Nonaqueous Deep Eutectic Solvent-Based Double Emulsions as a Novel Platform for Synthesizing Macroporous Beads with Tailored Morphologies

Nonaqueous deep eutectic solvent (DES)-based high internal phase double emulsions (double-HIPEs) were employed as a novel platform for synthesizing polyHIPE beads with a well-defined spherical shape and a 3D-interconnected macroporous structure. A primary DES₁/O HIPE was prepared using the monomer styrene and the cross-linker divinylbenzene (10:1 molar ratio) as the oil (continuous) phase (20 vol %) and stabilized with the nonionic Span 60 surfactant (10, 20, or 30 wt %). It was then dispersed at a controlled addition rate (0.9 or 1.8 mL/min) into DES₂, a surfactant and water-free suspension medium, forming stable DES₁/O/DES₂ double-HIPEs that were polymerized via free-radical polymerization. Different DESs were used as the polar phases (DES₁ and DES₂) and consisted of choline chloride with urea (ChCl), glycerol (G), or ethylene glycol (EG) (1:2 molar ratio), yielding viscosities of 750, 250, and 37 cP, respectively. The porosity and shape of the resulting spherical polyHIPE beads were finely tailored by adjusting the surfactant concentration, DES₁ and DES₂ viscosities, and primary HIPE addition rate, avoiding the complex fabrication techniques typically required in aqueous-based systems. Under these controlled conditions, uniform spherical beads were easily obtained with average bead diameters (D_b) ranging from 2.6 ± 0.2 to 1.5 ± 0.1 mm, average pore diameters (D_p) from 20.2 ± 0.2 to 10.2 ± 0.1 μm, degree of openness (O) from 18.2 to 12.3%, and specific surface areas (<i>S</i> _BET) from 5.78 to 2.8 m²g^-1. By contrast, aqueous W/O/W double emulsions prepared under analogous conditions produced irregular beads with collapsed porosity, reflecting insufficient stability during polymerization. In addition, the millimeter-sized spherical polyHIPE beads with the highest <i>S</i> _BET= 5.78 m²g^-1 (D_b = 1.8 ± 0.2 mm, O = 18.2%, D_p = 10.8 ± 0.3 μm) exhibited mechanical robustness (elastic modulus = 1.01 ± 0.21 MPa), a hydrophobic surface (contact angle of 95°), and easy packability into a glass column, enabling their use in a flow-through sorption process for the removal of emulsified engine oil from water. These materials achieved an oil sorption capacity (Q) of 383.04 mgg^{- 1} and oil removal efficiency (%R) of 92% at a high water-flow rate through the packed column (F_rate = 288.5 mLmin^-1) and were reused for over five sorption/desorption cycles without losing their oil uptake capacity. By comparison, a monolithic polyHIPE obtained by direct polymerization of the primary emulsion exhibited a lower <i>S</i> _BET = 3.2 m²g^-1 (O = 16.4%, D_p = 13.9 ± 0.6 μm), difficult packability, lower oil uptake capacity (%R = 81% and Q = 337.8 mgg^-1) and flowability (F_rate = 40 mLmin^-1), underscoring the advantages of the spherical beads. These results revealed that nonaqueous DES-based double emulsions constitute a novel platform for templating spherical polyHIPE beads, paving the way for the incorporation of a wide range of monomers and functional nanoparticles (<i>e.g.</i>, via Pickering templating) to produce application-tailored materials where spherical geometry, controlled pore architecture, and surface functionality are critical.