Can Composite Janus Membranes with an Ultrathin Dense Hydrophilic Layer Resist Wetting in Membrane Distillation?

Abstract

Tackling membrane wetting is an ongoing challenge for largescale applications of membrane distillation (MD). Herein, composite Janus MD membranes comprising an ultrathin dense hydrophilic layer are developed by layer-by-layer assembling cationic polyethyleneimine and anionic poly(sodium 4-styrenesulfonate) polyelectrolytes on a hydrophobic polyvinylidene fluoride substrate. Using surfactant-containing saline water as the feed with low surface tension, experiments reveal that the number of polyelectrolyte layers, rather than surface wettability or surface charge, determines the anti-wetting performance of the composite Janus membranes. More deposited layers yield higher wetting resistance. With the aid of positron annihilation spectroscopy, this study, for the first time, demonstrates the origin of the excellent wetting resistance of the composite Janus membranes. The effective pore size of the polyelectrolyte multilayer decreases with an increase in the number of the deposited layer. The membrane with an ultrathin hydrophilic multilayer of 48 nm has a sufficiently small pore size to sieve out surfactant molecules from the feed solution via a size exclusion mechanism, thus protecting the hydrophobic substrate from being wetted by the low-surface-tension feed water. This study may pave the way for developing next-generation anti-wetting Janus membranes for robust membrane distillation.

Janus Membrane with a Dense Hydrophilic Surface Layer for Robust Fouling and Wetting Resistance in Membrane Distillation: New Insights into Wetting Resistance

“…In the recent literature, the size exclusion effect is considered as the major mechanism of wetting resistance for MD membranes with a dense surface layer. , Specifically, it was argued that the permeation of SDS through the dense layer is prevented because the pores in the dense layer are too small for SDS to enter. To verify this explanation, the diffusion experiment was conducted to probe the SDS permeability of the pristine PVDF membrane and dense PVA/PVDF Janus membrane.…”

Janus membranes for membrane distillation: Recent advances and challenges

“…In this method, one side of the hydrophobic membrane (like PVDF) is sealed and then the membrane is periodically immersed in various solutions containing intermediate and hydrophilic solutions. This subsequent immersions is termed as layer-by-layer coagulation of dope solutions [104].…”

A Mini Review on Antiwetting Studies in Membrane Distillation for Textile Wastewater Treatment

“…Thus, material with desired antiwetting properties is beneficial to control membrane wetting. Although significantly improved antiwetting properties have been achieved by applying dual layer membranes containing hydrophilic surface layer and hydrophobic layer [72][73][74][75], there are only a few applied in the textile wastewater treatment. Polymers including PVDF, PP, and PTFE are the dominating materials for MD membranes.…”