Characterizing Depth-Dependent Crosslinking and Its Influence on Water and Solute Transport in Polyamide Membranes

Thin-film composite (TFC) membranes with polyamide (PA) active layers are the state-of-the-art in desalination and water purification, yet their performance strongly depends on nanoscale structural features that remain difficult to quantify. Among these, the distribution of crosslinking degree (CD) within PA layers plays a decisive role in governing the trade-off between water permeability and salt rejection. Conventional surface-sensitive techniques, such as X-ray photoelectron spectroscopy (XPS, ~5 nm probing depth), systematically underestimate the true crosslinking profile, masking critical structure–property relationships.

In this work, we employed XPS depth profiling, combining ion sputtering with chemical analysis, to characterize heterogeneous CD distributions across PA layers synthesized from different amine monomer chemistries. Depth-resolved analyses revealed significant gradients, with consistently lower CDs near membrane surfaces and higher CDs at the interior. These distributions strongly correlated with membrane performance: higher overall CDs yielded superior salt rejection but reduced water permeability, while lower CDs produced the opposite trend (M1: 99.2%, M2: 94.5%, M3: 87.2% rejection under seawater testing). Complementary characterizations, including contact angle, scanning electron microscopy (SEM), and atomic force microscopy (AFM), further confirmed that surface properties reflect underlying crosslinking heterogeneity.

To validate and extend these findings, non-equilibrium molecular dynamics (NEMD) simulations were conducted to model PA films with varying thicknesses and crosslinking densities. The simulations reproduced the observed density and crosslinking distributions and captured the associated transport behavior, providing molecular-level insights into how structural heterogeneity governs water and ion transport.

Overall, this study highlights the importance of depth-resolved analytical approaches for accurately assessing PA membranes. By establishing a direct connection between nanoscale crosslinking gradients and macroscopic membrane performance, these results provide new design principles to guide the development of next-generation membranes for sustainable desalination and water reuse.