Initial stages of soot formation in thermal pyrolysis of acetylene. II. A model for the incipience and growth of soot particles

A model is developed which describes simultaneously occurring processes of the initial hydrocarbon pyrolysis, nucleation, surface growth, and coagulation of soot particles. The model permits one to find the size distribution of the primary soot particles up to size 30–40 nm using a relatively small set of equations. The computed time dependence of soot particle concentration agrees satisfactorily with available experimental data. The existence of two limiting stages of the soot formation is revealed.     

Mode of Action and Design Rules for Additives That Modulate Crystal Nucleation

Molecular dynamics simulations reveal that the key factors that determine the ability of an additive to modulate crystal nucleation are the strength of its interaction with the solute, its disruptive ability (which may be based on steric, entropic, or energetic effects), and interfacial properties, along with its ability to serve as a template for nucleation (see snapshot of an emerging nucleus with a single‐particle additive: black spheres).

     

Crystallization of polymers in variable external conditions

General equations of crystallization in variable conditions derived in the former paper [1] have been applied to non-isothermal crystallization of unstressed and unoriented polymers. Crystallization rate involving transient and athermal effects is controlled by temperature,T, and cooling rate,. Experimental procedures leading to determination of three temperature-dependent material functions: steady-state crystallization rate,K _st(T), relaxation time, (T), and athermal nucleation function, Z(T), are outlined.     

Synthesis of Goethite by Separation of the Nucleation and Growth Processes of Ferrihydrite Nanoparticles Using Microfluidics

Microfluidic synthesis is used to form nanoparticles by separate nucleation and growth processes using two microreactors (see picture) operating under different temperature and flow conditions. Ferrihydrite nanoparticles precipitated in the first microreactor are aged under continuous flow in a second microtubular reactor, leading to goethite nanoparticles. TMAOH=tetramethylammonium hydroxide.

     

Nucleation: What Happens at the Initial Stage?

Crystallizing growth : The initial structure of crystal nuclei is supersaturation‐dependent. At low degrees of supersaturation, liquid‐like nuclei are formed initially, which undergo a continuous structure transition from liquid‐like to crystal‐like as the size N increases. This gradual structure evolution substantially lowers the nucleation barrier ΔG* and facilitates the nucleation relative to the formation of crystal‐like clusters from the beginning.

     

Dielectric study of poly(methyl methacrylate) microcapsules in particular reference to the effect of the frequency dependence of the permittivity of the capsule wall

The dielectric behavior of poly(methyl methacrylate) (PMMA) microcapsules was analyzed theoretically by taking account of the frequency dependence of the relative permittivity _s and the electrical conductivity _s of the capsule wall. For comparison, similar analysis was applied to polystyrene (PS) microcapsules which have frequency-independent _s and _s . The behavior of _s and _s of the PMMA microcapsules was assumed to be represented by a dielectric relaxation of them-th power type on the basis of dielectric data on underwater PMMA films. By means of a theory of the interfacial polarization, some consideration was carried out regarding the effect of the frequency dependence of _s and _s on the dielectric behavior of the PMMA microcapsules. The consideration led to predictions that the frequency dependence of _s and _s causes (i) the variation in the relaxation intensity with the change in the conductivity of suspending medium, (ii) the frequency dependence of the permittivity at low frequencies and (iii) the frequency dependence of the conductivity at high frequencies. The theoretical prediction (i) was verified by observed data, (ii) and (iii) remaining unverified owing to the experimental difficulty.     

A Monte Carlo Simulation of Homogeneous Crystallization in Confined Spaces: Effect of Crystallization Kinetics on the Avrami Exponent

Homogeneous nucleation in polymers and subsequent crystallization in spaces confined relative to spherulitic dimensions in one direction has been simulated for a wide range of nucleation and growth rates. An empirical equation based on simulation results relating an Avrami exponent of crystallization to growth and nucleation rates along with the characteristic dimension of crystallizing volume has been proposed. It was found that for ideal homogeneous crystallization, the Avrami exponent can be significantly lower than the anticipated values for free unconstrained growth; therefore, all crystallization conditions and parameters must be taken into account in order to make a conclusion on the character of the observed crystallization.

     

Effect of pressure on the decomposition of 2-tert-butyl peroxide in solution

A study was carried out on the effect of pressure up to 1400 MPa on the homolytic decomposition of di-tert-butyl peroxide (DTBP) at 403 K in 2-methoxy- (I) and 2-ethoxytetrahydropyran (II) as well as in a mixture of 30 mole % (I) and 70 mole % benzene. Spline approximation of the experimental pressure dependence of the logarithm of the decomposition rate constant (k_a) gave the continuous dependence of the volumetric activation effect (V) on pressure. The value of V at atmospheric pressure (V ₀) and the nature of the change of AV with increasing pressure were found to depend on the nature of the solvent. This dependence is difficult to explain in the framework of the accepted transition state theory.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1911–1914, August, 1990.     

Growth Mechanism of Metal–Organic Frameworks: Insights into the Nucleation by Employing a Step‐by‐Step Route

One step at a time : The in situ monitoring of the step‐by‐step formation of metal–organic frameworks (MOFs) by using surface plasmon resonance (SPR), allows the nucleation process and the formation of the secondary building units to be investigated. Growth rates on functionalized organic surfaces with different crystallographic orientations can also be studied.

     

Effects of the potential barrier fluctuations in non-adiabatic long-distance electron transfer

Equations for the transition probability of non-adiabatic electron transfer are derived which take into account both the effect of reorganization of non-symmetric classical or quantum vibrational modes and the effects which are due to the dependence of the electron matrix element on the normal coordinates of promoting vibrational modes. Any number of vibrational modes with an arbitrary frequency spectrum are incorporated in a simple manner. The results are represented in a simple form convenient for the analysis of the dependence of the transition probability on the temperature, transfer distance, and driving force.Dedicated to the 70^th birthday of Professor Z. Galus     

Correlation between the rotational mobility and the kinetics of the intramolecular exchange of the unpaired electron and bond in a tin-containing free radical with two pyrocatechol ligands

EPR spectroscopy was used to study the effect of the mobility of the medium on the dynamics of the interligand exchange of the unpaired electron and bond in the free radical formed upon the reaction of stannous bis-3,6-di-tert-butyl-o-semiquinolate with 3,6-di-tert-butyl-2-hydroxyphenoxyl in the presence of triphenylphosphine. A dependence exists between the characteristic exchange time _ex and the correlation time for the rotational mobility of the radical _c; _ex= _c. A temperature dependence was found for coefficient .Translated from Izvestiya Akademii Nauk SSSR. Seriya Khimicheskaya, No. 7, pp. 1663–1666, July, 1990.     

In‐Reactor Compounding Metallocenic Isotactic Poly(propylene)/Nucleation Agent Compositions by Employing the Nucleation Agent as a Catalyst Support

A nucleation agent, 1,3,2,4‐dimethylbenzylidene sorbitol (DMBS), was used as a support for C₂‐symmetric metallocene rac‐Me₂Si[2‐Me‐4‐Naph‐Ind]₂ZrCl₂. Propylene polymerization with the supported catalyst resulted in i‐PP polymers with granular morphology. The role of a catalyst support ensures a good dispersion of the nucleation agent in the formed i‐PP matrix. The employment of a nucleation agent as catalyst support provides a unique solution to in‐reactor compounding metallocenic i‐PP/nucleation agent compositions.

     

The effects of ionizing radiation on the thermophysical properties of substituted Sr-ferrites

Different samples of Sr-ferrites have been synthesized by the usual ceramic and high temperature sintering technique. On these samples, several measurements have been undertaken namely, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), the temperature dependence of DC electric conductivity before and after -absorbed dose, and the temperature dependence of dielectric constant. Results obtained were explained, correlated, interpreted and discussed in detail on the basis of the interaction of temperature, dopant cations and -radiation with the Sr-ferrite lattice. The activation energy (E), energy gap for semiconduction and activation energy for -radiation-induced carrier liberation () were evaluated for the first time. Finally, a mechanism for semi-conductivity in terms of energy band picture in intrinsic ferrite semiconductors is suggested and discussed.     

Dynamics of Phase Separation in Polymer Blends Revisited: Morphology,Spinodal, Noise,and Nucleation

The properties of the model B of mesoscopic dynamic with the Flory–Huggins free energy for the homopolymer blend are discussed. We focus on the rescaling of the spatial coordinates in the model and demonstrate that the commonly used rescaling of the spatial coordinates by the function vanishing at the spinodal leads to the unphysical pinning of the domains. The proper rescaling function for the spatial variables which avoids the unphysical pinning of the domain growth at the spinodal is proposed. We discuss in detail the evolution of morphological measures during separation in homopolymer blends and the problem of nucleation close to the spinodal.

     

Simultaneous Controllability of PSD and MWD in Emulsion Polymerisation

A sensitivity study of particle size distribution (PSD) and molecular weight distribution (MWD) responses to perturbations in initiator, surfactant, monomer and chain transfer agent in a semi‐batch emulsion polymerisation is presented. The objective is to provide a systematic study on the ability to simultaneously control both PSD and MWD, towards inferential control of end‐use product properties. This would lead towards identification of the practical feasible regions of operability. All inputs appeared to have an intrinsic and simultaneous influence on end‐time PSD and MWD. Trends shown in experimental results have been explained in a mechanistic sense and also compared to simulation results from a combined PSD/MWD population balance model. The preliminary comparison between experiment and simulation highlights areas to be focussed on with respect to model improvement.

     

A kinetic analysis of the photolysis of mixtures of acetone and propylene

The photolysis of mixtures of acetone and propylene at 308 nm has been studied kinetically from approximately 300 K to 580 K. Rate constants were calculated for the reactions at total pressures ranging from 110 torr to 750 torr and for [C₃H₆]/[(CH₃)₂CO] ratios in the range 0.03 to 3.3. No dependence of the rate constants on total pressure or on this concentration ratio could be detected within this range of conditions. The temperature dependence of the rate constants is given by © 1994 John Wiley & Sons, Inc.     

Ionic strength dependence of formation constants. Part II: potentiometric study of the copper(II)-malonate-glycinate system in the range 0.01≤I≤1.0

Summary The ternary copper(II)-malonate-glycinate system has been investigated by potentiometric measurements using a glass electrode, at 37° and at different ionic strengths (0.01I1.0 mol dm^–3), with KNO₃ as background salt. Complex formation between K⁺ and malonate has also been taken into account, and the parameters for the dependence on ionic strength have been obtained for each formation constant. A general equation, valid for all the complexes formed in this system, for the dependence, log K=f(I), is also proposed. The stabilization parameters of the ternary complex are also analyzed as a function of the ionic strength.     

Temperature dependence of the rate of reaction in thermal analysis

Special problems are encountered in modeling the temperature dependence of the kinetics of heterogeneous, condensed phase systems. In the division of the model for the reaction rate into two parts, a)f() which is physical (translational) and b)k(T) which is chemical (vibrational), complications arise in defining the temperature dependence of part a) which may take various mathematical forms and then in coupling it with the Arrhenius temperature dependence of part b). The role off() in thermal analysis systems is discussed. The concept of rate-controlling step is applied to the simplification of the temperature dependent term. The significance of the compensation effect in these systems is described and an heuristic rationalization for it is suggested. Maximum practical temperature ranges for thermal analysis experiments and the effect of temperature measurement imprecision on obtaining meaningful Arrhenius parameters are discussed. The WLF and other equations used to describe the temperature dependence off() are not found to couple compatibly with the Arrhenius equation.

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Zusammenfassung Bei der Modellierung der TemperaturabhÄngigkeit der Kinetik bei heterogenen, kondensierten Phasen treten spezielle Probleme auf. In der Unterteilung des Modelles für die Reaktionsgeschwindigkeit in zwei Teile: a)f() ist physikalisch (Translation) und b)k(T) ist chemisch (Vibration) treten bei der Definierung der TemperaturabhÄngigkeit von Teil a) und dann bei der Verknüpfung mit der Arrhenius-schen TemperaturabhÄngigkeit von Teil b) Komplikationen auf. Es wird die Rolle vonf() in thermoanalytischen Systemen besprochen. An einer Vereinfachung des temperaturabhÄngigen Termes wurde das Konzept des geschwindigkeitsbestimmenden Schrittes angewendet. Es wird die Bedeutsamkeit des Kompensationseffektes in diesen Systemen beschrieben und dafür eine heuristische Vereinfachung vorgeschlagen. Praktisch gesehen maximale Temperaturbereiche für thermoanalytische Experimente sowie der Einflu\ der Ungenauigkeit der Temperaturmessung für die erhaltenen Arrhenius-Parameter wird diskutiert. Die zur Beschreibung der TemperaturabhÄngigkeit vonf() benutzten WLF und anderen Gleichungen stellen keine KompatibilitÄt mit der Arrhenius-schen Gleichung her.

     

The application of thermodynamic perturbation theory to the calculation of excess free energy of small systems: 1. The study of the dimensional dependence of specific free energy of small droplets

Fundamentals of the application of the method of thermodynamic perturbation theory to finding the excess free energy of small objects were analyzed. The size dependence of the specific surface free energy of small droplets of a simple Lennard-Jones fluid, water, and metal melts was studied in the first and second approximations of the perturbation theory. It was established that, for all studied systems, the size dependence of the surface tension was satisfactorily described by Tolmans formula. At small particle radii, Rusanovs formula was well fulfilled.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 844–849.Original Russian Text Copyright © 2004 by Shcherbakov, Samsonov, Bazulev.Deceased.     

Styrene Emulsion Polymerization above the CMC: New Evidence on Particle Nucleation by means of AFFFF

Summary: The number (N) and size distribution of particles (PSD) of a styrene emulsion polymerization above the CMC were studied by means of asymmetric flow‐field flow fractionation (AF⁴). Bimodal PSDs were obtained, suggesting that coagulation of the primary particles is not as extensive as would be expected, according to the coagulative mechanism. AF⁴ allowed it to be demonstrated that N is constant during interval II, and that the resolution limit of other particle sizing techniques can lead to erroneous mechanistic inferences, from the evolution of N.

Particle size distribution measured at low conversion for the emulsion polymerization of styrene, obtained by AF⁴ and DLS. The initial surfactant (S₀), initiator (I₀) and monomer (M₀) concentrations are indicated in the figure.