## 3 Results and Discussion  
### Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) Measurements  
The typical ATR-FTIR spectra for pure mineral and ester oils are shown in Figure 5. Also, the FTIR spectra of pure and modified liquids with two types of nanofillers, C\({}_{60}\) and TiO\({}_{2}\), are presented in Figure 6. Additionally, spectra of liquids modified with surfactant Span(r) 20 are shown.  
Figure 5: Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of investigated dielectric liquids at room temperature (25 \({}^{\circ}\)C).  
The infrared spectra of investigated liquids (Figure 5) contain fundamental and characteristic bands whose frequencies and intensities can determine the relevant functional groups in the investigated liquids [82; 83; 84].  
Figure 6: ATR-FTIR spectra of investigated base dielectric liquids and dielectric liquids modified with C60, TiO\({}_{2}\), surfactant Span, or Span with TiO\({}_{2}\) at room temperature: (**a**) Mineral oil; (**b**) synthetic ester; (**c**) natural ester.  
Two of the studied liquids were esters (synthetic and natural esters), and absorption bands attributed to vibrations of esters were present at their spectra. Spectral bands in the regions of 1770-1715 cm\({}^{-1}\) (C=O stretching vibration of the ester carbonyl functional groups of the triglycerides), 1300-1100 cm\({}^{-1}\) (C-O stretching vibration of ester groups) and a peak with a maximum around 3460 cm\({}^{-1}\) (overtone of C=O of ester group) are attributed to vibrations of esters. Bands derived from aliphatic hydrocarbons vibrations were present in the spectra, too: asymmetric stretching vibration of C-H of aliphatic -CH\({}_{3}\) groups due to the alkyl rest of triglycerides (2953 cm\({}^{-1}\) in NE oil and 2958 cm\({}^{-1}\) in SE oil), C-H asymmetric stretching vibration of the aliphatic CH\({}_{2}\) group (2923 cm\({}^{-1}\) and 2929 cm\({}^{-1}\) for NE and SE oil, respectively), and C-H symmetric stretching vibration of the aliphatic CH\({}_{2}\) group (2858 cm\({}^{-1}\) and 2853 cm\({}^{-1}\) for NE and SE oil, respectively), C-H bending vibration of CH\({}_{2}\) and CH\({}_{3}\) aliphatic groups (1466 cm\({}^{-1}\) and 1459 cm\({}^{-1}\) for NE and SE oil), C-H bending symmetric vibration of CH\({}_{2}\) groups (1397 cm\({}^{-1}\) and 1389 cm\({}^{-1}\) for NE and SE oil), and overlapping of C-H rocking vibration of CH\({}_{2}\) and the out-of-plane vibration of the cis -HC=CH- group of disubstituted olefins (722 cm\({}^{-1}\) and 727 cm\({}^{-1}\) for NE and SE oil). In addition, the NE ester is an unsaturated ester, as evidenced by the presence of absorption bands with a maximum at 3009 cm\({}^{-1}\) (=C-H stretching symmetric vibration of the cis double bonds) and 1655 cm\({}^{-1}\) (C=C stretching vibration of cis-disubstituted olefins, RHC=CHR) on its spectrum.  
Mineral oils are high-boiling fractions obtained from the refining of crude oil. These oils mainly contain compounds with a ring structure, i.e., that which are unsaturated-aromatic and saturated-naphthenic, which may have additional saturated side-chains--paraffins. Depending on the predominance of a given structure, they are properly classified as aromatic, naphthenic, or paraffinic. The straight-chain paraffinic components are normal-alkanes, whereas the naphthenic group constitutes of cycloalkanes. Aromatics contain at least one ring of six-carbon atoms, with alternating single and double bonds [85]. On the mineral oil spectrum (Figure 5), absorption bands from saturated hydrocarbons are observed: 2952, 2921, 2853, 1456, and 1376 cm\({}^{-1}\), which indicates a naphthenic or paraffinic oil.  
The introduction of nanofillers (C\({}_{60}\), TiO\({}_{2}\)) or surfactant (Span(r) 20) did not cause any changes in the spectra of investigated liquids; the intensity or location of the bands of characteristic oil or modifier functional groups did not change. This result suggests the lack of interactions between the investigated liquids and the introduced modifiers.