Letter to the Editors Fullerene C 60 as an inhibitor of high temperature lipid oxidation Robert Czochara, Przemyslaw Ziaja, Piotr Piotrowski, Rafal Pokrop, Grzegorz Litwinienko * Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland ARTICLE INFO Article history: Received 2 February 2012 Accepted 1 March 2012 Available online 9 March 2012 ABSTRACT Non-isothermal oxidation of a model saturated lipid (stearic acid) in the bulk phase in the presence of fullerene C 60 at concentrations of 1–8 mM was monitored by differential scan- ning calorimetry in order to determine the kinetic parameters that describe the global first order oxidation process. The activation energies, pre-exponential factors, and rate con- stants calculated by the Ozawa–Flynn–Wall method for the first detectable exothermic effect of oxidation, indicate the antioxidant behavior of C 60 over a wide range of tempera- tures that makes it a non-corrosive agent effectively increasing the oxidative stability of lubricants and biolubricants. Ó 2012 Elsevier Ltd. All rights reserved. Autoxidation of hydrocarbons (RH) is a chain process mediated by free radicals. Formation of hydroperoxides (ROOH) occurs during a propagation step – two consecutive processes: oxygen addition to an alkyl radical, R Å +O 2 ! ROO Å (k  10 9 M 1 s 1 ) and hydrogen atom abstrac- tion from another RH, i.e., ROO Å + RH ! ROOH + R Å (k p from 0.1 to 10 2 M 1 s 1 , see Supplementary material for detailed description of the kinetics of the process). At higher tem- peratures, the reaction chain can be branched and new R Å , RO Å , ROO Å and HO Å radicals are generated. Autoxidation can be inhibited by the addition of small amount of com- pounds able to break the propagation chain and many nat- ural and synthetic phenols, amines and hydrocarbons (carotenoids) acting in this way are called chain-breaking antioxidants. Fullerenes are considered as ‘‘radical sponges’’ [1,2], and there is a number of evidences that fullerenes and their derivatives inhibit radical mediated processes in living systems [3,4]. Few of these reports concern the role of microenvironment (homo/heterogenic), temperature, partial pressure of oxygen on the activity of antioxidant and, in general, a mechanism of such interactions is not recognized. Detailed kinetic studies in model systems indi- cated that C 60 alone is not a powerful scavenger of peroxyl radicals [5,6]. Thus, we decided to study the antioxidant activity of C 60 under more extreme conditions, in bulk phase and in non-isothermal mode. In this letter we report how differential scanning calorimetry (DSC) can be used to obtain detailed information about antioxidant properties of fullerene C 60 and oxidative stability of the bulk lipid sys- tems (biolubricants) containing fullerene. Conditions of such measurements correspond to conditions of exploita- tion of oils and lubricant working sometimes in high tem- peratures. Recently, a great emphasis is stressed on novel materials that are friendly for environment due to biode- gradability and biolubricants which are based on modified renewable oils are becoming efficient replacements for con- ventional lubricants. Unfortunately, vegetable oils are more prone to oxidation process. Since market of biolubricants is still increasing, the idea of non-toxic and non-aggressive additives efficiently protecting biolubricants is worth to be studied in the context of development of new environment friendly materials. 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.03.002 * Corresponding author: Fax: +48 22 822 23 80. E-mail address: litwin@chem.uw.edu.pl (G. Litwinienko). CARBON 50 (2012) 3943 – 3960 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon