Ordering-Induced Fast Diffusion of Nanoscale Water Film on Graphene

We show that ordering in nanoscale water film on a hydrophobic surface gives rise to fast diffusion of water. Specifically, as the surface coverage of water increases, the diffusion coefficient of water increases until a critical surface coverage and a further increase in Surface coverage results in a decrease of water diffusion coefficient. For thin nanoscale films that form two layers of waters on a hydrophobic surface, the first layer of water forms a hexagonal structure, very similar to the ice Ih structure, that is independent of the surface coverage. As the surface coverage increases, the ordering of water molecules in the second layer increases and for a critical surface coverage the ordering in the second layer is maximized and the hydrogen bonding between first and second layers is minimal giving rise to fast diffusion. As the Surface coverage further increases, the hydrogen bonding between the first and second layers increases and the diffusion coefficient of water is reduced. This "ordering-induced diffusion enhancement" on hydrophobic Surfaces is contrary to the ordering-induced slow mobility in hydrophobic nanotubes (e.g., in a carbon nanotube).