Effects of Imidazolium-Based Ionic Liquids on Molecular Structural Transitions of Polysulfone Using Molecular...

Polysulfone (PSF)-ionic liquids (ILs) composites exhibit great potential for the development of advanced ultrafiltration membranes. Therefore, understanding the molecular-level interactions between PSF and IL is critical. In this work, the structure and intermolecular properties of PSF with two cations, 1-butyl-3-methylimidazolium ([BMIM]⁺) and 1-ethyl-3-methylimidazolium ([EMIM]⁺) with a combination of six anions ([NO₃]^-, [FRM]^-, [ACT]^-, [BF₄]^-, [TFS]^-, and [BEZ]^- are studied using molecular dynamics simulations. The conformational structures of the PSF chains are analyzed using the radius of gyration (R_g), which increases with increasing anion size. Radial distribution functions present strong PSF-ion interactions for [EMIM]⁺. Spatial distribution functions reveal the anion clustering around the PSF monomers for small anions compared to large anions. Reduced density gradient presents a combination of both van der Waals and steric repulsions for both large and small anions. Excess molar volume ( $V_m^E$ ) and solvent-accessible surface area capture the structural properties of the polymer and its favorable interactions with the ILs as anion size increases. The self-diffusion coefficients of ILs show minimal effect in the presence of PSF other than [BF₄]^-. Overall, the results provide a systematic framework of how anion-cation combinations regulate the polymer structure and interactions in the environment.