Soft layer charge modulation in nanofluidic membranes: A pathway to enhanced desalination

Water desalination is a critical challenge in addressing global freshwater scarcity, and the development of efficient membrane technologies is essential for sustainable water purification. While nanofluidic membranes have demonstrated great potential in energy conversion, their application in desalination remains underexplored. This study simulates the performance of cylindrical nanofluidic membranes in removing LiCl, NaCl, and KCl from water, focusing on the influence of soft layer charge distribution on ion rejection efficiency. Three distinct charge configurations were examined: unipolar (negatively charged), bipolar (negatively–positively charged), and tripolar (negatively–positively–negatively charged). The Poisson–Nernst–Planck and Navier–Stokes equations were numerically solved under steady-state conditions to analyze the coupled effects of electrostatic interactions, convective transport, and external field application on desalination performance. The results demonstrate that bipolar nanochannels achieve the highest desalination efficiency, benefiting from diode-like rectification and strong ion depletion zones. At an applied pressure of 30 bar, the bipolar nanochannel achieves 85 %–96 % salt rejection with voltage and 50 %–89 % without voltage, outperforming the unipolar and tripolar configurations. Additionally, the bipolar nanochannel exhibits superior current rectification behavior, with ionic currents reaching 1.08 nA for LiCl, 1.25 nA for NaCl, and 1.60 nA for KCl at 1 V, confirming enhanced selectivity. Further analysis of the Peclet number (Pe) and electrolyte concentration reveals that bipolar nanochannels maintain NaCl rejection above 95 % at Pe ≈ 0.05, whereas unipolar and tripolar configurations experience a sharper decline in selectivity, with rejection dropping below 50 % at Pe > 0.2 and TMP > 80 bar. These findings underscore the significant impact of charge modulation in nanofluidic membranes and highlight that bipolar nanochannels provide the most effective balance between ion rejection and water permeability.