In the rapidly evolving field of membrane technology, the quest for highly selective ion separation has long centered on the precise control of pore architectures within membranes. For decades, narrowing the pore size distribution (PSD) has been acknowledged as the fundamental strategy to enhance ion selectivity, particularly in polyamide nanofiltration membranes widely used for water purification, brine management, and resource extraction. However, a groundbreaking study now challenges this entrenched paradigm by illuminating an alternative, and perhaps more influential, factor dictating ion selectivity: the homogeneity of surface charge distribution at the nanoscale.
This pioneering work, led by Lu, Huang, Zhang, and their colleagues, shifts the focus from merely tuning physical pore sizes to engineering the electrochemical landscape across membrane surfaces. Utilizing advanced multimodal atomic force microscopy (AFM) techniques, the researchers visualized nanoscale charge heterogeneities, extracting detailed three-dimensional maps of surface potential, mechanical phase, and functional group distribution on polyamide membranes. The metrological innovations underpinning this methodology enable unprecedented quantitative insight into spatial variations of surface charge—a critical parameter previously shadowed by the dominant emphasis on pore size.
Their findings reveal a counterintuitive phenomenon: membranes with ostensibly “optimized” narrow PSDs may underperform in ion selectivity if the surface charge distribution remains heterogeneous. Conversely, membranes exhibiting enhanced charge homogeneity demonstrate sharper discrimination capabilities between ions, especially in complex separations like the lithium–magnesium mixtures crucial for next-generation resource recovery. This discovery overturns established thought, underscoring that homogenous electrostatic environments on membrane surfaces significantly modulate ion transport and rejection beyond what pore size alone can achieve.
Critically, the research team employed a polyethyleneimine (PEI)-based multivariate functionalization strategy to program stepwise enhancements in the spatial homogeneity of electropositive amine groups on polyamide membranes. This chemical engineering approach results in a progressively uniform surface charge distribution that directly correlates with improved ion selectivity. The membranes designed this way distinctly outperform those optimized solely via PSD manipulation, especially in challenging separations involving ions of similar size but differing charge characteristics.
The implications of this discovery extend wide across membrane science and engineering. By decoupling ion selectivity from the traditional constraint of pore size precision, the study opens new pathways for designing membranes with superior performance through facile surface charge modulation. This could expedite the development of next-generation nanofiltration devices that are simpler to manufacture, more robust, and highly selective, meeting urgent global needs in clean water production, waste brine valorization, and critical metal recovery.
One of the most remarkable aspects of this research is the integration of multimodal AFM for charge mapping, a cutting-edge metrological advance. By correlating surface potential images with phase shifts and functional group chemical signatures, the team created a holistic representation of the nanoelectrochemical landscape. This comprehensive mapping capability allows the differentiation of subtle charge patch distributions that conventional characterization methods would overlook, explaining why previously puzzling disparities in membrane performance often arose despite similar pore size specifications.
Furthermore, the study’s revelation that charge homogeneity outweighs pore size in selective ion transport challenges membrane developers to rethink fundamental design principles. The heterogeneous distribution of charges creates localized “hot spots” or “cold spots” that disrupt uniform ion partitioning, leading to less predictable and often degraded selectivity. Homogenizing this charge landscape mitigates such variability, enabling more consistent and controllable ion rejection behavior.
Experimentally, the researchers demonstrated these concepts by fabricating a series of polyamide membranes with varying degrees of charged surface uniformity but comparable pore size distributions. They rigorously quantified ion selectivity metrics and correlated them with nanoscale charge uniformity indices derived from AFM data. The data conclusively showed that membrane selectivity systematically improves with increasing charge homogeneity, even when PSD remains essentially constant. Such a clear decoupling of these two factors was previously unattainable in the field.
Beyond lithium–magnesium separations, the fundamental concept of charge homogeneity governing ion selectivity may have transformative applications across diverse membrane-based processes. These include desalination, wastewater treatment, resource recovery from brines, and selective electrolyte separation for energy applications. The ability to tune ion rejection profiles through electrostatic nanoengineering promises to catalyze innovation in sustainable membrane technologies worldwide.
Another significant advantage of focusing on charge homogeneity is the simplified manufacturing complexity it affords. Traditional approaches demand ultrafine control of membrane pore sizes—often at the nanometer scale—requiring sophisticated fabrication techniques that are difficult to scale. In contrast, chemical modulation of surface charge distribution via polymers like polyethyleneimine can be achieved through accessible, scalable processes such as layer-by-layer assembly or surface grafting. This could democratize access to high-performance nanofiltration membranes beyond specialized laboratories.
The study also sheds light on the fundamental ion transport mechanisms within charged membranes. Uniform surface charge distributions enhance electrostatic exclusion effects and uniform potential barriers, which work synergistically to differentiate ions not just by size exclusion but by ionic charge density and valence. This nuanced electrochemical interplay explains why traditional size-based models failed to fully capture the observed selectivity patterns.
Importantly, this research aligns with ongoing trends toward multifunctional membrane surfaces that combine tailored pore architectures with chemically active moieties to achieve superior selectivity, permeability, and anti-fouling properties. Understanding the predominant role of charge homogeneity enables more rational design principles, where surface chemistry and nanoelectrostatics are engineered in unison rather than in isolation.
Moreover, the authors’ multidisciplinary approach, combining materials chemistry, nanoscale metrology, and membrane engineering, exemplifies the integrative research needed to tackle complex separation challenges. Their methodology could inspire future studies to explore charge homogeneity effects in a wider range of membrane materials, including emerging 2D materials, ionomers, and biomimetic structures.
While this discovery propels membrane science forward, it also raises intriguing questions about how charge homogeneity evolves during membrane aging, fouling, or chemical degradation. Future work could focus on maintaining or dynamically tuning nanocharge uniformity under real-world operating conditions, further enhancing membrane lifespan and reliability.
In summary, this visionary study ushers in a paradigm shift by spotlighting nanoscale surface charge homogeneity as the dominant factor over pore size distribution in dictating ion selectivity of polyamide membranes. By advancing novel characterization tools and surface engineering strategies, the research provides both fundamental insights and practical routes to fabricate next-generation ion-selective membranes. The direct impact on critical applications—ranging from clean water to resource recovery—affirms its significance and potential to inspire transformative advancements in sustainable membrane technologies.
Subject of Research: Ion selectivity in polyamide nanofiltration membranes and the role of nanoscale surface charge homogeneity.
Article Title: Impact of charge homogeneity on ion selectivity in polyamide membranes.
Article References:
Lu, D., Huang, M., Zhang, C. et al. Impact of charge homogeneity on ion selectivity in polyamide membranes. Nat Water (2025). https://doi.org/10.1038/s44221-025-00498-5
Image Credits: AI Generated
