## 4 Conclusions  
Surfactants and polymers are promising EOR chemicals that are currently being combined in order to achieve the water/oil IFT reduction required to enhance the mobility of the oil retained in the pores of the rocks, while also avoiding adverse mobility ratios. The usual solid character of these chemicals at room conditions may hamper the preparation of the corresponding formulations. The main conclusion of this study is that polymers and surfactants can be adequately selected to form eutectics, thus facilitating the preparation of EOR formulations due to the liquid character of the mixture. This has been shown through the combination of AOT with PEG. The mixtures showed melting temperatures significantly lower than those corresponding to the surfactant and slightly lower than those corresponding to the polymers. The lower the molecular weight of the polymer, the greater the reduction in the melting point compared to that of the pure surfactant. The most promising combination was found for AOT with PEG-600 at a 40:60 molar ratio. Such combination formed a eutectic mixture with a melting point of 275 K, thermally stable up to 473 K (\(T_{d,5\%}\)). Based on the reduction of the IFT, an aqueous optimal formulation containing 1 wt% of the AOT + PEG-600 eutectic and 0.5 wt% NaCl was designed for EOR. The formulation was capable of reducing water/oil IFT to a value of 5.1 \(\times\) 10\({}^{-2}\) mN/m. Moreover, the combination of the polymer with the surfactant reduced the adsorption of the latter on \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Authors** & **Eutectic Mixture** & **Concentration** & **Rock Type** & **IFT (mN/m)** & **Adsorption (mg/g Rock)** & **AOR (\%OOIP)** \\ \hline [PERSON] et al. [15] & ChClClClovered (1.2) & 50 wt\% & Berea & 40 (298.15 K) & - & 29.8 (353.15 K) \\  & ChClClClue (1.2) & 50 wt\% & sandstone & 31.3 (301.15 K) & - & 30.8 (333.15 K) \\ \hline [PERSON] et al. [16] & ChClClethylene Glycol & 50 vol\% & Berea & 12 (298.15 K) & - & 15.8 (353.15 K) \\  & (1.2) & & & & \\ \hline [PERSON] et al. [18] & ChClClUrea (1.2) & & & 4.3 (313.15 K) & 9.5 & 25 \\  & ChClCl:Tho urea (1.2) & & & 0.57 (313.15 K) & Ca. 12 & 29 \\  & ChClCl:ethylene Glycol & 50 vol\% & sandstone & 5 (313.15 K) & Ca. 8.5 & 15 \\  & (1.2) & & & 1.52 (313.15 K) & Ca. 10.5 & 22 \\ \hline [PERSON] et al. [3] & ChClClAlonic acid (1.0.5) & & & 21.9 (318.15 K) & - & 4.1 (318.15 K), 8.0 \\  & ChClMalonic acid (1.1) & 50 vol\% & Berea & 9.8 (353.15 K) & - & (353.15 K), 8.2 \\  & ChClMalonic acid (1.1) & 50 vol\% & sandstone & 21.9 (318.15 K) & - & 6.6 (318.15 K), 8.2 \\  & & & & 8.3 (353.15 K) & & (353.15 K) \\ \hline [PERSON] et al. [19] & ChClClClric acid (1.1) & 1 vol\% & Carbonate rock & 0.5 (ambient) & Ca. 1 DES & 48.4 \\  & and 5 ppm of CTAB & & & & & \\ \hline Present study & AOT:PEG-600 (1.5:1) & 1 wt\% & Berea & Sandstone & 0.051 (298.15 K) & 4.75 & 34.18 (ambient) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of results obtained in this work and in previous studies. the rocks. According to the AOR achieved (34.18% of OOIP), it can be concluded that the formulation is promising for sandstone rocks and for reservoirs with low salinity conditions. Future outcomes are expected on eutectics stable in harsh environmental conditions, a challenge for any EOR method. The following are available online at [[https://www.mdpi.com/article/10.3390/app11178164/s1](https://www.mdpi.com/article/10.3390/app11178164/s1)]([https://www.mdpi.com/article/10.3390/app11178164/s1](https://www.mdpi.com/article/10.3390/app11178164/s1)). Figures S1-S4: 1H NMR spectra. Figures S5-S12: TGA and DSC thermograms. Table S1: Melting temperatures for AOT + PEG mixtures. Table S2: Physical properties for the AOT + PEG-600 eutectic. C.G. : experimental, analysis, original draft preparation. A.S. ([PERSON]): EOR experimental supervision, analysis, original draft preparation. H.R. : thermal studies' supervision, analysis, editing. A.S. ([PERSON]): conceptualization, supervision, writing, funding acquisition.