Co-surfactant based interfacial strategies for the formation and stabilization of multiple nanoemulsions

There has been significant recent interest in the production of multiple emulsions – i.e., droplets in droplets – of increasingly small size, especially at the nanoscale. However, the energetic cost of stabilizing many highly curved interfaces makes it challenging to produce and stabilize such multiple nanoemulsions. Here, we propose a general method for promoting and stabilizing complex nanodroplet structures by manipulating their interfacial mechanics through co-surfactants of opposing spontaneous curvature. Using asymmetric pairs of ethoxylated co-surfactants, we experimentally show that this strategy induces the preferential formation of droplets with multiple highly curved interfaces and develop a theoretical framework to predict the observed droplet structures. Using neutron spin echo experiments, we measure the equilibrium dynamics of these ultra-low surface tension systems to determine the effective elastic constants of the mixed surfactant membranes. Using the measured elastic constants, we develop equilibrium interfacial free energy models to quantify conditions under which multiple nanoemulsion structures are stabilized, providing a rational means for engineering co-surfactant systems to promote the formation and stability of multi-phase nanodroplets.