Controlling water transport between micelles and aqueous microdroplets during sample enrichment

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

Spontaneous emulsification is a phenomenon that forms nanometer-sized droplets (nanodroplets) without the application of any external force, and the mechanism has been actively studied for application to various technologies. In this study, we analyzed the kinetics of spontaneous emulsification induced by Span 80. The measurement of water concentration in Span 80 hexadecane solution indicated that the chemical potential of water in the nanodroplets decreased as the amount of water in the nanodroplets decreased. Based on this result, water transport between the aqueous phase and nanodroplets in which the chemical potential of water was controlled was quantitatively investigated by using a microfluidic device. The results demonstrate that the kinetics of water transport during spontaneous emulsification induced by Span 80 was described by a model of osmotic transport through an organic liquid film between the aqueous phase and nanodroplets.

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

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 HLD_mix. The influence of surfactant ratio, salinity, and oil type was investigated. Spontaneous emulsification could only be observed when the HLD_mix was between -0.96 and 1.04. Within this range, a negative HLD_mix resulted in a greater spontaneity to form o/w emulsion, and a w/o emulsion was more likely to form when the HLD_mix was positive. When the HLD_mix 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.

Transport of Oligopeptide from Aqueous Phase to Span 80 Reverse Micelles in Microdroplet Array

The partitioning of water and tetramethylrhodamine-conjugated-10-residue oligopeptides from the aqueous phase of microdroplets into Span 80 reverse micelles was observed by utilizing microdroplet arrays. Each peptide was dissolved in phosphate buffer saline, and initially encapsulated in arrayed droplets. An organic phase containing the reverse micelles was added to the microdroplets. Here, the hydration degree of the reverse micelle was adjusted by contact of the organic phase with a 1.0 M NaCl aqueous solution or with a phosphate buffer saline before combining it with the microdroplets. For micelles treated with a 1.0 M NaCl, significant water transport from the microdroplet to the micelle was observed, and peptide with low solubility in water was transported to the reverse micelles, while those with high solubility in water were not. For micelles treated with phosphate buffer saline, the water transport was minimal, and no significant peptide transport was observed. These results suggest that the partitioning of low-solubility oligopeptides requires accompanying water transport to the reverse micelle phase.