Pore Wetting and Compaction in Pressure-Driven Distillation: Insights from Impedance Spectroscopy

Abstract

Pressure-driven distillation (PD) is a desalination process that uses applied hydraulic pressure to drive water vapor through an air-trapping porous hydrophobic membrane. Unlike distillation processes that rely on heat, PD leverages applied pressure, making it more energy-efficient and allowing it to operate in a similar form factor as other pressure-driven processes like reverse osmosis. However, the high pressures required for PD operation─typically exceeding 10 bar─make membranes vulnerable to wetting and compaction. In this study, we employ electrochemical impedance spectroscopy to analyze compaction and wetting behavior in distillation membranes subjected to pressures up to 15.2 bar. We examine six different hydrophobic membranes made from poly(tetrafluoroethylene) and poly(vinylidene fluoride), identifying correlations between membrane morphology, applied pressure, and wetting mechanisms through highly sensitive impedance measurements. Our findings show significant compaction effects during the initial pressure increase, followed by progressive pressure-induced pore wetting as pressure rises, both in the presence and absence of surfactants. We also develop and validate an equivalent circuit model that represents air-trapping hydrophobic membranes. Overall, this research offers valuable insights into the dynamics of membrane wetting under pressure and demonstrates that impedance measurements can potentially serve as a critical control point for water treatment systems.