The membrane also maintained stable pervaporation performance during 14 days of continuous operation, including at elevated temperatures. Reported in the Journal of Materials Science , the research investigated how membrane-fabrication conditions influenced structure, characteristics and dehydration performance. Chitosan formed the principal polymer matrix, while Fe₃O₄ nanoparticles created a mixed matrix membrane.

The nanoparticles were oriented with magnetic assistance during preparation, and STP served as the cross-linking agent. The investigators varied three main parameters: magnetic-field application, Fe₃O₄ content and STP concentration. The best reported formulation incorporated 5 wt% Fe₃O₄ in the chitosan matrix under a magnetic field and used 3 wt% STP cross-linking on a porous support.

The study therefore examined a specific membrane design as well as the effects of the selected preparation variables. The team used an extensive analytical suite to characterize the modifier and resulting membranes. Fourier-transform infrared and nuclear magnetic resonance spectroscopy, X-ray powder diffraction, dynamic light scattering and X-ray photoelectron spectroscopy were employed to examine chemical and structural characteristics.

Swelling tests, contact-angle assessments and mechanical-property evaluations characterized other aspects of membrane performance. Together, these methods were used to validate the structure and properties of the chitosan-based membranes under different preparation conditions. Computational analysis was also conducted to investigate interactions between the membrane and components of the feed, adding an interpretation of the observed separation behavior to the experimental characterization.

The transport results were reported across a broad composition range of 12–90 wt% water in the isopropanol feed. The optimized cross-linked supported membrane combined a permeation flux range of 0.13–0.65 kg/(m² h) with a separation index of at least 725 kg/(m² h) and water permeance of at least 3,577 GPU. These are material-level measurements from laboratory pervaporation experiments.

The reported evidence concerns membrane preparation, characterization and operation under the tested conditions rather than performance in a complete industrial isopropanol-production system. Continuous testing showed that the membrane retained stable pervaporation performance for 14 days, and the test included operation at elevated temperatures. The results support magnetic-field-assisted Fe₃O₄ incorporation as a way to improve the measured pervaporation characteristics of chitosan membranes for isopropanol dehydration.

The optimized combination of 5 wt% Fe₃O₄, 3 wt% STP, a porous support and magnetic assistance produced the strongest reported separation parameters while preserving performance over the reported continuous test.

Subject of Research: Magnetic Fe₃O₄ mixed matrix membranes for pervaporative dehydration of isopropanol

Article Title: Chitosan-based mixed matrix membranes with magnetically oriented Fe3O4 nanoparticles for enhanced pervaporation dehydration of isopropanol

Article References: Kuzminova, A., Dmitrenko, M., Stepanova, A., Mazur, A., Lisovenko, A., & Penkova, A. (2026). Chitosan-based mixed matrix membranes with magnetically oriented Fe3O4 nanoparticles for enhanced pervaporation dehydration of isopropanol. Journal of Materials Science, 61(40), 31358-31394. https://doi.org/10.1007/s10853-026-13626-7

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13626-7

Keywords: chitosan membranes, Fe₃O₄ nanoparticles, magnetic alignment, mixed matrix membranes, pervaporation, isopropanol dehydration, water separation, sodium tripolyphosphate, membrane transport, polymer membranes, Chitosan-based, mixed

The solution–diffusion model was born in the 1960s, when researchers studying cellulose acetate reverse osmosis membranes had no way to see inside a polymer film at the molecular scale. Because no instrument could resolve the subnanometer structure, scientists assumed the membrane was a homogeneous, pore-free medium. The model describes transport in three steps: molecules partition from the feed into the polymer according to their solubility, diffuse independently through the matrix, and desorb into the permeate. Crucially, it assumes that free volume inside the polymer exists only as isolated, transient cavities created by thermal fluctuations of the polymer chains, never as continuous channels. This simplicity became the model’s greatest strength, allowing generations of researchers to compare membranes using just two numbers: solubility and diffusivity.

But the model’s experimental foundation, according to the new analysis, is shakier than textbooks suggest. The classic evidence came from stacked-membrane experiments, in which researchers measured water content layer by layer after permeation and found a gradient decreasing from the feed side to the permeate side. That gradient was interpreted as the concentration difference driving diffusion. The Rice-led team, drawing on their own molecular dynamics simulations and continuum poromechanical modeling, offers a different explanation: the gradient is a mechanical artifact of membrane compaction. Under hydraulic pressure, the pressure inside the membrane falls continuously from feed to permeate, so the polymer skeleton is compressed progressively harder toward the permeate side, squeezing out porosity along the way.

To settle the question experimentally, the researchers repeated stacked-membrane experiments with Nafion 117 ion-exchange membranes, the same material used in recent work by Freeman and co-workers. If the gradient were truly diffusion-driven, releasing the pressure should erase it within minutes as water redistributes. Instead, after more than 24 hours of permeation, depressurization, and a 10-minute relaxation, a pronounced porosity gradient remained. Soaking the individual layers in deionized water for three hours did not remove it either. Thickness measurements confirmed progressively greater irreversible compaction toward the permeate side, and short 15-minute permeation runs produced essentially no gradient at all. Flux–pressure cycling from 30 to 120 bar and back caused roughly a 20 percent irreversible loss in water permeance, independent evidence of permanent structural deformation. The gradient, the authors conclude, is not the driving force for transport but the structural fingerprint of pressure-driven flow through a compressible, interconnected pore network.

The structural evidence goes far beyond compaction experiments. Positron annihilation lifetime spectroscopy can measure the size and distribution of free-volume elements; small-angle neutron and X-ray scattering reveal nanoscale heterogeneity and solvent-rich domains; wide-angle X-ray scattering probes chain packing; and synchrotron-based X-ray absorption and emission spectroscopy show that the hydrogen-bond network of water confined in polyamide membranes is distorted relative to bulk water. Neutron-scattering studies have even resolved distinct populations of bound and mobile water coexisting within the active layer of reverse osmosis membranes. Molecular dynamics simulations complement these measurements by mapping the three-dimensional, solvent-accessible free volume and testing whether pathways actually connect across the membrane. Together, these techniques consistently indicate that membranes long classified as dense are riddled with connected, dynamic, subnanometer porosity.

Importantly, the authors stress that

The pervaporation case deserves particular attention because it forces a reconsideration of what happens inside a membrane when liquid and vapor coexist on either side. In the classical picture, a liquid feed dissolves into the polymer, diffuses across, and evaporates only upon reaching the downstream surface, where vacuum or sweep gas maintains near-zero partial pressure. The Perspective argues instead that spectroscopic and simulation evidence points to liquid-like and vapor-like domains coexisting within the membrane interior, meaning the liquid–vapor transition occurs inside the material rather than exclusively at its downstream boundary. This internal phase transition reframes pervaporation as a process whose selectivity is set partly by where and how that phase change unfolds within the pore network, a question the solution–diffusion framework was never designed to address.

Gas separation in glassy polymers offers a complementary line of evidence. Glassy polymers are frozen far below their relaxation timescales, so their free-volume elements are comparatively rigid and long-lived, making them ideal systems for probing whether transport proceeds through connected channels or isolated cavities. Recent molecular simulations described in the article show that permeability and selectivity in these materials emerge from pore connectivity, molecular sieving at constrictions, and the strength of gas–wall interactions, with adsorption-mediated hopping between sites replacing the notion of a homogeneous diffusion matrix. This mechanistic picture naturally explains long-standing empirical observations, such as the strong dependence of gas permeability on polymer chain rigidity and the distinctive sieving behavior of highly rigid ladder-type polymers, without requiring ad hoc corrections to diffusivity coefficients.

The practical stakes of this reframing are considerable. If transport proceeds through connected subnanometer pores, then the levers that matter most for membrane design are geometric and chemical rather than phenomenological: pore size distribution, network connectivity, tortuosity of the pathways, and the chemistry of the pore walls that governs solvent–polymer and gas–polymer interactions. This shifts the design paradigm from fitting solubility and diffusivity parameters after the fact toward deliberately engineering the architecture of the free-volume network itself. For polyamide reverse osmosis membranes, for example, it suggests that controlling the nanovoid structure formed during interfacial polymerization, and how that structure responds to sustained pressure, may matter as much as the polymer chemistry that determines water affinity.

The framework also carries implications for how membrane aging and compaction are understood. The irreversible permeance losses observed under cyclic pressurization, on the order of twenty percent in the experiments highlighted, become interpretable as progressive collapse of pore connectivity rather than as a mysterious drift in an effective diffusivity. Similarly, fouling and solvent-induced swelling can be reexamined as phenomena that alter pore geometry and wall chemistry in real time, which helps explain why membrane performance often depends strongly on operating history in ways that equilibrium sorption and diffusion coefficients cannot capture.

Methodologically, the convergence across PALS, SANS, SAXS, WAXS, synchrotron X-ray spectroscopy, and molecular dynamics is itself significant. Each technique probes a different length and timescale, from picosecond chain fluctuations to static nanoscale morphology, and their agreement on the existence of connected free-volume networks reduces the likelihood that any single measurement is an artifact. Continued progress will likely depend on coupling these experimental probes with simulations that can resolve how pore networks reconfigure under realistic operating pressures and solvent compositions, and on developing characterization methods capable of mapping connectivity, not merely cavity size, in operating membranes.

The authors are careful to note that the two conceptual frameworks need not be mutually exclusive at every scale. The solution–diffusion model remains a useful correlation tool, and continuum-level descriptions built on it have guided decades of engineering practice. The argument is instead about mechanism: the molecular picture underlying those correlations is better represented by viscous flow and hopping through connected nanoscale pathways than by diffusion through a truly continuous dense matrix. Recognizing this distinction matters because models that encode the wrong mechanism can mislead extrapolation, particularly as new membrane materials push into regimes of tighter sieving, higher pressures, and more aggressive solvents where the difference between the two pictures becomes experimentally consequential.

What emerges from the synthesis is a unifying thread across processes that have traditionally been treated separately. Reverse osmosis, organic solvent nanofiltration, pervaporation, and glassy polymer gas separation have each accumulated their own empirical models and design heuristics. Framing all of them around interconnected subnanometer pore networks provides a common physical substrate: pressure-driven viscous flow in liquid separations, internal phase coexistence in pervaporation, and sieving with adsorption-mediated transport in gas separation are all manifestations of the same underlying structural reality. Whether this perspective ultimately reshapes how next-generation membranes are designed will depend on translating the structural picture into quantitative, predictive tools, but the conceptual realignment it proposes is already substantial, touching everything from how compaction is interpreted to how the internal state of a pervaporation membrane is defined.

Subject of Research: Transport mechanisms and pore-network structure of dense polymer membranes used in water, solvent, and gas separations

Article Title: Interconnected pore networks as a unifying framework for transport in dense polymer membranes

Article References: Wang, R., Fan, H., Qian, J., Wang, L., Li, Y., & Elimelech, M. (2026). Interconnected pore networks as a unifying framework for transport in dense polymer membranes. Frontiers in Membrane Science and Technology, 5, Article 1955546. https://doi.org/10.3389/frmst.2026.1955546

Image Credits: AI Generated

DOI: 10.3389/frmst.2026.1955546

Keywords: polymer membranes, pore networks, solution-diffusion model, reverse osmosis, organic solvent nanofiltration, pervaporation, gas separation, molecular dynamics, free volume, membrane compaction, molecular sieving, desalination

The central obstacle the team set out to solve is a familiar one in membrane science. Mixed-matrix membranes combine the easy processing of polymers with the precisely engineered pores of crystalline materials, but the recipe has a built-in ceiling. When the fraction of porous filler climbs beyond a modest threshold, the particles aggregate, defects appear at the polymer–filler interface, and the membrane’s performance collapses rather than improves. Most strategies have therefore kept filler loadings low, which means the polymer dominates transport and the carefully designed pores of the filler contribute little. The intrinsic transport properties of the crystalline material are effectively wasted.

The Tianjin-led group overturned the conventional design philosophy. Rather than maximizing strong interactions between polymer and filler to glue the two phases together, they pursued what they call a weak-interaction inlaying strategy. By balancing the interaction energies between polymer and filler on one hand and between filler particles themselves on the other, they found that the single-crystal COFs could be dispersed uniformly at loadings reaching 75.5 percent by volume, an extraordinary fraction for a mixed-matrix membrane. At such loadings the filler no longer acts as a minor additive; it becomes the dominant, continuous transport network through which gas molecules travel.

The choice of filler matters as much as the loading. Covalent organic frameworks are a class of porous crystalline polymers built from light elements such as carbon, nitrogen, oxygen and hydrogen, linked by strong covalent bonds into periodic lattices with regularly sized pores. Unlike metal–organic frameworks, they contain no metal nodes, making them fully organic and compatible with an organic polymer matrix. The team used single-crystal COFs, whose well-defined, defect-minimized lattices preserve the exact pore geometry designed into the material. When these crystals are embedded intact, the membrane inherits their intrinsic transport channels rather than a degraded approximation of them.

The resulting membranes create what the authors describe as filler-dominated pathways for fast carbon dioxide transport. Gas molecules moving through the membrane encounter a continuous network of COF pores whose size and chemistry favor carbon dioxide over nitrogen, while the surrounding polymer simply holds the architecture together. Because the interfaces remain clean and compatible even at very high filler content, there are no leaky voids that would undermine selectivity and no dense polymer regions that would choke off flow. The combination of high permeability and maintained selectivity pushes the materials well beyond the conventional upper-bound trade-off line that has long constrained polymer membranes for carbon dioxide–nitrogen separations.

The numbers are striking in context. A permeability of 74,800 Barrer is orders of magnitude above typical commercial polymer membranes for post-combustion capture, which often sit in the tens to low hundreds of Barrer. High permeability translates directly into smaller membrane areas and lower driving pressures for a given capture duty, which in turn reduces the capital cost and energy consumption of a capture plant. Previous life-cycle assessments have suggested that membrane separation can compete with the amine-based chemical absorption processes used today, but only if membrane materials reach the permeability–selectivity combination that makes module sizes practical. This work moves that target substantially closer.

Performance on day one means little if a membrane degrades, and polymer membranes are notorious for physical ageing, the gradual collapse of free volume that throttles transport over months and years. The team addressed this concern directly with long-term tests. Membranes based on polyimide retained approximately 85 percent of their initial permeability after six months of ageing at room temperature, a result that suggests the rigid crystalline COF network helps stabilize the membrane structure against the densification that normally erodes polymer performance. For a material operating at such extreme filler loadings, this stability is itself a notable finding.

The work also carries a broader methodological lesson for materials chemistry. The weak-interaction inlaying strategy reframes the interface problem: instead of fighting aggregation with ever-stronger polymer–filler bonding, the researchers tuned the relative strengths of competing interactions so that dispersion remains thermodynamically comfortable. That principle, balancing polymer–filler against filler–filler interaction energies, could plausibly extend beyond COFs to other crystalline fillers and beyond carbon capture to other molecular separations, from hydrogen purification to hydrocarbon cracking. The study’s authors, including co-first authors Hanze Ma and Shilin Guo, with Guangwei He and Zhongyi Jiang as corresponding authors, frame the approach as a general route to exploiting the intrinsic properties of porous crystals inside processable membranes.

Scalability remains the decisive question for any laboratory membrane advance. The team emphasizes solution processability as a design requirement, and the fact that the membranes are all-organic, formulated from polymer and COF in solution, aligns with existing membrane fabrication methods rather than requiring exotic deposition techniques. Still, translating a laboratory coupon into square meters of defect-free membrane module, and validating performance on real flue gas with its humidity, sulfur impurities and temperature swings, will demand further engineering. The six-month ageing data and the robust interfacial design are encouraging early signals, but industrial pilots will be the true test.

Even with those caveats, the study represents one of the most compelling demonstrations yet that the long-standing permeability–selectivity trade-off can be beaten by design rather than merely nudged. By making the crystalline filler the membrane rather than a garnish on it, the researchers have effectively built a processable, polymer-locked single-crystal separation device. If the approach survives scale-up, membranes of this kind could shrink the footprint and energy bill of post-combustion carbon capture, a technology the Intergovernmental Panel on Climate Change and the International Energy Agency both identify as essential to limiting warming. In a field where incremental gains are the norm, a seventy-thousand-Barrer membrane with intact selectivity is the kind of result that redraws the map.

Subject of Research: All-organic mixed-matrix membranes with single-crystal covalent organic frameworks for energy-efficient carbon capture

Article Title: All-organic mixed-matrix membranes with single-crystal covalent organic frameworks for carbon capture

Article References: Ma, H., Guo, S., Su, P., Zhao, Q., Wang, S., Ren, Y., Fu, J., Zhang, H., Zhang, T., Yuan, S., Pan, Q., Villalobos, L. F., Li, Y., Ma, X., Agrawal, K. V., He, G., & Jiang, Z. (2026). All-organic mixed-matrix membranes with single-crystal covalent organic frameworks for carbon capture. Nature Energy. https://doi.org/10.1038/s41560-026-02149-9

Image Credits: AI Generated

DOI: 10.1038/s41560-026-02149-9

Keywords: carbon capture, covalent organic frameworks, mixed-matrix membranes, gas separation, CO2 permeability, polyimide, membrane technology, porous materials, post-combustion capture, polymer membranes, Nature Energy, materials science