(614i) The Structure, Dynamics and Relaxation of Water Confined in Graphene Oxide Slit Pores

Graphene oxide (GO) membranes have attracted a lot of interest due to ease of synthesis, exceptional water permeation properties and precise molecular sieving of ions. As a consequence, graphene oxide membranes may find its application in efficient water purification and desalination technologies. We have used molecular dynamics simulations to study the structure, dynamics and thermodynamics of confined water by constructing a slit pore made up of two surfaces of GO. In order to mimic the different confining situations that occur in GO membranes, we have constructed a GO surface consisting of strips of functionalized regions and studied pores made up of extended GO surfaces, where the functionalized regions are either in-registry (IR), out-of-registry (OR) as well as pores with fully functionalized surfaces (O) and a Janus pore (J) made up of one GO surface and a graphene surface. Inter-surface separations were varied from 0.8 nm to 1.5 nm and we have investigated translational and rotational dynamics, dipole-dipole relaxation as well as the entropy of confined water and compared these across the different GO confining environments. The in-plane diffusion coefficients were higher for the IR pores when compared with the OR pores and in all cases were found to be lower when compared to bulk water even at the largest separations of 1.5 nm. The highest values of diffusion coefficients were observed for the J surfaces. A distinct plateau regime in the mean squared displacements was observed for the smaller pores indicative of glass-like dynamics in these systems. The dipole moment distributions were peaked around 90^ofor all cases except for the O surface where two distinct peaks are observed. Relaxation times for the water dipole-dipole correlations were similar for both the IR and OR pores (10 - 300 ps) with distinct differences in the relaxation times only for the 0.8 nm pores. Water in the O and J surfaces showed the slowest and fastest relaxation times respectively. Similar trends were observed for the water translational entropies computed using the two phase thermodynamic model. Interestingly water rotational entropies for the O and J surfaces were found to be slightly higher than that of bulk water. Our study indicates that the structure and ordering of water in GO membranes is a strong function of the local environment of the water molecules and this study attempts to investigate these different situations by constructing different model systems. Clear signatures of extended and slow relaxation of water at the 0.8 – 1.0 nm pores are signatures of frustrated water dynamics at these separations.