Molecular mechanism of water permeation in helium impermeable graphene and graphene oxide membrane

TL;DR

This study investigates the microscopic mechanisms behind water permeation in graphene oxide membranes, revealing how environmental conditions influence interlayer spacing and permeation pathways through potential of mean force calculations.

Contribution

It provides a detailed molecular-level understanding of water and helium permeation in GO membranes by calculating the potential of mean force and thermodynamics of interlayer spacing.

Findings

Helium cannot permeate oxidized GO sheets at 4.8 Å interlayer distance.

Water can permeate oxidized GO sheets at larger interlayer distances due to two minima in PMF.

Electrostatic interactions facilitate water permeation by opening up the GO layers.

Abstract

The layers of graphene oxide (GO) are found to be good for permeation of water but not for helium (Science 2012 335 (6067): 442-444) suggesting that the GO layers are dynamic in the formation of permeation route depending on the environment they are in (i.e, water or helium). To probe the microscopic origin of this observation we calculate the potential of mean force (PMF) of GO sheets (oxidized and reduced parts), with inter-planar distance as reaction coordinate in helium and water. Our PMF calculation shows that equilibrium interlayer distance between oxidized part of GO sheets in helium is at 4.8 {\AA} leaving no space for helium permeation. In contrast PMF of oxidized part of GO in water shows two minima one at 4.8 {\AA} and another at 6.8 {\AA} corresponding to no water and water filled region and thus giving rise to permeation path. The increased electrostatic interaction between water with the oxidized part of the sheet helps the sheet opening up and pushing water inside. Based on the entropy calculations for water trapped between graphene sheets and oxidized graphene sheets at different inter-sheet spacing we also show the thermodynamics of filling.