Journal of Physical Chemistry B, Vol.105, No.39, 9557-9568, 2001
Influence of spatial redox distribution on the electrochemical behavior of electroactive self-assembled monolayers
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 pf 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)(CH2)(11)SH/CH3(CH2)(9)SH and Fc(CH2)(16)SH/CH3(CH2)(15)SH mixed monolayers.