Understanding the Influence of the Electrochemical Behavior of Potassium Hexacyanoferrate(II/III) on Acetonitrile/Water Binary Mixtures: An Experimental and Molecular Dynamics Study

Abstract

Microelectrode voltammetry was employed to gain information on the behavior of potassium hexacyanoferrate(II/III) in acetonitrile/water binary mixtures in order to understand the bulk properties of the mixture and the electrode/binary solution interface properties. The first analysis of the obtained data demonstrated that the diffusion coefficient values of hexacyanoferrate(III) apparently do not respect the Stokes-Einstein equation, considering the hydrodynamic radius does not change with acetonitrile content. Another interesting observation is the adsorption of electrogenerated potassium hexacyanoferrate(II) onto the electrode surface in binary mixtures with 50 mol % acetonitrile or higher. Molecular dynamics simulations showed that acetonitrile molecules accumulate on the surface of an electrode model and also that ionic species tend to form clusters in acetonitrile-rich mixtures. Since potassium hexacyanoferrate(II) is not soluble in acetonitrile, the adsorption of the iron complex onto the electrode is related to the formation of the acetonitrile layer on the surface of the electrode. The diffusion coefficient of hexacyanoferrate(III) in the mixtures decreases because of the ion clustering and the formation of an acetonitrile layer on the surface of the electrode, which is an obstacle that the ions have to overcome to reach and leave the electrode.