## 3 Results and Discussion
### Nanoparticles Characterization  
FTIR and PCS analyses were performed to investigate the presence of surfactant and sulfonate functional groups in the synthesized nanoparticles polymer structure, and their influence on the nanoparticles' particle size. Figure 8 shows the FTIR spectra for: 0 sluf10, 0 sluf20, 12 sulf20 and 30 sulf20 nanoparticles. Figure 6: Surface tension as a function of the logarithm of surfactant concentrations for the CMC determination. Figure 7: IFT as a function of surfactant concentration. Comparing the spectra for the nanoparticles with different amounts of OADA, it was noted that the bands referring to C-H stretching vibration (CH\({}_{3}\), CH\({}_{2}\) groups) at 2854 and 2924 cm\({}^{-1}\), C=O stretching vibration of tertiary amide at 1620 cm\({}^{-1}\) and C-N stretching vibrations at 1465 cm\({}^{-1}\) were present for all samples. However, it was possible to perceive some differences related to the amount of retained surfactant in the 0 sulf20 and 12 sulf20 spectra. It was observed that in the samples' spectra, especially for 12 sulf20, there are relatively greater intensities of bands at 1620 and 3400 cm\({}^{-1}\), which can be attributed to a larger OADA amount present in the nanoparticles. Additionally, the FTIR spectra are similar except for some bands, such as the one at 699 cm\({}^{-1}\) which is characteristic of the out-of-plane skeleton bending vibrations of benzene ring. Another is at 759 cm\({}^{-1}\) which refers to the out-of-plane bending vibration of the five -CH- groups in the ring of the monosubstituted benzene ring. It was noticed that the intensity of these bands is lower when increasing the sulfonation degree, as was also observed by [PERSON] et al [38]. It is also reported that the intensity of the band at 841 cm\({}^{-1}\), referring to a para substitution of the ring, decreased with SO\({}_{3}\) substitution [38; 39]. Effect of Crosslinking Agent and Surfactantconcentration on the Non-Sulfonated Polystyrene Nanoparticles (NPPS) Size and on the Oil/Water Interfacial Tension  
Some studies have shown that there is not a clear tendency of increasing or decreasing particle size obtained by polymerization reactions carried out with increasing of crosslinking agent content [31; 39]. Despite that, it could be expected that an increase of DVB amount could favor the production of polystyrene particles presenting smaller diameter and pore sizes. Based on the fact that small particle sizes with surfactant controlled release ability are preferred for EOR application, the aim of this study was the determination of the DVB amount that better satisfies this condition by means of monitoring particle size and IFT measurements. Also, the emulsion polymerization reactions were performed using an OADA concentration above the critical micelle concentration (CMC) in order to obtain the PSNP in appropriate sizes since  
Figure 8: Polystyrene nanoparticles FTIR spectra with different OADA and sulfonate contents: 30 sulf20, 12 sulf20, 0 sulf10 and 0 sulf20, whose polymerization formulations are described in Table 1.  
[PERSON] et al. (2018) noted that the PS nanoparticle sizes tended to decrease when it was used greater amounts of surfactant. So, in this work, a similar methodology was used, testing not only the influence of a more non-polar non-ionic surfactant, as OADA, but also the effect of greater or equal surfactant concentrations than the ones investigated in that work. Table 2 summarizes the particle size results obtained by PCS and the interfacial tension values for water/mineral oil systems measured by the Du Nouy ring method, after 24 h.  
As previously reported ([PERSON], 2018; [PERSON], 2018), there is an expected trend of particle size decrease with the increase of crosslink agent concentrations. Nevertheless, the reduction of nanoparticles size obtained with 4.5-9.0 (_v/v_) % of OADA was not significant. This might be due to the inter-crosslinking between the precursor nanoparticles ([PERSON], 2018).