JAVA applet version ('97, May 6)



Voltammetric Response Accompanied by Inclusion of Ion Pairs and Triple Ion Formation of Electrodes Coated with an Electroactive Monolayer Film

Anal. Chem., 1997, 69, 1045-1053.
Electrochemistry of a self-assembled monolayer (SAM) having redox active groups has been getting great interest in recent years. Theory on cyclic voltammograms of such surface-confined redox species has been developed with use of the Langmuir adsorption isotherm and the Nernst equation. However, the experimentally obtained voltammetric waves are often much broader than those expected by the theoretical predictions, or in some cases spiky redox waves appear. Furthermore, it has been found that the apparent formal potential is apt to shift with increasing the coverage of the surface-confined redox species. Such non-ideal voltammetric behaviors were first theoretically treated by Laviron who introduced empirical adjustable parameters to make good matching of experimentally obtained voltammograms to the theoretically obtained ones. Recently White and Smith analyzed voltammetric behaviors of metal electrodes coated with redox active SAM and showed that the broadening of voltammetric waves and shifts of their apparent formal potentials can be explained without any empirical adjustable parameters if the potential drop within the SAM is taken into account.

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This paper describes further refinements of the theoretical treatments made by Smith and White in terms of formation of ion-pairs and triple ions between the redox active SAM and electrolyte ions. The finding that voltammetric peak potential of the electrode coated with SAM of alkylthiol having ferrocenyl group was shifted with changes in the concentration of electrolyte solution suggests strongly that the electrolyte ions are involved in the redox reactions of the redox active SAM. It was revealed by in-situ measurements using the EQCM (electrochemical quartz crystal microbalance) technique of the electrodes coated with SAM of alkylthiol having ferrocenyl or viologen group that the association and the dissociation of electrolyte ions with surface-confined redox species occur during the course of the redox processes.

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It is easy to imagine that the potential distribution is affected by ion-pair and triple ion formation, resulting in the distortion of voltammetric wave form. You can ascertain such effect as a function of some physical and chemical parameter by the comparison with the ideal Nernstian-type wave.



O+ + e- = R0 (Reversible electrode reaction)

O+ + X- = AX0 (Ion-pair formation, K1)

OX + X- = OX2- (Triple ion formation, K2)



Figure 1. Model of solution/electrode interface.



MODEL
Figure 1 shows an interfacial structure model of a plane electrode coated with a redox active SAM having thickness d1 and dielectric constant e1. The electrochemically active species are located at the end of the SAM (x=d1). The distance between the inner and outer Helmholtz planes is d2 in which a medium of the dielectric constant e2 is filled. The dielectric constant of the solution phase is given by e3. The charge density on the metal electrode surface at x=0 is given by sM, and that on the inner Helmholtz plane at which the redox head groups are presented is given by sPET. The inner Helmholtz plane is denoted here as PET. It is assumed that both sM and sPET are delocalized in each plane, i.e., the discreteness-of-ion effect is ignored as assumed by Smith and White. This means if the ion-pairs and triple ions are formed, the charges of electrolyte ions which associated with surface-confined redox species are also delocalized on the PET at x=d1 and are included in sPET. Consequently, it is assumed that there is no net charges in the region of 0< x < d1 and d1 < x < d1 + d2 . The volume charge density in the diffuse layer (x > d1 + d2) is given by r(x).
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Only you have to do is putting parameter values and pushing the "Calc." button!

Parameters:
  • The parameter "e1" is a relative dielectric constant at 0 < x < d1.
  • The parameter "e2" is a relative dielectric constant at d1 < x < d1+d2.
  • The parameter "e3" is a relative dielectric constant at d1+d2< x, i.e., solution bulk.
  • The parameter "Eo" is formal potential of a redox couple O+/R0.
  • The "Sur.Cov." means total surface concentration of surface confined species.
  • The parameter "C" is a concentration of 1:1-type electrolyte solution.
  • The parameter "K1" and "K2" are formation constants of ion pair OX0 and triple ion OX2-, respectively.

    Blue wave is the ideal Nernstian wave which has FWHM of 90.6 mV. Red wave is the wave which corrected for the potential distribution. Zo means net ionic charges of surface-confined species in oxidized state.Temperature is fixed at 25 oC.

    Caution:
     It was assumed that the formation of ion pair and triple ion is independent on the potential. Consequently, if the irrational large value of K2 was used, the voltammetric wave would be inversed. This will be improved.

    Details are described in Anal. Chem., 1997, 69, 1045-1053, and you could mail me at momo@kanazawa-bidai.ac.jp