Radial diffusion in electrochemistry of two-dimensional systems at the air-water interface

Abstract

The formation of a non-linear diffusion zone of electroactive material in solution adjacent to a microelectrode is now a well-known phenomenon [ll. It is observed whenever at least one dimension of an electrode becomes small compared with the thickness of the diffusion layer. For example, a hemispherical diffusion zone develops in solution around a microdisk electrode and a hemicylindrical diffusion zone is known to form at microband electrodes [l]. (This statement reflects convenient approximations of the behavior of microdisk and microband electrodes by the current to hemisphere and hemicylinder respectively.1 These simple patterns also prevail when one examines a redox system in two-dimensional (2-D), space. However, in two dimensions, only one dimension of a working electrode is relevant and thus radial diffusion begins to dominate mass transport when the length of a line electrode becomes small compared with the diffusion layer thickness. We have recently described an electrochemical experiment in which redox species are confined to the air-water interface [2]. N-octadecyNferrocenecarboxamide) (C,,Fc), a water-insoluble and non-volatile surfactant, is spread at the water surface in a Langmuir trough. Its surface concentration can be controlled over a wide range by adjusting the position of a movable barrier which determines the area of water surface available to the monolayer assembly. The electrochemical experiments addressing the C,,Fc monolayer on the water surface are carried out with a one dimensional (1-D) microelectrode positioned in the plane of the air-water interface. The counter-electrode and reference electrode are immersed in the subphase behind the barrier as shown in Fig. 1. We showed that the 2-D nature of the surface monolayer and the effectively 1-D character of the microelectrode lead to the development of a linear diffusion layer in two dimensions against the line working electrode 121. The shape of the voltammetric curves is identical