Surfactant Partitioning in Nanoemulsions.

Using a fractionated silicone oil-in-water nanoemulsion (NEM), which has a high ratio of surface area-to-volume, we investigate surfactant partitioning between the bulk continuous phase and the adsorbed interfacial phase. By adjusting the droplet volume fraction of this fractionated NEM and by using gravimetric and electrical conductivity methods, we measure the bulk and the surface concentrations of an ionic surfactant (sodium dodecyl sulfate, SDS), thereby obtaining a raw adsorption isotherm of SDS on the interfaces of the nanodroplets. To overcome significant uncertainties in the total surface area of this nanoemulsion, we have also measured the macroscopic interfacial tension (IFT) of silicone oil in contact with aqueous SDS solutions using the du Noüy ring method. We then scale the surface concentration of this raw isotherm using an appropriate Gibbs derivative based on the IFT measurement, yielding an adjusted isotherm. We show that this adjusted isotherm can be described using a simple Langmuir equation. In addition, we show that a significant and non-negligible percentage of surfactant typically partitions to nanodroplet interfaces after high-flow-rate emulsification (HFRE) has transformed a microscale premix emulsion into a NEM. We develop a model for predicting the final bulk surfactant concentration after HFRE given the initial bulk surfactant concentration before HFRE. We show that this model can be used to predict trends for surfactant partitioning in polydisperse nanoemulsions after HFRE.