Revealing Wetting Patterns of Porous Hydrophobic Membranes for Desalination Using Electrochemical Impedance Spectroscopy

Membrane distillation (MD) holds great promise for high salinity wastewater desalination, but membrane pore wetting remains a major hurdle to the process. We develop a novel framework to dynamically monitor the wetting progression of hydrophobic MD membranes and quantitatively describe the wetting status changes by implementing four-electrode mode electrochemical impedance spectroscopy (EIS). We decouple the membrane impedance into resistance and capacitance components, which are then translated into air gap span and wetted pore fractions. Distinguishing the wetting progression and the occurrence of pore breakthroughs, this framework successfully depicts the disparate wetting patterns of hydrophobic membranes induced by surfactants and low-surface-tension liquids. We show that the ability to instantaneously capture the wetting progression makes our framework particularly attractive for MD operations accompanied by slow wetting. EIS applied in our framework further reveals that a membrane of a randomly porous structure incurs a stepwise wetting front advancing by low-surface-tension liquids, as opposed to the single pore breakthrough event in the membrane of straight pores. Combined with the internal surface area and the pore size analyses, this illustration by EIS is used to model a randomly porous membrane as tortuous pores with multiple 'necks' of local minimum pore sizes.