Plasma Etched Omniphobic Hydrophobic Porous Membranes for Application in Pressure-Driven Distillation

There is an increased interest in implementing hydrophobic membranes in desalination technologies due to their potential for high chemical resistance and ability to reject nonvolatile contaminants. When hydrophobic membranes are submerged into a wastewater stream, an air liquid interface is formed and driving forces such as heat, pressure or concentration cause the water to evaporate at the interface and transport through the membranes as vapor water. This mechanism allows for full rejection of nonvolatile contaminants from the feed stream. However, hydrophobic membranes are susceptible to water penetrating through the airgap in the presence of low surface tension liquids leading to pore wetting and the eventual loss of the highly selective nature of these membranes. As a result, the ability to create membranes with reentrant structures on the surface, otherwise known as omniphobic membranes, has been increasingly sought out to address these unfavorable interactions. Current methods to create these surfaces are multistep and complicated procedures that would be difficult to scale up. Much research has been done to find simpler methods to create nanotextures, and plasma etching has recently been found to create uniform textures on polymer surfaces. In this work, we created an omniphobic polymer membrane by treating PTFE membranes with a brief low power oxygen plasma and found that not only does the plasma treatment create uniform nanotextures on the surface, but also enhances the hydrophobicity. We found that the treated membranes have enhanced wetting behavior in the presence of low surface tension liquids in addition to enhanced flux values in pressure driven distillation. We also probed the polymer texturing mechanism with x-ray photoelectron spectroscopy, scanning electron microscopy and x-ray diffraction to provide further insights on how plasma processes can be used to design hydrophobic membranes for use in desalination.