  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.
