Packing restrictions and hydrophobic interactions are likely to lead to a spatial distribution of redox centers in electroactive monolayers. A mean field analysis of the electrochemical implications of spatial redox dispersion in SAMs, including the possibility of surface ion pair formation, has been carried out. The boundary value problem associated with a layered distribution of potential-induced charges has been solved by using the orthogonal collocation technique under equilibrium conditions. Spreading of the redox centers into a 3D dielectric slab results in broader and asymmetric voltammograms, reflecting a layer-by-layer redox conversion. It is also shown that the voltammetric shape is sensitive to the specific features of the spatial redox distribution, and theoretical requirements for the appearance of asymmetric broadening are examined in terms of the electrostatic properties of the monolayer. It is suggested that this type of spatial inhomogeneity may cause some of the broad and asymmetric voltammetric shapes that often characterize the electrochemical behavior of electroactive SAMs, and that some structural information can be gained from the analysis of these voltammograms, as long as electrolyte ions do not permeate the organic monolayer. The effect of surface ion association on the voltammetric features is also examined, and it is interpreted in terms of the distinct sensitivity of the potential at each redox plane with respect to the local counterion concentration. Comparison is made with the experimental results of Chidsey et al. (J. Am. Chem. Soc. 1990, 112, 4301) for the oxidation of FcCO 2 (CH 2 ) 11 SH/CH 3 (CH 2 ) 9 SH and Fc(CH 2 ) 16 SH/CH 3 (CH 2 ) 15 SH mixed monolayers.
The influence of ion pairing on the potential distribution at an electrode modified with a redox-active selfassembled monolayer is considered. The voltammetric response of the film is modeled as a function of its dielectric properties and the extent of association between the redox centers and counterions in solution. On the basis of the scheme of squares, a general treatment for any number of association steps is introduced. In addition, the effect of the counterion selective permeation on the voltammetric features is presented. It is shown that ion pairing and double-layer effects are strongly coupled, and may lead in some situations to the same variation of the peak potential with electrolyte concentration. The possibility of obtaining association parameters from voltammetric experiments is discussed. Comparison is made with Rowe and Creager's data on the oxidation of mixed monolayers of ferrocene-n-hexanethiol and n-alkanethiols.
The voltammetric response of an electrode covered with
an electroactive self-assembled monolayer is
modeled including discreteness of charge effects and interfacial ion
association. Discreteness of charge
potentials are estimated according to the hexagonal array model of
Macdonald and Barlow, and results
are compared with those obtained in previous work with the cutoff disk
model. As a consequence of the
slower variation of the discreteness of charge potential on the applied
electrode potential, voltammetric
waves are predicted to be asymmetrical and wider than those computed
from the cutoff disk model. For
relatively high redox coverage and/or small values of the integral
capacities of the inner and outer part
of the monolayer, a negative differential capacity is predicted.
This is a consequence of the additional
stabilization provided by the discreteness of charge effect, which
allows the charge density at the redox
plane to increase faster than the charge density on the electrode
surface when the monolayer is being
oxidized. Comparison with experimental results shows that
inclusion of the discreteness of charge effects
results in a variation of the absolute value of the interfacial
parameters, while their qualitative trends
are the same as those obtained on the basis of an average potential
model. Therefore, only the physical
significance of the fitting parameters may help to discriminate among
the different models.
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