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The layers of graphene oxide (GO) are found to be good for permeation ofwater but not for helium (Science 2012 335 (6067): 442-444) suggesting that theGO layers are dynamic in the formation of permeation route depending on theenvironment they are in (i.e, water or helium). To probe the microscopic originof this observation we calculate the potential of mean force (PMF) of GO sheets(oxidized and reduced parts), with inter-planar distance as reaction coordinatein helium and water. Our PMF calculation shows that equilibrium interlayerdistance between oxidized part of GO sheets in helium is at 4.8 {\AA} leavingno space for helium permeation. In contrast PMF of oxidized part of GO in watershows two minima one at 4.8 {\AA} and another at 6.8 {\AA} corresponding to nowater and water filled region and thus giving rise to permeation path. Theincreased electrostatic interaction between water with the oxidized part of thesheet helps the sheet opening up and pushing water inside. Based on the entropycalculations for water trapped between graphene sheets and oxidized graphenesheets at different inter-sheet spacing we also show the thermodynamics offilling.

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