Surface polarity modulation enables high-performance polyamide membranes for separation of polar/non-polar organic solvent mixtures

Achieving highly selective separation of organic mixtures is crucial to modern fine chemicals and pharmaceutical industries. The membrane-based organic solvent reverse osmosis (OSRO) technology is expected to break through this barrier. The key to achieving the separation of organic mixed liquids is to design the OSRO membrane as needed based on the physical and chemical properties of the target separation solvent. However, this poses a huge challenge to traditional OSRO membranes. In this work, we report composite polyamide (PA) OSRO membranes with ultrahigh polar/non-polar organic mixture selectivity. The surface polarity of the composite PA membrane is increased by coating a layer of Fe3+/tannic acid (Fe3+/TA) layer on the surface of the PA membrane. The enhanced surface polarity increases the interaction difference between polar and non-polar solvents and the membrane. And a polarization layer is formed on the membrane surface, which weakens the concentration polarization effect on the membrane surface. Under high-concentration feed liquid (the mass ratio of methanol/hexane is 7:3), the separation factor of the prepared membrane reaches 218, which is the highest value of all OSRO membranes reported so far. For the first time, we establish a partial flux analysis for the OSRO process. This analysis unravels the pressure, concentration dependence, and sensitivity of separation performance. It provides novel insights into the design of high-performance OSRO membranes.