Melting mechanism in monolayers of flexible rod-shaped molecules

The melting of butane and hexane monolayers adsorbed on a graphite basal-plane surface has been studied by molecular-dynamics simulations and experimentally by neutron diffraction. The simulation results are qualitatively consistent with the observed diffraction patterns and suggest a general mechanism for melting in monolayers of flexible rod-shaped molecules. Melting requires the formation of vacancies in the monolayer by molecular motion perpendicular to the surface. This ``footprint reduction'' mechanism implies that strictly two-dimensional theories of melting are inapplicable to these systems.