Structure of thin films of aliphatic esters and alcohols on metals

This paper describes an electron-diffraction study of the structure and orientation of thin films of aliphatic esters and alcohols. The effect of temperature has also been examined by transmission and reflexion methods. Transmission experiments on small crystals of the alcohols show that the orthorhombic structure observed at room temperature gives place to a hexagonal structure at temperatures below the bulk melting-point. These results are analogous to those observed by Muller (1932) with hydrocarbons using X-ray diffraction. When melting occurs there is a marked increase in the lattice parameter, but the packing appears to remain essentially hexagonal. The esters behave similarly. Reflexion experiments on thin films of alcohols and esters deposited on metal surfaces show that the first molecular layer is generally oriented with the hydrocarbon chains normal to the surface. With alcohols the monolayer loses its orientation at a temperature close to the bulk melting-point of the alcohol, and the disorientation temperature does not depend appreciably on the nature of the metallic substrate. With esters it is difficult to obtain diffraction patterns owing to evaporation, but the evidence again points to a disorientation of the ester at its bulk melting-point. On zinc and cadmium surfaces, however, good orientation may be obtained up to elevated temperatures, and the behaviour is similar to that observed with metallic soaps. The bearing of these results on the nature of the disorientation process is discussed and a comparison made with the boundary lubricating properties of these substances.