Influence of Surface Tension-Driven Convection on Cyclic Voltammograms of Langmuir Films of Redox-Active Amphiphiles

Simulations were conducted to elucidate the effects of Marangoni convection on the cyclic voltammograms of Langmuir films of redox-active amphiphiles. Oxidation and reduction of the amphiphiles occur at a line electrode that contacts the Langmuir film, and oxidation is assumed to introduce an electrostatic contribution to the surface equation of state of the film. A boundary element method is used to compute the bulk fluid velocities that are induced by the corresponding surface tension gradients and a finite element method used to compute the surface transport of the surfactants. When compared with the case of purely diffusional transport processes, the simulation results reveal that when Marangoni convection is included, the cyclic voltammograms become asymmetric. The anodic peaks tend to be larger and sharper than the cathodic and the ratio of cathodic peak to anodic peak is about 0.808. The results of the simulation are compared to past reports of cyclic voltammograms of Langmuir monolayers of a ferrocenyl amphiphile, and good agreement is found.