Forward osmosis works on a deceptively simple principle. Water spontaneously flows from a feed solution of lower osmotic pressure toward a concentrated draw solution on the other side of a semipermeable membrane. Because the process requires no external pumping pressure, it fouls less than pressure-driven reverse osmosis and can be paired with draw solutes that are easily recovered, or run as a pretreatment stage ahead of reverse osmosis. The heart of any such system is the membrane itself, typically a thin polyamide active layer formed on a porous support. The polyamide layer is created by interfacial polymerization, in which an aqueous amine monomer, usually m-phenylenediamine, meets an organic acyl chloride monomer, usually trimesoyl chloride, at the support’s surface. The reaction is fast and self-limiting, producing an ultrathin but dense film whose thickness, roughness and crosslinking density dictate how fast water moves and how well it rejects solutes.

For years, membrane scientists have tried to tune that film by embedding nanomaterials, a approach known as thin-film nanocomposite design. Nanoparticles of zeolites, metal-organic frameworks, carbon nanotubes, graphene oxide and MXenes have all been slipped into or beneath the polyamide layer, with mixed results. Additives can create extra water pathways and increase hydrophilicity, but they can also introduce defects, agglomerate into clumps that wreck selectivity, or leach out over time. The Iranian team chose a different class of additive altogether: a polyamide-based hyper-crosslinked polymer. Hyper-crosslinked polymers are amorphous, permanently porous networks built from rigid aromatic building blocks locked together by extensive covalent crosslinks. Their high internal surface area, tunable chemistry and chemical stability have made them popular in gas storage and catalysis, and this study puts them to work as a nanofiller for water membranes.

The synthesis itself was characterized with a battery of analytical techniques. Attenuated total reflectance Fourier-transform infrared spectroscopy, X-ray diffraction and transmission electron microscopy confirmed that the hyper-crosslinked polymer had formed successfully, revealing a layered morphology rather than a granular one. Elemental mapping of the material showed that nitrogen-bearing groups were distributed uniformly throughout the structure, a detail that matters because those amide and amine groups interact strongly with water and with the monomers used in membrane fabrication. The high nitrogen content and even dispersion suggested the polymer would integrate well into the polyamide chemistry rather than sitting inertly inside it as a foreign body.

The genuinely clever part of the study is how the polymer gets into the membrane. Rather than blending it into the support or into the aqueous monomer solution, the researchers used what they call an intermediate strategy. The hyper-crosslinked polymer was dispersed in hexane, an organic solvent in which the polymer can be spread as a coating, and that dispersion was applied to the membrane support surface precisely between the aqueous amine phase and the organic acyl chloride phase. The polymer thus occupies the very frontier where interfacial polymerization happens. As the two monomers diffuse toward each other and react, they must contend with a porous, nitrogen-rich, layered material sitting at the reaction zone. The result is a polyamide film whose growth has been sculpted from the inside out.

That sculpting shows up clearly in the membrane’s final properties. The presence of the hyper-crosslinked layer changed the dynamics of monomer diffusion and film nucleation, and the modified active layers came out smoother, thinner and more hydrophilic than films polymerized without the additive. Each of those three attributes pushes performance in the right direction. A thinner film shortens the path water must travel. Greater hydrophilicity improves wetting and reduces the energetic penalty water pays entering the polymer network. Reduced roughness matters for fouling, because contaminants preferentially snag on peaks and valleys, so a smoother surface resists the accumulation of organic matter and dye molecules during long filtration runs. Chemical characterization confirmed the hyper-crosslinked polymer was stably embedded within the polyamide matrix rather than loosely deposited on top of it.

The performance numbers are the study’s headline. An unmodified forward osmosis membrane delivered a water flux of 12.2 liters per square meter per hour, a common benchmark unit abbreviated LMH. At the optimal loading of hyper-crosslinked polymer, flux rose to 23.4 LMH, an increase of more than 90 percent. Pushing the additive loading higher actually backfired, with flux declining again because excess polymer drove the formation of a thicker polyamide layer, which added resistance faster than the extra pathways added permeability. That dose-dependent behavior is typical of nanocomposite membranes and underscores why loading optimization is the central design variable. Importantly, the modified membrane also showed enhanced specific reverse salt flux, meaning that the salt that inevitably leaks backward from the draw solution did so at a rate disproportionately low relative to the water gained, an indicator that selectivity had not been sacrificed for speed.

To test the membrane under realistic and demanding conditions, the team ran a 24-hour continuous process concentrating reactive black B, a widely used anionic dye responsible for much of the colored effluent discharged by the textile industry. The membrane rejected more than 99.9 percent of the dye throughout the run and displayed an anti-fouling tendency, resisting the flux decline that usually plagues dye concentration processes as molecules build up on the membrane surface. The authors attribute this resilience to the combination of smoothness, hydrophilicity and stable integration of the hyper-crosslinked polymer within the active layer. Sustained performance over a full day of operation is a meaningful demonstration that the additive does not wash out, degrade or progressively separate from the polyamide, concerns that have undermined other nanofiller systems.

Why does a porous polymer interlayer boost flux so dramatically? The mechanistic picture, supported by the characterization data, centers on how the intermediate layer reshapes interfacial polymerization itself. Interfacial polymerization is governed by the diffusion of monomers into the reaction zone; anything that moderates that diffusion changes the film’s architecture. The hexane-dispersed hyper-crosslinked coating likely slows the amine monomer’s transport toward the organic phase and provides abundant hydrogen-bonding and amide interaction sites, yielding a film that is simultaneously thinner and more uniformly crosslinked. Related interlayer strategies in the literature, using materials from chitosan and cyclodextrins to covalent organic frameworks and MXene-carbon nanotube assemblies, have shown similar gains, confirming that controlling the reaction frontier is one of the most powerful levers in membrane design. What distinguishes the present work is the use of a permanently porous, polymer-based network rather than an inorganic nanoparticle, which sidesteps problems of particle agglomeration and interfacial incompatibility.

The broader significance lies in what the result says about the permeability-selectivity trade-off, the长久-standing constraint that forces membrane designers to choose between fast and clean. By growing a thinner, smoother, more hydrophilic polyamide film without opening defects, the hyper-crosslinked interlayer improved water transport while preserving rejection of salts and dyes, suggesting the trade-off is not an immutable law but a consequence of how films are made. For desalination, higher flux at equal selectivity translates directly into less membrane area and lower capital cost. For textile wastewater treatment, a membrane that concentrates reactive dyes with near-total rejection and low fouling could enable water reuse and dye recovery in one of the world’s most water-intensive industries. The study, published in Polymer Bulletin with financial and instrumental support from the University of Tehran, adds a versatile new tool, a synthetically tunable porous polymer, to the membrane engineer’s toolkit, and points toward a generation of forward osmosis membranes that no longer ask designers to choose between speed and selectivity.

Subject of Research: Modification of thin-film nanocomposite forward osmosis membranes with a hyper-crosslinked polymer interlayer to enhance water flux, salt rejection and dye concentration

Article Title: Modification of thin film nanocomposite forward osmosis membranes using hyper-crosslinked polymer via intermediate strategy

Article References: Mirsaeidghazi, E., Shakeri, A., & Salehi, H. (2026). Modification of thin film nanocomposite forward osmosis membranes using hyper-crosslinked polymer via intermediate strategy. Polymer Bulletin, 83(12), Article 652. https://doi.org/10.1007/s00289-026-06710-0

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06710-0

Keywords: forward osmosis, thin-film nanocomposite membrane, hyper-crosslinked polymer, interfacial polymerization, polyamide, water desalination, dye concentration, anti-fouling, membrane modification, water flux, reverse salt flux, textile wastewater treatment

The challenge the team set out to solve is a familiar one in synthetic biology and materials chemistry. Living cells protect their precious molecular machinery behind selectively permeable membranes that admit nutrients, expel waste and shield fragile proteins from a hostile world. Synthetic chemists have long tried to imitate this compartmentalization with microcapsules — hollow polymer spheres a few tens of micrometres across — but the standard fabrication routes are fraught with problems. Emulsion templating demands vigorous mixing, surfactants and organic solvents that can denature proteins. Layer-by-layer assembly involves many sequential steps. And most critically, nearly every existing method is cargo-specific: the chemistry that gently encapsulates a small dye molecule may shred an enzyme, while conditions mild enough for enzymes often fail to form a robust shell at all.

The new approach, which the researchers call ice-mediated interfacial reaction, or IMIR, turns these constraints on their head by making the harsh step — freezing — the gentle one. The concept is elegantly simple. A water-soluble monomer, in this case p-phenylenediamine, is dissolved together with whatever cargo needs to be protected, and the whole aqueous mixture is frozen into tiny ice spheres. An organic phase containing a complementary monomer, trimesoyl chloride dissolved in ethyl acetate, is then brought into contact with the frozen droplets. Where the organic solvent meets the ice, the quasi-liquid layer becomes the reaction stage.

Here the physics of freezing does the heavy lifting. As water freezes, it excludes nearly everything that is not water: salts, monomers and proteins are pushed out of the growing crystal lattice and become concentrated in the channels and surfaces of remaining liquid. During ice recrystallization — the slow process by which small ice crystals merge into larger ones — the dissolved p-phenylenediamine is progressively enriched within the quasi-liquid layer at the ice surface. Molecular dynamics simulations performed by the team show that this nanometre-thin film behaves much like supercooled liquid water, providing an environment in which the amine monomer can position itself at the ice–oil boundary and make intimate contact with the reactive acyl chloride in the organic phase. The result is a confined interfacial amidation reaction that builds a conformal polyamide membrane, molecule by molecule, directly on the curved surface of the ice template.

When the ice is finally allowed to melt, the shell remains behind as an intact, closed microcapsule, and the cargo that was frozen inside is simply released into the watery interior — undamaged, because it never experienced anything warmer than a freezer and never touched an aggressive solvent. The encapsulation is genuinely cargo-independent: the membrane forms around whatever happens to be trapped in the ice, whether that is a fluorescent dye, a large protein, magnetic nanoparticles or a combination of species. This decoupling of shell formation from cargo chemistry is the method’s central innovation, and it sidesteps the formulation headaches that have plagued microencapsulation for decades.

The degree of control the researchers achieved is equally striking. By adjusting freezing conditions, monomer concentrations and reaction times, they tuned membrane thickness across more than an order of magnitude, from roughly 7 nanometres to 260 nanometres, while capsule diameters ranged from 30 to 500 micrometres. Just as importantly, the polyamide membranes are semipermeable: their pore structure can be programmed so that small substrate molecules diffuse in and out freely while large enzymes remain locked inside. That combination — robust confinement plus tunable permeability — is precisely what nature achieves with lipid bilayers and protein pores, and it is what synthetic systems have struggled to replicate in a single mild process.

To demonstrate the practical power of these artificial compartments, the team loaded them with multienzyme cascades — sequences of catalysts that pass intermediates from one to the next, much like an assembly line. In biology, such cascades work efficiently precisely because the enzymes sit close together inside confined spaces, keeping unstable intermediates at high local concentration and preventing them from leaking away. The ice-templated microcapsules recreate this principle synthetically. In a showcase experiment, the researchers encapsulated a photoenzymatic redox cascade designed to convert carbon dioxide into methanol under visible light. Confined within the polyamide shells, the cascade achieved up to an 80-fold enhancement in conversion compared with the same enzymes operating free in solution — a dramatic demonstration that physical compartmentalization alone can multiply catalytic output.

The choice of ice as the template is more than a laboratory convenience; it draws on a growing appreciation of ice as an active chemical medium. Environmental scientists have known for years that the quasi-liquid layer and brine channels of sea ice concentrate nutrients and contaminants, creating microhabitats where microbes thrive and unusual photochemistry unfolds. Materials chemists have recently exploited ice confinement to synthesize high-entropy alloys and to grow polyamide nanofiltration membranes with unusually high ionization. The new work extends this ‘ice chemistry’ programme into the third dimension, using recrystallizing ice spheres as sacrificial, perfectly spherical moulds whose surfaces simultaneously concentrate reactants and define geometry. Because the process relies on freezing rather than heating, harsh pH swings or toxic crosslinkers, the authors argue it is potentially scalable — frozen droplets could in principle be produced continuously with microfluidics, an operation the team has already begun exploring.

The implications reach well beyond biocatalysis. Cargo-independent encapsulation at near-neutral conditions is exactly what drug delivery needs: protein and mRNA therapeutics are notoriously unstable during formulation, and a shell that assembles around them without organic solvents or high shear could preserve activity where current methods fail. Semipermeable microcapsules are also attracting attention as tools for high-throughput single-cell omics, where individual cells must be housed in compartions that admit reagents but retain secreted molecules for analysis. And in the long-running effort to build artificial cells from scratch, the ability to wrap any combination of enzymes, DNA and synthetic organelles in a programmable polymer membrane offers a modular construction kit that lipid vesicles and coacervates have not fully provided.

There remain questions to resolve before ice-templated capsules reach industrial or clinical use. The reported diameters of 30 to 500 micrometres are large compared with the sub-micron capsules favored for injectable drug delivery, and extending the method to smaller length scales will require tighter control of ice nucleation. Long-term membrane stability, biodegradability and immune compatibility in vivo have yet to be assessed. But as a demonstration of principle, the study is a striking one: a material as humble as frozen water, guided by nothing more exotic than recrystallization and a well-known polymer reaction, can be coaxed into building cell-like compartments that boost artificial photosynthesis eighty-fold. Sometimes the most advanced manufacturing technology in the lab is the oldest one on Earth — ice, doing what ice has always done, concentrating the world at its surface.

Subject of Research: Ice-templated interfacial polymerization for cargo-independent microcapsule synthesis

Article Title: Ice-mediated interfacial membrane synthesis of cargo-independent microcapsules

Article References: Du, H., Wu, J., Yang, K., Zhang, C., Wang, D., Chen, X., Wang, S., Wu, Y., Jin, S., Chen, X., & He, Z. (2026). Ice-mediated interfacial membrane synthesis of cargo-independent microcapsules. Nature Synthesis. https://doi.org/10.1038/s44160-026-01164-8

Image Credits: AI Generated

DOI: 10.1038/s44160-026-01164-8

Keywords: microcapsules, ice templating, quasi-liquid layer, interfacial polymerization, polyamide membrane, artificial cells, biocatalysis, enzyme cascade, CO2-to-methanol conversion, encapsulation, Nature Synthesis, compartmentalization