Fluorine-free microporous membranes with high wetting resistance for membrane distillation

Abstract

Membrane distillation (MD) as a promising desalination technology requires the use of microporous membranes with high wetting resistance. Conventionally, fluorine-based membranes such as polyvinylidene difluoride (PVDF) and polytetrafluoroethylene (PTFE) membranes are used in MD, with their manufacturing involving per- and polyfluoroalkyl substances (PFAS) that are referred to as “forever chemicals” with severe adverse effects on living organisms and human health. These hydrophobic membranes are prone to pore wetting when the feed solution contains low-surface-tension contaminants. Accordingly, omniphobic membranes that resist both high-surface-tension and low-surface-tension liquids have attracted tremendous interest in enhancing the resilience of MD, but their fabrication also commonly uses PFAS. To mitigate the pressing issue of PFAS contamination, there is an urgent need to develop novel methods that enable the fabrication of fluorine-free MD membranes with high wetting resistance. We demonstrate that such membranes can be fabricated by creating a hierarchical structure on microporous membranes using polydimethylsiloxane (PDMS) in the presence of tetraethoxysilane (TEOS). Different hierarchical structures consisting of nanoparticles, nanofilaments, or nanohairs can be formed by tuning PDMS/TEOS ratio. Our results reveal that the hierarchically structured membrane decorated with nanohairs displays omniphobicity and can resist wetting of liquids with a wide range of surface tensions. Furthermore, we demonstrate that our fluorine-free omniphobic membranes have superior MD performance (i.e., higher water vapor permeance and higher wetting resistance) when compared to the commercial PVDF membranes.