Kinetics of limonene autooxidation

The kinetic parameters of the oxidizability of Iimonene,k _p/(2k _t)^0.5 = 6.0- 10^–3 L^{0 5} mol^–5 s^–0.5, and of the bimolecular radical decomposition of hydroperoxides 2ek _d = 6.0 · 10^–6 L mol^–1 s^–1 were determined at 60 °C The oxidation rate increases in the presence of micro additives of water. Average effective diameters of particles formed in the water-AOT-(n-decane + lirnonene) microemulsion were measured by the light scattering technique. The hydroperoxides were found to affect the size of the microemulsion particles.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 7, pp. 1682–1685, July. 1996.     

Liquid-phase autooxidation of retinyl polyenes

The autooxidation of retinyl acetate and methyl retinoate was investigated in chlorobenzene at 45°C. The rates of thermal initiation in the retinyl acetate solutions were measured, and a value was determined of the rate constant for the reaction of oxygen with retinyl acetate (RH + O₂ → R· + HO₂·): k_io = (1.3 ± 0.2) × 10^?5 L/mol · s. The number of moles of oxygen absorbed per mole of polyene depends on the substrate concentration. A kinetic scheme for the methyl retinoate autooxidation was proposed which takes into account the isomerization of primary peroxy radicals, and the rate constants for different elementary reactions were estimated. The partial rate constant for “allylic” hydrogen abstraction from retinyl acetate was estimated to be ≥ 1.65 × 10³ L/mol · s. A probable propagation sequence was proposed for the autooxidation of retinyl acetate.     

The role of peroxides in the mechanism of low-temperature autooxidation of methyl oleate and its solutions with lipids

The kinetics of low-temperature autooxidation of methyl oleate for a bimolecular mechanism of the degenerate branched reaction were analyzed taking into account changes in the methyl oleate concentration for different amounts of peroxides formed. This analysis made it possible to explain the experimental data. The role of the initial peroxide concentration in the mechanism and kinetics of the chain degenerate branched reaction of methyl oleate autooxidation was studied in the steady-state approximation and in the course of establishing a steady-state concentration of radicals. A systematic approach to estimating the antioxidant properties of lipids on the basis of the methyl oleate model was proposed.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 848–858.Original Russian Text Copyright © 2004 by Khrustova, Shishkina.     

Role of pyromucic acid in the autooxidation of furfural

Pyromucic acid (PA) in aqueous solution is oxidized by molecular oxygen with opening of the furan ring and formation of -formylacrylic, fumaric, and formic acids. The process is accompanied by the interlinked oxidative decarboxylation of PA and the acids formed from it. PA is the major intermediate in the autooxidation of furfural. When PA and furfural are cooxidized, the rate of transformation of each compound increases by a factor of two; this is a sign of the interlinking of two radical processes evolving in one system. A probable scheme for the oxidation of PA in water by molecular oxygen is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 9–13, January, 1971.     

Temperature and pressure oscillations during the autooxidation of benzaldehyde

The oscillatory behavior of the autooxidation of benzaldehyde catalyzed by cobalt(II) and bromide ions can be monitored besides potentiometry and/or spectrophotometry also by measuring the temperature of the reaction system and the pressure of oxygen above the solution as a function of time. The temperature and pressure oscillations can be explained only qualitatively so far.

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Mechanism for the inhibition of the autooxidation of cyclohexanone by alkylammonium salts

Conclusions Tetraalkylammonium salts may efficiently hinder the oxidation of ketones by catalyzing the decomposition of -ketohydroperoxides, mainly through a pathway not involving the formation of free radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2100–2102, September, 1987.     

Synthesis and autooxidation of 2-amino-1,3-dialkylindoles

Autooxidation occurs during the liberation of the bases from the salts of 1,3-dialkyl-2-aminoindoles, and stable 3-hydroperoxides are formed. The same compounds can also be obtained without prior isolation of the salts via heterocyclization of 1-acyl-2-arylhydrazines. If one of the nitrogen atoms is not alkylated, the corresponding 3-hydroxy compounds are obtained.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1217–1221, September, 1980.     

Desorption-limited mechanism of release from polymer nanofibers

This work examines the release of a model water-soluble compound from electrospun polymer nanofiber assemblies. Such release attracts attention in relation to biomedical applications, such as controlled drug delivery. It is also important for stem cell attachment and differentiation on biocompatible electrospun nanofiber scaffolds containing growth factors, which have been encapsulated by means of electrospinning. Typically, the release mechanism has been attributed to solid-state diffusion of the encapsulated compound from the fibers into the surrounding aqueous bath. Under this assumption, a 100% release of the encapsulated compound is expected in a certain (long) time. The present work focuses on certain cases where complete release does not happen, which suggests that solid-state diffusion may not be the primary mechanism at play. We show that in such cases the release rate can be explained by desorption of the embedded compound from nanopores in the fibers or from the outer surface of the fibers in contact with the water bath. After release, the water-soluble compound rapidly diffuses in water, whereas the release rate is determined by the limiting desorption stage. A model system of Rhodamine 610 chloride fluorescent dye embedded in electrospun monolithic poly(methylmethacrylate) (PMMA) or poly(caprolactone) (PCL) nanofibers, in nanofibers electrospun from PMMA/PCL blends, or in core-shell PMMA/PCL nanofibers is studied. Both the experimental results and theory point at the above mentioned desorption-related mechanism, and the predicted characteristic time, release rate, and effective diffusion coefficient agree fairly well with the experimental data. A practically important outcome of this surface release mechanism is that only the compound on the fiber and pore surfaces can be released, whereas the material encapsulated in the bulk cannot be freed within the time scales characteristic of the present experiments (days to months). Consequently, in such cases, complete release is impossible. We also demonstrate how the release rate can be manipulated by the polymer content and molecular weight affecting nanoporosity and the desorption enthalpy, as well as by the nanofiber structure (monolithic fibers, fibers from polymer blends, and core-shell fibers). In particular, it is shown that, by manipulating the above parameters, release times from tens of hours to months can be attained.     

Detailed structures and mechanism of polymer solvation

In the present study, we simulated a model system, PE in biphenyl, to gain the insight into the detailed solvation structures and the molecular mechanism of polymer chain solvation. Using atomistic molecular dynamics (MD) simulation, it was found that when the biphenyl is far from PE chain or in the bulk, the dihedral angle of the two rings in the solvent molecule are approximately 32 degrees. But, the dihedral angel is about 27 degrees when the biphenyls are very close to the PE chain. In the first solvation shell, the orientation angle of the biphenyl long axis to the chain segment backbone was found to be enhanced around two values: approximately 0 and approximately 60 degrees. The detailed solvation structures found here include all dyad conformations (TT, TG, TG', GT, GG, GG', G'T, G'G, and G'G') and vary as a function of the distance between PE chain and biphenyls in the first solvation shell. The closer the the solvent molecule to the PE segment, the higher the TT conformation fraction response is. The other dyad conformations such as TG, GG', etc. undergo different decreases, respectively. This study shows that the solvation even in the Theta condition makes the overall size expansion or the chain stretched. Such a cooperative change was examined here and found not due to generating or losing a conformational state but due to a change in conformational distribution. This change occurs in the middle location of the chain instead of the chain end locations.     

Confinement mechanism of electrospun polymer nanofiber reinforcement

The sharp increase in elastic modulus of electrospun polymer nanofibers with decrease in their diameters is now a well-known phenomenon. Unfortunately, up to now, the physical reasons resulting in the above size-dependent behavior are unclear. The proposed explanation is based on the confinement concept. A manifesting mechanism of the confinement effect which provides the size-dependent elastic modulus of electrospun polymer nanofibers is discussed. According to this model, the nanofiber polymer matrix contains anisotropic regions consisting of correlated worm-like subchains, partially oriented along the fiber. A fiber elongation is accompanied by relative rotations of the above regions. Confinement effect is that these rotations are hindered by the fiber boundary. As a result, the elastic modulus depends on the fiber diameter. This restriction is dominant for the thin fibers, is decreasing with increase of the fiber diameter, and becomes negligible for thick fibers. Such a behavior is in good agreement with experimental observations. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

On the mechanism of crystalline polymorph selection by polymer heteronuclei

The phase-selective crystallization of acetaminophen (ACM) using insoluble polymers as heteronuclei was investigated in a combined experimental and computational effort to elucidate the mechanism of polymer-induced heteronucleation (PIHn). ACM heteronucleates from supersaturated aqueous solution in its most thermodynamically stable monoclinic form on poly(n-butyl methacrylate), whereas the metastable orthorhombic form is observed on poly(methyl methacrylate). When ACM crystals were grown through vapor deposition, only the monoclinic polymorph was observed on each polymer. Each crystallization condition leads to a unique powder X-ray diffraction pattern with the major preferred orientation corresponding to the crystallographic faces in which these crystal phases nucleate from surfaces of the polymers. The molecular recognition events leading to these outcomes are elucidated with the aid of computed polymer-crystal binding energies using docking simulations. This investigation illuminates the mechanism by which phase selection occurs during the crystallization of ACM using polymers as heteronuclei, paving the way for the improvement of methods for polymorph selection and discovery based on heterogeneous nucleation promoters.     

Insight into the inversion mechanism of an inverse polymer emulsion

A study of the mechanism of inversion of inverse polymer emulsions showed that the likely key step in the inversion process is the swelling of polymer particles caused by added water diffusing through the oil phase. Therefore, we propose that inversion occurs as a result of polymer particle crowding caused by water diffusion and subsequent droplet rupture that results in the release of the polymer into the water phase.     

Kinetics of the autooxidation of iodine with regard to the bray-liebhafsky oscillatory reaction

The kinetics of autooxidation of iodine in acidic solutions has been studied and its autocatalytic character has been observed. The proposed reaction scheme, based on the experimental results, is in accordance with the Treindl-Noyes skeleton mechanism for the Bray-Liebhafsky oscillatory reaction.