Separation of water/ethanol azeotropic mixtures using bipolar membranes

The dissociation of solvents (ROH → RO − + H + ) within the interfacial layer of negatively and positively charged membranes is a defining feature of bipolar membranes (BPMs). In this work, we systematically investigated the solvent dissociation behavior of BPMs in water/ethanol mixtures. Our results reveal a clear trend of sequential dissociation in water–alcohol systems. At ethanol concentrations below 92% (EtOH ≤ 92%), only water dissociation was observed, whereas at higher ethanol contents (EtOH > 92%), both water and ethanol underwent concurrent dissociation. Leveraging this cascade dissociation of mixed solvents in BPMs enables the separation of water–ethanol azeotropic mixtures, increasing the ethanol content from 95.5% to 97.3%. Molecular dynamics simulations further revealed that in low-ethanol systems, water has a higher diffusion coefficient, forms stronger hydrogen bonds, and maintains a more extensive hydrogen-bond network among water molecules, which facilitates its preferential diffusion into the BPMs and subsequent dissociation. In contrast, in high-ethanol solutions, ethanol has a relatively large diffusion coefficient and a progressively strengthened hydrogen-bond network, favouring its preferential dissociation. The cascade dissociation of different solvents in BPMs thus offers a promising strategy for separating various azeotropic and homologous mixtures, including water/alcohol and alcohol/ether systems.