One-pot synthesis of rationally-designed flexible, robust, and hydrophobic ambient-dried molecularly-bridged silica aerogels with efficient and versatile oil/water separation applications

Silica aerogels (SAs) combine low density, high porosity, and large surface area, enabling applications from insulation to environmental remediation. However, their commercialization is hindered by brittleness, poor mechanical robustness, hydrophilicity/hygroscopicity, and energy-intensive supercritical drying. The research question addresses whether molecularly bridged silica networks, synthesized via a green, one-pot process and dried under ambient pressure, can deliver flexible, robust, and intrinsically hydrophobic aerogels with high performance in oil sorption and oil/water separation. The study’s purpose is to rationally design bis-silane precursors bearing flexible spacers to reduce crosslinking density and introduce rotatable bonds and methyl groups for enhanced flexibility and hydrophobicity, while maintaining high porosity and surface area. The work aims to establish structure–property–performance relationships and demonstrate feasibility for practical oil spill cleanup and oily wastewater treatment, including under harsh conditions.

Prior efforts to improve SA mechanics include tuning sol–gel conditions (pH, temperature, solvent, precursor selection), lowering crosslinking density via di-/trialkoxy silanes, adjusting pore size and skeletal ratios, reinforcing with polymer foams or fillers, and introducing organosilane co-precursors. While effective, these strategies can involve complex procedures, added cost, or limited control. Molecular bridging has emerged as a promising approach: flexible linkers (C–C, C–O, C–S) with low rotational barriers are incorporated between silicon centers to reduce network rigidity. Bis-silane monomers have been synthesized via various chemistries (Schiff base, Heck, isocyanate reactions, thiol-isocyanate), but the thiol-ene click reaction offers selectivity, operational simplicity, mild conditions, and green processing without costly catalysts, while also introducing flexible thioether bonds. Existing bridged silsesquioxane aerogels show enhanced flexibility but often rely on vacuum or supercritical drying. The present work builds on this by using UV-assisted thiol-ene to craft new bis-silane precursors and employing ambient pressure drying to further simplify and green the process, targeting improved mechanics, hydrophobicity, porosity, and scalability.

Synthesis: Two symmetric bis-silane precursors were prepared in situ via UV-initiated thiol-ene click reaction using (3-mercaptopropyl)methyldimethoxysilane (MPMDMS), a divinyl ether spacer, and photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA) at a molar ratio of 2:1:3×10^3. The spacers were 1,4-butanediol divinyl ether (BDVE) and 1,4-cyclohexanedimethanol divinyl ether (CDVE), yielding precursors Si_pr-C4 and Si_pr-cyH, respectively. Reactions proceeded for 1 h at −25 °C under UV. Without isolation (one-pot), the precursors underwent a two-step acid–base catalyzed sol–gel process (ethanol as green solvent): acid-catalyzed hydrolysis of Si–O–CH3 to silanols (Si–OH), followed by base-promoted polycondensation to form Si–O–Si linkages. Gels were briefly aged, washed, and dried via ambient pressure drying (APD). Final aerogels are denoted BSA-C4 and BSA-cyH. The methyl groups and flexible ether/thioether bridges were intended to reduce capillary stresses during APD and impart hydrophobicity and flexibility. Characterization: Chemical structure was analyzed by FTIR (consumption of thiol/vinyl; emergence of C–S–C and ether signatures; Si–O–Si bands), solid-state 29Si and 13C MAS NMR (D1/D2 species and degree of condensation; carbon environments), and XPS (elemental composition and bonding states; confirmation of thioether). Morphology and microstructure were examined via SEM; skeletal/void metrics via pycnometry (porosity), and N2 adsorption–desorption (SSA). Mechanical properties were determined by compression (including 200-cycle fatigue at 80% strain) and tensile tests. Wettability was assessed by static water contact angle (WCA) and droplet tests with various oils/solvents. Oil sorption and separation: Sorption kinetics and equilibrium were measured up to 200 min for multiple organic solvents and oils (17 tested; properties in Supplementary Table S1). Saturation capacities were obtained by soaking–weighing. Recyclability was tested by manual squeezing and vacuum drying across cycles. Static oil/water separations were demonstrated for heavy (e.g., chloroform underwater) and light (e.g., toluene floating) oils. Performance under harsh simulated conditions was assessed (continuous shaking to mimic waves/turbulence; vigorous stirring for whirlpools). Dynamic continuous separation employed a custom vacuum-assisted pumping setup for in situ oil recovery through the aerogel. Oil flux was quantified over 5 cycles with various fluids. Demulsification tests for oil-in-water emulsions were performed for toluene-in-water (T/W) and hexane-in-water (H/W) emulsions stabilized by surfactant (Tween 80); microscopy quantified droplet sizes and separation efficiencies over time.

- Successful green, one-pot synthesis of new symmetric bis-silane precursors via UV thiol-ene coupling (MPMDMS + BDVE or CDVE), followed by acid–base sol–gel and APD to yield monolithic bridged silica aerogels (BSA-C4, BSA-cyH). - Chemical/structural validation: FTIR confirmed consumption of thiol/vinyl groups and formation of ether/thioether linkages; strong Si–O–Si bands. 29Si MAS NMR showed exclusively D1 (~−14 ppm) and D2 (~−22 ppm) species with high degrees of condensation: ~94% (BSA-C4) and ~95% (BSA-cyH). 13C MAS NMR resolved carbons near O/S in bridges; no residual vinyl or alkoxy carbons. XPS confirmed C, O, Si, S; dominant thioether S 2p (~163.1 eV); higher C:Si ratio for BSA-cyH consistent with bulkier cyclohexane spacer. - Physicochemical properties (Table 1): Gelation times 49±2 min (BSA-C4) and 54±2 min (BSA-cyH). Linear shrinkage 4.83±1.27% (BSA-C4) and 6.17±0.59% (BSA-cyH). Bulk density 0.098±0.006 g cm⁻3 (BSA-C4) and 0.12±0.03 g cm⁻3 (BSA-cyH). Porosity 90.30±2.60% (BSA-C4) and 91.70±2.10% (BSA-cyH). SSA 77.31 m² g⁻1 (BSA-C4) and 117.64 m² g⁻1 (BSA-cyH). WCA 140.9°±1.9° (BSA-C4) and 143.6°±0.9° (BSA-cyH). - Microstructure: Interconnected 3D macroporous networks with strong interparticle necks. Average particle size 3.49±0.82 µm (BSA-C4) and 4.51±1.17 µm (BSA-cyH). Average pore diameter 3.57±2.58 µm (BSA-C4) and 5.51±3.03 µm (BSA-cyH). After 200 compression cycles at 80% strain, particle sizes remained comparable; pore size distribution shifted modestly to wider pores without structural collapse. - Mechanics: Exceptional flexibility and resilience. BSA-cyH withstood up to 99% compressive strain at ~12 MPa with full recovery; BSA-C4 up to 90% at ~670 kPa. Under 200 compression cycles at 80% strain, BSA-cyH showed no fracture/plastic deformation; BSA-C4 showed ~6% plastic deformation. Compressive stress at 80% strain: ~127 kPa (BSA-C4), ~143 kPa (BSA-cyH). Tensile elongation at break: ~38% (BSA-C4) and ~47% (BSA-cyH); tensile strength ~7 and ~8.1 kPa, respectively. - Wettability: Superoleophilic and highly hydrophobic; water droplets retained spherical shape; average WCA >140°. Aerogels float on water for at least 8 weeks. - Oil sorption: Fast uptake for low-viscosity solvents (saturation within ~150 s); slower for high-viscosity oils (~30 min). Saturation capacity scaled with liquid density; up to ~19 g g⁻1 for chloroform. BSA-cyH generally showed higher capacities (higher porosity, larger pores), while BSA-C4 retained some low-viscosity solvents better (smaller pores increase capillarity and reduce drainage). - Recyclability (static): BSA-cyH maintained high capacities over 10 squeeze cycles for toluene, DMF, diesel; near-100% desorption after the first cycle; WCA remained >140°; SEM showed intact structure. - Static oil/water separation: Selective removal of heavy (underwater chloroform) and light (floating toluene) oils; underwater, aerogels showed silver mirror effect due to trapped air; rapid capillary-driven uptake without water intrusion. - Performance under harsh conditions: Efficient separation under continuous shaking (waves/turbulence) and stirring (whirlpool); aerogels retained oil and continued to float. - Dynamic continuous separation: Vacuum-assisted setup enabled continuous oil suction and in situ recovery with high selectivity; oil flux up to ~10,392 L m⁻2 h⁻1 depending on viscosity/density (lowest for viscous diesel; reduced for dense DMF). High flux retained over 5 cycles with minor declines attributed to residual pre-wetting. - Emulsion demulsification: For stabilized toluene-in-water (avg droplet ~7.3 µm) and hexane-in-water (avg ~4.6 µm) emulsions, separation efficiency reached up to 99.8% within 30 min. T/W demulsified faster (~90% in 15 min) than H/W (~90% in 30 min). Mechanism: superoleophilic capture, droplet flattening/attachment, coalescence on pore walls, capillary storage; pore size and SSA modulated kinetics and final efficiency.

The results confirm that molecularly bridged silica networks synthesized via UV thiol-ene coupling and processed with ambient pressure drying can overcome the traditional brittleness and hydrophilicity of silica aerogels. Flexible ether/thioether bridges and abundant methyl groups lower the effective crosslinking density and introduce rotatable bonds, yielding high degrees of freedom in the network and intrinsic hydrophobicity. Structural analyses (FTIR, NMR, XPS) validate the intended chemistry, while SEM reveals thick interparticle necks and interconnected macroporosity that underpin mechanical robustness and energy dissipation under large strains. The cyclohexane-bridged BSA-cyH, featuring bulkier linkers and slightly higher condensation, forms more cyclic siloxane motifs and thicker neck regions, leading to larger particles/pores, slightly higher density, higher SSA, and superior mechanical resilience compared to BSA-C4. These structure–property relationships translate directly into application performance. Hydrophobicity and superoleophilicity ensure selectivity for oil over water in both static and dynamic settings, including harsh wave/whirlpool conditions. Macroporosity and pore tortuosity enable rapid capillary-driven uptake, strong retention, and high through-flux under vacuum. Pore size governs kinetics and retention: smaller pores enhance capillary forces and reduce drainage for low-viscosity solvents; larger pores facilitate faster transport of viscous oils and higher dynamic flux. High SSA and suitable pore dimensions allow efficient demulsification by promoting droplet collision, attachment, and coalescence on oleophilic surfaces. Overall, the findings demonstrate that rational molecular design affords tunable networks that meet mechanical and separation performance targets while enabling scalable, greener processing (APD).

This work introduces two previously unreported symmetric bis-silane precursors and their corresponding bridged silica aerogels (BSA-C4, BSA-cyH) synthesized via a green, one-pot UV thiol-ene/sol–gel route with ambient pressure drying. The materials combine ultra-low density (0.098–0.12 g cm⁻3), high porosity (~90–92%), large SSA (up to ~118 m² g⁻1), intrinsic hydrophobicity (WCA >140°), and exceptional flexibility (withstanding up to 99% compressive strain; 38–47% tensile elongation). Robust recyclability and stability under simulated harsh conditions were demonstrated. The aerogels delivered fast oil uptake, high sorption capacities (up to ~19 g g⁻1 for dense solvents), selective static oil/water separation, efficient demulsification (η up to 99.8%), and ultrahigh dynamic oil flux (up to ~10,392 L m⁻2 h⁻1), with sustained performance across cycles. The study establishes clear structure–property–performance relationships: linker identity (linear vs cyclic) modulates condensation pathways, neck thickness, pore/particle size, SSA, and mechanics, enabling property tailoring. The approach provides a scalable path to families of flexible, hydrophobic silica aerogels for environmental remediation and beyond. Future work could expand the molecular linker library (length, rigidity, functionality), optimize pore architecture for targeted fluids (viscosity/droplet sizes), integrate functionalities (e.g., magnetic recovery, catalysis, antifouling), and scale device-level implementations for continuous separation in industrial settings.

- Relative to traditional TEOS-derived aerogels, bridged networks generally show lower specific surface areas due to larger particle sizes and macroporosity. - Ambient pressure drying, while green, still induces some shrinkage and can cause partial pore collapse or structural rearrangement, especially for the more malleable BSA-C4 (lower SSA). - Sorption of highly viscous oils is slower and may not fully access available porosity within practical timescales; dynamic flux decreases with viscosity and, to a lesser extent, with higher liquid density. - Gravitational drainage reduces measured capacities for low-viscosity solvents in larger-pore aerogels (BSA-cyH) immediately after removal from baths. - Recyclability by squeezing does not achieve complete desorption in the first cycle and may leave small residuals; performance depends on fluid viscosity and network pre-wetting. - Minor oxidized carbon and sulfur species were detected (≤~2%), arising from air/UV exposure; although minimal, they indicate some sensitivity of thioether/ether segments to oxidation. - Gelation times, though short for bridged systems, are longer than some mono-silane routes; process parameter windows for different linkers may require optimization for scale-up.