A procedure for estimating the surface dipole potential of monolayers adsorbed on electrodes
The extrathermodynamic potential difference Δϕ across an electrified interface enclosed between a bulk metal M and a bulk aqueous phase does not depend on the content of the interface, at constant applied potential E. By equating at constant E the expression of Δϕ for an electrode coated by a self-assembled monolayer (SAM) to that for the corresponding bare electrode immersed in the aqueous solution of a nonspecifically adsorbed electrolyte, it is possible to estimate the surface dipole potential χSAM of the SAM. If the molecules of the SAM form a bond M–X with the metal M, this procedure requires an independent knowledge of the surface dipole potential χM–XSAM due to such a bond. The other way round, if the χSAM value is known by independent means, the procedure allows an estimate of χM–XSAM. The self-consistency of this procedure was tested with SAMs of ten different thiolated peptides covalently bound to a mercury electrode, where χSAM can be determined independently by expanding a mercury drop. The procedure was then applied to the estimate of the χSAM value of a peptide SAM on a polycrystalline gold electrode.