Solid monolayers adsorbed at the solid-liquid interface studied by incoherent elastic neutron scattering

Incoherent elastic neutron scattering has been used to demonstrate the presence of an immobile monolayer adsorbed at the interface between liquid alkanes (methane, heptane, dodecane), an alkanol (dodecanol), and a carboxylic acid (dodecanoic acid) and a graphite substrate. The derived areas per molecule in the solid layers are consistent with the molecules lying with their long axis parallel to the surface. Ia every case studied the layers were found to melt approximately 10% higher than the melting points of the bulk materials. Diffraction data are used to confirm the two-dimensional nature of the solid monolayer in the case of methane. In every case there is also evidence for premelting of the bulk materials as the melting points are approached from below.