Soap film electrochemistry

In this paper we present preliminary electrochemical investigations into the transport properties of free standing ultra-thin surfactant films and the associated meniscus. We describe a new electrochemical cell composed of a 25 μm diameter gold wire placed through a stable surfactant film which served as the electrolyte. Solutions containing anionic sodium dodecyl sulphate (SDS) or non-ionic Triton-X100 surfactants, with background electrolyte NaCl and with electroactive probe ferrocyanide or ferrocene methanol, were used to create the surfactant films. The electrolyte was an ultra-thin surfactant film creating a two dimensional solution with a thickness between 300 and 1000 nm, and its meniscus at the gold wire, within which the electroactive probe was free to diffuse. Cyclic voltammetry was used to oxidise and reduce the electroactive probe within the surfactant film and meniscus. It was shown that films and the associated meniscus formed from SDS solution almost completely excluded negatively charged ferrocyanide. A finite difference simulation showed that the voltammetry was dominated by the meniscus region, the unusual spatially-varying bounded geometry of which resulted in an unusual dependence on potential scan rate of the peak to peak separation (decreasing with increasing scan rate) and anodic:cathodic peak current ratio (increasing with increasing scan rate).