The structure and polarization of the water-graphene interface from molecular dynamics simulations and X-ray reflectivity experiments*

Nanconfined water is found in numerous applications across natural and technological systems where water mediates chemical reactions, adsorption, diffusion, ion transport, among other processes. Water affects the interactions among ions, molecules, and surfaces. The electrostatic forces in the systems and processes are modified through polarization which is the collective orientation of the molecules' dipole moment. The polarization of water is different in bulk, in confinement, and at interfaces. Here we study the water structure and polarization next to uncharged graphene surfaces by means of molecular dynamics simulations and X-ray reflectivity experiments. Despite graphene being hydrophobic, our simulations and experiments show an adsorbed water layer on graphene. Even in the absence of an external electric field this adsorbed water layer has a persistent polarization and an induced electrostatic potential. In addition, the adsorbed water molecules form a hydrogen bond network and make the graphene surface repulsive to ions.