Spontaneous emulsion behavior has been difficult to predict and could be influenced by many variables including salinity, temperature, and chemical composition of the oil and surfactant. The formation of spontaneous emulsion has been of concern in wastewater remediation. In this work, the hydrophilic-lipophilic difference (HLD) framework was used to predict the formation of spontaneous emulsions using a mixture of Span-80 and SLES surfactants, different oils, and salinity conditions. The spontaneity and emulsion behavior of different systems were modeled by estimating the HLDmix with a simple mixing rule. The influence of surfactant ratio, salinity, and oil type were investigated. Spontaneous emulsification could only be observed when the HLDmix was between -0.96 and 1.04 in our systems. Within this range, a negative HLDmix resulted in a greater spontaneity to form o/w emulsion, and a w/o emulsion was more likely to form when the HLDmix was positive. When the HLDmix was close to 0 (between -0.22 and 0.56 in our systems), emulsions were formed in both the oil and aqueous phases with high spontaneity. This trend was also confirmed by estimating the dispersed phase content with emulsion turbidity. A combined effect of ultra-low interfacial tension, Span-80 micelle swelling, and interfacial turbulence due to Marangoni effects are likely the main mechanisms of the spontaneous emulsification observed in this study. A synergistic reduction in interfacial tension was observed between Span-80 and SLES (&lt; 1 mN/m). When the HLD of the system was close to 0, a bicontinuous emulsion phase was formed at the oil-water interface. The bicontinuous emulsion broke up over time due to the ultra-low interfacial tension and interfacial turbulence, forming dispersed oil and water droplets. Results from this work provide a practical method to suggest what surfactant composition, salinity, and oil type could promote (or eliminate) the conditions favorable for spontaneous emulsification. These findings are valuable in many fields such as bilge water treatment and where nanometer-sized emulsions are used.

Spontaneous emulsions: Adjusting spontaneity and phase behavior by Hydrophilic-Lipophilic difference-guided surfactant, salt, and oil selection

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