Molecular dynamics study of PFOA behaviour in water silica systems

Perfluorooctanoic acid (PFOA), a persistent long-chain per- and polyfluoroalkyl substance (PFAS), exhibits complex aggregation and transport behaviour in aqueous environments. This study employed classical molecular dynamics (MD) simulations using GROMACS to investigate the structural, dynamic, and energetic behaviour of PFOA in water in the absence and presence of a fully hydroxylated silica nanoparticle. Two systems, namely PFOA in pure water and PFOA in a silica–water system, were examined to evaluate PFOA aggregation and adsorption behaviour. In pure water, PFOA molecules rapidly aggregated into a compact cluster within 3–10 ns, primarily driven by strong intermolecular van der Waals interactions (−284.1 kcal mol⁻¹). The presence of a 5 nm silica nanoparticle significantly suppressed PFOA aggregation, reducing PFOA–PFOA van der Waals interactions to −72.3 kcal mol⁻¹ (approximately 74.6%) while increasing electrostatic interactions to −67.1 kcal mol⁻¹. Radial distribution function analysis revealed enhanced molecular ordering at the silica interface, accompanied by increased hydrogen bonding (22–25 to 28–32) and altered diffusion behaviour. These findings demonstrate that silica nanoparticles promote PFOA adsorption and interfacial organization, providing molecular-level insight into PFAS adsorption mechanisms and supporting the design of efficient silica-based adsorbents for water treatment.