AC admittance of the metal/insulator/electrolyte interface

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Abstract
An approximate equivalent circuit is presented which describes the electrical characteristics of a metal electrode coated with a weakly conducting material and placed in an electrolyte solution. The model is compared with experimental results obtained over the frequency range 10-2 to 106 Hz. For the tests, copper electrodes were coated in PVC and immersed in KCl solutions. Experiments on this model system showed that over several decades of frequency the total circuit admittance was dominated by the properties of the coating material, which in turn were sensitive to the electrolyte molarity. For example, the results displayed a loss process attributable to the absorption of solute ions by the film. The AC admittance technique also appears to be sensitive enough to allow an investigation of very thin Langmuir-Blodgett (LB) films. Sputtered copper electrodes coated in three monolayers of barium stearate and immersed in KCl solution are shown to have a dielectric constant of about 2.7. This value compares favourably with previously quoted values for 'dry' films, suggesting that overall film quality is good. Such measurements can be used to determine the stability of LB films in aqueous media; an important consideration for film deposition and for some sensor applications.

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