Bulk-Interfacial Partitioning of Surfactant in Concentrated Nanoemulsions

We explore the partitioning of amphiphilic dodecyl sulfate (DS^-) surfactant anions onto interfaces of nanodroplets in a fractionated silicone oil-in-water nanoemulsion that has a well characterized size distribution and a large surface-area-to-volume ratio. We quantify this partitioning by diluting a concentrated nanoemulsion and measuring bulk concentration across a semi-permeable membrane and measuring the concentration of DS^- using conductivity and gravimetric methods. We calculate and obtain an adsorption isotherm, which compares reasonably well with one obtained through interfacial tension measurments on a macroscopic interface at different bulk concentrations of DS^-. Using high flow rate emulsification, we also show that an appreciable amount of surfactant leaves the bulk continuous phase and adsorbs onto the newly formed interfacial area as microscale droplets are broken down into nanodroplets by the flow. We have developed a model to describe the change in the bulk continuous surfactant concentration before and after high flow rate emulsification. Creating a nanoemulsion at high droplet volume fractions can effectively consume surfactant from the bulk as it partitions onto the high surface area of the nanodroplets.