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.

Herein, we present the direct observation via liquidphase transmission electron microscopy (LPTEM) of the nucleation and growth pathways of structures formed by the so-called "ouzo effect", which is a classic example of surfactant-free, spontaneous emulsification. Such liquid−liquid phase separation occurs in ternary systems with an appropriate cosolvent such that the addition of the third component extracts the cosolvent and makes the other component insoluble. Such droplets are homogeneously sized, stable, and require minimal energy to disperse compared to conventional emulsification methods. Thus, ouzo precipitation processes are an attractive, straightforward, and energy-efficient technique for preparing dispersions, especially those made on an industrial scale. While this process and the resulting emulsions have been studied by numerous indirect techniques (e.g., X-ray and light scattering), direct observation of such structures and their formation at the nanoscale has remained elusive. Here, we employed the nascent technique of LPTEM to simultaneously evaluate droplet growth and nanostructure. Observation of such emulsification and its rate dependence is a promising indication that similar LPTEM methodologies may be used to investigate emulsion formation and kinetics.