Pickering Stabilization of Emulsions and High Internal Phase Structures in Immiscible Deep Eutectic Solvents

Deep eutectic solvents (DESs) have emerged as versatile and sustainable liquids with highly tunable physicochemical properties; however, interfacial phenomena between immiscible DES pairs remain largely unexplored. Here, we investigate the formation and stabilization of fully nonaqueous DES-in-DES emulsions using a hydrophilic choline chloride-glycerol DES and a hydrophobic menthol-dodecanol DES. Fourier transform infrared spectroscopy and Raman spectroscopy reveal distinct hydrogen-bonding environments within the two DES phases and indicate weak interfacial reorganization of hydrogen-bond networks upon emulsification. Interfacial tension (IFT) measurements show that the DES pair possesses a relatively low IFT (∼10 mN m–1), which is only slightly reduced by the addition of surfactant or hydrophobic silica nanoparticles (NPs). Despite this reduction, surfactant-stabilized emulsions undergo rapid phase separation, demonstrating that lowering of IFT alone is insufficient to stabilize DES-DES interfaces. In contrast, hydrophobic silica NPs produce long-lived Pickering emulsions that remain stable for more than 250 days. Microscopy and macroscopic stability studies indicate that particle adsorption suppresses catastrophic coalescence and promotes the formation of stable emulsion structures. Increasing NP concentration improves stability and reduces droplet size, highlighting the importance of interfacial coverage and particle-mediated jamming. Extending this approach, we demonstrate the first stable DES-in-DES high internal phase Pickering emulsions (HIPPEs) containing up to 80 vol % dispersed phase. These results establish DES-in-DES Pickering emulsions as a new class of supramolecular soft material and suggest that mechanical interfacial reinforcement, rather than interfacial tension reduction, governs emulsion stability in hydrogen-bond-rich DES systems. The findings provide a foundation for developing high-loading DES formulations for encapsulation, separations, catalysis, and interfacial reaction engineering.