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Polar paradox revisited: analogous pairs of hydrophilic and lipophilic antioxidants in linoleic acid emulsion containing Cu(II)


Correspondence to: Reşat Apak,Chemistry Department, Faculty of Engineering, Istanbul University, Avcilar 34320 Istanbul, Turkey. E-mail:



Literature data are scarce on the activities of analogous pairs of hydrophilic and lipophilic antioxidants related to the ‘polar paradox’ distinguishing antioxidants based on their partitioning between lipids and water. The peroxidation of linoleic acid (LA) in the presence of either Cu(II) ions alone or Cu(II) ions combined with Trolox (TR), ascorbic acid (AA), hydroquinone (HQ) and gallic acid (GA), as hydrophilic antioxidants, or with α-tocopherol (TocH), ascorbyl palmitate (AP), tert-butyl hydroquinone (TBHQ) and propyl gallate (PG), as their respective lipophilic analogues, was investigated in aerated and incubated emulsions at 37 °C and pH 7.


LA peroxidation induced by Cu(II) followed pseudo-first-order kinetics with respect to the formation of primary (hydroperoxides) and secondary (aldehyde- and ketone-like) oxidation products, which were determined by ferric thiocyanate (Fe(III)-SCN) and thiobarbituric acid-reactive substances (TBARS) methods respectively. With the exception of TocH at certain concentrations, the tested compounds showed antioxidant behaviour depending on their polarities. The results were evaluated in the light of structure–activity relationships and the polar paradox.


The results of this study partly confirm the hypothesis that the polar paradox experiences limitations in oil-in-water emulsions and that its validity is also dependent on the concentrations of the antioxidants employed. © 2013 Society of Chemical Industry