RESUMO
Surface modification with small-molecule zwitterions is experimentally proved to be an effective solution to improve the antifouling performance of polyamide membranes. However, there is no comprehensive understanding of their microscopic mechanism. In order to address this issue, in this work we constructed two atomistic models, PA (a pure polyamide membrane) and QDAP-PA (a polyamide membrane surface-modified with QDAP), where QDAP was a zwitterion that was prepared by 2,6-daaminopyridine quaternized with 3-bromopropionic acid experimentally. Density functional theory was adopted to elucidate the variations in the electrostatic potential before and after modification. Then, equilibrium molecular dynamics (EMD) simulations were conducted to investigate the structure and hydrophobic/hydrophilic nature of the membrane surface in the two models. Finally, we introduced two typical organic foulants, sodium dodecyl sulfonate (SDS) and dodecyl trimethyl ammonium chloride (DTAC), to evaluate the antifouling performance of the membranes with the umbrella sampling method. The analyses of the membrane structure and properties show that surface modification with small-molecule zwitterions can densify the membrane surface as well as enlarge the distribution of electrostatic potential on the membrane surface. Water molecules tend to have more interactions with the membrane and more hydrogen bonds near the membrane surface are observed in QDAP-PA. The antifouling test supports that QDAP-PA shows a better antifouling performance, as the surface-modified membrane exhibits a stronger resistance to SDS and DTAC. Even if the foulant is adsorbed to the membrane surface, the denser interface region can prevent a further pollution of the foulant. Also, the free energy needed during the process for QDAP-PA to desorb a foulant is relatively small, indicating that this kind of membrane is easy to clean. The current work might provide a comprehensive understanding of the enhanced fouling resistance of polyamide membranes after surface modification with small-molecule zwitterions.
RESUMO
CNT-based reverse osmosis membranes have long been regarded as one of the most promising candidates for water desalination. However, it is a pity that there is no complete understanding of the exact role of CNTs in those nanocomposite membranes. To address this issue, three atomistic models of PA (pure polyamide membrane), PA-CNT1 (polyamide nanocomposite membrane with an embedded carbon nanotube oriented vertical to the membrane surface) and PA-CNT2 (polyamide nanocomposite with an embedded carbon nanotube oriented parallel to the membrane surface) were constructed respectively in this work. Then, equilibrium molecular dynamics (EMD) and non-equilibrium molecular dynamics (NEMD) simulations were conducted to investigate the structure and water dynamics in these three models. The EMD simulations revealed a better stacking of the PA matrix due to the addition of the CNT and this impact was more significant in PA-CNT1 than in PA-CNT2. Meanwhile, PA matrix near the mouth of the CNT was found to behave as an obstruction that hindered the exchange of water molecules inside and outside the CNT. In NEMD simulations, we found that water molecules were guided away from the CNT because of the better stacked surrounding PA matrix. The partially covered CNT might not help to increase water flux in PA-CNT1 while guided water molecules and the smaller polymer region afftected by the CNT contributed to a relatively high flux in PA-CNT2. The current work might serve as a comprehensive understanding of the role of CNTs in the reverse osmosis process.
