Hierarchical stabilization of emulsions with multi-scale interconnected droplets and ultra-low nanoparticle loadings

Abstract

Abstract Pickering stabilization by colloidal particles is a common strategy to disperse droplets of one fluid into another fluid in food, cosmetics and chemical industries1-3. For over a century, this kind of stabilization has been governed by constant surface coverage concepts in which particles irreversibly attach to the fluid–fluid interface. The need to cover sufficient interfacial area to prevent coalescence typically results in large loadings of particles, uniform droplet size, creation of rigid interface and closed-cell structure with small total area4-7. Here we report a stabilization mechanism that yields hierarchically structured oil-in-brine emulsions with high interfacial area, deformability, connectivity and long-term stability at unprecedentedly low nanoparticle loadings. The hierarchy in structure is achieved via dynamic cation-particle-droplet interactions in cascaded emulsification, which consists of i) formation of submicron oil droplets (~250 nm) lightly covered by hydrophilic polymer-coated iron oxide nanoparticles and polyvalent metal ions; ii) spontaneous formation of small droplets of nonpolar oil (~1 μm) stabilized by the nanodroplets and cations and iii) attachment of nanodroplet/small droplet clusters to bridge large unarmoured oil droplets (5-50 μm) in macroemulsions. This new mode of stabilization enables much more efficient use of nanoparticles, stabilizing a given size macroemulsion droplet at an order of magnitude smaller particle loading. Moreover, particle loading decreases with the 5/3 power of droplet size, rather than the first power typical of Pickering emulsions. Finally, cations play a novel and essential role in this mechanism, which cannot be accommodated in the conventional Pickering model. Our approach provides a new pathway for templating materials with better control over the structure, and for exploiting applications that are currently inaccessible for Pickering and surfactant stabilized emulsions.