Characterization of n-alkanethiol self-assembled monolayers on mercury by impedance spectroscopy and potentiometric measurements

Long chain n-alkanethiol self-assembled monolayers (SAMs) on a hanging mercury drop electrode were studied in detail for the first time by impedance spectroscopy (IS) and potentiometry. IS reveals the detailed behavior of the monolayers in the absence and presence of two one-electron redox couple: hexaamineruthenium(III), Ru (NH₃)₆^{3 +} and tris(4,4′-bipyridine)cobalt(III), Co (bpy)₃^{3 +}. The pinholes-free layers behave as almost ideal capacitors and are permeable to some extent to redox species, depending on their hydrophobicity. Interestingly, Co (bpy)₃^{3 +} that exhibits sluggish kinetics on a bare Hg electrode reveals a more facile kinetics than Ru (NH₃)₆^{3 +} on an n-octadecanethiol/Hg interface. Potentiometric measurements recorded upon immersing the bare and SAM modified Hg electrode into a solution containing a mixture of the oxidized and reduced forms of the redox couples provide invaluable information on the charge transfer across the monolayer and its ohmic resistance. While Ru (NH₃)₆^{3 +} cannot freely penetrate into the monolayer and therefore establishes a potential difference across the monolayer; penetration of Co (bpy)₃^{2 +} into the film causes the Fermi level of the Hg surface to attain the Nernst potential of the solution. Finally, we find that increasing the length of the alkane chain of the thiols increases linearly the ohmic resistance of the layer.