The role of early stages of dioxygen activation in the biological and catalytic activity of phenol-type natural antioxidants

The main aspect of the A.N. Bakh peroxide theory of slow oxidation is the development of concepts on the role of peroxide compounds and the early stages of activation of molecular oxygen in reactions of biological oxidation. The use of modern physicochemical methods, namely, spectroscopy, pulse radiolysis, and ac polarography in studying the catalytic and antioxidant properties of natural compounds, flavonoids, makes it possible to detect the oxidation reaction intermediates which represent the partial-charge-transfer complexes [nQr^δ+…mO₂^∞−] and also to explain the possibility of formation of nanostructured metal particles in reverse micelles in the presence of oxygen with quercetin Qr acting as reducing agent.     

含有BaMoO4纳米粒子的反胶束溶液与罗丹明B的相互作用

在三种带有不同电荷的表面活性剂构建的反胶束体系中(AOT/异辛烷、Oπ-10/环已烷、CTAB/正已醇)合成了BaMoO₄的纳米粒子, 采用透射电镜(TEM)观察其粒子呈球形, 粒径在17～46 nm范围内, 分布均匀; 使用染料罗丹明B作为探针, 采用紫外-可见光谱(UV-vis)和荧光光谱研究反胶束水池中罗丹明B与BaMoO₄纳米粒子的相互作用; 由于反胶束水池的空间和极性的限定, 染料的光谱特征与其在纯水中发生很大变化, 不同的反胶束体系中, 由于染料分子所处的微观环境不同, 导致其光谱特征也有较大区别.     

Structural characterization of frozen n-heptane solutions of metal-containing reverse micelles

The microstructure of temperature-quenched solutions of reverse micelles formed by sodium, cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate in n-heptane has been investigated by SAXS and EXAFS. Some changes in the X-ray absorption spectra with respect to the same systems at room temperature have been observed. The analysis of the SAXS spectra leads to the hypothesis that at 77 K the closed spherical structure of reverse micelles is retained and that during the temperature quench they undergo a clustering process involving the transition from a quite random dispersion to the formation of more or less large clusters of strongly packed reverse micelles. This behavior is attributed to competitive effects caused by the temperature decrease. The prevalence of intermicellar attractive interactions with respect to Brownian motions leading to a collapse to more compact structure is in competition with the rapid decrease of reverse micelle diffusion rate involving a freezing of the local structures. In the case of cobalt, ytterbium, and cobalt/ytterbium bis(2-ethylhexyl)sulfosuccinate reverse micelles, further information from EXAFS measurements indicates that within the reverse micelle core exists a quite ordered nanosized domain composed of water, surfactant counterions, and oxygen atoms of the SO3- head groups. The conservation of local order and inverse structure during the clustering phenomenon that results from the fast freezing with liquid nitrogen of solutions of reverse micelles could have biological implications, i.e., the preservation of tissue samples at cryogenic temperatures.     

Interaction between hydrogen peroxide and cobalt(II) acetylacetonate in the system of reverse micelles of triton X-100 nonionic surfactant in cyclohexane

The yield of free radicals upon the decomposition of hydrogen peroxide catalyzed by cobalt acetylacetonate (Co(acac)₂) in the systems of reverse micelles of TX-100/n-hexanol and AOT in cyclohexane at 37°C was studied with the inhibitor method using a stable nitroxyl radical as a spin trap. It is shown that, in micellar AOT solutions in cyclohexane as well as in n-decane, H₂O₂ and Co(acac)₂ in practice do not react, because H₂O₂ is localized in a micelle water pool and Co(acac)₂, in the organic phase. Therefore, the generation of radicals is not observed in AOT solutions in cyclohexane, whereas, in aqueous solution, Co(acac)₂ catalyzes the radical decomposition of H₂O₂. In the system of mixed reverse micelles of TX-100 and n-hexanol in cyclohexane, at equal overall concentrations of H₂O₂ and Co(acac)₂, the rate of radical formation is much higher than in aqueous solution; i.e., the micellar catalysis of the radical decomposition of H₂O₂ takes place. It follows from measurements of UV and ESR spectra and the kinetics of changes in the content of peroxides in the reaction mixture that TX-100 and n-hexanol react with free radicals formed upon H₂O₂ decomposition and with atmospheric oxygen.     

AOT/Triton X-100混合反胶束体系的导电性能

采用聚乙二醇辛基苯基醚(TritonX-100)和二(2-乙基己基)琥珀酸磺酸钠(AOT)双表面活性剂,与正己烷、正己醇和水构成混合反胶束体系;研究了表面活性剂质量比、助表面活性剂含量、水油体积比和温度等因素对反胶束体系导电性能的影响,同时采用循环伏安法研究了K3Fe(CN)6/K4Fe(CN)6在该体系中的电化学行为.结果表明:由两种表面活性剂构成的反胶束体系电导率σ明显大于单一表面活性剂反胶束体系电导率;体系电导率随AOT与TritonX-100的质量比w(w=mAOT∶mTritonX-100)的变化而变化,w为0-0.4时,电导率随w增大而线性增大,之后增加趋势变缓;w=0.96时,σ达到稳定值576μS·cm-1.混合体系电导率随溶水量的增大及温度的上升而提高;而增加助表面活性剂可显著降低体系的电导率.在所研究体系中,Fe(CN)36-/Fe(CN)46-电化学反应对的氧化还原峰电位几乎不随扫描速率变化,峰电位差约为75mV,峰电流的比值约为1,氧化峰电流与扫描速率的平方根成正比,说明K3Fe(CN)6/K4Fe(CN)6在混合反胶束体系中显示出良好的氧化还原可逆性,反应由扩散步骤控制.     

Anodic film formation on osmium electrodes in strong acid solutions: I. Voltammetric studies

Anodic film formation and reduction on electrodeposited Os was studied in strong HCl and HClO₄ solutions. Significant voltammograms were obtained only after long pre-reduction of the electrode and at slow potential sweeps. The potentiodynamic curves reveal strong adsorption of oxygen with formation of OsO₂ and further oxidation to OsO₄ which goes into solution. In strong HCl solutions a corrosion step at high potential with formation of OsO₂Cl₄^2? is detected. The influence of scan rate on peak potential and current and the charge capacity of the electrode surface was investigated. Anion adsorption and increase in acid concentration was found to reduce the extent of the anodic film. The results are discussed in terms of the formation of thick surface oxides and compared with those for Ir and Ru.     

Influence of Confined Water on the Photophysics of Dissolved Solutes in Reverse Micelles

The photophysical parameters of two probes with largely different hydrophobic character, namely, coumarin 1 and coumarin 343, are investigated in sodium bis‐(2‐ethylhexyl)sulfosuccinate (AOT)/hexane/water reverse micelles at various water/AOT molar ratio w₀. Correlation of photophysical parameters such as fluorescence quantum yield, fluorescence lifetime, and emission maxima with w₀ indicate distinctly different trends below and above w₀≈7 for both probes. The variation of the average rotational correlation times obtained from fluorescence anisotropy decays for both probes in reverse micelles further corroborate the above observation. Similar studies were also performed in nonaqueous reverse micelles with acetonitrile as polar solvent. Similar to aqueous reverse micelles, breaks in the photophysical parameters with increasing acetonitrile/AOT molar ratios w′₀ were also observed in these cases, although at a much lower w′₀ value of 3. The present results indicate that around w₀≈7 for aqueous reverse micelles (and around w′₀≈3 for nonaqueous reverse micelles) a distinct change occurs in the probe microenvironment, which is rationalized on the basis of the relative populations of interfacial and core water. We propose that until the ionic head groups and counterions are fully solvated by polar solvents, that is, up to w₀≈7 (or w′₀≈3), the interfacial water population dominates. Above these molar ratios coalescence of excess water molecules with each other to form truncated H‐bonded water clusters leads to a sizable population of core water. This is further substantiated by changes in the IR absorption spectra for the O? D stretching mode of diluted D₂O in reverse micelles with varying w₀. Critical comparison of the present results with relevant literature reports provide clear support for the proposals made on water structure in reverse micelles. The role of relative size of the probe and the reverse micelles for differences in polar solvent to AOT ratios (w₀=7 and w′₀=3) in the observed breaks in the two types of reverse micelles is also discussed.     

Self-assembly of reverse micelle nanoreactors by zwitterionic polyoxometalate-based surfactants for high selective production of β‑hydroxyl peroxides

Surfactants with polyoxometalates (POMs) as polar head groups have shown fascinating self-assembly behaviors and various functional applications. However, self-assembly them into reverse micelles is still challenging owing to the large molecular size and intermolecular strong electrostatic repulsions of POM heads. In this work, a zwitterionic POM-based surfactant was synthesized by covalently grafting two cationic long alkyl tails onto the lacunary site of [PW₁₁O₃₉]^7?. With decreased electrostatic repulsions and increased hydrophobic effect, the POM-based reverse micelles with an average diameter of 5 nm were obtained. Interestingly, when these reverse micelles were applied for catalyzing the oxidation of styrene, an unprecedented β?hydroxyl peroxide compound of 2?hydroxyl-2-phenylethan-1?tert-butylperoxide was produced in high selectivity of 95.2%. In comparison, the cetyltrimethylammonium electrostatically encapsulated POMs mainly generated the epoxides or 1,2-diols. A free radical mechanism was proposed for the oxidation reaction catalyzed by the zwitterionic POM surfactants.     

Aggregation Behaviors of Tricarbocyanine Dye in Water and in AOT Reverse Micelles

The photophysical property of the tricarbocyanine dye IR144 has been extensively studied in non‐aqueous solvents. However, as a potential near‐infrared biomedical imaging probe, the photophysical property of IR144 in water is still little known. So, the aggregation behaviors of IR144 in water with steady‐state absorption spectroscopy and integrated polarization dependent femtosecond pump‐probe spectroscopy were investigated. Through comparing the absorption spectral bandshape of IR144 in water and in water pool of AOT reverse micelles, It is found that IR144 form dimer aggregates in water even at very low concentration (<1.0×10^?7 mol·L^?1). And the absorption spectrum of the IR144 aggregates always displays a bimodal feature, which is independent of the dye concentration ranging from 1.0×10^?7 to 1.0×10^?4 mol·L^?1. For better understanding the aggregation behaviors of IR144 in water, we measured the ground state recovery kinetics and the reorientation kinetics of IR144 in water and in water pool of AOT reverse micelles (W₀=[H₂O]/[AOT], W₀=40). It is found that the fluorescence quantum yield of IR144 in water is lower than that in water pool of AOT reverse micelles, and the reorientation time of IR144 in water is slower than that in water pool of AOT reverse micelles. Those kinetic measurements also verify that IR144 exists as dimer aggregates in water.     

Highly viscoelastic reverse worm-like micelles formed in a lecithin/urea/oil system

While lecithin alone can form spherical or ellipsoidal reverse micelles in oil, we found that urea can promote the growth of lecithin reverse worm-like micelles in oil. In a mixed system of urea and lecithin, the urea binds to the phosphate group of lecithin, thus reducing the interface curvature of the molecular assembly and inducing the formation of reverse worm-like micelles. The regions in which these micelles form increased with lecithin concentration. In addition, the zero-shear viscosity (η ₀) of the reverse worm-like micelles rapidly increased upon the addition of urea, reaching a maximum of 2 million times the viscosity of n-decane. We examined the change in η ₀ in detail by performing dynamic viscoelasticity measurements. Values for η ₀ increased with urea concentration because the disentanglement time of reverse worm-like micelles increased with micellar growth.     

Humidity dependence of carbon dioxide generation during photodegradation of biaxially oriented polypropylene in oxygen

UV generation of CO₂ gas from a range of biaxially oriented polypropylene (BOPP) films exposed in oxygen gas has been measured by in situ infrared spectrometry and compared with the development of absorption at 1713 cm⁻¹ due to carbonyl formation in the same films. As in studies of other polyalkene films, the CO₂ measurements, which take only 3 h, correlate well with carbonyl development measurements that require hundreds of hours.Upon UV irradiation of BOPP in dry oxygen an induction time of ∼60 min preceded a linear increase of photogenerated CO₂. In wet oxygen, no noticeable induction time preceded the linear increase of CO₂. The rate of CO₂ increase was greater when the oxygen was humidified. This pattern was observed for four different types of film and for films of different thickness. The increase in the rate of CO₂ photogeneration with increased humidity was greater for thicker films. The correlation between the amount of CO₂ generated and the strength of the carbonyl absorption developed under standard conditions was better for BOPP oxidation in dry than in humidified oxygen. The inclusion of recycled polymer caused an increase in the rate of photodegradation.Although measurements on each subset of films demonstrated a clear increase in the photogeneration of CO₂ at high humidity, the mechanism by which the CO₂ generation from BOPP is enhanced remains unclear and further work is necessary to resolve this puzzle.     

Kinetics and Mechanism of low-Temperature Oxidation of H2S with Oxygen in the gas Phase

The possibility of the formation of polysulfides during oxidation of H₂ S with oxygen on oxide catalysts has been checked, and the sequence of the reaction stages at temperatures below the sulfur dew point determined. The amount of polysulfides formed during H₂ S oxidation has been found to exceed significantly that obtained in the reaction of sulfur with H₂ S. Polysulfides are concluded to be intermediates in H₂ S oxidation to sulfur. The rate of formation of SO₂ from sulfur vapor is shown to be negligibly low at 100-200°C. A reaction scheme involving the formation of sulfur from polysulfides and the formation of sulfur dioxide by direct oxidation of H₂ S is suggested.     

Reducing the Cost and Preserving the Reactivity in Noble‐Metal‐Based Catalysts: Oxidation of CO by Pt and Al–Pt Alloy Clusters Supported on Graphene

The oxidation mechanisms of CO to CO₂ on graphene‐supported Pt and Pt‐Al alloy clusters are elucidated by reactive dynamical simulations. The general mechanism evidenced is a Langmuir–Hinshelwood (LH) pathway in which O₂ is adsorbed on the cluster prior to the CO oxidation. The adsorbed O₂ dissociates into two atomic oxygen atoms thus promoting the CO oxidation. Auxiliary simulations on alloy clusters in which other metals (Al, Co, Cr, Cu, Fe, Ni) replace a Pt atom have pointed to the aluminum doped cluster as a special case. In the nanoalloy, the reaction mechanism for CO oxidation is still a LH pathway with an activation barrier sufficiently low to be overcome at room temperature, thus preserving the catalyst efficiency. This provides a generalizable strategy for the design of efficient, yet sustainable, Pt‐based catalysts at reduced cost.     

Specific Behavior of Hydrogen Peroxide in a System of AOT Reverse Micelles

It is revealed that, in contrast to organic hydroperoxides, hydrogen peroxide (H₂O₂) is rapidly decomposed in a system of reverse micelles of sodium bis(2-ehylhexyl)sulfosuccinate (AOT) in n-decane. The yield of free radicals upon the decomposition of H₂O₂ in a system of reverse micelles upon the interaction between AOT and cobalt acetyl acetonate (Co(acac)₂) is studied by the inhibitor method using an original spin trap. It is established that the interaction between H₂O₂ and AOT proceeds with no radical formation. Co(acac)₂ catalyses the radical decomposition of H₂O₂ in an aqueous solution. In micellar AOT solutions in n-decane, H₂O₂ and Co(acac)₂ in practice do not react, because H₂O₂ is localized in a micelle water pool, Co(acac)₂ and the spin trap, in the organic phase. In this case, the generation of radicals is not observed.     

Anodic film formation on osmium electrodes: Part II. Galvanostatic and potentiostatic measurements

The formation and reduction of anodic films on Os electrodes in 2 M HCl and HClO₄ solutions were studied by anodic and cathodic charging curves. The galvanostatic oxidation of Os in HClO₄ shows the formation of OsO₂ as an intermediate step to OsO₄ that goes in the solution. The cathodic charging curves at Os electrodes previously oxidized at constant potential reveal the anodic film to be made up of a reversibly desorbed oxygen layer and an oxide phase reduced irreversibly. Both layers increase with time under potentiostatic conditions following a logarithmic equation until a constant value is reached. At all times, the content of OsO₂ in the anodic film at high potentials is larger than that of chemisorbed oxygen.In HCl solutions only the reversible reduction of an oxygen layer is observed. The growth of this film also complies with a direct logarithmic law before attaining a limiting coverage. The charge involved in the reduction increases linearly with the potential at a given time of formation. The results are discussed in terms of a Temkin-type isotherm and a place-exchange mechanism.     

Photoinduced Oxidation of Sterically Hindered Amines in Acetonitrile Solutions and Titania Suspensions (An EPR Study)

The reactions of sterically hindered amines (SHA) were investigated in acetonitrile solutions and TiO₂ suspensions upon exposure to monochromatic radiation, λ = 365 nm, by means of in situ EPR spectroscopy. The formation of singlet oxygen, as one of the possible oxidation agents for SHA, in these systems is affected significantly by solvent used and the experimental conditions. Experiments in homogeneous media evidenced alternative pathways for the SHA oxidation with a variety reactive oxygen species involved. In anhydrous acetonitrile solutions containing KO₂, the SHA oxidation was negligible not only in the dark but also on continuous exposure. However, the presence of water, even at low concentrations, led to the transformation of O₂^?? to singlet oxygen and hydrogen peroxide, which served as a source of hydroxyl radicals. These species participated in oxidation of SHA resulting in the generation of nitroxide radicals. To investigate the influence of different competitive reactions of SHA with other ROS formed upon TiO₂ photoexcitation, a series of experiments using different additives (e.g. KO₂, H₂O₂, NaN₃, dimethylsulfoxide, methanol as organic cosolvents) under air or argon were performed. The detailed analysis of paramagnetic intermediates formed upon the irradiation of the studied systems was accomplished using EPR spin trapping technique.     

Gold clusters and nanoparticles in reverse micelles formed by tritons X-100, X-114, and X-45

The formation of gold nanoparticles and clusters in solutions of reverse micelles formed by Tritons X-45, X-114, and X-100 (polyoxyethylene isooctylphenyl ethers with degrees of polymerization n = 5, 7–8 and 9–10, respectively) in the presence of sodium sulfite is studied. The stability of these particles is shown to depend on the length of ethylene oxide chains in surfactant molecules, water content in micelles, and the presence of special additives. It is found that, on one hand, L-ascorbic acid favors a reduction of HAuCl₄ and, on the other hand, affects the stability of resultant particles.     

Micropolarity of sodium bis(2-ethylhexyl) sulfosuccinate reverse micelles prepared in supercritical ethane and near-critical propane

The micropolarity of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles prepared in supercritical ethane and near-critical propane has been determined in terms of a solvent polarity parameter, E _T(30) values, by using absorption probes, 1-ethyl-4-methoxycarbonyl pyridinium iodide and 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate as a functions of pressure and the molar ratio of water to AOT, W ₀, at a constant temperature of 310 K. The micropolarity comparable to that of ethanol was observed for reverse micelles containing water of W ₀ = 2. The micropolarity increased with the water content and became independent of pressure after the system changed to a one-phase reverse micelle solution. For a given W ₀ value, no difference in the micropolarity was noticed in the micelles prepared in ethane and propane. Phase behaviour investigations have revealed that complete dissolution of 50 mM AOT occurred at 20 MPa in supercritical ethane, while a much lower pressure of 1 MPa was required in near-critical propane. The amount of water solubilized in reverse micelles formed in supercritical ethane was relatively low, reaching a W ₀ value of 7 at 36 MPa. In contrast, the amount of water solubilized in near-critical propane reverse micelles was W ₀ = 11 at a much lower pressure of 6 MPa. A higher pressure was required to solubilize larger amount of water in reverse micelles prepared in both ethane and propane. Received: 9 October 1998 Accepted in revised form: 12 February 1999     

Kinetics and mechanism of oxidation of dimethyl sulfide with molecular oxygen catalysed by K[RuIII(EDTA-H)Cl] complex in water-dioxan medium

Oxidation of dimethyl sulfide (Me₂S) with molecular oxygen catalysed by [Ru^III(EDTA)(H₂O)]⁻1a (EDTA = ethylenediaminetetraacetate anion) was studied spectrophotometrically in water-dioxan medium at constant pH 5.0 (acetic acid-acetate buffer) and ionic strength 0.2 M (KCl). The reaction proceeds through the formation of a [Ru^III(EDTA)(Me₂S)]⁻2 intermediate which undergoes oxidation with molecular oxygen to give dimethyl sulfoxide (Me₂SO) as the oxidation product. The rate of formation of 2 and its decomposition was followed spectrophotometrically by monitoring the reactions at 528 nm the characteristic peak of 2. The rate of formation of 2 was found to be first order in the concentrations of 1a and Me₂S. The rate of decomposition of 2 is independent of the concentration of Me₂S and is half-order with respect to oxygen concentration. Both the formation and decomposition reactions of 2 were studied at different temperatures, and the activation parameters ΔH≠ and ΔS≠ were determined. A suitable mechanism was proposed for the catalytic oxidation of dimethyl sulfide to dimethyl sulfoxide with molecular oxygen.     

Influence of negatively charged interfaces on the ground and excited state properties of methylene blue

Properties of the ground and excited states of methylene blue (MB) were studied in negatively charged vesicles, normal and reverse micelles and sodium chloride solutions. All these systems induce dimer formation as attested by the appearance of the dimer band in the absorption spectra (lamdaD approximately 600 nm). In reverse micelles the dimerization constant (KD) corrected for the aqueous pseudophase volume fraction is two-three orders of magnitude smaller than KD of MB in water, and it does not change when W0 is increased from 0.5 to 10. Differences in the fluorescence intensity as a function of dimer-monomer ratio as well as in the resonance light scattering spectra indicate that distinct types of dimers are induced in sodium dodecyl sulfate (SDS) micelles and aerosol-OT (sodium dioctyl sulfoxinate, AOT) reversed micelles. The properties of the photoinduced transient species of MB in these systems were studied by time-resolved near infrared (NIR) emission (efficiency of singlet oxygen generation), by laser flash photolysis (transient spectra, yield and decay rate of triplets) and by thermal lensing (amount of heat deposited in the medium). The competition between electron transfer (dye*-dye) and energy transfer (dye*-O2) reactions was accessed as a function of the dimer-monomer ratio. The lower yield of electron transfer observed for dimers in AOT reverse micelles and intact vesicles compared with SDS micelles and frozen vesicles at similar dimer-monomer ratios is related with the different types of aggregates induced by each interface.