CONSEQUENCES OF KINETIC DISPERSION ON THE ELECTROCHEMISTRY OF AN ADSORBED REDOX-ACTIVE MONOLAYER

Mixed monolayers of (ferrocenylcarboxy)alkanethiol + n-alkanethiol hav e been investigated electrochemically in 2:1 (v:v) chloroethane:butyro nitrile solvent in the temperature range of 120-150 K. Cyclic voltamme try (CV) of these monolayers shows large oxidation-reduction peak pote ntial separations indicative of electron transfer rate control. The vo ltammetric wave shapes are also broadened; this and curved log i vs ti me transients observed in potential step experiments are interpreted a s a dispersion in the reaction rates of the ferrocene sites. This pape r considers origins and three models for such kinetic dispersion: (i) Using simulations, the observed kinetic dispersion effects can be succ essfully represented by a Gaussian distribution among the formal poten tials (E(0')) of the surface redox sites. While only an apparent kinet ic dispersion (having a thermodynamic origin), we show by simulations that its presence affects potential step log k(APP,eta) vs overpotenti al (eta) plots, depressing the apparent reorganizational barrier energ ies (lambda) and elevating the apparent rate constants (k(0)), consist ent with previous experimental obsenrations. Similarly, cyclic voltamm etric simulations with a Gaussian distribution of E(0') give excellent fits to experimental voltammograms with midpoint average rates (that with voltammograms can be simulated to fit both the experimental wave shape and Delta E(PEAK) that are roughly 6-fold smaller than the avera ge rate (determined from a fit to the experimental Delta E(PEAK) assum ing a homogeneous population). The temperature and chain length depend ence of CV simulations are also consistent with experimental observati ons and indicate that the dispersion has little effect on the accurate determination of lambda from an activation analysis) or the electroni c coupling coefficient (beta) (from a plot of log k(0) 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 Gau ssian distribution of tunneling distances (or equivalently the electro nic coupling parameter) from the electrode surface is also evaluated. This model predicts curved potential step log i us time plots and, in analysis of log k(APP,eta) vs eta plots, undistorted results for lambd a but alteration of the apparent k(0).