Salt-bridge pinning as the dominant mechanism of protein fouling on FT-30 reverse osmosis membranes: Insights from all-atom molecular dynamics simulations

Protein fouling of reverse osmosis (RO) membranes is a critical challenge for seawater desalination, causing flux decline, increased energy consumption, and premature membrane failure. Despite its operational significance, the molecular mechanisms governing early foulant adhesion remain poorly understood. All-atom molecular dynamics (MD) simulations investigated the early-stage adsorption of bovine serum albumin (BSA), a model protein foulant, onto FT-30 polyamide RO membranes under desalination-relevant saline conditions (0.5 M NaCl). In the simulated trajectory, adsorption proceeded via a reorient–detach–re-adsorb sequence, stabilizing a final partially hydrated interfacial contact of 2.4 nm 2 . Salt bridges between polyamide carbonyl oxygens and cationic Arg/Lys residues — histidine remains neutral and contributes no cationic charge — constitute the primary adhesive mechanism, while a robust interfacial hydration layer suppresses direct hydrogen bonding. The adsorbed BSA centre-of-mass diffusion coefficient decreased by approximately 75–80% relative to the Stokes–Einstein estimate for freely diffusing BSA under identical ionic conditions, confirming strong electrostatic surface pinning, while interfacial water retained high lateral fluidity. MM-PBSA calculations indicated a binding free energy of −7.19 ± 1.88 kcal·mol −1 , consistent with favourable but moderate adsorption. Energy decomposition showed that van der Waals interactions dominated (−8.71 kcal·mol −1 ), complemented by electrostatics (−2.46 kcal·mol −1 ) and partly offset by polar solvation penalties (+5.17 kcal·mol −1 ). These findings demonstrate that short-range electrostatic anchoring and dispersive forces, rather than hydrophobic collapse, govern early-stage biofouling, providing a molecular basis for designing next-generation low-fouling RO membranes.