Arrangement of Molecules in a Single Layer and in Multiple Layers

Abstract

Electron diffraction studies have been made upon layers of molecules of barium stearate and of stearic acid built upon surfaces by the Langmuir-Blodgett method. Upon clean metal surfaces odd numbers of layers have been built, and investigated by the reflection method; upon thin organic supporting foils even numbers of layers have been built, and investigated by the transmission method. On foils of Resoglaz the molecules are regularly arranged. The hydrocarbon chains of barium stearate molecules form hexagonal arrays with their axes normal to the supporting surface and separated by distances of 4.85A. Stearic acid molecules form crystals of monoclinic form with a=8.27A, b=4.96A, β=70°, the a and b axes lying in the plane of the supporting surface. In built-up films we previously observed a different polymorphic form of stearic acid; a correction to the data on this earlier structure is given here. On a clean metal surface molecules of the first layer stand with the axes of the hydrocarbon chains normal to the surface and the chains closely packed together laterally but irregularly arranged. The precision of alignment of the axes normal to the surface is greater for barium stearate than for stearic acid. In layers on top of the first, molecules of barium stearate and of stearic acid are regularly arranged in the way the same molecules are arranged on organic foils. In the case of barium stearate, the arrangement is random about the surface normal, although large single arrays are often formed; in the case of stearic acid, crystals are built with the axes of the hydrocarbon chains approximately in the plane defined by the surface normal and the dipping direction and pointing upward from the surface. Two applications or continuations of these experiments are suggested. 1. The methods of the experiments can probably be applied to fundamental studies of boundary lubrication. 2. Interpretable diffraction patterns can be obtained from single layers of molecules which are considerably shorter than stearic acid, and comparable in scattering power with films of adsorbed gas; thus studies of films of adsorbed gas can be carried out as soon as adequate vacuum technique is available.