Assembly and jamming of polar additive-swollen microgels at liquid-liquid interfaces: From inverse Pickering emulsions to functional materials.

Hypothesis: Poly-N-isopropylacrylamide (PNIPAM)-based microgels have garnered significant interest as effective soft particulate stabilizers because of their deformability and functionality. However, the inherent hydrophilic nature of microgel restricts their potential use in stabilizing water-in-oil (W/O) Pickering emulsions. Employing diverse polar additives can improve the hydrophobicity of microgels, thus unlocking new possibilities in inverse Pickering emulsion formation and materials fabrication.

Experiments: Different types of microgels were generated using free-radical precipitation polymerization with tailored physiochemical properties. The effect of various polar additives on the wettability, adsorption kinetics, and interfacial coverage of microgels was systematically investigated. Additive-swollen microgels were utilized to stabilize inverse W/O Pickering emulsions, which served as templates to develop functional materials with stimuli responsiveness and hierarchical structures.

Findings: Additive-swollen PNIPAM-based microgels exhibited enhanced hydrophobicity and superior emulsifying capability, which spontaneously assembled and jammed at oil-water interfaces, resulting in a significant interfacial energy decrease. The additive-swollen microgels formed a tightly packed, elastic, and responsive microgel monolayer. The feasibility of the strategy was verified by preparing various inverse W/O Pickering emulsions and high internal phase Pickering emulsions (HIPPEs). More importantly, this straightforward formation strategy of microgel-stabilized inverse W/O Pickering emulsions offered a novel platform to create functional materials with customized inner structures from microscale (e.g., responsive core-shell hydrogel microspheres and colloidosomes) to macroscale (e.g., hierarchical porous materials) that can be used for potential applications, such as recyclable contaminant removal and droplet manipulation.