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Abstract
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.
Supplementary materials
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Supporting Information
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Supporting figures, tables, video descriptions, and methods
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Video S1
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Video S1. SDS addition influences nonionic surfactant partitioning. Addition of 100 µL of 0.5 w/v% SDS to monodisperse droplets of m-xylene and HFE-7500 (3:1 volume ratio) produced in 0.5 w/v% Tergitol NP-9 (0.5 mL). SDS reduces the partitioning of the Tergitol into the drop, facilitating the mixing of the droplet oils to produce a single-phase droplet. Once the droplets were a single phase, 200 μL of 2.0 w/v% Tergitol NP-9 was added to induce phase separation again. Real time.
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Video S2
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Video S2. Using UV light to modulate surfactant partitioning. Monodisperse droplets containing m-xylene and HFE-7500 in a 3:1 volume ratio were produced in 0.25 w/v% Makon TD-9 and 0.25 w/v% AzoTAB mixed surfactant solution. Over time, Makon TD-9 accumulated inside the droplets and caused phase separation. After phase separation, samples were illuminated with UV light to photoswitch the AzoTAB surfactant. Upon photoswitching, mixing of the oil inside the droplets proceeded (i.e. Makon left the drop). Droplets phase separated when the UV light was turned off. 3x speed.
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Video S3
Description
Video S3. pH responsive modulation of surfactant partitioning. Monodisperse drops containing a 1:1 volume ratio of dibutyl phthalate and ethoxynonafluorobutane were produced in a 1.0 w/v% Triton X-100 and 0.5 w/v% N-dodecylpropane-1,3-diamine mixed surfactant solution (0.5 mL). Phase separation inside the droplets subsequently was observed due to partitioning of the Triton X-100. After phase separation, 50 μL of aqueous HCl solution (pH=1) was added to the sample. Mixing of the oils inside the droplets was observed due to Triton X-100 leaving the drops. After droplets were again a single phase, 50 μL of aqueous KOH solution (pH=14) was then added to the sample to induce phase separation once more. 2x speed.
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