Inverse Pickering Emulsion Polymerization of Water-Soluble Monomers: Toward Hydrophilic Microspheres with Tunable Morphology

Abstract The growing demand for efficient recovery of metal ions from secondary aqueous resources requires high-performance hydrophilic adsorbents. Here, Pickering emulsion polymerization technology is extended to inverse water-in-oil (w/o) emulsions, enabling the surfactant-free synthesis of hydrophilic polymer microspheres directly from water-soluble vinyl monomers. Thiol-functionalized silica nanoparticles (NP-SH) were used to stabilize inverse Pickering emulsions containing methacrylic acid (MA), 1-vinylimidazole (VIm), and N,N′-methylenebis(acrylamide) (MBA). Fluorescence microscopy confirmed the formation of stable inverse emulsions, while the precursor droplet size increased systematically with increasing MA content, demonstrating that particle size is established during emulsification and governed by Pickering stabilization. Polymerization yielded surface-nanostructured hydrophilic microspheres with composition-dependent internal architectures. Intermediate VIm:MA compositions formed the most compact polymer networks, consistent with enhanced intermolecular MA/VIm interactions, whereas composition extremes produced more porous structures. Cu2+ adsorption was governed primarily by the density of imidazole N-donor sites rather than by particle morphology, reaching equilibrium adsorption capacities of approximately 53 mg·g–1 and theoretical Langmuir capacities of 114 mg·g–1, comparable with state-of-the-art VIm-based adsorbents. These results establish inverse Pickering emulsion polymerization as a versatile platform for producing hydrophilic, surface-nanostructured polymer microspheres from water-soluble monomers and substantially broaden the scope of Pickering emulsion polymerization for metal-ion recovery and water-remediation applications.