Polarographic maxima and dipole repulsion forces

Abstract

Strong dipole repulsion forces at the mercury interface can give rise to polarographic maxima of the first kind even in the absence of net free surface charge. The dipole energy and forces yield an expression for the surface velocity of the mercury droplet. It is shown that movement of the mercury surface should cease when the dipole concentration is uniformly depleted by electrolytic action even though the potential increases. Maxima of the first kind may be interpreted in terms of dipole repulsion forces. The force is given by the product of the dipole moment and the tangential derivative of the electric intensity field. The force appears to be substantial because of the size of the tangential derivatives of the field. The field arises from the dipoles and quadrupoles created by “bond” formation between the electrode site and adsorbed species. Non-uniform surface concentration of the adsorbed species is responsible for the non-uniform fields and large tangential field derivatives. Non-uniform surface concentrations occur whenever the current distribution is asymmetric as in polarographic systems. Here the dipole repulsion forces bring about the migration of matter at the interface. An expression for the velocity of the migration has been derived. The velocity may be determined from reasonable assumptions about the hydrodynamic boundary layer thickness. The expression for the velocity indicates an association of polarographic maxima with electrocapillary maxima and it predicts the cessation of polarographic maxima with increasing potential because of dipole depletion.