Measurement of Aqueous Film Thickness between Charged Mercury and Mica Surfaces: A Direct Experimental Probe of the Poisson−Boltzmann Distribution

A stable aqueous electrolyte film is formed between a mercury drop and a flat mica surface due to electrical double-layer repulsion when a negative potential is applied to the mercury. Film thickness has been measured as a function of applied potential while keeping the film pressure constant. By making measurements in this way, it is possible to map the data directly according to the Poisson−Boltzmann equation. An excellent fit to the data is obtained, providing direct evidence for this classical equation and its use as the basis of the Gouy−Chapman model of the diffuse double layer in electrolyte solutions.