## 3 Results and Discussion
### Transport Test Evaluation  
Transport tests were performed to evaluate the surfactant and nanoparticles losses that may occur during transport, as can be seen by the results in Figure 15. Also, nanoparticles may cause pores clogging depending on their size and interactions with the porous media. The hydroxyl groups present in the OADA structure and sulfonate groups present in the nanoparticles could interact with silanol groups of silica, which represents the main component of sandstone reservoirs, and, therefore, the adsorption of these materials would be favored. Some studies have already reported that anionic surfactants containing sulfonates as head groups can adsorb on positively charged clay edges contained in sandstones [3; 57; 58; 59]. In a less extent, nonionic surfactants, that contain polar groups, can adsorb  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**System** & **Particle Size (nm)** & **Retention\%** & \(Q_{\text{e}}\) **(mg/g)** \\ \hline
0 sulf10 & 162 & 56.2 & 17.2 \\
0 sulf20 & 114 & 35.0 & 21.6 \\
12 sulf20 & 66 & 34.4 & 21.2 \\
30 sulf20 & 132 & 19.8 & 12.2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Effect of sulfonate groups and OADA amount, used in the polymerization reaction, on the surfactant retention capacity of nanoparticles.  
onto sand surface due to hydrogen bonds and weak hydrophobic interactions between them, causing surfactant loss during transport through the medium [60; 61; 62; 63]. For that reason, this experiment must be conducted before the oil displacement test, when several conditions may be applied.  
The analysis of the Breakthrough curves (Figure 15) shows that the systems have different elution profiles. The free surfactant (OADA) concentration at the plateau did not reach the initial concentration of the injected system, which means that a fraction of it was retained on the porous sand surface. In the nanoparticles' fluids curves, it is noted that the lower the sulfonate group content in the nanoparticles, greater was its percolation capacity in the porous medium. In the same way as it was observed for the curve related to the OADA solution, the concentrations of the 30 sulf20 nanoparticle suspension at the plateau was inferior to the injected concentration and also showed a delay in its elution from the porous medium, similar to a tail [64], probably as a consequence of its larger particles size (132 nm). Differently, the 12 sulf20 nanoparticle curve has reached the plateau rapidly due probably to both these factors: Its particle size (66 nm), which is almost a half of 30 sulf20 nanoparticles size, which favors its permeation through rock pores, and its lower sulfonate group content which may have contributed to decrease its interaction with the sand.  
The cumulative active substance recovered in the total effluent of those tests were 99.5%, 94.6% and 79.2% for 12 sulf20, 30 sulf20 nanoparticles and OADA surfactant solution, respectively. In other words, the losses in terms of mg of active/ mg of sand were 0.002 mg/g for 12 sulf20, 0.025 mg/g for 30 sulf20 and 0.096 mg/g for OADA. The adsorption of about 20% of OADA in the porous medium corroborates the concerns about surfactant adsorption and reinforced the necessity of nanocarripemployment. These results also proved that sulfonated polystyrene nanocarriers acted successfully, inhibiting the OADA adsorption onto sand particles surfaces. However, the increase of sulfonate content in the nanoparticles' surface caused more adsorption onto the porous medium, but still, it was inferior to that observed for the only-surfactant solution. Due to that and of other properties explored in the previous sections, it was concluded that the 12 sulf20 SPSNP forms the most effective system to percolate the porous medium in an unconsolidated sand column. Accordingly, these nanoparticles were further applied to an oil displacement test, which is discussed in the next section.