## 3 Results and Discussion  
### Determination of Eutectics  
Table 2 shows water content and melting temperatures for the pure compounds used in this work. DSC thermograms can be found in the Supplementary Materials. Some differences are found with previously published data. It should be noted that not all the results coming from previous works indicate the water content of the samples and, in most cases, chemicals were not dried prior to the experiments. Moisture content has a substantial impact on thermal and mechanical properties of polymers [30] and organic salts [31], thus providing a reason for the differences observed.  
AOT is solid at ambient conditions, and melting points of the polymers range from 285 K (PEG-600) to 319 K (PEG-2000). The existence of a eutectic for these mixtures could allow for easy manipulation of the surfactant while two chemicals of interest for EOR are combined. For that reason, solid-liquid equilibria of the systems AOT + PEG-2000, AOT + PEG-1000, and AOT + PEG-600 were determined using DSC. Figure 1 shows the melting temperatures of the mixtures as a function of AOT mole fraction (numerical data are shown in Table S1 in the Supplementary Materials).  
The system AOT + PEG-2000 exhibited a minimum melting temperature of 314 K, when the surfactant mole fraction was ca. 0.80, a value very close to the melting point of the PEG-2000 (319 K). In fact, all the melting points found for this system were similar to the melting point of the polymer, even for mixtures with a high proportion of surfactant. The  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \(T_{d}\) (K) & \(T_{d,5\%}\) (K) & \(T_{m\_onset}\) (K) & \begin{tabular}{c} **Water Content** \\ **(ppm)** \\ \end{tabular} \\ \hline AOT & \begin{tabular}{c} Exp. \\ Lit. \\ \end{tabular} & \begin{tabular}{c} 510 \\ 533 [32] \\ \end{tabular} & - & \begin{tabular}{c} 485 \\ 428 [33] \\ \end{tabular} & - \\ \hline PEG-600 & \begin{tabular}{c} Exp. \\ Lit. \\ \end{tabular} & \begin{tabular}{c} 597 \\ 623–650 [34] \\ \end{tabular} * & - & \begin{tabular}{c} 518 \\ 283 [35] \\ \end{tabular} & - \\ \hline PEG-1000 & \begin{tabular}{c} Exp. \\ Lit. \\ \end{tabular} & \begin{tabular}{c} 645 \\ 650–675 [36; 37] \\ \end{tabular} * & - & \begin{tabular}{c} 575 \\ 304 [38] \\ \end{tabular} & - \\ \hline PEG-2000 & \begin{tabular}{c} Exp. \\ Lit. \\ \end{tabular} & \
\begin{tabular}{c} 638 \\ 650–675 [36; 39] \\ \end{tabular} * & - &
\begin{tabular}{c} 319 \\ 323 [38] \\ \end{tabular} & - \\ \hline \hline \end{tabular}
* Ranges estimated graphically from plots of the TGA curves.  
\end{table}
Table 2: Water content and thermal characterization of pure compounds.  
Figure 1: Melting curves of the systems AOT + PEG: \(\bullet\) AOT + PEG-2000, \(\blacktriangle\) AOT + PEG-1000, \(\blacklozenge\) AOT + PEG-600.  
system AOT + PEG-1000 exhibited a minimum of 290 K (14 K lower than that corresponding to the polymer) for a mixture with AOT mole fraction of ca. 0.60. Depending on the ambient conditions, this mixture could be in a solid or liquid state. For the system AOT + PEG-600, a minimum melting point of 275 K was found for a mixture with surfactant mole fraction of ca. 0.40. This temperature represents a significant decrease (129 K) from the melting point of the pure surfactant, and a slight decrease (10 K) compared to the pure polymer. Consequently, this mixture of AOT and PEG-600 was considered as an interesting eutectic for EOR applications.