(19h) Interfacial Demixing of Co-Surfactant “Frenemies” Stabilizes Complex Nanoemulsions

Stabilization of emulsion droplets at the nanoscale – whether as thermodynamically stable microemulsions or metastable nanoemulsions – requires careful selection of surfactants to promote their formation and stabilization. Specifically, the spontaneous curvature of individual surfactants is often used as a guide for this purpose, including engineering of low-energy emulsification processes and control of resulting droplet sizes. In this work, we demonstrate how relatively simple mixtures of surfactant “frenemies” – co-surfactants with similar chemistry but dissimilar spontaneous curvature – can be used to stabilize complex multi-phase nanodroplet structures. First, we show that nonionic co-surfactant pairs with increasing differences in spontaneous curvature lead to state diagrams for both oil-in-water and water-in-oil nanoemulsions that contains an increasing window of multi-phase nanodroplet structures including core-shell and multi-shell vesicle-like nanodroplets. Using contrast-variation neutron scattering, we directly show that the appearance of these structures coincides with interfacial demixing of the co-surfactants, whereby surfactants preferentially partition to interfaces that better match their spontaneous curvature. Based on this idea, we develop a simple interfacial thermodynamic model to predict the conditions of droplet morphology and size over which interfacial demixing occurs, and compare it to experimental measurements. The results show considerable promise and new tools for using co-surfactant mixtures to engineer complex nanostructured emulsions and solution self-assemblies.