Orientation of fatty acid and soap films on metal surfaces

Abstract

An electron-diffraction study has been made of the structure and orientation of fatty acid films and soap films adsorbed on metal surfaces. In particular, an examination has been made of the effect of temperature on the orientation, using the reflexion method and an improved experimental technique. The results show, as previous workers have found, that at room temperature the first layer is generally oriented with the hydrocarbon chains normal to the surface, though other orientations may be obtained by ‘rubbing’ the film. As the temperature is raised the 'layer lines’ become increasingly less distinct, and at a characteristic temperature only the pattern of the substrate is observed. This transition is a true disorientation effect, since on cooling orientation is again observed. At higher temperatures the film may evaporate completely from the surface. With fatty acids on non-reactive metals such as platinum the disorientation temperature is close to the bulk melting-point of the fatty acid. On reactive metals such as zinc and cadmium the disorientation occurs at higher temperature which is close to the bulk melting point of the appropriate metallic soap suggesting the formation of the soap by chemical reaction. This is supported by the similar behaviour of thin films of metallic soap deposited directly on the metal substrate. These observations are compared with the lubricating properties of fatty acids and soaps on metal surfaces.