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

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Abstract
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 (Db) ranging from 2.6 ± 0.2 to 1.5 ± 0.1 mm, average pore diameters (Dp) 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 (S 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 S BET= 5.78 m2g–1 (Db = 1.8 ± 0.2 mm, O = 18.2%, Dp = 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 (Frate = 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 S BET = 3.2 m2g–1 (O = 16.4%, Dp = 13.9 ± 0.6 μm), difficult packability, lower oil uptake capacity (%R = 81% and Q = 337.8 mgg–1) and flowability (Frate = 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 (e.g., via Pickering templating) to produce application-tailored materials where spherical geometry, controlled pore architecture, and surface functionality are critical.
Funding Information
  • Universidad Nacional Aut?noma de M?xico (IN115624)
  • Consejo Nacional de Humanidades, Ciencias y Tecnolog?as
  • Secretar?a de Ciencia, Humanidades, Tecnolog?a e Innovaci?n (CF-2023-I-1339)

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