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 operando 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. 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 operando experimental evidence for dynamically evolving transport states during PV operation. The observed transport behavior is inconsistent with the solution-diffusion mechanism, which assumes transport through a homogeneous dense membrane, and instead supports a pore-mediated transport mechanism in which dynamic liquid-vapor phase transitions within an interconnected pore network govern transport in PV membranes.