Dynamic partitioning of surfactants into non-equilibrium emulsion droplets

Emulsion droplets, such as oil-in-water droplets stabilized by surfactant, are ubiquitous in products ranging from food to pharmaceuticals to paints. However, emulsion droplets are often thermodynamically unstable and thus persist under non-equilibrium conditions for extended times. As such, equilibrium properties like partition coefficients or interfacial tensions may be inadequate to describe the properties of an out-of-equilibrium droplet that can potentially experience conditions not accessible at equilibrium. Here, the partitioning of nonionic surfactants between microscale oil droplets and water is investigated under non-equilibrium conditions wherein the droplets are shrinking in volume over time via solubilization. Quantitative mass spectrometry is used to analyze the composition of individual micro-droplets as a function of time under conditions of varying droplet diameter, surfactant molecular structure and concentration, and oil molecular structure. We find that common nonionic surfactants partition into the oil droplets over a timescale of minutes and reach a non-equilibrium steady state; this steady state concentration can be orders of magnitude higher than the aqueous phase surfactant concentration and higher than what is accessible under equilibrium partitioning conditions. Using kinetic data and steady state apparent partition coefficients, we describe the surfactant distribution between the water and droplet using a mass transfer model. Over longer timescales of hours, the droplet sheds accumulated surfactant back into the water, creating transiently high concentrations of oil and surfactant near the droplet interface which leads to the evolution of ultralow interfacial tension. Introduction of an ionic surfactant that forms mixed micelles with the nonionic surfactant reduces the nonionic surfactant transfer into oil; based on this observation, we use stimuli-responsive ionic surfactants to trigger phase separation and mixing inside droplets via modulation of the nonionic surfactant partitioning. This study thus reveals generalizable non-equilibrium states and conditions experienced by solubilizing droplets which govern emulsion properties.