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

M Rajasekaran and KG Ayappa, JOURNAL OF PHYSICAL CHEMISTRY B, 123, 2978-2993 (2019).

DOI: 10.1021/acs.jpcb.8b12341

Graphene oxide membranes have been widely studied for their potential applications in water desalination applications. 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 angstrom) made up of different surface types are carried out. The greatest differences are observed at 8 angstrom, 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 2 order decrease in the dipole-dipole relaxation time observed adjacent to a graphene surface. The translational and rotational density of states show distinct blue shifts and red shifts, 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 angstrom, suggestive of glasslike dynamics, and extended alpha-relaxations were observed for the other surfaces. Although 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 (similar to 4 kJ/mol) for the Janus surfaces, which have 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.

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