## 4 Conclusions  
In this study, the mixed surfactant systems of anionic and extended nonionic surfactants containing EO and PO groups were analyzed to enhance the solubilization of the  
Figure 9: Optimal salt concentrations (S*) from the HLD, and HLDN prediction of SDHS-AAE(6 EO4 PO) systems for commercial diesel (EACN = 9.10).  
n-alkane oils with different EACNs. The optimal salt concentrations were determined from the microemulsion phase scan and predicted from the HLD/HLDN equation; the findings from the study can be summarized as follows.  
In the SDHS-AAE(6 EO4 PO) systems, the intermediated polar molecules (i.e., EO and PO) of AAE(6 EO4 PO) were key to the formation of microemulsions. The ethylene oxide (EO) affected the phase inversion by increasing the salt concentration for the Winsor Type I-III transition when the mole fraction of AAE(6 EO4 PO) in the mixed system increased. Concurrently, the presence of the PO group in the extended nonionic surfactant resulted in salt concentration reduction. Thus, the salt concentration range for Winsor Type III was narrower with the addition of AAE(6 EO4 PO). When the EACN increased, the PO became the dominant factor that could reduce the salt concentration required for phase transitions (Winsor Type I-III and III-II) compared with the single SDHS system. For optimal salt concentration (S*) and the minimum interfacial tension (IFT*), the SDHS-AAE(6 EO4 PO) mixed system showed strong synergism relative to the single SDHS system. Especially in the case of long-chain carbons (i.e., n-decane), SDHS-AAE(6 EO4 PO) definitely reduced the optimal salinity (~36%) and the IFT* (up to 86%). Moreover, the salt concentration range forming Winsor Type III for the n-decane was expanded. Remarkably, the SDHS-AAE(6 EO4 PO) system was suitable for high-hydrophobic oil starting at EACN = 10.  
At the reference temperature (25 \(\pm\) 2 \({}^{\circ}\)C), the HLDN equation of the SDHS-AAE(6 EO4 PO) mixture was proposed as a tool for determining a suitable surfactant formulation in cleaning applications, including oil decontamination. The accuracy was investigated to determine the optimal salt concentrations and compared to those obtained from the experiment. The results were remarkable and showed that the error difference was less than 10%. This result demonstrated that even when many steps (e.g., the phase behavior experiment, the optimal condition, the interaction term) were required to determine the HLDN, SDHS-AAE(6 EO4 PO), the HLDN equation could be applied as a guideline for establishing the formulators. The preparation of optimal conditions is a goal to reduce the length of time and cost required for conducting experiments.  
Conceptualization, V.K., A.C. and S.K.; methodology, V.K. and A.C.; investigation, V.K.; writing--original draft preparation, V.K.; writing--review and editing, A.C. and S.K. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the 100 th Anniversary Chulalongkorn University Fund for Doctoral Scholarship, the 90 th Anniversary Chulalongkorn University Fund, and the Ratchaaphisek Somphot Endowment Fund (2019), grant number CU-GR_62_56_21_18.  
Not applicable.  
This work was funded by Chulalongkorn University, namely, the 100 th Anniversary Chulalongkorn University Fund for Doctoral Scholarship, the 90 th Anniversary Chulalongkorn University Fund, and the Ratchaaphisek Somphot Endowment Fund (2019) (Grant no. CU-GR_62_56_21_18). The authors thank the research program of Industrial Waste Management-Policies and Practices and the Center of Excellence on Hazardous Substance Management (Chulalongkorn University, Thailand) for the invaluable support received in the forms of financing and experimental equipment.  
The authors declare no conflict of interest.