## 3 Results and Discussion
### Wetting Properties of Unsaturated Fatty Acids  
Wetting properties of unsaturated fatty acids were tested on the surface of apolar PTFE, monopolarPMMA, and bipolar part of a car engine [34]. In the case of PTFE, the best wettability is shown by linolenic acid (Figure 2) and the worst by oleic acid. However, any acid spreads completely over the PTFE surface. Over the PMMA surface, the complete spreading is not observed only for oleic acid.  
Taking into account the contact angle of oleic acid, the surface tension of PMMA was calculated from Equation (3). Its calculated value is equal to 31.57 mN/m and it is lower than that of PMMA calculated from the contact angle measurements for the model liquids [47]. However, the PMMA surface tension value calculated based on oleic acid data is higher than that of linoleic and linolenic acids. This justifies the total spread of these acids over the PMMA surface. On the other hand, it is possible that during the fatty acids spreading, the film is first created and the surface tension of PMMA covered by this film, only in the case of oleic acids, is lower than its surface tension. In such case, a contact angle higher than zero is formed. This phenomenon can be similar to benzene spreading over the water surface [37]. Similar to PTFE, the best wetting agent of the engine part is linoleic acid, and the worst oleic. The engine part wettability by oleic acid is similar to that of canola oil (Figure 2) [34]. The n-hexane addition to oleic acid improves its wetting properties in relation to the part of the car engine similarly to PTFE (Figure 4). To account for the influence of n-hexane on the wetting properties of oleic acid, the values of the contact angle of oleic acid and the n-hexane mixture were determined from the following equation:  
\[\theta_{1}X_{1}+\theta_{2}X_{2}=\theta \tag{9}\]  
Indeed, this equation reflects the changes of the contact angle as the function of the ideal mixture composition. Comparing the calculated values of the contact angle to those of the measured ones, a high synergetic effect in the contact angle values is observed. This points out that the measured values of the contact angle at a given composition of oleic acid and the n-hexane mixture are considerably lower than those calculated from Equation (9). This is probably a result of the fact that the concentration of n-hexane at the PTFE-oleic acid interface, similar to the oleic acid-air interface, is higher than in the  
Figure 4: A plot of the contact angle (\(\theta\)) for n-hexane and oleic acid mixtures on the PTFE surface vs. n-hexane percentage concentration (\(C_{\text{p}}\)).  
bulk phase. The coverage of the PTFE-oleic acid interface by n-hexane molecules can be determined from the following equation:  
\[\theta_{1}X_{1}^{\mathrm{S}}+\theta_{2}(1-X_{1}^{\mathrm{S}})=\theta \tag{10}\]  
The fraction of area occupied by n-hexane calculated from Equation (10) is larger than the mole fraction of n-hexane in the bulk phase, but lower than that of n-hexane at the oleic acid-air interface (Figure 5).  
This points out that n-hexane adsorption at the PTFE-air interface is lower than that at the oleic acid-air one. It should be mentioned that the adhesion work of oleic acid to the PTFE surface (46.54 mJ/m\({}^{2}\)) is higher than that of n-hexane (37.63 mJ/m\({}^{2}\)). On the other hand, the cohesion work of oleic acid is also higher than that of n-hexane. The balance of adhesion and cohesion work of oleic acid and n-hexane decides about n-hexane adsorption at the PTFE-oleic acid interface. As follows, the difference between oleic acid adhesion to the PTFE surface and its cohesion work is equal to -17.32 mJ/m\({}^{2}\), and that for n-hexane is 2.65 mJ/m\({}^{2}\).