Rubbed Films of Barium Stearate and Stearic Acid

Abstract

Films of barium stearate and of stearic acid have been prepared on polished chromium and on smooth natural faces of silicon carbide crystals. After these films have been rubbed with clean lens paper, electron diffraction patterns are obtained from them by the reflection method. Well rubbed films give patterns characteristic of a single layer of molecules standing with their axes approximately normal to the surface; the hydrocarbon chains of barium stearate are found to be more precisely oriented than those of stearic acid; exactly the same difference exists between unrubbed single layers of molecules of barium stearate and of stearic acid deposited by the Langmuir-Blodgett method. Thickness of rubbed films on chromium has been found, by the Blodgett optical method, to be the same as that of unrubbed single layers of molecules. Lightly rubbed films may be thicker than a single layer of molecules. The arrangement of barium stearate in such thicker films has been found to have been somewhat altered by the rubbing. The axes of the hydrocarbon chains still stand normal to the surface, but lateral arrangement is less regular than it is in unrubbed films of equal thickness. In the case of stearic acid, molecules left on top of the first layer after light rubbing in one direction are found to lie inclined by about 8\ifmmode^\circ\else\textdegree\fi{} to the surface and to point outward against the rubbing direction (Fig. 7); they are arranged in crystals having a structure different from that of the film before rubbing. Such "upset" films of stearic acid are completely removed by very light rubbing in the direction opposite to that of the original rubbing, but they are rather resistant to light rubbing in the same direction.