## 3 Results and Discussion
### Surface Tension of Unsaturated Fatty Acids
[42; 43; 44] proposed a relationship associating the interface tension with the components and parameters of the surface tension of the contacting phases. This dependence for the solid-liquid system has the form:  
\[\gamma_{\text{SL}}=\gamma_{LV}+\gamma_{SV}-2\sqrt{\gamma_{LV}^{LW}\gamma_{SV}^ {LW}}+2\sqrt{\gamma_{LV}^{+}\gamma_{SV}^{-}}+2\sqrt{\gamma_{LV}^{-}\gamma_{ SV}^{+}} \tag{4}\]  
If the liquid contacts apolar solid, whose surface tension results only from the dispersion intermolecular interactions, then from Equations (1) and (4) the following is obtained:  
\[\gamma_{LV}^{LW}=\frac{\left[\gamma_{LV}(\cos\theta+1))\right]^{2}}{4\gamma_{ SV}^{LW}} \tag{5}\]  
By substituting the values of the contact angle (Figure 2) of unsaturated fatty acids measured on the PTFE surface, as well as that of PTFE surface tension which results only from the dispersion  
Figure 2: Values of the contact angle (\(\theta\)) for oleic, linoleic, and linoleic acids on the polytetrafluoroethylene (PTFE) and engine valve surface. intermolecular interactions [41] into Equation (3), the Lifshitz-van der Waals component of fatty acids was calculated (Figure 1). Next the acid-base component of this tension was determined (Figure 3). Figures 1 and 3 show that the Lifshitz-van der Waals components of oleic and linolenic acids are almost the same, only differing in acid-base components. The values of the Lifshitz-van der Waals and the acid-base components of surface tension of linoleic acid are the lowest of the tested fatty acids. It should be noted that the values of components of the acids' surface tension considerably differ from those of canola oil [34]. From apractical point of view, a decrease of liquids' surface tension by the addition of some substances which have a low surface tension is very advantageous. Therefore, the surface tension of oleic acid and the n-hexane mixture was measured (\(C_{\text{p}}\) n-hexane 5% \(\gamma_{\text{LY}}\) = 21.2 mN/m, 10%--19.1 mN/m, 15%--18.6 mN/m, and 20%--18.3 mN/m). It appear that the measured values of the surface tension are considerably lower than those calculated from the equation for ideal mixtures, which has the form:  
\[\gamma_{LV}=\gamma_{1}X_{1}+\gamma_{2}X_{2} \tag{6}\]  
These differences indicate that n-hexane can adsorb at the oleic acid-water interface. Thus, the composition of the surface region of oleic acid-n-hexane is quite different from that in the bulk phase. As follows from our earlier studies, the surface tension of the mixture can be predicted based on that of components and the surface fraction occupied by the given component of the mixture (\(X_{1}^{S}\), \(X_{2}^{S}\)) [45]. If so, the fraction of the interface area occupied by n-hexane (\(X_{1}^{S}\)) can be determined from the following equation:  
\[\gamma_{1}X_{1}^{S}+\gamma_{2}(1-X_{1}^{S})=\gamma_{LV} \tag{7}\]  
Knowing the fraction area of n-hexane occupying the oleic acid-air interface, it is possible to determine n-hexane concentration in the surface layer. It is commonly known that the area occupied by n-hexane molecule at the perpendicular orientation is equal to 21A\({}^{2}\)[46]. This corresponds to the n-hexane concentration in the surface region equal to \(7.91\times 10^{-6}\) mol/m\({}^{2}\). This value should be treated as the limiting concentration of n-hexane at the surface layer (\(\Gamma_{1}^{\infty}\)). Thus:  
\[X_{1}^{S}=\Gamma_{1}/\Gamma_{1}^{\infty} \tag{8}\]  
Calculated from Equation (8), values of \(\Gamma_{1}\) indicate that at the concentration of n-hexane in the bulk phase equal to 10%, there is almost 90% of n-hexane molecules at the oleic acid-air interface. \[\gamma_{SL}=\gamma_{LV}+\gamma_{SV}-2\sqrt{\gamma_{LV}\gamma_{SV}}\exp^{- \beta(\gamma_{LV}-\gamma_{SV})^{2}} \tag{2}\]0 The values of the acid-base component (\(\gamma_{LV}^{AB}\)) of oleic, linoleic, and linolenic acids’ surface tension. This means that the addition of n-hexane to oleic acid causes a considerable decrease in the surface tension because of its high adsorption at the oleic acid-air interface.