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Mixed monolayers of (ferrocenylcarboxy)-alkanethiol/n-alkanethiol have been investigated electrochemically in 2:1 (v:v) chloroethane:butyronitrile solvent in the temperature range of 120K to 150K. Cyclic voltammetry of these monolayers shows large oxidation-reduction peak potential separations indicative of electron transfer rate control.The voltammetric waveshapes are also broadened; this and curved log[i] vs. time transients observed in potential step experiments are interpreted as a dispersion in the reaction rates of the ferrocene sites. This paper considers origins and three models for such kinetic dispersion: (i) Using simulations, the observed kinetic dispersion effects can be successfully represented by a Gaussian distribution among the formal potentials E 0 ' of the surface redox sites. While only an apparent kinetic dispersion (having a thermodynamic origin), we show by simulations that its presence affects potential step log[k APP ,] vs. tj plots, depressing the apparent reorganizational barrier energies (X) and elevating the apparent rate constants (k°), consistent with previous experimental observations. Similarly, cyclic voltammetric simulations with a Gaussian E 0 ' distribution give excellent fits to experimental 2 voltammograms with mid-point average rates (that with voltammograms can be simulated to fit both the experimental waveshape and AE PEAK ) that are roughly 6-fold smaller than the average rate (determined from a fit to the experimental AE PEAK assuming a homogeneous population). The temperature and chain length dependence of simulations are also consistent with experimental observations and indicate that the dispersion has little effect on accurate determination of X (from an activation analysis) or ß (from a plot of log(k°) vs. chain length), (ii) A Gaussian distribution of reorganizational energies, which is a real kinetic dispersion, has consequences on the appearance and the analysis of data quantitatively equivalent to those of a distribution of formal potentials, (iii) A kinetic dispersion model based on a Gaussian distribution of tunneling distances (or equivalently electronic coupling parameter) from the electrode surface is also evaluated. This model predicts curved potential step log[i] vs. time plots, and in analysis of log[k A p Pl) ] vs. rj plots, undistorted results for X but alteration of the apparent k°.

Formal potentials for ferrocene in self-assembled monolayers of N-(7-mercaptoheptyl)ferrocenecarboxamide coadsorbed with n-alkanethiol derivatives of variable chain length and terminal functionality are substantially more positive than the corresponding potentials for the same ferrocene compound in bulk solution. The differences in formal potential are a strong function of the chain length and terminal functionality of the alkanethiol coadsorbate, the nature and concentration of supporting electrolyte, the coverage of ferrocene on the electrode, and the solvent. Two physical models for the electrode/monolayer/solution interface are invoked to explain these differences. One model is based on ion solvation energetics in the interfacial microenvironment relative to that in bulk solution and describes essentially a solvent effect on the formal potential for the immobilized redox-active moieties. The other is based on the spatial distribution of ions in the interfacial region and describes essentially a double-layer effect on the apparent formal potential for the immobilized redox-active moieties. Quantitative predictions are developed from these models that specifically address the effects of electrolyte type and concentration, solvent, ferrocene surface coverage, and coadsorbate chain length. It is concluded that both interfacial solvation and ion spatial distribution effects must be considered to adequately explain the data.

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