Polymer Nanofilm Composite Membranes for Ionic and Molecular Separation: History, Challenges and Future Perspectives

The membrane-based separation process is one of the energy-efficient technology to overcome the demand for a large quantity of fresh water and is also known for its use in water treatment, desalination, solvent recycling, and environmental remediation. Most conventional membranes used for separation-based applications are mainly polymer-derived thin-film composite (TFC) membranes, where the top selective layer of the composite membrane is made by using the interfacial polymerization (IP) process at the aqueous-organic interface. Different fabrication strategies are introduced for their applications in the aqueous feed to produce a high-specification selective layer of membranes with high liquid permeance that are selective to specific ions or molecules. It is particularly interesting to develop ultrathin polymer nanofilm as a selective layer of composite membranes, which provides ultrahigh liquid permeance as the liquid permeance is inversely proportional to their thickness. Additionally, to achieve the required high ions/molecular selectivity and make them stable in organic solvents, highly crosslinked polymer nanofilms (e.g., polyamide, polyester, polyimine, etc.) are developed. Here, we discussed the fabrication of conventional high-specification polymer membranes made from ultrathin polymer nanofilm and their ionic or molecular separation performance. The possible strategy to improve membrane fabrication techniques, applications in seawater desalination, and aqueous nanofiltration performance are discussed. A summary of the overview, challenges and future needs in the membrane-based separation process are discussed.