Verwey-Niessen model

FIGURE 32.3 Modified Verwey-Niessen model of an ITIES with an inner layer (shaded area) separating two space-charge regions. 

The simplest model for the ionic distribution at liquid-liquid interfaces is the Verwey-Niessen model [10], which consists of two Gouy-Chapman space-charge layers back to... 

The interfacial capacity is then obtained by calculating the profiles for various potential drops A0 and subsequent differentiation. Figure 7 shows several examples of capacity-potential characteristics for several widths of the interface. Obviously, the wider the interface, the higher the capacity. In all cases investigated it was higher than that calculated from the Verwey-Niessen model, in which ... 

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... 

III. MICROSCOPIC MODELS A. Modified Verwey-Niessen Model (MVN)... 

FIG. 1 Structure of the ITIES (a) Verwey-Niessen model [11], (b) mixed solvent layer model [4], and (c) molecular dynamics simulation [24]. 

The static - double-layer effect has been accounted for by assuming an equilibrium ionic distribution up to the positions located close to the interface in phases w and o, respectively, presumably at the corresponding outer Helmholtz plane (-> Frumkin correction) [iii], see also -> Verwey-Niessen model. Significance of the Frumkin correction was discussed critically to show that it applies only at equilibrium, that is, in the absence of faradaic current [vi]. Instead, the dynamic Levich correction should be used if the system is not at equilibrium [vi, vii]. Theoretical description of the ion transfer has remained a matter of continuing discussion. It has not been clear whether ion transfer across ITIES is better described as an activated (Butler-Volmer) process [viii], as a mass transport (Nernst-Planck) phenomenon [ix, x], or as a combination of both [xi]. Evidence has been also provided that the Frumkin correction overestimates the effect of electric double layer [xii]. Molecular dynamics (MD) computer simulations highlighted the dynamic role of the water protrusions (fingers) and friction effects [xiii, xiv], which has been further studied theoretically [xv,xvi]. 

Homogeneous kinetics may be considerably more MVN (modified Verwey-Niessen) model - Verwey-comphcated than the simple first-order reactions de- Niessen model... 

Verwey-Niessen model — Earliest theoretical model of the - interface between two immiscible electrolyte solutions (ITIES) assuming the existence of a diffuse double layer with one phase containing an excess of the positive space charge and the other phase an equal excess of the negative space charge [i] (Figure). The difference of - inner electric potentials, Afcj> = (f>w - [Pg.692]

Enhancement of Capacitance. The agreement between (modified) Verwey-Niessen models and experiment is less satisfactory for lower-polarity organic media (e.g., DCE, as opposed to NB) and for lower electrolyte concentrations [13]. What is the physical origin of the higher experimental capacitances seen for these conditions As noted by Schmickler and co-workers [59], this enhancement of capacitance at the ITIES relative to the classical model stands in contrast to the response of electrode-electrolyte interface, where the capacitance is often found to be lower than the Gouy-Chapman function. 

The Verwey-Niessen model gives a reasonable first approximation, but at a closer glance significant deviations are observed even at low ionic concentrations, where this theory is expected to hold. Surprisingly, at low (<10 M) concentrations the experimental capacity is often higher than that predicted by Verwey-Niessen theory - this is just the opposite to the behavior of metal-solution interfaces, where the capacity is always ioiver than the GC value. 

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... 

Gavach and coworkers [10] extended the Verwey-Niessen model (MVN) by introducing an ion-free transition layer... 

Figure 17.3.3 The modified Verwey-Niessen model and potential distribution. (Reprinted with permission from (20). Copyright 1977 Elsevier Science).

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