Using a simple and efficient way to optimize a chemically specific bead-and-spring model for polymer/surface systems, we analyze the structural properties of high molecular weight polyisoprene (PI) in contact with graphite. We find that in the vicinity of the graphite the adsorbed PI chains assume a pancake structure and are highly packed and highly entangled. The addition of plasticizers even with moderate surface affinity guarantees an almost complete surface coverage and forces the polymer chains to detach from the surface and to become less entangled. The softening effect of the plasticizers is observed also in bulk when they are added to the system but are not adsorbed on the surface. Finally, we show that the definition of the thickness of the interface is not unambiguous but depends on the observable used to characterized the melt: it is function of the polymer molecular weight if defined looking at the chain conformation, but it becomes independent of the polymer chain length if defined looking at the entanglement density.