Monday, March 13, 2023 - 11:15am to 1:00pm
ESB 2001 Zoom link: https://ucsb.zoom.us/j/9022337323
An Interfacial Strategy to Stabilize Multiple Nanoemulsions with Mixtures of Asymmetric Co-Surfactants
Multiple nanoemulsions consist of solutions with droplets on the nanoscale (by convention, < 200 nm in diameter) that comprise multiple entrained phases (typically distinct hydrophilic and hydrophobic regions). Although typically unstable and difficult to control in terms of droplet yield and internal structure, multiple nanoemulsions remain of great interest to a number of emerging applications in which nano-sized carrier systems that potentially encapsulate and protect higher quantities of otherwise insoluble materials are desired. Recent advancements in the production of these solutions to create complex, structured nanomaterials have demonstrated remarkable potential to utilize multiple nanoemulsions to template compartmental nanoparticles and nanocapsules. Moreover, these emerging techniques demonstrate the possibility of leveraging co-surfactant design and their resulting interfacial behavior to control and stabilize complex droplet morphologies. Here, we propose and theoretically validate a general method for promoting and stabilizing multiple nanoemulsion structures by manipulating their interfacial mechanics via co-surfactant selection. Using small angle neutron scattering, we experimentally characterize nanodroplets made using several asymmetric pairs of ethoxylated co-surfactants. We demonstrate that increased co-surfactant asymmetry favors the formation and stabilization of multiple nanoemulsions. Neutron scattering measurements also enable us to probe the equilibrium dynamics of mixed surfactant layers across nanoscale droplets as well as quantify the preferred partitioning of co-surfactants across interfaces in core-shell nanodroplets. The results of these studies suggest mixtures of asymmetric co-surfactant exhibit frustrated interfacial dynamics and nonideal mixing. Using equilibrium interfacial free energy models, we establish a theoretical basis for the preferred stabilization of multi-phase core-shell nanodroplet structures formed with co-surfactants of opposing spontaneous curvatures and demonstrate these models may be used to predict and quantify conditions under which multiple nanoemulsions may be produced. We experimentally verify the predictions made by these models using small-angle neutron scattering to characterize the droplet morphologies of nanoemulsions made using asymmetric pairs of ethoxylated co-surfactants. We show that the co-surfactant based interfacial strategy allows for some degree of control over the internal droplet morphology and provides a rational means of engineering desirable multi-phase nanodroplets.