## 3 Results and Discussion
### Interfacial Tension (IFT) and Optimal Salt Concentration of the SDHS-AAE(6 EO4 PO) Mixed System  
The IFT of microemulsion samples from the phase behavior studies were measured to determine the optimal salt concentration for each model oil, as presented in Figure 4. The IFT at the optimal condition is well known as an optimal IFT (IFT*) [1]. Regarding the effect of the SDHS-AAE(6 EO4 PO) mixing ratio, the IFT* tended to decrease with the increase of AAE(6 EO4 PO). The nonionic surfactants could reduce the repulsive forces of the head groups of the anionic surfactant in the mixed system, which resulted in closer surfactant packing [63; 64]. Thus, the 5:5 ratio could solubilize both water and oil molecules better than the other ratios. Regarding the effect of the oil's EACN at the same surfactant ratio, the IFT* was not observed in any trends of these systems. In the mixed surfactant with 9:1 ratio, the IFT* tended to decrease. For the other ratios (i.e., 8:2, 7:3, 6:4, 5:5 ratio), the IFT* was not observed, as shown in Table 2.  
Figure 4: Interfacial tensions (IFTs) of SDHS-AAE(6 EO4 PO) systems with (**a**) n-pentane, (**b**) n-heptane, (**c**) n-octane, and (**d**) n-decane.  
In contrast to the single SDHS system, the presence of AAE(6 EO4 PO) in the mixture could reduce the IFT*, especially in the n-decane system (see Table 2). The IFT* of the single anionic system tended to increase when the hydrophobicity of the oil increased (i.e., when the EACN increased). In the mixed system, the nonionic surfactant helps the anionic surfactant in solubilizing the oil molecules in the middle phase by reducing the electrostatic repulsive forces between the anionic heads [1,15,27]. In addition, the EO and PO, as an intermediate polarity group, facilitate the nonionic surfactant molecules to balance the solubilization of the long-chain alkane oils (i.e., high EACN). Thus, the presence of the nonionic extended surfactant in the mixed system showed the synergism of the microemulsions that affected the long-chain alkane oil system by making them stronger than the short-chain alkane oil system. It is interesting to note that the mixed system of nonionic extended surfactant decreased the IFT*, ranging about 0.25-48%, 18-63%, 44-70%, and 73-86% for the n-pentane, the n-heptane, the n-octane, and the n-decane, respectively, compared with the single SDHS system. However, the IFT* of SDHS-AAE(6 EO4 PO) could not reach ultralow interfacial tension (<0.01 mN/m) [17,18].  
The optimal salt concentrations of the microemulsion system are shown in Table 2. No trends were observed for the effect of the mixing ratios in the mixed system. Comparing the mixed system and the single SDHS system, the optimal salt concentration for the n-pentane, the n-heptane, and the n-octane were not significantly different. For the n-decane, the optimal salt concentration was reduced from 14% wt/v NaCl (for the single SDHS) to 9% wt/v NaCl (for the mixed system). At the same anionic-nonionic ratio, the optimal salt concentration was reduced from 14% wt/v NaCl (for the single SDHS) to 9% wt/v NaCl (for the mixed system).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Anionic–Nonionic Ratio**} & \multicolumn{1}{c}{**Oil**} & \multicolumn{1}{c}{**EACN**} & \multicolumn{1}{c}{**Optimal NaCl concentration (S*)**} & \multicolumn{1}{c}{**IFT* (mN/m)**} \\ \hline  
\hline \hline \end{tabular} \({}^{1}\) Obtained from our previous study [51].  
\end{table}
Table 2: Optimal salt concentrations (S*) and optimal interfacial tensions (IFT*) of SDHS-AAE(6 EO4 PO) systems.  
concentration tended to increase when additional nonionic surfactant was presented. This is likely due to the stronger interactions between anionic head moieties and the EO and PO group in the nonionic extended surfactant, which were needed to solubilize the long-chain oil molecules by increasing the salinity in the mixed system.