Contact Potentials of Reversible Soluble Films of Lauric Acid

Abstract

WHEN a nearly saturated solution of lauric acid in distilled water is placed in an Adam-Jessop film-pressure trough, the soluble adsorbed film may be compressed or swept in the same manner as an in soluble film of a higher fatty acid. However, the surface is rapidly replenished by diffusion from below, after sweeping, or the compressed film diffuses down-ward. In either case the reading of the film-balance returns to zero, in a period of about thirty minutes at 25° C. Similar effects have been noticed with benzopurpurin solutions¹ and with soap solutions². Since the contact potential of solutions is proportional to the concentration of the surface-active solute, simultaneous contact potential measurements with a polonium-covered silver electrode on the above surfaces might be expected to yield potential-time curves nearly parallel to the force-time curves. Such proved to be the case, with the following notable differences.