ELSEVIER Thermochimica Acta 307 (1997) 97-106 thermochimica acta Effects of selected phenol derivatives on the autoxidation of linolenic acid investigated by DSC non-isothermal methods Grzegorz Litwinienko*, Teresa Kasprzycka-Guttman, Maciej Studzinski Department of Chemistry, University of Warsaw, 02-093 Warsaw, Pasteura 1, Poland Received 29 April 1997; received in revised form 25 August 1997; accepted 8 September 1997 Abstract The kinetic features of the thermoxidation of linolenic acid (LNA) was investigated under non-isothermal conditions (50-- 300°C) in the presence of the phenolic compounds: o-cresol, p-cresol, 2,4-dimethylphenol, 2,5-dimethylphenol, 3,5- dimethylphenol, 2,4,6-trimethylphenol and 2,6-di-t-butyl-4-methylphenol at concentrations 0.26--20 mmol of compound per mol of LNA. Temperatures of reaction onset at different heating rates were used to evaluate kinetic parameters of linolenic acid thermoxidation by the Ozawa-Flynn-Wall method. Activation energy (Ea) of the pure linolenic acid thermoxidation was 70.4±0.5 kJ/mol and at the tested concentrations range of the phenolic compound, Ea increased maximally to 95.8±5.1 kJ/ mol for 2,6-di-t-butyl-4-methylphenol, 101.1±3.0kJ/mol for o-cresol, 90.9±l.5kJ/mol for 2,4,6-trimethylphenol, 89.9+4.4 kJ/mol (p-cresol), 80.7±4.5 kJ/mol (2,5-dimethylphenol) 86.1±3.6 kJ/mol (3,5-dimethylphenol) and 75.1-t-3.0 kJ/ mol for 2,4-dimethylphenol. At concentrations of the phenolic compound greater than 10 mmol per mol of LNA, a decrease of inhibitory effect was observed. © 1997 Elsevier Science B.V. Keywords: Activation energy; Antioxidants; DSC; Phenolic compounds; Linolenic acid 1. Introduction Atmospheric oxygen is the most accessible and economically important oxidising agent in many che- mical processes in the petroleum and fat industries. The production of large quantities of organic chemi- cals, drying oils and modified polymers is based on the controlled oxidation of hydrocarbons. However, auto- xidation is an undesirable process since rubber, plas- tics, fuel oil, fat and materials containing fat can undergo autoxidative deterioration or rancidification [1]. Unfavourable effects of oxidation are also *Corresponding author. Fax: 0048 22 8225996; e-mail: litwin @chem.uw.edu.pl 0040-6031/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0040-6031(97)00366-3 observed in biological systems, where pathological events such as various diseases, cancer and ageing are possibly caused by free radical oxidation of polyun- saturated fatty acids, natural phospholipid membranes and nucleic acids. The autoxidation of lipids and other oxidizable substrates is a free-radical chain reaction and after initiation (catalysed by light, transition metal ions or water molecules), propagation of free hydro- peroxy radicals occurs. This process can be repre- sented by reaction: ROO ° + RH + 02 -~ ROOH + ROO" (1) An induction period is observed on the addition of certain phenols [2], aminophenols [3] or carotenoids [4], known as antioxidants, during the oxidation of