New approach for measuring lateral diffusion in Langmuir monolayers by scanning electrochemical microscopy (SECM): Theory and application

A new SECM approach for studying the lateral diffusion of redox-active amphiphiles in Langmuir monolayers at an air/water (A/W) interface is described. To apply this technique practically, a triple potential step transient measurement is utilized at a submarine ultramicroelectrode (UME) placed in the water phase close (1-2 mum) to the monolayer. In the first potential step, an electroactive species is generated at the UME by diffusion-controlled electrolysis of a precursor. This species diffuses to, and reacts with, the redox-active amphiphile at the A/W interface resulting in the formation of the initial solution precursor, which undergoes diffusional feedback to the UME. In this first step, the rate constant for electron transfer between the solution mediator and the surface-confined species can be measured from the UME current-time transient. In the second period, the potential step is reversed to convert the electrogenerated species to its initial form. Lateral diffusion of electroactive amphiphile into the interfacial zone probed by the UME occurs simultaneously in this recovery period. In the third step, the potential is jumped in the same direction as for the first step and the corresponding UME current-time transient can be used to determine either the distance between the UME tip and the monolayer at the water surface, or the lateral diffusion coefficient of the amphiphile. A theoretical treatment for this technique is developed and discussed in detail. The method is demonstrated experimentally with measurements of the lateral diffusion of N-octadecylferrocenecarboxamide in a 1:1 Langmuir monolayer with 1-octadecanol.