Synergism between phenolic antioxidants in autoxidation

Abstract

Combinations of two types of phenolic antioxidants were examined to assess synergistic or antagonistic interaction from both radical scavenging rate constant (kinh) and radical scavenging number (n). The synergism of two types of phenols was observed especially with comparatively lower and similar oxidation potentials. In synergism of kinh, a phenol with lower oxidation potential simply acts as a peroxy radical scavenger, whereas the other phenol with higher oxidation potential acts as a hydrogen donor in the regeneration of the former phenol from its phenoxy radical. On the other hand, synergism of n is seen for less or non-hindered phenols, especially with 4-methoxy substituent, and is controlled by the reaction conditions, such as solvent polarity. This is interpreted as a molecular association of phenols formed through hydrogen bonds and hydrophilic interactions, resulting in obstruction of coupling of the phenoxy radicals. As a result, 3,5-di-t-butyl-4-hydroxy-toluene (BHT) and 2-t-butyl-4-methoxyphenol (3 : 2 by mole ratio), and BHT and 4-methoxyphenol (3 : 2 by mole ratio) were estimated to show total synergism of 1.41 and 1.35 times, respectively, compared with the simple sum of the antioxidant activities in a hydrophobic system, such as chlorobenzene. These results are important for reduction of costs as well as environmental pollution by allowing decreased use of phenols.