Dynamic Partitioning of Surfactants into Non-Equilibrium Emulsion Droplets

Characterizing the propensity of molecules to distribute between fluid phases is key to describing chemical concentrations in heterogenous mixtures and the corresponding physiochemical properties of a system. Usually, partitioning is studied under equilibrium multiphase conditions. However, there are situations where a mixture may form only a single phase at equilibrium but can exist in multiple phases when out-of-equilibrium. Emulsion droplets, such as oil-in-water droplets stabilized by surfactant, are one such example. Droplets are often thermodynamically unstable and persist under non-equilibrium conditions for some time while disappearing by dissolution or solubilization over longer timescales. Consequently, equilibrium properties like partition coefficients or interfacial tensions may not necessarily describe the properties of an out-of-equilibrium droplet. Here, the partitioning of nonionic surfactants between microscale oil droplets and water is investigated under non-equilibrium conditions wherein the droplets shrink in volume over time via solubilization and only a single phase exists at equilibrium. Quantitative mass spectrometry is used to analyze the composition of individual micro-droplets as a function of time under conditions of varying surfactant composition, concentration, and oil molecular structure. Common nonionic surfactants partition into oil micro-droplets within minutes and reach a non-equilibrium steady state concentration that can be orders of magnitude higher than the aqueous phase. Over timescales of hours, the droplet solubilizes and sheds accumulated surfactant back into the water. Transiently high concentrations of oil and surfactant accumulate near the oil droplet - water interface over time, which leads to the localized evolution of a microemulsion phase and unexpectedly low interfacial tension. The 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 binary oil droplets via modulation of the nonionic surfactant partitioning. This study thus reveals generalizable non-equilibrium states and conditions experienced by solubilizing droplets that govern emulsion properties.