CHARACTERIZATION OF EXTREMELY LOW DEFECT DENSITY HEXADECANETHIOL MONOLAYERS ON HG SURFACES

Hexadecanethiol (HDT) forms self-assembled monolayers on static Hg drop electrode (SMDE) surfaces. Films are formed in less than 1 min and are extremely impermeable, indicating the absence of defects. The onset of Hg oxidation at about 0.4 V vs SCE on a bare SMDE in 0.1 M NaF is shifted anodically by 1 V on an HDT-coated SMDE. The current for Ru(NH3)63+, methyviologen, benzoquinone, and Fe(CN)63-reduction at an HDT-coated SMDE is <0.005% of that at a bare electrode, and is principally due to capacitive effects. The reduction in differential capacitance upon HDT coating is consistent with the formation of a monolayer film 20 angstrom thick. An upper limit estimate of the apparent electron transfer rate constant for Ru(NH3)63+ at a coated electrode is 3 x 10(-13) cm/s, giving an estimate of the tunneling parameter of 1.5 angstrom-1 and a barrier to tunneling of 2.1 eV. The Pb2+ aquo ion is able to permeate the HDT film, giving currents only 25-fold lower than those at a bare SMDE on the first cycle.