Radical scavenging efficiency of different fullerenes C60-C70 and fullerene soot |
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Authors: | Eldar B Zeynalov Nazilya I Salmanova |
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Institution: | a National Academy of Sciences of Azerbaijan, Institute of Petrochemical Processes, Khojaly Avenue. 30, Baku Az1025, Azerbaijan b Biology, Chemistry and Health Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK c Azerbaijan State Oil Academy, Azadlig Avenue 20, Baku Az1010, Azerbaijan |
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Abstract: | This investigation was undertaken to determine the antioxidant activity of a range of fullerenes C60 and C70 in order to rank them according to their comparative efficiency. The model reaction of initiated (2,2′- azobisisobutyronitrile, AIBN) cumene oxidation was used to determine rate constants for addition of radicals to fullerenes. Measurements of oxidation rates in the presence of different fullerenes showed that the antioxidant activity as well as the mechanism and mode of inhibition were different for fullerenes C60 and C70 and fullerene soot. All fullerenes - C60 of gold grade, C60/C70 (93/7, mix 1), C60/C70 (80 ± 5/20 ± 5, mix 2) and C70 operated as alkyl radical acceptora, whereas fullerene soot surprisingly retarded the model reaction by a dual mode similar to that for the fullerenes and with an induction period like many of the sterically hindered phenolic and amine antioxidants. For the C60 and C70 the oxidation rates were found to depend linearly on the reciprocal square root of the concentration over a sufficiently wide range thereby fitting the mechanism for the addition of cumylalkyl radicals to the fullerene core. This is consistent with literature data on the more ready and rapid addition of alkyl and alkoxy radicals to the fullerenes compared with peroxy radicals. Rate constants for the addition of cumyl radicals to the fullerenes were determined to be k(333K) = (1.9 ± 0.2) × 108 (C60); (2.3 ± 0.2) × 108 (C60/C70, mix 1); (2.7 ± 0.2) × 108 (C60/C70, mix 2); (3.0 ± 0.3) × 108 (C70), M−1 s−1. The increasing C70 constituent in the fullerenes leads to a corresponding increase in the rate constant.The fullerene soot inhibits the model reaction according to the mechanism of trapping of peroxy radicals; the oxidation proceeds with a pronounced induction period and kinetic curves are linear in semi-logarithmic coordinates.For the first time the effective concentration of inhibiting centres and inhibition rate constants for the fullerene soot have been determined to be fnC60−soot] = (2.0 ± 0.1) × 10−4 mol g−1 and kinh = (6.5 ± 1.5) × 103 M−1 s−1 respectively.The kinetic data obtained specify the level of antioxidant activity for the commercial fullerenes and scope for their rational use in different composites. The results may be helpful for designing an optimal profile of composites containing fullerenes. |
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Keywords: | Model oxidation Fullerene Fullerene soot Kinetics Oxidation rate Induction period |
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