Built-Up Films of Barium Stearate and Their Optical Properties

Abstract

The technique of depositing successive single layers of molecules of various stearates on a solid surface is described. Films containing 3001 layers have been built of barium-copper stearate. A photograph shows films built in a series of steps having intervals of 2 molecular layers. The contrast of the steps is plainly visible when the slide is illuminated by polarized light at angles near grazing incidence. By measuring the angles at which films containing known numbers of layers reflect minimum intensity of monochromatic light for the first five interference fringes, the thickness per layer and refractive index can be calculated with great accuracy. The thickness per layer of barium stearate was found to be 24.40A. The presence of traces of foreign substances in the water affect the spacing by 1 to 3 percent. The films are uniaxial crystals, the optic axis being perpendicular to the surface on which the films are built. The refractive index of the ordinary ray, ${n}_{1}$, and of the extraordinary ray in a direction perpendicular to the axis, ${n}_{3}$, are ${n}_{1}=1.491$, ${n}_{3}=1.551$. Equations are given which describe the refraction of the extraordinary ray, the intensity of the rays reflected from the upper surface and from the film solid boundary, the phase change at the boundaries, Brewster's angle, and other special properties of birefringent films. The intensity of the light and dark fringes reflected by films built on a series of glasses of known refractive indices is used as a measure of ${n}_{1}$. Skeleton films. Barium stearate films built at $\mathrm{pH}<7.0$ are composed of a mixture of stearic acid and neutral stearate. The stearic acid can be dissolved by benzene leaving a skeleton of stearate which is birefringent and has refractive indices much lower than those of the normal film. Measurements are given of a skeleton for which ${n}_{1}=1.30$, others for which ${n}_{1}=1.32$, ${n}_{3}=1.39$. Skeletons have been built having ${n}_{1}=1.25$ and 1.22. The skeleton for which ${n}_{1}=1.30$ had 99.2 percent of the thickness of the original film, although only 63.7 percent of its density.