Enhancing Dynamics of Water Confined between Graphene Oxide Surfaces with Janus Interfaces: A Molecular Dynamics Study.

Graphene oxide membranes have been widely studied for their potential applications in water desalination applications. In order to understand the influence of surface oxidation and the inherent heterogeneity imposed by opposing surfaces formed in macroscopic membranes, molecular dynamics simulations of water confined in nanopores (8 - 15 Å) made up of different surface types are carried out. The greatest differences are observed at 8 Å which is the optimal separation distance for molecular sieving of ions. The dipole-dipole relaxation and HH rotational relaxation of confined water are the slowest between fully oxidized (OO) surfaces with a two order decrease in the dipole-dipole relaxation time observed for the Janus confinement consisting of an oxidized surface adjacent to a graphene surface. The translational and rotational density of states show distinct blueshifts and redshifts respectively at the smaller separations, with the extent of the shifts dependent on the surface type. Self-intermediate scattering functions show a pronounced plateau region for the OO surfaces at 8 Å suggestive of glass-like dynamics and extended α-relaxations were observed for the other surfaces. Although the water diffusivity is an order of magnitude smaller than bulk diffusivities at the smaller surface separations, water between the Janus surfaces always had the highest diffusivities. The free energy to transfer a water molecule from bulk water was found to be the smallest (∼ 4 kJ/mol) for the Janus surfaces which has the lowest number of hydrophilic groups among the different systems studied. Thus the Janus interface appears to provide the optimal environment for water transport providing a design strategy while assembling graphene oxide based membrane stacks for water purification.