## 4 Results and Discussion  
### Phase Behavior Test  
Figure 3 presents the results of the aqueous stability test. The test group included the nonionic surfactant (LB-95), while the reference group had heavy alcohol (IBA) added into the base the ASP mixture (0.25 wt% 1:4 (wt/wt) of the LAS and DOSS anionic surfactant pair, 0.2 wt% Na\({}_{2}\)CO\({}_{3}\) as the alkali, and 0.15 wt% HPAM as the polymer). The amount of chemical added was 0.2 wt%. The proposed mixture (nonionic surfactant) did not show any precipitation until 2.5 wt% NaCl (Figure 3b) while the reference mixture (heavy alcohol) started precipitation from 2.0 wt% NaCl (see the solid-like white materials at 2.0 and 2.5 wt% NaCl in Figure 3a). This aqueous stability test showed that to compare the two cases, we have to maintain a salinity below 2.0 wt% NaCl, and thereby the proposed nonionic surfactant could apply up to 2.5 wt% NaCl, but the applicability of heavy alcohol would be limited below 2.0 wt% NaCl. Figure 4 shows a salinity scan test with Winsor type III microemulsion and solubilization ratio to determine the optimum salinity. The proposed mixture with a nonionic surfactant showed a lower optimum salinity and wider range of the type III zone than the reference case. The optimum salinity with the same solubilization ratio of oil and water was 1.1 wt% NaCl for the proposed mixture and  
Figure 3: Results of the aqueous stability test of (**a**) 0.2 wt% heavy alcohol (IBA; the reference case); (**b**) 0.2 wt% nonionic surfactant (LB-95; the proposed case) mixing with 0.25 wt% 1:4 (wt/wt) of LAS and DOSS anionic surfactant pair, 0.2 wt% Na\({}_{2}\)CO\({}_{3}\) as the alkali, and 0.15 wt% HPAM as the polymer. 1.7 wt% NaCl for the reference case. The figures on the left side of Figure 4 (Figure 4a,c) revealed that the add-on of the nonionic surfactant resulted in a larger amount of Winsor type III than the reference case, which could be effective in making the residual oil moveable. The solubilization ratio at the optimum salinity was 16 mL/mL for the proposed mixture and 18 mL/mL for the reference. They were over the criterion to obtain an ultralow interfacial tension, i.e., \(1.17\times 10^{-3}\) mN/m and \(0.93\times 10^{-3}\) mN/m (Table 4; [20; 36]). Table 4 summarizes the results of the salinity scan test; a notable result is the difference of optimum salinity. The proposed case (AS with nonionic surfactant) showed the reduced value of optimum salinity favorable at the low salinity environment. The coreflooding test is required to evaluate the difference of solubilization ratios, as well as the optimum salinities influencing residual oil recovery. Figure 4: Salinity scan test: (**a,b**) as the reference case (heavy alcohol); (**c,d**) as the proposed mixture (nonionic surfactant). The left-hand side (**a,c**) shows microemulsions with the Winsor type III and the right-hand side (**b,d**) describes the solubilization ratio with the salinities. ‘ME’ stands for microemulsion. The hydrophobicity was deemed to be the reason why the nonionic surfactant produced a reduction in the optimum salinity. Theoretically, PO units broadened the ultralow interfacial tension and made the surfactant mixture more hydrophobic. The optimum condition with hydrophobic surfactant would increase the dispersing tendency of the surfactant in water by decreasing the salinity. The hydrophobicity was not quantitatively validated using an HLB value, but the hydrophobicity was indirectly estimated from the solubilization ratio and the optimum salinity. The nonionic surfactant contributed to making the ASP and oil system more hydrophobic, and it reduced the optimum salinity. [PERSON] et al. [20] showed that a single anionic surfactant such as LAS and DOSS failed to reach the equilibrium condition and was not suitable as a surfactant for low salinity conditions. The mixture of the two with glycol ethers solvent produced an optimum salinity higher than 2 wt% NaCl at 25 \({}^{\circ}\)C. Increasing the temperature will increase the interfacial tension and the optimum salinity [38,39].