Emulsions, which are mixtures of immiscible fluids, are widely used in pharmaceutical formulation, coatings, and oil recovery. Surfactants are used to stabilize emulsion droplets, and typically, surfactants are expected to be preferentially soluble in the continuous phase. However, if the surfactant is also soluble in the dispersed phase, then surfactant partitioning can occur. It has been shown previously with bulk-scale equilibrium partitioning studies that some water-soluble non-ionic surfactants can reach higher concentration in the oil phase than in the aqueous phase, even though such surfactants preferentially stabilize oil-in-water emulsions. In this work, we report on the partitioning of surfactants into microscale oil droplets of varying size and oil composition in non-equilibrium conditions, wherein the droplet itself is solubilizing and has a finite lifetime. We conduct single-droplet chemical analysis based on quantitative mass spectrometry and analyze the partitioning of surfactant into droplets over time. We also explore how surfactant partitioning can be exploited to induce phase separation inside droplets to create complex emulsions.

Nonequilibrium partitioning of surfactants into oil droplets

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