## 3 Results and Discussion
### Surface Tension of Unsaturated Fatty Acids  
The average measured value of oleic acid surface tension is equal to 31.92 mN/m (Figure 1). This value is slightly lower than that reported in the literature (32.79 mN/m) [36] and the surface tension of canola oil [34]. The surface tension of linoleic and linoleic acids was measured and determined from the contact angle on the PTFE surface (Figure 1). The contact angle is closely related to the surface tension of the liquid and solid, as well as solid-liquid interface tension. The relationship between these values is presented by the Young equation, which has the form [37]:  
\[\gamma_{SV}-\gamma_{SL}=\gamma_{LV}\cos\theta \tag{1}\]  
where \(\gamma_{SV}\), \(\gamma_{LV}\) and \(\gamma_{SL}\) are the solid surface tension, liquid surface tension, and solid-liquid interface tension, respectively, and \(\theta\) is the contact angle in the solid-liquid drop-air system. Based on the Young equation, it is possible to determine the surface tension of the solid and the liquid wetting, depending which surface tension is known. The application of the Young equation in relation to the surface tension of a liquid or solid is possible when the dependence of the solid-liquid interface tension on the surface tension of a solid and a liquid is known. Analyzing the contact angles for many different types of liquids on the surface of commonly used polymers, [PERSON] et al. [38; 39; 40] determined a semi-empirical relationship between these parameters. This dependence has the form [38; 39; 40]:  
\[\gamma_{SL}=\gamma_{LV}+\gamma_{SV}-2\sqrt{\gamma_{LV}\gamma_{SV}}\exp^{- \beta(\gamma_{LV}-\gamma_{SV})^{2}} \tag{2}\]  
where \(\beta\) is the constant independent of the kind of the solid-liquid system. By substituting Equation (2) into Equation (1), one can obtain [38; 39; 40]:  
\[\cos\theta=-1+2\sqrt{\frac{\gamma_{SV}}{\gamma_{LV}}}\exp^{-\beta(\gamma_{LV} -\gamma_{SV})^{2}} \tag{3}\]  
Figure 1: The values of the surface tension (\(\gamma_{LV}\)) and the Lifshitz-van der Waals component of this tension. (\(\gamma_{LV}^{LW}\)) for oleic, linoleic, and linoleic acids. Using the value of the surface tension of oleic acid (Figure 1) in Equation (3) and its contact angle on the surface of PTFE (Figure 2), the surface tension of PTFE was calculated. This value is equal to 17.75 mN/m, being lower than the PTFE surface tension determined from the contact angle of the n-alkanes (20.24 mN/m) [41]. On the other hand, this value is similar to that of PTFE surface tension determined by [PERSON] et al. applying their equation for many polar liquids (18 mN/m) [40]. Assuming the value of PTFE surface tension calculated based on the contact angle of oleic acid and its surface tension, and taking into account the contact angle of linoleic and linoleic acid, their surface tension was calculated from Equation (3) (Figure 1). It appears that the surface tension determined in this way is similar to that measured. The difference does not exceed the accuracy of the measurements and therefore, in Figure 1, the values of the surface tension obtained from the contact angle are shown. As follows from Figure 1, the surface tension of linoleic acid is close to that of oleic acid. The lowest surface tension value is exhibited by linoleic acid. The surface tension values of all tested unsaturated fatty acids are lower than that of canola oil (34.15 mN/m) [34]. [PERSON] et al. [42; 43; 44] argue that wettability of solids by liquids depends not only on their total surface tension, but also on their components and parameters resulting from different types of intermolecular interactions. The authors divide the surface tension into two components: Lifshitz-van der Waals (\(\gamma^{\text{LW}}\)) and acid-base (\(\gamma^{\text{AB}}\)). On the other hand, the acid-base component is a function of electron-acceptor (\(\gamma^{+}\)) and electron-donor (\(\gamma^{-}\)) parameters. Taking into account such division of the surface tension, [PERSON] et al.