At Least 10-fold Higher Lubricity of Molecularly Thin D₂O vs H₂O Films at Single-Layer Graphene-Mica Interfaces

Interfacial water is a widespread lubricant down to the nanometer scale. We investigate the lubricities of molecularly thin H₂O and D₂O films confined between mica and graphene, via the relaxation of initially applied strain in graphene employing Raman spectroscopy. Surprisingly, the D₂O films are at least 1 order of magnitude more lubricant than H₂O films, despite the similar bulk viscosities of the two liquids. We propose a mechanism based on the known selective permeation of protons vs deuterons through graphene. Permeated protons and left behind hydroxides may form ion pairs clamping across the graphene sheet and thereby hindering the graphene from sliding on the water layer. This explains the lower lubricity but also the hindering diffusivity of the water layer, which yields a high effective viscosity in accordance with findings in dewetting experiments. Our work elucidates an unexpected effect and provides clues to the behavior of graphene on hydrous surfaces.