The Effect of Surface Polarity on the Electrochemical Double Layer and Its Influence on the Measurement of the Standard Rate Constant of Electron Transfer

The influence of surface polarity and, hence, the electrical double layer on long-range electron transfer was investigated using 35 different electrode constructs. These constructs were prepared using five different lengths of ferrocene-modified alkanethiols (of general formula HS(CH 2 ) n CONHCH 2 Fc where n was 7, 10, 11, 14, and 17 and Fc refers to ferrocene) mixed with an appropriate ratio of hydroxyl-terminated to methyl-terminated alkanethiols as diluents. The mixtures of diluents in different ratios served not only to separate and dilute the redox-active species but also to control the surface polarity of the self-assembled monolayer (SAM). The ratios of the three components in each SAM were 1:20:0, 1:16.6:3.4, 1:13.4:6.6, 1:10:10, 1:6.6:13.4, 1:16.6:3.4, and 1:0:20 of the ferrocene-, hydroxyl-, and methyl-terminated species, respectively. The formal redox potential and electron transfer rate constant were measured for each construct. It was found, first, that formal potentials changed according to the theory of interfacial potential distribution and, second, that rate constants measured using cyclic voltammetry are strongly influenced by the Stern layer of the electrical double layer, which forms at the SAM−solution interface.