Atmospheric water and graphite lubrication: Insights into surface intercalation and nanoscale confined spaces

The role of environmental water in graphite lubrication has generated conflicting observations and considerable confusion. Early hypotheses suggested water intercalation between graphene layers, but x-ray diffraction studies revealed no change in interlayer spacing with changing humidity. Conversely, recent Raman spectroscopy studies indicated water intercalation in bilayer graphene under humid conditions. To reconcile these discrepancies, we report density functional theory (DFT) and molecular dynamics (MD) simulations coupled with a reinterpretation of experimental measurements. While DFT calculations reveal softer surface interlayers suggesting potential for water intercalation, MD simulations indicate that such intercalation requires a significant reduction in van der Waals interactions, beyond what surface-induced softening can achieve. Alternatively, if water does intercalate, it likely occurs between the surface layers where graphene interactions are over 80% weaker than in bulk graphite, making such intercalation plausible. Instead, we demonstrate that water forms clusters on the graphene surface, maintaining a stable two-atomic layer structure across a wide temperature range, emphasizing the importance of surface water adsorption. We therefore propose that the lubrication effect is primarily related to the graphene-water interface rather than the graphene-graphene interface. While we do not completely rule out water intercalation, our findings suggest that graphene-water interactions dominate lubrication. These results resolve previous discrepancies by highlighting the role of surface or near-surface water layers over bulk intercalation, underscoring the significance of molecular interactions at the graphene-water interface in confined nanoscale spaces.