Molecular Simulation Study on Molecularly Mixed Porous Organic Cage/Polymer Composite Membranes for Water esalination and Solvent Recovery

Emerging as a distinct type of molecular cage possessing both intrinsic and extrinsic porosity, porous organic cages (POCs) have been regarded as intriguing nanofillers to produce molecularly mixed composite membranes (MMCMs). Here, we report a molecular simulation study to examine the swelling of three MMCMs in water, methanol, and acetonitrile and then explore their applications for water desalination and solvent recovery. The MMCMs are formed by mixing three POCs (CC1, CC3, and CC17, respectively) with a polymer of intrinsic microporosity (PIM-1). The POC/PIM membranes exhibit the greatest swelling in methanol, followed by acetonitrile and water. It is revealed that the periphery groups on the POCs have different interactions with the polymer and solvent during swelling, which plays a significant role in determining the structures of swollen membranes. Remarkably, water and methanol permeabilities through PIM-1 are enhanced 1-2-fold by all three POCs, while acetonitrile permeability is enhanced 1-fold by CC17. The enhancement is primarily attributed to the large voids formed in swollen membranes during swelling. Within the simulation durations in this study, all three POC/PIM membranes exhibit perfect rejection toward salt (NaCl) and a small solute (paracetamol). Furthermore, the POCs are observed to provide an additional pathway for solvent transport. A majority of water molecules can reside within the POCs for less than 400 ps, while methanol and acetonitrile molecules reside for approximately 1000 and 2000 ps. The residence time of the solvent is unravelled to depend on not only the solvent-cage interaction but also the solvent size and shape. From the bottom up, this comprehensive simulation study provides microscopic insights into the crucial effects of the POCs on the swelling and solvent permeation of POC/PIM membranes, and it facilitates the judicious selection of appropriate POCs to produce high-performance MMCMs for water desalination, solvent recovery, and other industrially important molecular separation.