Graphene Oxide Liquid Crystal Membranes in Protic Ionic Liquid for Nanofiltration

Abstract

Graphene oxide (GO) liquid crystals are of great interest for membrane preparation. Vacuum filtration has been frequently adopted as a small-scale manufacturing method. The main challenge is to obtain thin and robust layers with high permeation and selectivity by methods that could be applied in large scale. GO liquid crystals are mostly formed by dispersion in water. For the first time, we demonstrate that GO can form lyotropic liquid crystalline nematic phase dispersions in protic ionic liquid and be fabricated as membranes for nanofiltration. The well-balanced electrostatic interaction between ionic liquid and GO promotes and stabilizes the alignment of GO nanosheets even when concentrations as low as 9 mg GO/mL are used, providing the ideal rheology for the dispersion casting and membrane preparation. Robust membranes with GO layers as thick as 1 μm with high permeance (37 L/(m2 h bar)) and 99.9% rejection of dyes with molecular weight 697 g/mol were obtained. We confirmed the liquid crystal formation by the detection of birefringence and the rheological behavior and explained the liquid crystal formation as an interplay between hydrogen bonding and electrostatic interactions.