Membrane technology is a very sustainable approach to separation, as it requires much less energy than conventional separation approaches. However, the sustainable image of membranes becomes substantially tarnished when you realize that nearly all membranes are prepared using large quantities of toxic and unsustainable aprotic solvents (NMP, DMF, etc.). Moreover, these aprotic solvents are under serious scrutiny by governments, with the European Union recently agreeing on substantial restrictions for these solvents. To secure the future of membrane technology, it becomes critical to develop more sustainable approaches to membrane fabrication. An Aqueous Phase Separation (APS) technique has recently been proposed by the group of the PI, Membrane Surface Science (MSuS), as a green and sustainable alternative to the currently dominant non-solvent induced phase separation (NIPS) process.APS utilizes polyelectrolytes such as poly(sodium 4-styrenesulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMAC), poly(allyl amine hydrochloride) (PAH), and polyethyleneimine (PEI) to obtain sustainable polyelectrolyte complex (PEC) membranes in a completely water-based process. The structure and morphology of these APS membranes can easily be controlled to produce excellent separation properties. Although APS membranes show high solute retentions, the water permeability is much lower than NIPS membranes now utilized for the same application. For example, APS based nanofiltration (NF) membranes showed pure water permeability in the range of ~1–4 L·m–2·h–1·bar–1¸ while the permeability of most commercial NF membranes lie in the range of 5–20 L·m–2·h–1·bar–1.The lower water permeability of the existing APS membranes is now the major obstacle preventing their commercial production and large scale industrial acceptance. This originates from the fact that the separation in case of APS membranes is performed by the same material that gives the membrane its mechanical strength. Indeed, the water permeability is significantly compromised when utilizing dense and mechanically strong membranes, while more swollen an permeable materials are simply took weak. For NIPS based membranes this problem is typically alleviated by coating a thin layer of dense and selective material on top of a porous support membrane either by interfacial polymerization (IP) or dip-coating. But this second step is laborious and again relies on dangerous and harmful organic solvents, for example n-hexane, a known neurotoxin. In this project, we have studied an advanced APS procedure, employing Interfacial Complexation (IC) during the phase inversion step, to produce composite membranes in a highly sustainable one-step approach that will lead to the required high performances i.e. excellent permeability and solute retentions.
