Partial charge transfer and surface dipole potential of the Hg–S bond layer at thiol monolayers tethered to mercury

Abstract

The surface dipole potential χ_SAM due to the molecules of a monolayer self-assembled on a mercury drop can be estimated by tilting them via gradual expansion of the drop surface. Moreover, the potential difference due to distortional polarization of the self-assembled monolayer (SAM) can be estimated by measuring the charge experienced by the diffuse layer ions on the basis of the Gouy–Chapman theory. In the case of a physisorbed SAM, the sum Δϕ′ of these two potential differences differs from the extrathermodynamic potential difference Δϕ across the whole electrified interface by the surface dipole potential, χ^Hg_SAM, due to electron spillover; in the case of a chemisorbed thiol, this sum, herein denoted by Δϕ″, differs from Δϕ by χ^Hg_SAM plus the surface dipole potential χ^Hg–S_SAM due to partial charge transfer along the Hg–S bond. Comparing the two “partial” potential differences Δϕ′ and Δϕ″ at the same applied potential allows an estimate of χ^Hg–S_SAM. The magnitude of χ^Hg–S_SAM depends heavily upon the orientation of the dipole of the adsorbed molecules. If the χ_SAM value for a given thiol SAM estimated on Hg can be assumed not to vary in passing to Au or Ag, the above procedure can also be extended to the latter noble metals.