Operando Impedance Spectroscopy Reveals Dynamic Liquid–Vapor Phase Transitions in Pervaporation Membranes

Pervaporation (PV) is increasingly applied to environmentally relevant separations, including hypersaline brine treatment, solvent recovery, and resource recovery, yet the fundamental transport behavior governing membrane performance remains poorly understood. In particular, permeating molecules may undergo liquid–vapor transition during PV operation, but direct experimental evidence of the existence and evolution of such internal transport states remains largely unavailable. Here, we developed an operando electrochemical impedance spectroscopy (EIS) platform integrated with PV to probe dynamic transport-state evolution within polymer membranes under controlled operating conditions. Distinct impedance responses under vacuum-on and vacuum-off conditions revealed heterogeneous internal membrane states. Continuous modulation of downstream vacuum produced progressive impedance evolution, indicating gradual redistribution of liquid-filled and vapor-filled regions rather than abrupt switching between discrete states. Vacuum cycling showed reproducible behavior after an initial conditioning cycle, while feed salinity and temperature systematically altered impedance responses and fitted transport parameters. Equivalent-circuit analysis showed substantial changes in conductive and ionic diffusion-associated transport parameters, whereas dielectric characteristics remained comparatively stable, indicating that operating conditions primarily reorganize internal transport connectivity rather than fundamentally altering polymer properties. These findings provide experimental evidence for dynamically evolving transport states during PV operation. More broadly, this work suggests that controlling dynamic membrane transport states is a potential strategy for improving membrane design and process optimization for environmental separations and resource recovery