Probing Pore Wetting in Membrane Distillation Using Impedance: Early Detection and Mechanism of Surfactant-Induced Wetting

Pore wetting is an important failure mechanism unique to membrane distillation (MD). The existing approach of wetting detection based on distillate conductivity works only when a membrane has failed in the presence of fully wicked-through pores. In this study, we develop a novel and simple method, based on measurement of cross-membrane impedance, for monitoring the dynamics of membrane pore wetting and enabling early detection of imminent wetting-based membrane failure. Using Triton X-100 to induce pore wetting in direct contact MD experiments, we demonstrated the rapid response of single-frequency impedance to partial pore wetting long before any change in distillate conductivity was observed. We also conducted an MD experiment using alternating feed solutions with and without surfactants to elucidate the mechanism of surfactant-induced pore wetting. Our experimental observations suggest that surfactant-induced pore wetting occurred via progressive movement of the water–air interface and that adsorption of surfactants to the membrane pore surface plays an important role in controlling the kinetics of progressive wetting.