In biodiesel and fossil diesel blends, highly viscous phase-separated products were generated as the oxidation reaction proceeded, but there is no previous research on this phenomenon. In this study, the phase-separated products were characterized by various chemical analyses. In addition, oxidative degradation tests were conducted using binary mixtures of n-heptadecane and methyl linoleate. The results showed that the phase-separated products were mainly oxidation products from fatty acid methyl esters with carboxyl groups. It is thought that the formation of carboxyl groups by oxidation increased the polarity and cause phase separation from the hydrophobic fossil diesel. It was also suggested that alkanes in fossil diesel, which are inherently stable, contribute to the radical chain oxidation of biodiesel.
