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

Abstract

The surface dipole potential χSAM 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, χHgSAM, due to electron spillover; in the case of a chemisorbed thiol, this sum, herein denoted by Δϕ″, differs from Δϕ by χHgSAM plus the surface dipole potential χHg–SSAM 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 χHg–SSAM. The magnitude of χHg–SSAM depends heavily upon the orientation of the dipole of the adsorbed molecules. If the χSAM 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.