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.