(403a) Pressure-Controlled Nanochannels in Polymer Cross-Linked Graphene Oxide Membranes for Water Treatment: A Combined Experimental – Molecular Simulation Approach

During the past years, graphene oxide (GO) has been started to be exploited as membrane for water treatment because of its simple synthetic method and chemical tenability [1]. The flexible 2D structure of GO layers facilitates to have an orderly deposited structure via interlayer interaction, resulting in the formation of unique sub-nanometer channels which can even filter monovalent ions [2]. However, maintaining the laminated GO structure for permeation processes is dramatically challenging.

The interlayer channel distance formed by the assembly of GO nanosheets can be regulated through various approaches such as cross-linking [3], pressure compression [4] and molecular intercalation [5], precisely tuning the channel size by establishing an energy barrier originated from steric hindrance during permeation. It is extremely difficult to design and synthetize a membrane that is excellent in both permeability and selectivity; however, optimal interlayer spacing combined with appropriate charged properties could make possible the attainment of outstanding performance in terms of both parameters.

Hence, in this contribution, pressure-controlled nanochannels in polymer-crosslinked GO membranes are designed for improving ion selectivity together with high water permeability. The layer-connected architecture is created by amide bonds consisting of intercalated cationic hyper-branched polyethylenimine (HPEI) and anionic polyacrylic acid (PAA) polymers cross-linked within the GO membrane. Owing to the bulky structure of grafted HPEI onto the GO basal plane, the layer-stacked channel of the HPEI-GO (HGO) membrane is vulnerable to the applied pressure that can forcibly diminish broadened graphitic domains, modifying the morphology of the inserted PAA during the crosslinking process. Moreover, the linearly compressed PAA can form wide hydrogen bond networks that enable rapid water transport within nanochannels, leading to increased water flux even in a narrow channel contrary to the trade-off relationship.

Membrane characterization, ion and water transport behavior of the composite material were studied to validate the strategy that simultaneously controls physicochemical and electrostatic properties on the diffusion process, along with the role of the inserted polymers in the anti-swelling property and diffusion coefficients. Complementary quantum and molecular simulations were performed to understand the interactions between GO, polymers, ions and water, further confirming the experimental results.

By virtue of the modified nanochannel, it was found that ions diffusion can be reduced up to five times by applying pressure in the composite membrane compared to the neat GO membrane, since permeating ions are strictly suppressed mainly by electrostatic interactions with locally existing cationic-HPEI in nanocapillaries. Consequently, it is expected that this strategy can be utilized to control the nanochannel of 2D materials through the combination of physicochemical and electrostatic modification for numerous promising applications.

This work was developed in the framework of a collaborative bilateral project between the Korea Advanced Institute of Science and Technology (KAIST) and Khalifa University (KU). This research was supported by the KAI-NEET Institute, KAIST, and also by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07048233). Additional partial support of this work was provided by Khalifa University through project RC2-2019-007. Computational resources from the Research and Innovation Center on CO2 and H2 (RICH) and the HPC cluster at KU are also acknowledged.

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Figure. Schematic illustrations of cation-PAA interaction in terms of the morphology of the inserted PAA into the HGO-PAA membrane.