Heterostructure particles enable omnidispersible in water and oil towards organic dye recycle

The study addresses the challenge of dispersing colloidal particles across mutually incompatible liquids, particularly water and nonpolar oils, which is crucial for applications in self-assembly, catalysis, inks, and biomedical uses. Conventional dispersion relies on surfactant modification (monomeric, polymeric, biomacromolecular) or adding nanoprotrusions (hedgehog- or raspberry-like), which can shield native surfaces and alter intrinsic activity, often with complex fabrication. Inspired by 2D chemically heterogeneous surfaces that promote both water and oil wetting via alternating hydrophilic/hydrophobic domains, the authors hypothesize that constructing 3D colloidal particles with nanoscale hydrophilic-hydrophobic surface heterogeneity will enable omnidispersion without added surfactants or protrusions. The purpose is to develop such heterostructure particles (HL-HBPs), elucidate their dispersion mechanism, and leverage their omnidispersity for rapid adsorption and recyclable recovery of organic dyes from wastewater through simple solvent exchange. The importance lies in providing a broadly applicable, simpler route to stable dispersions in diverse solvents and introducing a fast, regenerable dye recycling platform.

Prior work to achieve stable colloidal dispersions has centered on: (1) surface modification using surfactants or polymers to enhance solid–liquid affinity and provide electrostatic or steric stabilization; and (2) engineering particle topography with nanoprotrusions (e.g., hedgehog, raspberry particles) to reduce contact area and van der Waals attraction. While effective, these methods import foreign species, can mask active surfaces, and involve complex syntheses. On planar materials, chemical heterogeneity (alternating hydrophilic/hydrophobic domains) has produced superamphiphilic behavior (e.g., TiO2, silicon surfaces) by capillary-driven spreading of both water and organics. Translating this concept to 3D particles for omnidispersion remains largely unexplored. In wastewater treatment, dye removal strategies include coagulation–flocculation, biodegradation, catalytic oxidation, membrane nanofiltration, and porous adsorbents (activated carbons, MOFs). These focus on removal rather than recyclable recovery of dyes, and elution typically requires inorganic acids/bases/salts, complicating downstream purification. Thus, there is a gap for materials that disperse in both aqueous and organic phases and enable rapid, reversible dye capture and recovery with simple organic solvents.

Synthesis of HL-HBPs: Hydrophilic–hydrophobic heterostructure particles were synthesized by emulsion interfacial polymerization using polystyrene (PS) templates to create porous structures. For negatively charged HL-HBPs (PSS-PSDVB), an oil-in-water emulsion contained hydrophilic monomer sodium 4-styrenesulfonate (SS) in water and hydrophobic monomers styrene (St) and divinyl benzene (DVB) in oil. Steps: (i) Emulsify 1-chlorododecane in SDS solution, disperse uniform PS particles in SDS solution, stir at 40 °C for 20 h. (ii) Dissolve SS in SDS solution and mix with St, DVB, and AIBN; ultrasonicate to form oil-in-water emulsion; add to flask and stir at 40 °C for 6 h. (iii) Add PVA solution, deoxygenate with nitrogen, polymerize at 70 °C for ≥10 h. Wash with deionized water and ethanol. For positively charged HL-HBPs (PTMAEMC-PSDVB), similar steps used PVA as stabilizer and 2-(trimethylammonium)ethyl methacrylate chloride (TMAEMC) as hydrophilic monomer with St/DVB in oil. Post-treatment: Improve pore interconnectivity by dissolving/removing linear PS templates in dichloromethane (DCM), three cycles (1 h each), then dry. Characterization: SEM (Hitachi SU8010) for morphology; TEM (FEI Tecnai G2 20) on epoxy-embedded ultrathin sections (~100 nm) to visualize core–shell-like domains; PiFM (VistaScope, Molecular Vista) for nanoscale chemical imaging and infrared spectra at 1102, 1451/1491, and 1735 cm⁻¹ to map PSS, PSDVB, and epoxy; AFM (Bruker Dimension FastScan Bio-ICON) PeakForce QNM mode for topography and adhesion force mapping to distinguish hydrophilic/hydrophobic domains. Particle structural metrics (diameter, porosity, BET surface area) were tuned via template size, monomer type, and porogen volume. Dispersion mechanism calculations: Interaction potentials between two HL-HBPs were computed via DLVO theory treating particles as core–shell spheres with pores filled by the suspending medium. van der Waals potential Vvdw calculated using Hamaker model with effective Hamaker constants accounting for porosity and medium (water or octane). Electrostatic potential VDL computed via Poisson–Boltzmann expression using solvent dielectric constant, measured zeta potentials (water: ζ −45.8 mV for PSS-PSDVB, +42.9 mV for PTMAEMC-PSDVB; octane: ζ −19.9 mV and +7.1 mV), and Debye lengths (water κ⁻1 ≈ 100 nm; octane κ⁻1 ≈ 100 µm assumed for weak ionic screening). Total potential Vtotal = Vvdw + VDL. Dispersion tests: Visual dispersion in solvents spanning polarity: water, DMSO, acetonitrile (ACN), ethanol, ethyl acetate (EA), toluene, tetrachloromethane (TCM), octane. Comparators: purely hydrophilic particles (sulfonated PSDVB) and hydrophobic PSDVB particles. Dye adsorption/desorption experiments: Equal volumes of HL-HBP suspension (concentration CHL-HBPs) and dye solution (initial concentration C0) were mixed for a set time t at room temperature, then filtered (2 µm). UV–vis spectroscopy (Shimadzu UV-2600) quantified dye concentration C to compute adsorption efficiency (%) = (C0 − 2C)/C0 × 100 and adsorption amount (µg mg⁻1) = (C0 − 2C)/CHL-HBPs. Tested cationic dyes (e.g., MG, MV, RB, CV, MLB) with negatively charged PSS-PSDVB and anionic dyes (e.g., MB, AF, CR, EB, MO) with positively charged PTMAEMC-PSDVB. An adsorption index nsum/Mw was devised by counting dye interaction sites (electrostatic, hydrogen-bonding, hydrophobic/π–π) normalized by molecular weight. Kinetics and mechanism visualization: In situ laser scanning confocal microscopy (Nikon Eclipse Ti LSCM) imaged RB uptake by PSS-PSDVB HL-HBPs at t = 5 s to 3 min; intensity profiles quantified penetration. Particle diameter series (≈3.9, 4.3, 6.2, 12.6 µm) and BET surface area series (≈7.6, 10.2, 13.0, 14.8 m² g⁻1) assessed effects on adsorption depth and capacity. pH stability tested from ≤0 to 14. Desorption tested by adding organic solvents and monitoring rapid dye release. Solvent-exchange dye recycle: Process flow: (1) Adsorb dyes from water with HL-HBPs (optimized CHL-HBPs ≈ 10 mg mL⁻1 for 20 ppm RB). (2) Filter to separate clean water and dyed particles. (3) Redisperse dyed particles in low-boiling organic solvents (e.g., DCM, methanol, ethanol; or mixtures with n-pentane/n-hexane) to desorb. (4) Filter to collect dyed organic phase and regenerated particles. (5) Distill organics at 40–80 °C to recover dye powders; reuse HL-HBPs. Recycling performance evaluated over 10 cycles; scale-up demonstrated to 2 L using a 5 L reactor.

• Structure and heterogeneity: HL-HBPs (PSS-PSDVB) are uniform, porous spheres with mean diameter 3.87 ± 0.14 µm (n > 200), pore sizes from a few nanometers to hundreds of nanometers. Cross-sectional TEM confirms hydrophilic PSS outer domains and hydrophobic PSDVB inner domains accessible via pores. PiFM maps at 1102 (PSS), 1491/1451 (PSDVB), and 1735 cm⁻1 (epoxy) and AFM adhesion mapping verify alternating hydrophilic/hydrophobic domains on particle surfaces.
• Omnidispersity: HL-HBPs disperse stably in solvents spanning high to low polarity: water, DMSO, ACN, ethanol, EA, toluene, TCM, octane. In contrast, hydrophilic-only particles agglomerate in low-polarity solvents, and hydrophobic-only particles agglomerate in water. Gentle shaking re-disperses HL-HBPs if sedimentation/floatation occurs due to density mismatch.
• DLVO analysis: Calculations indicate stable dispersion in both water and octane due to combined effects of medium-wetted pores and charged hydrophilic domains. Representative zeta potentials: in water, ζ = −45.8 mV (PSS-PSDVB) and +42.9 mV (PTMAEMC-PSDVB); in octane, ζ = −19.9 mV and +7.1 mV. In oil, both Vvdw and VDL are reduced in magnitude but long-range electrostatic effects persist due to large Debye length, supporting stability.
• Dye adsorption selectivity and capacity: Negatively charged HL-HBPs (PSS-PSDVB) capture cationic dyes; positively charged HL-HBPs (PTMAEMC-PSDVB) capture anionic dyes. Adsorption amounts vary by dye and correlate positively with an adsorption index (nsum/Mw) accounting for electrostatic, hydrogen-bonding, and hydrophobic/π–π interaction sites, with minor deviations (e.g., RB vs MV/CV).
• Ultrafast kinetics: LSCM shows RB adsorption within ≈5 s with negligible increase up to 3 min. Batch tests across CHL-HBPs = 1.33–13.33 mg mL⁻1 reach equilibrium by 5 s.
• Depth and size effects: For smaller particles (≈3.9–4.3 µm), dyes penetrate to the innermost regions; for larger particles (6.2 and 12.6 µm), adsorption depth λ ≈ 1.4–1.6 µm, consistent with Debye-length-limited electrostatic attraction plus additional interactions.
• Surface area dependence: Increasing BET surface area from 7.6 to 10.2, 13.0, and 14.8 m² g⁻1 increases RB adsorption amount from 3.44 ± 0.04 to 3.83 ± 0.06, 4.46 ± 0.05, and 5.56 ± 0.06 µg mg⁻1 (Mean ± SD, n = 3).
• pH robustness: Adsorption efficiency remains >92% from pH 1–14; slightly decreases to 86.15 ± 1.09% when pH ≤ 0 (Mean ± SD, n = 3).
• Rapid desorption and recyclability: Dyes desorb within seconds upon adding suitable organic solvents (e.g., DCM, ethanol, methanol, with n-hexane/n-pentane mixtures), enabling solvent-exchange dye recovery. Over 10 cycles, adsorption efficiency remains 93–96% and desorption efficiency 94–98%. HL-HBP regeneration ratio ≈95.5% (minor losses on filtration). Single-cycle dye recovery ratio ≈92.2%. Process scales from milliliters to 2 L.

The findings validate that nanoscale surface heterogeneity—alternating hydrophilic (charged) and hydrophobic domains on porous particles—enables omnidispersion across solvents by facilitating wetting of both water and oils and establishing electrostatic repulsion between particles. DLVO modeling incorporating medium-filled pores explains stable dispersion in both high- and low-dielectric media. This omnidispersity is leveraged to couple two otherwise incompatible steps: aqueous-phase dye adsorption (driven by electrostatic and hydrogen-bonding interactions on hydrophilic domains and hydrophobic/π–π interactions on hydrophobic domains) and organic-phase dye desorption (where interactions weaken), allowing rapid solvent-exchange-based dye recovery. The approach circumvents the need for surfactant coatings or nanoprotrusions, preserving intrinsic surface activity. Although adsorption capacities are modest relative to high-surface-area materials (e.g., MOFs), the adsorption rates are orders of magnitude faster (equilibrium in seconds), with broad pH tolerance and excellent recyclability, highlighting practical relevance for industrial wastewater treatment and resource recovery. The proposed adsorption index offers a qualitative framework to rationalize dye-dependent uptake by accounting for multiple interaction modalities, acknowledging some deviations for structurally similar dyes.

The study introduces a surface heterogeneous nanostructuring strategy to fabricate hydrophilic–hydrophobic heterostructure particles (HL-HBPs) that disperse in both water and oils without added surfactants or protrusions. Comprehensive imaging (TEM, PiFM, AFM) confirms alternating domains, and DLVO analysis supports stable dispersion across media. HL-HBPs enable ultrafast dye adsorption (≈5 s) and rapid desorption in organic solvents, facilitating efficient solvent-exchange dye recycling with high adsorption/desorption efficiencies maintained over at least 10 cycles and scalability to liter volumes. This work advances both the scientific understanding of dispersion via chemical heterogeneity and a practical route for dye recovery and environmental remediation. Future research should generalize the strategy to other material systems, increase adsorption capacity while preserving rapid kinetics (e.g., by tuning porosity/chemistry), and refine predictive models (beyond the preliminary adsorption index) to quantify multi-interaction contributions.

• Generality of the surface heterogeneous nanostructuring strategy has been demonstrated primarily with emulsion interfacial polymerization of PSS-PSDVB and PTMAEMC-PSDVB; applicability to other chemistries and inorganic systems remains to be established.
• Adsorption capacities are lower than some high-surface-area adsorbents (e.g., MOFs), despite superior kinetics; further optimization of surface area and interaction site density is needed.
• The adsorption index (nsum/Mw) is an initial, qualitative descriptor and does not fully capture all structural subtleties (e.g., methyl/ethyl groups), leading to deviations for certain dyes; more rigorous mechanistic quantification is warranted.
• DLVO modeling simplifies complex pore/medium interactions (e.g., assumed Debye length in nonpolar solvents); experimental validation of long-range electrostatics in oils and detailed pore filling effects would strengthen mechanistic claims.