Polyamide desalination membranes: Formation, structure, and properties

Empirical discovery, four decades ago, of polyamide membranes made by interfacial polymerization revolutionized large-scale desalination and has made desalinated water affordable to millions of people worldwide. The path to better understanding of the exceptional performance exhibited by these polymeric films, critical for rational membrane design and the search for alternative materials, begins from understanding their formation and the resultantmolecular and nanostructure and has posed numerous questions. The self-limited, ultra-small thickness, irregular multiscale nanostructure, and the need for polyamide films to be formed on a support rather than in a bulk process pose formidable challenges for structural characterization and molecular modeling. Further challenges arise from insufficient understanding of the relations between polymer chemistry, multi-scale structure, and their impact on transport and mechanical characteristics. Extensive research conducted over the last decades, using dedicated experimental and theoretical approaches, have highlighted many remarkable features of polyamide thin films, and yet many questions still await conclusive answers. The present paper reviews the current state of understanding of polyamide thin films, and, in particular, fully and semi-aromatic “winner” chemistries used for membrane separation, based on recent advances in the nanoscale characterization and theoretical investigations of their formation, chemistry, structure, morphology and barrier characteristics down to the molecular scale. These are reviewed in light of the recent developments in understanding of the interfacial polymerization process, reaction, packing and crosslinking of polymeric building blocks, formation and distribution of charge groups, and interaction of the resultant polymer network with water and ions. The proposed unified picture that links the emerging picture of the multiscale nano- and molecular structure of polyamide thin films with macro-scale characteristics, enables a consistent rationalization of their superior barrier characteristics. Furthermore, such a framework provides insight on inherent weaknesses of polyamides and the challenges of overcoming these limitations and developing viable alternatives. With membranes fast becoming the choice for a growing number of challenging separations, and with the need to replace current materials with sustainable, environmentally benign alternatives, drawing on established knowledge will provide a solid foundation from which better controlled, tunable membranes can be fabricated from next-generation building blocks.