Chemical Diffusion Processes Research Articles

Microphasic structure of oligoesteracrylate networks obtained in a medium of different solvents by the mechanism of anionic polymerization

Wide and small angle X-ray analysis has been used to study the microphasic structure of reticular polymers obtained by the anionic mechanism from oligoesteracrylates. It has been established that the microheterogeneity of the polymer networks essentially depends on the nature of the solvent and is already laid down in the structure of the gels and is also connected with the special features of the course of the chemical reactions and diffusion processes in the polymerizing system. The removal of the solvent causes rearrangement of the structure of the gel as a result of phase separation. The paper proposes a model of the heterogeneous structure of oligoesteracrylate gels on the basis of which the parameters of microheterogeneity are evaluated.

Asymptotic stability of reaction-diffusion systems in chemical reactor and combustion theory

Some coupled reaction-diffusion systems arising from chemical diffusion processes and combustion theory are analyzed. This analysis includes the existence and uniqueness of positive time-dependent solutions, upper and lower bounds of the solution, asymptotic behavior and invariant sets, and the stability of steady-state solutions, including an estimate of the stability region. Explicit conditions for the asymptotic behavior and the stability of a steady-state solution are given. These conditions establish some interrelationship among the physical parameters of the diffusion medium, the reaction mechanism, the initial function and the type of boundary condition. Under the same set of physical parameters and reaction function, a comparison between the Neumann type and Dirichlet or third type boundary condition exhibits quite different asymptotic behavior of the solution. For the general nonhomogeneous system, multiple steady-state solutions may exist and only local stability results are obtained. However, for certain models it is possible to obtain global stability of a steady-state solution by either increasing the diffusion coefficients or decreasing the size of the diffusion medium. This fact is demonstrated by a one-dimensional tubular reactor model commonly discussed in the literature.

Propriétés de diffusion de l'alliage eutectique liquide AgGe

Knowledge of the chemical diffusion process in a eutectic liquid close to a moving solid-liquid interface is essential for understanding the formation of the solid structure. We made measurements of the two self-diffusion coefficients in the eutectic alloy AgGe from 800 to 658 °C ( T E + 7 °C). Chemical diffusion coefficients were then calculated using the Darken relationship which has been previously experimentally verified. All these coefficients follow an Arrhenius-type relationship in the temperature region explored.

The Kinetics of Metal Solvent Extraction

The kinetics of solvent extraction of metal species by organic solution of acidic, basic, and neutral reagents has been reviewed by considering the contribution of both chemical reactions and transport processes on the rate of extraction. The relative predominance of slow chemical reactions and diffusion processes has been shown to depend, in a critical way, both on the hydrodynamic conditions present in the extracting apparatus and on the specific nature and composition of the chemical system involved in the extraction. The general criteria that can help to distinguish between extraction kinetics controlled either by mass transport or chemical reactions have been reviewed. These criteria involve the experimental determination of the influence of a rather large number of physical and chemical variables on the rate of extraction and do not always lead to unambiguous conclusions. The reviewed literature has allowed to draw the following conclusions: (1) because of a lack of complete characterization (especially in terms of hydrodynamics, interfacial pressure, and sufficiently wide concentration ranges), much of the early work on solvent extraction kinetics has rarely led to the definite identification of the rate determining processes; as a consequence, correct extrapolation to the different experimental conditions that are met in practicalmore » analytical or technological separations has been often impossible; and (2) more recently a certain number of extraction kinetics studies have appeared which have been carried out in hydrodynamically controlled conditions, exploring a large range of compositions of the species and with knowledge of the interfacial properties of the biphasic system; such studies have contributed to the emergence of a general pattern for the solvent extraction kinetics which is summarized.« less

The effect of desorption in micellar particle nucleation in emulsion polymerization

AbstractThe effect of desorption of monomer radicals from particles on the particle number generated in interval I of an emulsion polymerization is calculated by means of a semi steady state model. It is shown that desorption leads to an increase in the particle number, and at the same time the orders of particle number with respect to emulsifier and initiator are changing. The general equation of Nomura N∝Sz0ϱ1−zi is found to apply, z increasing from 0,6 to 1,0 with increasing desorption. The values of z and N are dependent on the exact reaction mechanisms in the monomer/polymer system and on rate constants of chemical reactions and diffusion processes involved in the particle formation.

Detonation chemistry: Diffusion control in non‐ideal explosives

AbstractThe performance of an explosive is a function of both the peak energy released near the detonation front, and the remainder of the energy that is released during the Taylor wave. The relative partitioning of energy between the front and the expansion, and the rate of energy release in the latter, may be controlled by either chemical kinetic or diffusion processes. Detonation calorimetry has been the principal experimental technique used to investigate these processes. Both the total energy release, and the quantitative analysis of the detonation products at a point or region on the expansion isentrope have been determined for formulations of ammonium nitrate (AN) and TNT in which both the composition and particle size of the AN have been varied. Isotopic labeling of selected explosives has also been used to give further insight into the reactions taking place in or near the reaction zone. Similar experiments have been performed in an ideal, homogeneous explosive.

Effect of physicochemical processes in fireproof materials on their thermal conductivity

The relationship between the effective thermal conductivity of fireproof materials with heterogeneous phase transitions and chemical transformations and diffusion processes in the pores and microfissures is demonstrated.

Transfer rate of some tervalent cations in the biphasic system HClO 4, water-dinonylnaphthalenesulfonic acid, toluene—III : Transfer rate at t≠0 determined by both chemical reactions and diffusion processes

Concentration vs time data for the transfer of Fe(III), Ce(III), Eu(III), Gd(III), Tm(III) from aqueous perchloric acid solutions to toluene solutions of dinonylnaphthalenesulfonic acid have been obtained as function of the chemical composition of the system and in some cases of the stirring speed of the phases. A model, coupling together both chemical reactions and diffusion processes, obtained through the elaboration of models previously developed has been proved to explain well the experimental data in a wide range of stirring speeds and for times ranging from zero to about 300 min (near equilibrium). A complete description of the diffusion processes and interfacial chemical reactions taking place in the system as well as numerical values of the rate constants are reported.

Transfer rate of some tervalent cations in the biphasic system HClO 4, water-dinonylnaphthalenesulfonic acid, toluene—II : Transfer rate of europium(III) at t = 0 determined by both chemical reactions and diffusion processes

The transfer rate of Eu(III) ions between aqueous perchloric acid solutions and toluene solutions of dinonylnaphthalenesulfonic acid has been studied. The initial forward transfer rates have been evaluated as function of the chemical composition of the system and the stirring speed of the phases. A model for the mass transfer rate, coupling together both chemical reactions and diffusion processes, has been elaborated. This model explains well the experimental data.

Influence de la forme de l'interface liquide-solide sur la diffusion chimique pendant une solidification eutectique lamellaire

In-situ observations have yet shown that the liquid-solid interface during eutectic growth can be markedly corrugated. The influence of the interface shape on the chemical diffusion process in the liquid is evaluated in this paper. The periodicity of the boundary conditions allows a Fourier analysis of the problem of the mass transfer in the liquid. It leads to a matricial equation that could be solved analytically. Computer calculations were carried out for the Pb-Sn eutectic system assuming circular shapes of the cross section of the interface between the liquid and each lamella. Even when the volume fraction satisfies the lever rule, the mean liquid composition along the interface can be strongly different from the eutectic composition for a corrugated interface. The discrepancy depends mainly on the bulk composition of the liquid and on the leading distance of one phase on the other. Moreover, the more convex the interface is, the lower are the lateral composition gradients.

Internal Relaxation in Chemically Reacting Fluids

The coupling of an internal relaxation to both the chemical reaction and diffusion processes is investigated for a fluid mixture. A microscopic approach beginning with a generalized Langevin equation is used to derive relaxation equations for the concentration fluctuations of the species in specified internal states as well as total concentration fluctuations. These equations are used to calculate the matrix of concentration correlation functions which are then related to the intensity of the light scattered from the fluid. The circumstances under which such a generalized hydrodynamics approach must be used are examined. The possibility of extracting information about both the chemical and internal relaxation processes from a measurement of the width of the polarized component of the Rayleigh line is considered.

Light Scattering from Reactive Fluids. III. Intensity Calculation

Using standard perturbation theory for degenerate matrices, an expansion for the eigenvectors corresponding to the thermodynamic fluctuation modes in a chemically reactive fluid is obtained. The expansion is valid for small scattering angles, that is, vanishing spatial gradients. These eigenvectors give rather complicated expressions for the intensities of the individual lines, making it rather impractical to get these intensities. We have therefore evaluated partial sums of integrated intensities, which depend only on thermodynamic equilibrium parameters and stoichiometric coefficients. We evaluate explicitly the ratio of the intensity contribution due to the chemical reactions and diffusion processes (Berne–Frisch approximation). For the case of the reaction AB⇆A + B, the intensity is proportional to the net change in the dielectric polarizability in the reaction, in accordance with the result obtained by Berne, Deutch, Hynes, and Frisch for a simpler case. Our method is, however, general and can be used for any number of components and reactions.

Studies of the initial reactions of carbonization

The transformation of organic materials to coke, carbon and ultimately, graphite involves a complex combination of chemical reactions and diffusion processes in both the liquid and solid state. The chemical structures of the starting organic compounds, their initial chemical reactions, as well as the structure, conformation, and reactivity of free radical intermediates determine the course of carbonization and ultimate structure of the final graphite. The electron spin resonance technique has been particularly useful in identifying the radical intermediates and in clarifying the early reaction mechanisms.

Self-diffusion and interdiffusion in gold-nickel alloys

Measurements are described on the self-diffusion and interdiffusiori coefficients in gold-nickel alloys as a function of composition and temperature. These data are combined with the thermodynamic properties of the gold-nickel system to test the validity of the Darken equation: D ̃ = (X 1D 2 ∗ + X 2D 1 ∗) ( 1 + d ln ƒ 1 d ln X 1 ). This relationship is found to hold within the experimental accuracy of the measurements, and the results may be considered to demonstrate the concept that the chemical potential gradient is the driving force for the chemical diffusion process. The influence of the thermodynamic factor ( 1 + d ln ƒ 1 d ln X 1 ) on the activation, energy for interdiffusion is also shown.