  The interaction of interfacial water with graphitic carbon at the atomic
scale is studied as a function of the hydrophobicity of epitaxial graphene.
High resolution X-ray reflectivity shows that the graphene-water contact angle
is controlled by the average graphene thickness, due to the fraction of the
film surface expressed as the epitaxial buffer layer whose contact angle
(contact angle \theta_c = 73{\deg}) is substantially smaller than that of
multilayer graphene (\theta_c = 93{\deg}). Classical and ab initio molecular
dynamics simulations show that the reduced contact angle of the buffer layer is
due to both its epitaxy with the SiC substrate and the presence of interfacial
defects. This insight clarifies the relationship between interfacial water
structure and hydrophobicity, in general, and suggests new routes to control
interface properties of epitaxial graphene.
