Mechanism of Surfactant-Induced Pore Wetting in Membrane Distillation

Pore wetting is a unique and important technical challenge that can lead to process failure in membrane distillation (MD) using hydrophobic membranes. While it is well known that amphiphilic molecules, such as surfactants, are effective wetting agents, the detailed mechanism of surfactant-induced pore wetting remain unclear. In particular, the role of surface adsorption in surfactant-induced wetting remains to be elucidated. This study provides fundamental insights to understanding the mechanism of pore wetting induced by surfactants. We develop a theoretical model, based on the assumption of quasi-equilibrium adsorption, to successfully explain the important features experimentally observed in surfactant-induced wetting. It was found that the kinetics of surfactant-induced wetting strongly depends on the vapor flux, the bulk concentration of surfactants in the feed solution, and the transmembrane hydraulic pressure difference. The results from our study also suggest that while the presence of surfactants promotes wetting, adsorption of surfactants onto pore surface actually deters pore wetting instead of promoting it by rendering the surface hydrophilic.