## 3 Results and Discussion
### Phase Behavior of the SDHS-AAE(6 EO4 PO) Mixed System  
The phase transitions of all the oils are shown in Figure 3. In the SDHS-AAE(6 EO4 PO) mixed system, the same trends as those of the anionic-nonionic ratios were observed. The salt concentration of the Winsor Type I-III transition tended to remain the same or increase slightly when AAE(6 EO4 PO) was added, changing from the anionic-nonionic ratio of 9:1 to 5:5. The six-ethylene oxide (EO) of AAE(6 EO4 PO) played a key role in the Winsor Type I-III inversion. The EO in the extended surfactant exhibited hydrophilic characteristics and could increase the AAE(6 EO4 PO)-water interactions, while the salt concentration for the Winsor Type III-II transition of all the oils was reduced. When AAE(6 EO4 PO) increased, propylene oxide (PO) played a key role relative to the effect of the EO. The PO enhanced the AAE(6 EO4 PO)-oil interactions because of the hydrophobic characteristics of the PO. Therefore, the phase behavior of anionic-nonionic surfactant was influenced by both the EO and the PO. For this reason, the range of the salt concentration for Winsor Type III (middle phase) tended to decrease. Figure 3d presents SDHS-AAE(6 EO4 PO) with n-decane. The salt concentration for the Winsor Type I-III transformation increased from 6.5 (9:1 ratio) to 7.5 (5:5 ratio) % wt/v. The salt concentration for the Winsor Type III-II transition decreased from 19 (9:1 ratio) to 15 (5:5 ratio) % wt/v. The salt concentration range for the middle phase decreased from a ratio of 9:1 to 5:5, which was around 40% in the n-decane system. The results of the phase behavior of the single SDHS (10:0 ratio) are shown in Figure 3. The phase behavior of the mixed system and the SDHS-only system were divided into two cases, which included the short-chain oils (n-pentane, n-heptane, and n-octane) and the long-chain oil (n-decane). In the short-chain oil system, the range of salinity to form Winsor Type III tended to decrease, compared to the single SDHS system when the AAE(6 EO4 PO) ratio increased. Even with a small amount of AAE(6 EO4 PO) (0.2 %wt/v from 2% wt/v total surfactant concentration, 9:1 anionic-nonionic ratio), the salt concentration range for the middle phase formation was reduced (see Figure 3b). Due to the lipophilicity of the PO in the AAE(6 EO4 PO) molecules, the oil-surfactant interactions tended to increase. Hence, Winsor Type III would transform to Winsor Type II in the mixed system more easily than in the single SDHS system. However, the salt concentration required for the Winsor Type I-III transformation of the mixed system was still the same and close to that of the single SDHS system because the EO in AAE(6 EO4 PO) helped to maintain the water-surfactant interactions. For the long-chain oil system (i.e., EACN > 8), the salt concentration required for the Winsor Type I-III and III-II inversions decreased, relative to the single SDHS system. In this case, the PO was the predominant factor leading to the mixed system being more hydrophobic than the single SDHS system. Moreover, the salt concentration range of the middle phase expanded relative to the single SDHS system. Therefore, the conclusion is that the intermediate molecules in AAE(6 EO4 PO) strongly affected the n-alkane oil. Both the EO and the PO were involved in the phase behavior of the mixed system for the short-chain oils, whereas for the long-chain oil, the role of the PO was mainly related to the phase inversions of the mixed system. With regards to the effect of the n-alkane oils, the salt concentration required for phase inversion (both Winsor Type I-III and Winsor Type III-II) tended to increase when the EACN of the oil increased at the same anionic-nonionic ratio. As the hydrophobicity of the oil increased, the interactions between the oil molecules increased as well. To solubilize the oil molecules with long chains into micelles (Winsor Type I), the surfactant head interactions should be increased by increasing the salt concentration in the system [1].