Origin of hydration lubrication of zwitterions on graphene
Formation of a hydration layer on charge sites can support normal pressure, and meanwhile it retains excellent fluidity to provide efficient boundary lubrication; however, it is limited to the sliding system between two similarly charged surfaces. In the present study, we report extremely low friction as the zwitterions in a lipid bilayer slide on the topmost graphene layer of graphite across pure water, with the friction coefficient falling to the level of 0.001, which provides direct evidence that hydration lubrication is effective even between such dissimilar surfaces. The origin of hydration lubrication on graphene was studied by atomic force microscopy and molecular dynamics simulation simultaneously. It reveals that a subnanometer hydration layer is confined between zwitterions and graphene, which remains as a liquid phase under normal pressure. The shear occurs between water molecules and graphene because of the extremely low shear strength of the water/graphene interface, which contributes to extremely low friction. Our finding demonstrates that the formation of a hydration layer is possible to lubricate layered materials efficiently, which has potential implications for designing efficient boundary lubrication with layered materials.