formation of bimetallic nanoparticles via the microemulsion route. The motivation stems from the need to understand the kinetics of nanoparticle formation in microemulsion droplets in order to determine the best experimental conditions to synthesize a nanoparticle with a given structure. We focus our study on the influence of the homogeneous and heterogeneous critical nucleus sizes of both metals on nanoparticle structure, as well as the role played by the surfactant film flexibility. The study reveals that the final structure is sensitive to changes in the critical nucleus numbers, because these parameters determine the rate of nucleation. An increase in the difference between nucleation rates of both metals gives rise to a better segregation of metals in the final nanoparticle. Likewise, as long as the formation of heterogeneous seeds is faster, the degree of alloying is greater. Finally, a fast material intermicellar exchange leads to a better mixture of metals, so the influence of the critical nucleus sizes on nanoparticle structure becomes less pronounced as the flexibility of surfactant film is increased.     

Studies of Lead Zirconate Titanate Sol Aging Part II: Particle Growth Mechanisms and Kinetics

The kinetics of particle growth in lead zirconate titanate sol gel precursor solutions has been investigated. It was found that chemical reaction limited aggregation was responsible for most of the sol aging, followed by diffusion limited aggregation in the later stage. At the beginning, particles grow by reaction between initial polymers and particles. As the particle number-density increases, a particle-particle aggregation characterised by an exponential growth law becomes the predominant mechanism. When particles grow larger, towards gel formation, the aggregation changes to diffusion limited. Mathematical models derived according to the above mechanisms agreed well with experimentally measured particle growth profiles.     

How realistic is the pore size distribution calculated from adsorption isotherms if activated carbon is composed of fullerene-like fragments?

A plausible model for the structure of non-graphitizing carbon is one which consists of curved, fullerene-like fragments grouped together in a random arrangement. Although this model was proposed several years ago, there have been no attempts to calculate the properties of such a structure. Here, we determine the density, pore size distribution and adsorption properties of a model porous carbon constructed from fullerene-like elements. Using the method proposed recently by Bhattacharya and Gubbins (BG), which was tested in this study for ideal and defective carbon slits, the pore size distributions (PSDs) of the initial model and two related carbon models are calculated. The obtained PSD curves show that two structures are micro-mesoporous (with different ratio of micro/mesopores) and the third is strictly microporous. Using the grand canonical Monte Carlo (GCMC) method, adsorption isotherms of Ar (87 K) are simulated for all the structures. Finally PSD curves are calculated using the Horvath-Kawazoe, non-local density functional theory (NLDFT), Nguyen and Do, and Barrett-Joyner-Halenda (BJH) approaches, and compared with those predicted by the BG method. This is the first study in which different methods of calculation of PSDs for carbons from adsorption data can be really verified, since absolute (i.e. true) PSDs are obtained using the BG method. This is also the first study reporting the results of computer simulations of adsorption on fullerene-like carbon models.     

Synthesis of palladium nanoparticles by sonochemical reduction of palladium(II) nitrate in aqueous solution

The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of palladium(II) nitrate solution. The starting solutions were prepared by the addition of different concentrations of palladium(II) nitrate in ethylene glycol and poly(vinylpyrrolidone) (PVP). The resulting mixtures were irradiated with ultrasonic 50 kHz waves in a glass vessel for 180 min. The UV-visible absorption spectroscopy and pH measurements revealed that the reduction of Pd(II) to metallic Pd has been successfully achieved and that the obtained suspensions have a long shelf life. The protective effect of PVP was studied using Fourier transform infrared (FT-IR) spectroscopy. It has been found that, in the presence of ethylene glycol, the stabilization of the nanoparticles results from the adsorption of the PVP chain on the palladium particle surface via the coordination of the PVP carbonyl group to the palladium atoms. The effect of the initial Pd(II) concentration on the Pd nanoparticle morphology has been investigated by transmission electron microscopy. It has been shown that the increase of the Pd(II)/PVP molar ratio from 0.13 x 10(-3) to 0.53 x 10(-3) decreases the number of palladium nanoparticles with a slight increase in particle size. For the highest Pd(II)/PVP value, 0.53 x 10(-3), the reduction reaction leads to the unexpected smallest nanoparticles in the form of aggregates.     

On the Chemical Processes Accompanying Silver Reduction from Solutions of Its Salts in Organic Media

Silver reduction from its nitrate has been studied in the media of DGEBA-based and aliphatic epoxy resins. It has been found that, under the experimental conditions that were employed, silver ions can be reduced without chemical interaction with the medium only due to photochemical processes. The reduction rate is determined by the rate of dissolution of the initial silver salt in an organic medium and the possibility of the formation of solvates by the system components. Refractometry has been proposed for use in monitoring the kinetics of silver nitrate dissolution in epoxy resins and the formation of silver nanoparticles. Stable dispersions of silver nanoparticles in epoxy resins have been obtained.     

Role of adsorption in formation of inorganic nanoparticles: Kinetic model

A simple kinetic model is proposed for the formation of inorganic nanoparticles in the presence of additives of readily adsorbing organic compounds. Additives and monomers may occupy the same sites on the surface of a growing particle. The maximum sizes and size distribution of formed particles are estimated under the assumption that the surface curvature of a growing particle has equivalent effects on the rate constants of the growth and adsorption. Equations are derived that relate the polydispersity indices for particle mass and radius distributions to the variances of particle radius distribution. The conditions are determined for the formation of virtually monodisperse nanoparticles.     

反应条件对聚苯乙烯复合硫化铅纳米微粒粒度及分布的影响

用新的方法将ＰｂＳ纳米微粒复合在聚苯乙烯光学塑料中，并利用吸收光谱、小角Ｘ－光散射和透射电镜等方法研究了不同反应条件对复合的硫化铅（ＰｂＳ）纳米微粒粒度及分布的影响．结果表明，在一定范围内Ｈ２Ｓ的量和初始含铅聚合物浓度对纳米微粒的粒度影响很小，但对其粒度分布影响较大；当Ｈ２Ｓ与Ｐｂ２＋的摩尔比及初始含铅聚合物浓度均较大时，纳米微粒的粒度分布变宽．实验结果表明复合于聚苯乙烯中的ＰｂＳ的粒度分布是由溶液中反应决定的，本体聚合反应时对ＰｂＳ微粒的粒度分布影响很小．因此控制适当反应条件，可以得到粒度分布均一、分散均匀且透明性好的ＰｂＳ纳米微粒复合有机光学材料．