## 3 Results and Discussion
### Surfactant Controlled Release Study  
In order to better evaluate if the prepared systems could be applied as surfactant-controlled release nanocarriers, surface and interfacial tension measurements were performed to evaluate their dependence with time. For that, suspensions containing 0.1 (_m/v_) % of 0 sulf10, 0 sulf20, 12 sulf20 and 30 sulf20 nanoparticles were prepared and surface tension values were collected until they reached an equilibrium. Then, this property was monitored over the first 96 h, as revealed in Figure 11. As expected, the surface tension values for all samples did not present any significant variation, meaning that a premature surfactant loss in the aqueous phase during an EOR process would be negligible. This can be explained by the existence of hydrophobic interactions between the polystyrene chains and the oleic acid diethanolamide molecules, especially through their long fatty chain, that turns difficult their release in a polar medium. In other studies by our Group [17], it was achieved similar results using nonionic surfactants in mesoporous silica systems. Figure 11: Surface tension behavior over the time for systems containing 0.1 (_m/v_) %. Measurements were performed using the Wilhem plate method (temperature: 25 \(\pm\) 0.5 \({}^{\circ}\)C). Figure 10: Influence of St-S content on the interfacial tension between 0.1 (_m/v_) % SPSNP suspensions in deionized water and mineral oil. Afterwards, these aqueous systems were placed in contact with oil to proceed with the interfacial tension (IFT) measurements. Figure 12 shows a typical L-shape where an IFT reduction occurs during the first two hours of analysis and, after that period, the variations were not relevant. [PERSON] et al. [17] and [PERSON] et al. [14] identified a similar behavior in which the IFT decreased gradually during the first hours probably due to the migration of the surfactant molecules away from the nanoparticles to the water/oil interface where they replace the solvent molecules until the IFT reached an equilibrium plateau. The system containing 12 sulf20 nanoparticles readily reached IFT values lower than 1.0 mN-m\({}^{-1}\) when it was measured by the Du Nouy method. Since the Du Nouy method is not able to measure values below 1.0 mN-m\({}^{-1}\), the IFT measurements for the 12 sulf20 system were carried out in the spinning drop tensiometer and the results are displayed in Figure 13.  
The Figure 13 shows that the 12 sulf20 resulted in lower IFT values. Also, it is possible to note that this system acts at the interface and reaches an equilibrium after a shorter time interval in comparison with the other systems. Figure 12: IFT behavior over time for systems containing 0.1 (_m/v_) % suspensions and mineral oil. Measurements were carried out using the Du Nouy method (temperature: 25 \(\pm\) 0.5 \({}^{\circ}\)C). Figure 13: Interfacial tension (IFT) for the 12 sulf20 SPSSNP suspension (0.1 _m/v_ %) and mineral oil as a function of time. Measurements were performed using Spinning Drop method. Besides, it was prepared an OADA solution containing 0.006% of free surfactant that corresponds to a theoretical system where all surfactant used in the polymerization would have been retained and then liberated at the interface. The spinning drop measurements for the system formed by this solution in contact with the mineral oil show an equilibrium IFT of 9.3 mN\(\cdot\)m\({}^{-1}\), obtained after 3 h. Comparing its performance with the results for the nanoparticles systems, shown in Figures 12 and 13, it is possible to suppose that a synergistic effect did occur in these systems, since the 12 sulf20 nanoparticles presented a much reduced IFT values and shortened the time necessary to establish the equilibrium. Similar results were observed in the literature [9; 10; 17]. Therefore, the results have shown that the sulfonated polystyrene nanoparticles (12 sulf20) can act as efficient nanocarriers, causing an interfacial tension decrease along the time when in contact with the oil, reaching very low IFT values in a short time.