Spontaneous Emulsions: Adjusting Spontaneity and Phase Behavior by Hydrophilic–Lipophilic Difference-Guided Surfactant, Salt, and Oil Selection

Abstract

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. 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. The spontaneity and emulsion behavior of different systems were modeled by estimating the HLDmix. The influence of surfactant ratio, salinity, and oil type was investigated. Spontaneous emulsification could only be observed when the HLDmix was between −0.96 and 1.04. 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. A combined effect of ultralow interfacial tension, Span-80 micelle swelling, and interfacial turbulence due to Marangoni effects is likely the main mechanism of the spontaneous emulsification observed in this study. A synergistic reduction in interfacial tension was observed between Span-80 and SLES (<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 ultralow 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.

Kinetic description of water transport during spontaneous emulsification induced by Span 80

“…3,20 The mechanism of spontaneous emulsification is different depending on the surfactants. Several types of mechanisms have been reported based on ultralow interfacial tension, 2,20 solubility changes, 21 phase transition of the organic phase 11,16 and the interface induced by temperature changes. 15 Recently, spontaneous emulsification induced by Span 80, a well-known non-ionic surfactant, has been reported (Fig.…”

“…3,19 Also, it is known that the mixture of surfactants efficiently induces spontaneous emulsification due to low surface tension dynamic effects such as the Marangoni effect. 3,20 The mechanism of spontaneous emulsification is different depending on the surfactants. Several types of mechanisms have been reported based on ultralow interfacial tension, 2,20 solubility changes, 21 phase transition of the organic phase 11,16 and the interface induced by temperature changes.…”

Integrated Thermoelectric Design Inspired by Ionic Liquid Microemulsion-Based Gel with Regulatable Dual-Temperature Responsiveness

“…Microemulsion is a transparent thermodynamically stable system, composed of an oil phase, water phase, surfactant, and cosurfactant. , When exposed to external stimuli, such as extreme temperature, CO 2 , inorganic salts, and pH, the microemulsion droplets will aggregate or coagulate, resulting in turbidity changes. − However, due to the inherent thermodynamic stability, it will spontaneously return to the uniform and transparent state after removing the external stimuli. , Hence, the microemulsion exhibits dual-temperature sensitivity, which will take place noticeable transmittance changes of opacity–transparency–opacity with the increase of temperature. Also, this phenomenon has stable repeatability.…”

Quantifying the Influence of Electrolytes on the Kinetics of Spontaneous Emulsification

“…Another study with ionic emulsifiers revealed that the thickness of the intermediate layer correlates positively with the rate of spontaneous emulsion . In the case of well-mixed W/O and O/W emulsions, the intermediate layer can contain both emulsions . The intermediate layer was proposed to consist of the emulsified droplets and surfactant layer (Figure ).…”

“…27 In the case of well-mixed W/O and O/W emulsions, the intermediate layer can contain both emulsions. 28 The intermediate layer was proposed to consist of the emulsified droplets and surfactant layer (Figure 1). More recently, we have shown a spontaneous emulsification process, 29 in which surfactant movement through the intermediate layer becomes critical.…”