Transition state theory unveils enthalpy–entropy interplay in polyamide nanocomposite membranes for enhanced water–salt selectivity

Polyamide nanocomposite membranes are promising for seawater desalination, yet the molecular mechanisms governing water and solute transport remain unclear. Here, we synthesized a conjugated microporous polymer (IPA–PPA) and embedded it within the interlayer of polyamide thin-film nanocomposite (CMP–TFN) membranes. Using transition state theory and the Eyring equation, we deconvoluted intrinsic water and ion permeability into enthalpic and entropic contributions to probe the molecular basis of water–salt selectivity. The partially protruding, intrinsically microporous IPA–PPA generated additional nanoscale transport channels and modified polyamide morphology and surface charge, thereby lowering mass-transfer resistance. As a result, CMP–TFN membranes achieved high Na 2 SO 4 rejection (98.4 %) alongside enhanced water flux, antifouling performance, and long-term stability. Transition state analysis revealed that increased water transport was driven by higher activation entropy ( ΔS ‡ ), whereas stronger solute rejection correlated with elevated activation enthalpy ( ΔH ‡ ). The synergy between enlarged pore structures (increasing ΔS ‡ ) and stronger surface charge (increasing ΔH ‡ ) collectively modulated ion transport, ultimately improving water–salt selectivity. Our findings demonstrate that enthalpy–entropy interplay underpins water–ion discrimination in polyamide nanocomposite membranes, enabling high selectivity.