Interfacial Performance in Supramolecular Oxidation
According to the association colloids hypothesis, reverse micelles are efficient nano-reactors that significantly alter the chemical reaction rate between water- and oil-soluble components by creating the water-oil interface. The LOOH molecules are surface-active and tend to migrate to the water-oil interface of reverse micelles. Therefore, the accessibility of surface-active phenolic antioxidants to reactive free radicals increases with partitioning at the water-oil interface created by reverse micelles (Chen et al., 2010).
As shown in Table 4, the water content in the oil treatments increased during the oxidation steps (0.02-0.1%). The increase in water content is due to the decomposition of LOOH molecules during lipid peroxidation. In the initiation phase, LOOH molecules take part in the monomolecular decomposition reactions (LOOH + LH\(\rightarrow\) LO• + L• + H2O), while in the propagation phase, extensive micelle collisions increase the bimolecular decomposition reaction of LOOH (2LO2H \(\rightarrow\) LO2• + LO• + H2O) (Budilarto and Kamal‐Eldin, 2015a). The LOOH reverse micelles size, and interfacial tension increased in parallel with the increase amount of water during lipid oxidation phases. As the amount of water and LOOH molecules increases, the reverse micelles grow in size and number until their concentration is high enough to flocculate as mesophases, which are then disintegrated and decomposed to form a range of secondary oxidation products in the final oxidation steps (Budilarto and Kamal‐Eldin, 2015b).
As can be seen in Table 4, with increasing of LOOH molecules, the interfacial tension of purified soybean oil decreased from 36.02 to 33.26 mN/m at the induction period (IP), then decreased from 33.26 to 30.19 mN/m at IPm due to the intensification of the production of LOOH which are incorporated into the reverse micelles. Finally, the interfacial tension at Pe decreased from 30.19 to 24.71 mN/m due to the flocculates breakdown that causing to release of LOOH molecules along with numerous secondary oxidation products. The addition of phenolic antioxidants was significantly decreased the interfacial tension of bulk oils. Due to the effect of steric structure on amphiphilic properties, the levels of interfacial tension at t = 0 in bulk oils containing gallic acid alkyl ester derivatives were lower than the gallic acid. After that, the values were more steeply increased in parallel with the progress of lipid oxidation. The results clearly showed that esterification with the aim of increasing the alkyl chain length had a nonlinear impact on the amphiphilic property of alkyl gallates, which was in agreement with Elder et al. (). The significantly increased levels of interfacial tension in the presence of phenolic antioxidants, causing to the formation of mixed reverse micelles of a quite larger droplet size compared to the reverse micelles only consisting of LOOH molecules, which was in accordance with the results reported by Mansouri et al. (2020). The quite higher interfacial tension for methyl gallate at IP can also confirm its higher ability in packing the free LOOH molecules as organized reveres micelles. Furthermore, the soybean oil containing methyl gallate had lower values of the CMC and larger micellar size, which confirms the methyl gallate, could more effectively decrease the concentration of free LOOH molecules and inhibit them in an interfacial way.
As shown in Table 4, the p -anisidine value (AV), which is a useful index for measuring secondary oxidation products, in all of the oil samples increased during initiation and propagation stages much more slowly than the LOOH. The protective impact of alkyl gallates except dodecyl and stearyl gallates against the production of carbonyl compounds was much better than the gallic acid.
In general, considering the water content, micellar size, and interfacial tension results, the antiradical potency of the phenolic inhibitors was significantly promoted by increasing the length of the alkyl chain (until a critical chain length) due to the increased interfacial activity. The interfacial performance of antioxidative compounds in supramolecular oxidation of bulk oil was in the order of methyl gallate > propyl gallate > octyl gallate > gallic acid > dodecyl gallate > stearyl gallate, which was completely in line with the values of the AV, CMC (Table 3), and the corresponding physical parameters shown in Table 4. The reason that methyl, propyl, and octyl gallates (hydrophobic antioxidants) were more effective than gallic acid (hydrophilic antioxidant) in bulk oil could be due to their increased partitioning at the water-oil interface created by reverse micelles and, thereby, increasing the accessibility of antioxidative compounds to reactive free radicals.