An optical examination of thin films. II.—The behaviour of thin films of fatty acids on mercury

Abstract

The optical method of examining plane surfaces, described in I, p. 115, was applied to fatty acid films on mercury, in the hope that it would be possible to determine the optical properties of monolayers. The difficulties of making and keeping large clean surfaces of mercury are well known, but the optical method possesses this advantage over the surface-pressure and surface-potential methods, that it requires only a relatively small surface. Mercury possesses advantages over water as a subject for the optical method, in that it reflects most of the light that falls on it, and no light is reflected or scattered from the bottom of the liquid, as with transparent media. An analogous optical method has been most successfully applied to the study by Bouhet. He studied the Gibbs-layer on saturated solutions of the lower fatty acids, and found that it merged continuously, as he went up the series of acids, into the solid surface-films of the higher acids. Observations on films on mercury have been carried out by Schulman and Fahir, both working in currents of nitrogen. Schulman distilled mercury into a large dish, and found that films could readily be spread and that they caused large changes in the air-mercury interface potential. Fahir used a trough edged with sheets of cellophane, and cellophane barriers, and was able to measure surface-pressures, which he found to be about twice as great as the corresponding pressures on water. Neither of these workers, however, makes any claim to have had dry surfaces.