Introduction

Liquid-liquid separation is an essential process with ubiquitous applications encompassing fine chemical synthesis, wastewater treatment, and petrochemical manufacturing1,2,3,4,5. Based on mutual solubility, liquid mixtures can be divided into two broad categories: immiscible and miscible liquids6. Depending on the surface energy disparity of the liquids to be separated7,8,9, each liquid will exhibit different dissolution states among them. These systems can be classified into four types: layered liquids, emulsions, partially miscible liquids, and fully miscible liquids10,11. In recent years, there has been increased focus on understanding liquid systems requiring separation and developing more efficient separation techniques. Traditional pervaporation (PV) membranes12,13,14,15,16,17 can achieve effective separation of miscible liquid mixtures through a solution-diffusion mechanism but face challenges such as low flux, high energy consumption, and membrane swelling issues18,19,20. In contrast, superwetting membranes (SWMs) achieve liquid separation based on differences in wettability, offering advantages such as low energy consumption, high efficiency, and high flux21,22,23,24. As depicted in Fig. 1, SWMs were initially primarily employed for the separation of immiscible layered liquids and emulsions through alterations in multiscale structures or surface chemical compositions. For partially and fully miscible liquids, the separation task is more arduous but can be accomplished through the synergistic application of an inductive agent and the meticulous control of molecular interactions within the porous membrane. Separation of moderately polar miscible liquids is predominantly achieved through the interplay of polar and nonpolar interactions between the solution and the inductive agent. For highly polar miscible liquids, effective separation can be achieved through salt-induced hydration.

Fig. 1: Overview diagram of liquid-liquid separation by SWMs.
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Reproduced with permission from ref. 30. Copyright 2011 Wiley–VCH. Reproduced with permission from ref. 65. Copyright 2012 Wiley–VCH. Reproduced with permission from ref. 50. Copyright 2013 Wiley–VCH. Reproduced with permission from ref. 43. Copyright 2018 Wiley–VCH. Reproduced with permission from ref. 51. Copyright 2022 Elsevier. Reproduced with permission from ref. 54. Copyright 2024 Elsevier. Reproduced with permission from ref. 53. Copyright 2022 Elsevier. Reproduced with permission from ref. 55. Copyright 2024 Elsevier.

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The separation principle of SWMs relies on their exceptional wetting properties25. The wetting behavior of the membrane surface is tailored to create opposing Laplace forces between the separated liquids, enabling separation without the need for additional energy. As shown in Fig. 2, for the separation of layered liquids, the two liquids can be separated by adjusting the polarity of the membrane26,27,28,29,30,31,32,33,34,35. The pioneering research on SWMs was the superoleophilic (SOPI)/superhydrophobic (SHPO) porous membranes reported by Jiang et al.36 in 2004, which utilized capillary forces and liquid bridges to facilitate the passage of oil while effectively blocking water. When separating emulsions, precise control of the membrane pore size is required37,38,39,40,41,42,43,44,45,46,47,48,49. Via an ammonia water-assisted phase-inversion method, an SHPO/SOPI poly(vinylidene fluoride) (PVDF) membrane with fiber-like connected rough microparticles has been designed for efficient separation of surfactant-stabilized water-in-oil emulsions, which provides a promising strategy for purification of wastewater produced in industry and daily life50. Although SWMs have achieved great success in the separation of layered liquids and emulsions, the separation of miscible liquids has remained a significant challenge until recently, when we achieved the separation of miscible liquids by synergistically regulating the polar/nonpolar interactions between the liquid inductive agents, the solid porous membranes, and the miscible components51,52,53,54,55.

Fig. 2: Key achievements in the development history of SWMs with different separation systems.
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Reproduced with permission from ref. 36. Copyright 2004 Wiley–VCH. Reproduced with permission from ref. 30. Copyright 2011 Wiley–VCH. Reproduced with permission from ref. 31. Copyright 2012 Springer Nature. Reproduced with permission from ref. 32. Copyright 2015 Springer Nature. Reproduced with permission from ref. 35. Copyright 2017 Wiley–VCH. Reproduced with permission from ref. 50. Copyright 2013 Wiley–VCH. Reproduced with permission from ref. 41. Copyright 2018 Wiley–VCH. Reproduced with permission from ref. 49. Copyright 2018 Wiley–VCH. Reproduced with permission from ref. 47. Copyright 2024 Wiley–VCH. Reproduced with permission from ref. 51. Copyright 2022 Elsevier. Reproduced with permission from ref. 52. Copyright 2024 Elsevier. Reproduced with permission from ref. 54. Copyright 2024 Elsevier. Reproduced with permission from ref. 53. Copyright 2022 Frontiers.

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In this perspective, we systematically present recent advancements in the development of SWMs for liquid-liquid separation applications. Unlike previous reviews that primarily focus on membrane materials and fabrication techniques, our work offers a novel perspective on separation mechanisms for diverse separation systems from the standpoint of liquid properties, specifically tracing the evolution from immiscible to miscible liquid mixtures, which provides a more thorough understanding of the separation mechanisms and membrane design principles across different liquid systems. Finally, we critically analyze the current challenges and future directions in this rapidly evolving field, offering insights into potential breakthroughs and applications of SWMs in advanced liquid-liquid separation processes.

SWMs for immiscible liquids separation

SWMs for layered liquids separation

Extensive research has been conducted on immiscible liquid-liquid separation through the regulation of superwettability in solid porous membranes56,57,58,59,60. Based on their distinct wettability characteristics, SWMs can be divided into the following five primary categories: SHPO/SOPI membranes (Fig. 3a), superhydrophilic (SHPI)/underwater superoleophobic (SOPO) membranes (Fig. 3b), SHPI/SOPO membranes (Fig. 3c), liquid gating membranes (Fig. 3d), and responsive SWMs (Fig. 3e). The design of SHPO/SOPI membranes is primarily driven by the fundamental principle that most oils exhibit lower surface tension than water61,62. Through precise regulation of surface energy and structural parameters of porous substrates, these membranes enable selective oil permeation, making them particularly effective for separating heavy oils with densities higher than water. However, the inherent high viscosity of oils often results in limited flux and membrane fouling. Recently, a robust polytetrafluoroethylene (PTFE) nanofibrous membrane was prepared for viscous crude oil at a high speed (Fig. 3a). This membrane exhibits outstanding corrosion resistance and mechanical stability63, withstanding the harsh conditions present in industrial oil-water separation processes while maintaining a high flux of 1215 L m−2 h−1. An alternative strategy to overcome viscosity-related flux limitations involves the development of SHPI/underwater SOPO membranes, which simultaneously address the challenge of membrane fouling. A notable example is the fabrication of nanoparticle-engineered superwettable membranes, demonstrating exceptional oil-water separation efficiency through selective water permeation. Hierarchical nanoparticle structures are developed on the surface of PVDF microfiltration (MF) membranes by co-depositing proanthocyanidins (PC) and γ-aminopropyltriethoxysilane (APTES). The hierarchical nanoparticle architecture on the membrane surface confers SHPI/underwater SOPO, along with resistance to crude oil contamination, chemical and mechanical stability, and salt tolerance (Fig. 3b)64. Moreover, the direct design of SHPI/SOPO membranes through the incorporation of compounds featuring mobile fluorinated oleophobic groups and charged hydrophilic groups onto micro/nano-structured substrates can also achieve selective water permeation, thereby significantly enhancing oil-water separation efficiency (Fig. 3c)65. Liquid gating membranes represent a significant innovation in membrane technology, with applications spanning microfluidics66,67, drug delivery56,57, filtration and separation58,59,60. These membranes utilize capillary-stabilized liquids as reversible gating elements, enabling precise control over fluid transport through the manipulation of interfacial properties. For instance, superamphiphilic porous membranes can preferentially capture liquid with high polar components, forming a relatively stable liquid-infused surface that facilitates selective permeation while repelling immiscible low-polarity liquids. The infused liquid can be easily replaced by another immiscible liquid with higher polarity, thus achieving continuous separation of multiphase liquids (Fig. 3d)61. Furthermore, the development of responsive SWMs has opened new possibilities for smart separation systems. A representative example is the fabrication of CO2/N2-responsive membranes through the incorporation of poly(methyl methacrylate-co-diethylaminoethyl methacrylate) (PMMA-co-PDEAEMA) copolymers onto polyester fabrics. These membranes demonstrate reversible wettability switching between high hydrophobicity/underwater superoleophilicity and superhydrophilicity/underwater superoleophobicity, enabling adaptive separation capabilities (Fig. 3e)62.

Fig. 3: SWM systems for layered liquid-liquid separation.
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Schematic of the mechanism and membrane modification, and the photographs of a SHPO/SOPI, b SHPI/underwater SOPO, c SHPI/SOPO, d liquid gating, and e responsive SWMs for layered liquids separation. Reproduced with permission from ref. 63. Copyright 2017 Elsevier. Reproduced with permission from ref. 64. Copyright 2021 Elsevier. Reproduced with permission from ref. 65. Copyright 2012 Royal Society of Chemistry. Reproduced with permission from ref. 61. Copyright 2017 Springer Nature. Reproduced with permission from ref. 62. Copyright 2023 Springer Nature.

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SWMs for emulsion separation

Emulsions, characterized by the dispersion of one liquid in the form of minute droplets throughout another immiscible liquid, present a more complex separation challenge compared to layered liquids68. SWMs, however, demonstrate remarkable efficacy in various emulsion separations. For instance, chemically treated balsa wood, with partial removal of lignin and hemicellulose, has been used to fabricate SHPI/underwater SOPO wood films. These films are capable of separating surfactant-stabilized oil-in-water emulsions, allowing water to selectively pass through while blocking oil (Fig. 4a)69. In contrast, a separation method suitable for water-in-oil emulsions is indispensable. Owing to the complementary effect of the tetraethyl orthosilicate-derived hierarchical bead-on-string SiO₂ nanofiber structure and the superior hydrophobicity of octadecyltriethoxysilane, the resulting SOPI/SHPO nanofiber membrane exhibited a lotus leaf-like micro/nanostructure and superhydrophobicity. This synergy not only provided excellent self-cleaning and antifouling performance but also enabled the membrane to effectively separate water-in-oil emulsions (Fig. 4b)70. To further separate complex systems such as oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W) emulsions, precise control over membrane wettability, response time, and pore structure is required. Dual CO2- and photothermal-responsive membranes prepared by two steps can effectively separate double emulsions (Fig. 4c). The membrane exhibits wettability that rapidly switches between SHPI (under CO₂ stimulation) and SHPO (under near-infrared stimulation), enabling on-demand separation of various O/W/O and W/O/W emulsions with separation efficiencies exceeding 99.6%47. However, current SWMs face challenges in simultaneously recovering both oil and water from surfactant-stabilized emulsions to avoid liquid discharge. Recently, a novel method utilizes a Janus membrane channel (JMC), featuring narrow channels created by hydrophilic and hydrophobic membranes, to achieve simultaneous and highly efficient recovery of both oil and water from surfactant-stabilized emulsions (Fig. 4d). This design facilitates the rapid enrichment of local emulsions and enhances droplet collisions due to the confining effect of the narrow channels. Additionally, the feedback loop in JMC effectively mitigates the generation of concentration polarization, enabling concurrent recovery of oil and water from both water-in-oil and oil-in-water emulsions71.

Fig. 4: SWM systems for separation of emulsions.
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Schematic of the mechanism and membrane modification, and the photographs of a SHPI/SOPO, b SHPO/SOPI, and c responsive SWMs for emulsion separation. d Schematic of the mechanism and the photographs for oil and water recovery from emulsion. Reproduced with permission from ref. 69. Copyright 2025 American Chemical Society. Reproduced with permission from ref. 70. Copyright 2023 Elsevier. Reproduced with permission from ref. 47. Copyright 2024 Wiley–VCH. Reproduced with permission from ref. 71. Copyright 2024 Science.

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SWM systems for miscible liquids separation

SWMs for partially miscible liquids separation

SWMs have demonstrated remarkable success in separating immiscible liquid systems, particularly oil-water mixtures that encompass layered oil-water systems, water-in-oil emulsions, and oil-in-water emulsions, through precise regulation of liquid wettability at gas/liquid/solid or liquid/liquid/solid three-phase interfaces. However, the separation of miscible mixtures without distinct macroscopic liquid-liquid interface remains a formidable challenge for conventional SWMs. To solve this challenge, our research group has pioneered a novel SWM system capable of effectively separating partially miscible liquid mixtures51. This approach leverages synergistic control of polar/nonpolar interactions among the porous membrane materials, inductive agents, and miscible liquid components, enabling energy-efficient, high-flux separation of organic liquid mixtures containing medium-low polar liquids and medium-high polar liquids. The separation mechanism operates through polarity-matched interactions: when employing a low-polarity porous membrane (C16-modified hexadecyltrimethoxysilane) in conjunction with a high-polarity inductive agent (formamide, FA), middle low polar toluene (TL) can be selectively separated from its mixture with dimethyl sulfoxide (DMSO) (Fig. 5a). Conversely, the combination of a high-polarity membrane (PMPC-modified poly(2-methacryloyloxyethylphosphorylcholine)) with a low-polarity inductive agent (n-hexane, HE) facilitates the separation of middle high-polarity DMSO from the same mixture. By extending the principle of polar/nonpolar interactions to nonpolar/hydrogen bonding interactions, we have also successfully separated dimethyl carbonate (DMC) from methanol (MeOH) using a low-polarity PTFE membrane combined with water as a strong hydrogen-bonding agent (Fig. 5b)52.

Fig. 5: SWM strategy for separation of partially and fully miscible liquids.
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a Design principle of SWMs for miscible liquid-liquid separation. Schematic of the mechanism, chemical structure, and the wetting behaviors of SWM strategy for b partially miscible MeOH/DMC and c fully miscible aqueous separation. Reproduced with permission from ref. 51. Copyright 2022 Elsevier. Reproduced with permission from ref. 52. Copyright 2024 Elsevier. Reproduced with permission from ref. 53. Copyright 2022 Frontiers.

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SWMs for fully miscible liquids separation

The separation efficiency of the SWM systems is highly dependent on the polarity difference between the miscible liquids. A larger polarity difference between the separated liquids facilitates the selection of suitable inductive agents and porous membranes to disrupt the molecular interactions between miscible liquids, thereby enabling effective separation. In contrast, for liquid mixtures composed of highly polar components with small polarity difference—typically fully miscible in any ratio (Fig. 5c), such as ethanol/water mixtures, it remains challenging to identify appropriate inductive agents capable of overcoming the strong intermolecular interactions between highly polar liquids. Fortunately, in such cases, inorganic salts that form strong hydration with water molecules are chosen as the inductive agents. By utilizing a SiO2-TiO2 composite membrane (STM) modified with 1H, 1H, 2H, 2H-perfluorooctyltrimethoxysilane as the porous SWMs and potassium pyrophosphate (K4P2O7) as the inductive agent, the purification of ethanol from fully miscible ethanol/water mixture has been successfully achieved53.

SWMs-based membrane modules for miscible liquids separation

Although the synergistic regulation of the interactions among the porous solid membrane, inductive agents, and miscible liquid components at the molecular level has successfully enabled the separation of miscible liquids using SWM systems, the purity of separated components remains constrained by the thermodynamic three-phase equilibrium involving the inductive agents and miscible liquid components. Fortunately, high-purity separation of miscible liquids can be achieved through rational integration of SWM systems with conventional membrane separation technologies, such as PV membrane72,73,74, membrane distillation (MD)75,76,77 to further enhance the permeate purity. For instance, in the separation of partially miscible DMC/MeOH, the purity of DMC was improved 93 wt% to 99.4 wt% by combining our designed SWM system with a hydrophilic PV membrane (Fig. 6a). Moreover, in the production of biofuel ethanol, which involves the separation of fully miscible ethanol/water mixture, over 99.5% anhydrous ethanol was obtained through coupling a SWM system with a hydrophilic PV membrane, where the inductive agent, concentrated K4P2O7 solution, can be recycled through a MD process (Fig. 6b). Thus, by rationally integrating our designed SWM systems with conventional membrane technologies, effective separation of miscible liquid mixtures can be achieved for practical applications, offering the combined advantages of high flux and high purity. In addition, fermentation of biobutanol typically involves the separation of quaternary mixture composed of acetone (A), n-butanol (B), ethanol (E), and water by combining permeation fractionation coagulation (PV-FC) with SWM (Fig. 6c). The SWM & PV & FC-based membrane module is capable of highly efficiently extracting ABE from solution and efficiently separating n-butanol from acetone and ethanol, yielding more than 99.2% n-butanol78.

Fig. 6: Membrane modules for efficient miscible liquid-liquid separation.
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a SWM & PV membrane module for DMC/MeOH separation. b SWM & PV & MD membrane module for EtOH/H2O separation. c SWM & PV & FC membrane module for A-B-E aqueous solution separation.

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Summary and outlook

This perspective aims to provide readers with a comprehensive understanding of the revolutionary development of SWMs in liquid-liquid separation, which has fundamentally transformed membrane separation technology over the past two decades. The evolution of SWM systems has demonstrated remarkable progress, transitioning from the separation of immiscible liquids (including layered systems and emulsions) to the more challenging separation of miscible liquid mixtures. The integration of SWMs with conventional membrane processes, such as PV and MD, has enabled the design of advanced membrane modules that achieve both high flux and exceptional purity in targeted liquid separations, paving the way for practical industrial applications. However, it is crucial to recognize that SWM systems, particularly those designed for miscible liquid mixtures, remain in their nascent stages of development. The research advancements summarized in this perspective represent only the initial breakthroughs in this rapidly evolving field. There exists substantial potential for further exploration and innovation in SWM systems for miscible liquid separation, particularly through optimizing membrane materials, developing novel inductive agents, and rationally designing membrane modules. These advancements will undoubtedly expand the frontiers of liquid-liquid separation technology, opening new possibilities for diverse industrial applications.