Measurement of Contact Potentials at the Oil–Water Interface

Abstract

ALTHOUGH the contact potential of the air–water interface is easily measured¹, this is not so for the oil–water interface. Many measurements have been made on the latter system; but in all cases the change in contact potential, ΔV, caused by the spreading or adsorption of a monolayer, decreases with time^2–4, or is but “moderately reproducible”⁵. Indeed, ΔV often reaches zero after only a few minutes. These difficulties have been shown⁶ to be due to the diffusion of traces of ions from the aqueous phase into the oil, until at the equilibrium concentrations these ‘counter-ions’ balance out the potential due to the film.