Formation and reductive desorption of mercaptohexanol monolayers on mercury

The formation and reductive desorption of self-assembled monolayers of 6-me rcaptohexanol on mercury has been studied by using cathodic stripping volta mmetry and capacitative transients, including the possibility of expanding or contracting the electrode area at the end of the preconcentration step. Experimental evidence shows the existence of three sequential stages during the formation of a thiol self-assembled monolayer. Each of these stages ca n be associated to the presence of (i) a low surface density state of oxidi zed thiol molecules, characterized by a single electrodimerization wave, (i i) a high surface density state, characterized by the emergence of a second voltammetric wave, and (iii) an ordered monolayer, which gives rise to a v oltammetric spike. On the basis of electrode expansion experiments, a metho d is described to determine the surface concentrations of oxidized products , which does not require a baseline subtraction of the voltammograms to acc ount for the nonfaradaic current. Quantitative voltammetric fits are consis tent with the initial formation of a mixture of noninteracting monomers and dimers of oxidized thiol. The value of the maximum surface concentration a nd the ability to block the RU(NH3)(6)(3+) electron transfer reveal that ox idized thiol molecules adopt a nearly perpendicular orientation in the high surface density state, which hampers ionic permeation. A theoretical model is proposed to account for the observed voltammetric behavior. The transit ion from the lower to the higher surface density states is modeled as a che mical step involving the exchange of metal free sites. Capacitative transie nts are also interpreted in terms of the three-stages model.