A Normalized HLD (HLD\({}_{\text{N}}\)) Tool for Optimal Salt-Concentration Prediction of Microemulsions  
[PERSON]  
1 Department of Environmental Engineering, Chulalongkorn University, Bangkok 10330, Thailand; [EMAIL_ADDRESS]  
2 The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand; [EMAIL_ADDRESS]  
[PERSON]  
2 The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Environmental Engineering, Chulalongkorn University, Bangkok 10330, Thailand; [EMAIL_ADDRESS]  
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example, contain polyethylene oxide (EO), polypropylene oxide (PO), or polyethylene-polypropylene oxide (EO-PO) [11; 15]. The EO (C\({}_{2}\)H\({}_{4}\)O) group can enhance the interactions between the water and the surfactant. Influenced by oxygen molecules, the EO group acts as the hydrophilic part, while the PO (C\({}_{3}\)H\({}_{6}\)O) acts as the lipophilic part by extending further into the oil phase [11; 13]. Regarding the intermediate structure, extended surfactants can expand the thickness of the interfacial region, which helps to reach ultralow interfacial tension (\(<\) 0.01 mN/m) and increase surfactant-oil or surfactant-water interactions [13; 14; 16; 17; 18]. Relative to conventional surfactants, extended surfactants also show a good solubility of oil and water. [PERSON] et al. (1995) first introduced extended surfactants, the alkyl polypropylene oxide ether sulfates [12]. The addition of PO resulted in a reduction in the optimal salt concentration level for all the SOW systems. Moreover, extended surfactants could form the middle-phase microemulsion, reduce the interfacial tension to ultralow, and increase the oil solubilization for various oils [15]. Microemulsions are isotropic, transparent and thermodynamically stable liquid systems of water, oil, and amphiphile (i.e., SOW system) [1; 19]. Hence, microemulsions have been used in many industries or products, especially for cleaning processes, due to the assembled capability of immiscible liquids [20]. The dispersed size and zeta potential of the emulsion depend on the conditions of the SOW, such as pH, temperature, and salinity [21; 22; 23]. In the SOW system, the middle phase (Winsor Type III) at an equal volume of oil and water phases shows a low interfacial tension (IFT) and high solubilization among four types of microemulsions (i.e., Winsor Type I oil in water, Winsor Type II water in oil, Winsor Type III bi-continuous phase, and Winsor Type IV single-phase microemulsion with high surfactant concentration [24; 25; 26]. The optimal condition is defined as the lowest IFT in the middle phase region [1]. The SOW system transforms the Winsor Type I-III-II by increasing the salinity or the temperature for the ionic or the nonionic surfactants (i.e., ethylene oxide-based surfactants), respectively. Therefore, when optimizing the performance, the characteristics of the surfactants are key to designing the phase behaviors and, as a result, determining the appropriate condition of any SOW system. The determination of optimal conditions in the SOW system is a time-consuming process, achieved through a conventional phase scan experiment [27]. Many empirical equations have been proposed to describe a surfactant characteristic and predict a microemulsion formation, which can save time in an experiment [26; 28; 29; 30; 31; 32]. The hydrophilic-lipophilic deviation (HLD), a pioneer work introduced by [PERSON] et al. (1979), is one of the empirical equations [28]. The HLD is normalized from the change of the chemical potential for the surfactant transfer that is derived from the surfactant affinity difference (SAD) [28; 32]. The value of the HLD can be used to identify the type of microemulsions [1; 28; 32].