## 3 Results and Discussion
### Degradation Behaviour Identified by Advanced Analytical Methods
Most probably, cleavage of the C-O ester bond and the combination with a CH\({}_{3}\)CO moiety results in the identified ester. The CH\({}_{3}\)CO moiety is most possibly derived by the cleavage of the ethyl side chain of 2-ethylhexanol and subsequent oxidation. Summarizing the above described results of the neat base oil, relevant fragments of the oxidation mechanism could be identified: preferential sites of cleavage are between the C-O ester bond, resulting in products such as 2-ethylhexyl alcohol (d) and 2-ethylhexyl fatty acid esters with various acid chain lengths (e, g-j), and a bond cleavage between the O atom and C1 of the alcohol, providing 2-ethylhexanoic acid (f). Such a structure can be further oxidized to 3-heptanone (c) and, after a second cleavage step, to propionic and butanoic acid (a, b). For one of the main degradation products, adipic acid (k), the two \({}^{18}\)O atoms are present in each of the ester bonds (C-\({}^{18}\)O), whereas \({}^{16}\)O was  
Figure 9: Positive ion mass spectra of 2-ethylhexyl acetic acid ester (g) obtained after alteration under \({}^{16}\)O\({}_{2}\) (**A**) and \({}^{18}\)O\({}_{2}\) (**B**) atmosphere, respectively. Peaks of fragments where a change of the \(m/z\) value due to isotope labelling occurred are labelled with the respective values. A simplified oxidation reaction is suggested for the respective molecule. found in the carbonyl groups (C=\({}^{16}\)O) of the acid chain of the initial ester. Hence, a combination of oxidative cleavage of the C-O ester bond and subsequent acid formation occurs on both sides. Moreover, the highly abundant 2-ethylhexyl 2-oxobutyl adipate (m) results from bond dissociation and oxidation between C2 and C3 of the alcohol chain, whereas the rest of the initial molecule stays intact. The formation of short-chain fatty acids as found in esters (e, g-j) is assigned to the cleavage and oxidation of 2-ethylhexyl alcohol. In contrast to the expected oxidation mechanism, formic acid (in e) and pentanoic acid (in j) do not carry \({}^{18}\)O. Furthermore, the oxidation mechanism of 2-ethylhexyl 6-oxohexanoate (l) could not be revealed. An alteration of the neat base oil was not only performed at 1 bar, but also at a pressure of 2 bar, to estimate the influence of pressure on the formation of degradation products. Figure S3 illustrates chromatograms of alteration under 1 bar \({}^{16}\)O\({}_{2}\) atmosphere (A) and 2 bar \({}^{16}\)O\({}_{2}\) atmosphere (B). For both alteration samples, the main products were marked and given in Table 1. Based on the degradation products obtained, it can be concluded that there is hardly any influence of the enhanced pressure on the structure of the degradation products formed, at least in the time frame and temperature regime applied. However, particular peaks show a noticeably lower abundance, e.g., (2-ethylhexyl)oxy TMS silane (d) at retention time \(t_{\text{R}}\) = 8.55 min, and 2-ethylhexyl 2-oxobutyl adipate (m) at \(t_{\text{R}}\) = 24.74 min, while some of them increased with increasing pressure, e.g., adip acid bis(TMS) ester (k) at \(t_{\text{R}}\) = 17.90 min. Therefore, an effect of pressure on the degradation mechanism and, consequently, on the quantities of the products formed can be assumed, whereas the quality of the degradation products is not influenced. In addition to the neat base oil, blends with 1 (_w/w_) % AO were subjected to artificial alteration. GC-MS analysis obtained from these samples altered under \({}^{16}\)O\({}_{2}\) atmosphere and \({}^{18}\)O\({}_{2}\) atmosphere, respectively, showed identical products and distribution. The most abundant degradation products in the artificially altered samples have already been given in Table 1. Additionally, the degradation products of AO under both atmospheres are listed in Table 3.  
Compared to alteration without AO (see Figure 10A), not all of the products were formed in similarly significant amounts and are therefore not marked as main degradation products.