Tuning the Charge and Hydrophobicity of Graphene Oxide Membranes by Functionalization with Ionic Liquids at Epoxide Sites

Abstract

Functionalization of graphene oxide (GO) membranes is generally achieved using carboxyl groups as binding sites for ligands. Herein, by taking advantage of the ability of imidazolium-based ionic liquids (ILs) to undergo an epoxide ring-opening reaction, a new approach of GO modification was established, in which ILs were bonded to the abundant epoxides on GO without sacrificing the carboxyl groups. Computational methods confirmed this unique configuration of ILs on GO, which enabled the dispersion of IL/GO flakes in water for facile casting into laminate membranes. Compared with neat GO, the ILs in IL/GO membranes served as spacers that substantially reduced the multi-valent cation mobility, simultaneously facilitated ion desolvation, and increased the water flux across the membrane. Our studies found that the higher separation efficiency of IL/GO membranes may be attributed to the synergistic modification of the hydrophobicity and surface charge. Specifically, the protonated nitrogen of the imidazolium cations altered the surface charge of GO, thereby generating electrostatic repulsion that enhanced the selectivity of cation rejection. On the other hand, the increased length of the alkyl chains bound to the imidazolium rings was found to increase the hydrophobicity of GO, which, in turn, aided the fine-tuning of the water desolvation/transport dynamics at the GO/IL interface to achieve a high water flux. Additionally, the water retention was reduced on the hydrophobic planes, which inhibited GO swelling during aqueous separations. Molecular dynamics simulations revealed increased water diffusivity when ILs were intercalated within GO layers. We establish that without requiring a high energy input, functionalization of GO membranes with ILs may be a promising approach to achieve efficient ion separation and critical material recovery.

Convenient Confinement: Interplay of Solution Conditions and Graphene Oxide Film Structure on Rare Earth Separations

“…The ion–GO interactions that guide these separations and insertions occur in the confined region between the GO surface and the subphase. However, few works have specifically considered ion and water interactions with GO in this interfacial region, , and fewer have probed the interface directly experimentally. − Additionally, molecule behaviors at interfaces greatly differ from the bulk. − Nanoscale, detailed information about both water and ion behavior near model GO surfaces is imperative for groundbreaking membrane and sorbent advances, including improvements in permeability and selectivity and the realization of membrane regeneration.…”

Ion and water adsorption to graphene and graphene oxide surfaces

“…We note electrochemical impedance spectroscopy measurements of GO modified with different ILs revealed variations in cation and anion diffusion through GO membranes made from stacked GO flakes, which was connected to changes in the hydrophobicity of the GO. 99 Additional simulations showed an increase in water diffusion across GO depending on the IL used during GO modification.…”

Ultrahigh stable laminar graphene membranes for effective ionic and molecular nanofiltration with a machine learning-assisted study

“…This describes a surface charge in the graphene channels, which corresponds to the previous literature studying ion transport through nanofluidic channels, 53,64 graphene membranes, 65 and laminar graphene membranes. 3,66 As compared to the deviation point, the charged confining surface is more pronounced for the 3 µm-thick membrane (high surface charge) due to the deviation in fluidic conductance was observed at a higher ionic strength (see the dashed line for bulk conductance). Moreover, the effect of charge density inside the graphene nanochannels was also determined using the role of the adsorption of hydronium (H 3 O + ) and hydroxide ions (OH − ) on the graphene nanochannel surface (see Fig.…”