Adsorption of propionitrile at the mercury/aqueous solution interface

Abstract

The adsorption of propionitrile on polarized Hg electrodes has been studied from 0.25 mol dm^–3 NaF aqueous solutions by means of both electrocapillary and capacity curves. The results show that the adsorption isotherms are apparently congruent with respect to both charge and potential on the negative side of the maximum in adsorption. In this region, a Frumkin isotherm is obeyed with saturation coverage Γ_s= 5.9 × 10^–10 mol cm^–2, corresponding to a projected area A= 28 Ų per molecule, and an interaction parameter corresponding to intermolecular attraction. On the positive side of the maximum in adsorption, the isotherms are congruent with respect to neither of the two electrical variables. The adsorption may be described by a Frumkin isotherm either with constant Γ_s and interaction parameter a changing quadratically with charge, or with constant a and Γ_s linearly decreasing with charge on the metal, although at the more positive charges the latter description appears more adequate for describing the experimental results.

Results are discussed in terms of polarization of the CN group giving rise to a net positive charge on the propionitrile molecule. This generates repulsion between particles depending on charge and therefore non-congruence. Some implications about the physical meaning of the parameters in the isotherms are discussed. A model for the adsorbed layer is proposed.