## 4 Conclusions  
The results obtained from the measurements of the surface tension show that tension for unsaturated fatty acids is lower than that of canola oil and depends on the kind of acid. The value of the surface tension of oleic acid (31.92 mN/m) is comparable to that which can be found in the literature. The surface tension of linoleic and linoleic acids is equal to 25.02 mN/m and 30.35 mN/m, respectively. Unfortunately, for these acids, it is difficult to find these data in the literature; therefore, it is impossible to make a comparison between the literature values and those measured by us. Taking into accounthe measured values of the surface tension and the contact angle of the fatty acids on the PTFE and PMMA surfaces, as well as applying the [PERSON] and co-workers' concept for the interface tension, it was established that the surface tension of these acids results from the Lifshitz-van der Waals and Lewis acid-base intermolecular interactions. This indicates that the surface tension of fatty acids can be divided for two components--apolar resulting from the Lifshitz van der Waals intermolecular interactions and polar resulting from the Lewis acid-base interactions. However, the apolar component of surface tension is significantly higher than that of the polar one, and represents 83.2%, 86.7%, and 84.6% of the total values of oleic, linoleic, and linoleic acids'  
Figure 6: The values of the adhesion work (\(W_{a}\)) for oleic, linoleic, and linoleic acids to the PTFE and engine valve surface. Figure 7: The values of the adhesion work (\(W_{a}\)) for oleic acid to the PTFE, PMMA, and engine valve surface. surface tension, respectively. The Lewis acid-base intermolecular interactions are a function of two parameters: Electron-acceptor and electron-donor. For all unsaturated fatty acids, the electron-donor parameter is higher than that of electron-acceptor which is connected with the presence of oxygen in the acids molecules. The measured values of the contact angle of oleic, linoleic, and linoleic acids on PTFE, PMMA, and the engine valve show that the best wetting properties are exhibited by linoleic acid (PTFE--47.58\({}^{\circ}\), PMMA--0\({}^{\circ}\); and the engine valve--17.57\({}^{\circ}\)). However, from the analysis of the Young-Dupre equation, it results that the best adhesion properties are shown by oleic acid (\(W_{\text{a}}\) = 46.52, 63.5, and 60.0 mJ/m\({}^{2}\) for PTFE, PMMA, and the engine valve, respectively). The values of the contact angle and adhesion work of fatty acids obtained by us are difficult to compare to those in the literature because it is difficult to find such values. The addition of n-hexane to oleic acid causes a considerable decrease of its surface tension from 31.92 mN/m to 18.3 mN/m in the range of n-hexane concentration from 0 to 20% due to its high adsorption at the oleic acid-water interface. The changes of the surface tension of oleic acid and the n-hexane mixture as a function of the compositionare not linear and synergism in the surface tension reduction is observed. As follows, the oleic acid-n-hexane mixture is not ideal. The addition of the n-hexane to oleic acid causes a non-linear decrease of the contact angle on the PTFE surface from 62.77\({}^{\circ}\) to 18.84\({}^{\circ}\)in the range of n-hexane concentration from 0 to 20%. Similarly to the surface tension of the mixture, synergism in the contact angle reduction is observed. The synergism in the surface tension and contact angle reduction by oleic acid and then-hexane mixture results from a higher concentration of n-hexane at the oleic acid-air and PTFE-oleic acid interfaces, rather than in the bulk phase. However, n-hexane adsorption at the oleic acid-air interface is higher than that at the PTFE-oleic acid one. The decrease of surface tension and improvement of the wetting properties of oleic acid being the main component of canola oil by the addition of n-hexane suggests that it is possible to improve canola oil applied for the diesel engine.