Non-Monotonic Effect of Asphaltenes on Gas Sorption in Polysulfone-Based Nanocomposite Membranes: Molecular Dynamics...

Asphaltenes represent a potentially useful class of naturally derived carbonaceous nanofillers for modifying the gas-transport properties of polymer membranes. Here, we combine all-atom molecular dynamics simulations with a recently developed analytical theory of free-volume relaxation to investigate the influence of asphaltenes on the transport properties of bisphenol A-based polysulfone and its nanocomposites containing 0-30 wt.% of asphaltenes. The estimated room-temperature density of the nanocomposites decreases with increasing asphaltene content, whereas the mean solubility coefficients of He, O₂, N₂, CH₄, and CO₂ exhibit a non-monotonic dependence on filler loading, reaching a maximum around 10 wt.% asphaltenes. Relative to neat polysulfone, the mean solubility coefficients of CO₂, CH₄, O₂, N₂, and He at this loading are higher by approximately 24%, 14%, 10%, 5%, and 3%, respectively. Further increases in asphaltene loading reduce gas solubility relative to this maximum. The He diffusion coefficient also varies non-monotonically, reaching a maximum increase of more than 30% at 10-20 wt.% of asphaltenes. Analysis of the nanocomposite structure, free-volume characteristics, and gas interactions suggests that the solubility trend is primarily associated with changes in fractional free volume accompanying asphaltene aggregation. These results show that moderate asphaltene loading can enhance gas sorption in polysulfone and provide a molecular-level basis for further computational and experimental studies of asphaltene-filled polymer gas-separation membranes.