Ion-free layer

The Gouy-Chapman theory for metal-solution interfaces predicts interfacial capacities which are too high for more concentrated electrolyte solutions. It has therefore been amended by introducing an ion-free layer, the so-called Helmholtz layer, in contract with the metal surface. Although the resulting model has been somewhat discredited [30], it has been transferred to liquid-liquid interfaces [31] by postulating a double layer of solvent molecules into which the ions cannot penetrate (see Fig. 17) this is known as the modified Verwey-Niessen model. Since the interfacial capacity of liquid-liquid interfaces is... 

The first controversial point in this mechanism is the nature of the reaction planes where the precursor formation and the ET reaction take place. Samec assumed that the ET step occurs across an ion-free layer composed of oriented solvent molecules [1]. By contrast, Girault and Schiffrin considered a mixed solvent region where electrochemical potentials are dependent on the position of the reactants at the interface [60]. From a general perspective, the phenomenological ET rate constant can be expressed in terms of... 

FIG. 3 Different models of interfacial ET. (A) Aqueous and organic redox species are separated by the sharp interfacial boundary. (B) Interfacial potential drop across a thin ion-free layer between redox reactants. (C) ET reaction occurs within a nm-thick mixed solvent layer. No potential drops between reactant molecules. 

Surprisingly this important experimental information on the properties of the compact layer has been ignored in modern microscopic theories of the interfacial structure, while the thickness of the ion-free layer at the electrode surface could be calculated on the basis of the distribution functions for the solvent and ions. 

Gouy-Chapman (GC), that is, to consider the whole interphase as consisting of two layers, a compact (or inner or Helmholtz ) and a diffuse one. The former corresponds to the aforementioned hypothesis of an ion-free layer of the solvent at the metal surface. The diffuse layer is located between the compact one and the bulk solution and the whole counterion charge is distributed inside this region in accordance with the GC theory. 

Let us consider first a surface-inactive solution, whose properties have already been outlined in Sects. 2.1.3 to 2.1.6. By definition, this term means that the compact layer is solely composed of solvent molecules. The distribution of components within the diffuse layer is determined mostly by the electrostatic forces. If the image forces (see Sect. 2.1.11.3) can be disregarded the concentrations of solute species, ionic or neutral, at the p.z.c. are identical to their bulk values, and the integral (66) over the diffuse layer vanishes. Inside the compact layer the concentration, Ci(z), is zero, and the integration in (66) yields Eq. (12) for the Gibbs adsorption of surface-inactive components, F = —zhc, which allows one to measure the thickness of the ion-free layer of the solvent, zh-... 

These deviations were first explained by the presence of a compact, ion-free layer at the interface this is known as the modified Verwey-Niessen model. Obviously, the presence of an ion-free layer can only reduce the capacity, so the theory had to be modified further. For a few systems a consistent interpretation of the experimental capacity was achieved [78-80] by combining this model with the soolled modified Poisson-Boltzmann (MPB) theory [81], which attempts to correct the GC theory by accounting for the finite size of the ions and for image effects, while the solvent is still treated as a dielectric continuum. The combined model has an adjustable parameter, so it is difficult to judge whether the agreement with experimental data is significant. The existence... 

The first term of the r.h.s. of this equation represents the contribution of the ion-free layer, as can be shown simply by use of the Gibbs equation for the solvent... 

They analyzed their results using a thermodynamic approach based on the Gibbs adsorption equation and the main conclusion of their work was that relative surface excesses of the ionic species were well described by the Gouy-Chapman theory. They adopted the MVN model of the ideally polarized interface stating that the compact layer is an ion-free layer consisting of laminated layers of water and nitrobenzene sandwiched between two diffuse layers. The potential difference across this inner layer was estimated to be about 20 mV at the PZC but was found to vary with the surface charge density. 

The participants of the ET reaction are separated by a thin interfacial boundary which they do not significantly penetrate (Figure 8.3A). Otherwise, the potential drop between two redox molecules would be much smaller than the total A°q>, and a would be much smaller than 0.5. Similarly, the experimental results do not support a picture of ET occurring within a fairly thick mixed solvent layer (Eigure 8.3C). This does not exclude the possibility of a thin ion-free layer at the interface separating participants of the redox reaction (Eigure 8.3B). Such a layer would result in the smaller ET rate constant, but would not affect the a value. 

---