## 3 Results and Discussion  
### OADA Characterization  
The FTIR spectrum for OADA is shown in Figure 3. A broad-band at 3400 cm\({}^{-1}\) was attributed to stretching modes of OH group of alcohol and a band at 1065 cm\({}^{-1}\), for stretching modes of C-O bond of primary alcohols. The stretching vibrations of amide carbonyl were observed at 1620 cm\({}^{-1}\) and C-N stretching at 1465 cm\({}^{-1}\). Asymmetrical and symmetrical stretchings of -CH\({}_{2}\) and -CH\({}_{3}\) of long chain fatty acid appeared at 2924 and 2854 cm\({}^{-1}\), respectively. The disappearance of -NH- angular vibration at 1550 cm\({}^{-1}\) confirmed an efficient elimination of amine species that were used in excess in the reaction.  
The \({}^{1}\)H NMR spectra of OADA is displayed in Figure 4, where some characteristic peaks were identified: Terminal proton (a) of -CH\({}_{3}\) at \(\delta\) = 0.89 ppm, -CH\({}_{2}\) proton (b) present in the long fatty chain at \(\delta\) = 1.27 ppm, -CH\({}_{2}\) proton (e) adjacent to amide -C=O observed at \(\delta\) = 2.4 ppm, proton (g) of -CH\({}_{2}\) near hydroxyl appeared at \(\delta\) = 3.9 ppm, proton (h) of -C=C- of unsaturated fatty acid chain at about \(\delta\) = 5.35 ppm and the peak at about \(\delta\) = 3.53 ppm is assigned to -CH\({}_{2}\) adjacent to amide nitrogen. These peaks were also confirmed in the literature [28; 35].  
Figure 3: FTIR spectrum of OADA.  
Figure 2: Oil displacement set-up scheme.  
Figure 5 shows the \({}^{13}\)C NMR spectrum of OADA, using attached proton test (APT) technique, and the structure obtained by \({}^{1}\)H NMR analysis was confirmed by the results from \({}^{13}\)C NMR. The characteristic peak of -CH\({}_{3}\), present in the ester methyl group (methyl oleate), which was used as a reagent, was not present, and that confirms the completion of the reaction.  
Since OADA was used in this work as a surfactant for both polystyrene polymerization and enhanced oil recovery processes, it was necessary to determine its critical micelle concentration. For that, surface tension measurements were performed, and a graph of surface tension versus OADA concentration was plotted, as shown in Figure 6. The critical micelle concentration corresponds to the inflection of the curve in the graph. The oleic acid diethanolamide produced a relevant reduction of the water surface tension (72.3 mN\(\cdot\)m\({}^{-1}\)), reaching values less than 40 mN\(\cdot\)m\({}^{-1}\), which is related to a superior wetting agents category [30; 36]. As expected, the surface tension decreased as concentration increased, and it was observed a clear breakpoint related to this condition of critical micelle concentration (CMC):  
Figure 4: \({}^{1}\)H NMR spectrum of OADA.  
Figure 5: \({}^{13}\)C NMR-APT spectrum of OADA.  
\(\gamma_{\rm CMC}=36.6\) mN/m and CMC \(=3.13\times 10^{-4}\) M or 115.3 mg/L. Greater concentration values caused a lower reduction of surface tension.  
The important effect of the OADA concentration on the water/mineral oil interfacial tension (IFT) is shown in Figure 7.  
It was observed that at very low OADA concentrations occurred a drastic IFT reduction when compared to water/mineral oil interfacial tension (12.6 mN\(\cdot\)m\({}^{-1}\)), which is in accordance with other surfactants previously studied [37]. At a concentration of 0.10 mM (IFT = 0.070 mN\(\cdot\)m\({}^{-1}\)), the curve seems to have reached an equilibrium plateau, although greater concentrations still caused a discrete IFT decrease until reaching a minimum at 0.017 mN\(\cdot\)m\({}^{-1}\) for the solution containing 2.0 mM of OADA.