Mixed solvent layer

Assuming that the two space-charge regions are separated by a layer of solvent molecules (inner layer or mixed solvent layer), the Galvani potential difference can be expressed as the sum of three contributions ... 

For the mixed solvent layer formalism, the preexponential parameter was estimated by extension of the Newton and Sutin approach [61],... 

Marcus model for flat interfaces [Eqs. (17)-(19)], it was realized that the continuum model for a mixed solvent layer also approaches these experimental values [3-6]. 

The non-steady-state optical analysis introduced by Ding et al. also featured deviations from the Butler-Volmer behavior under identical conditions [43]. In this case, the large potential range accessible with these techniques allows measurements of the rate constant in the vicinity of the potential of zero charge (k j). The potential dependence of the ET rate constant normalized by as obtained from the optical analysis of the TCNQ reduction by ferrocyanide is displayed in Fig. 10(a) [43]. This dependence was analyzed in terms of the preencounter equilibrium model associated with a mixed-solvent layer type of interfacial structure [see Eqs. (14) and (16)]. The experimental results were compared to the theoretical curve obtained from Eq. (14) assuming that the potential drop between the reaction planes (A 0) is zero. The potential drop in the aqueous side was estimated by the Gouy-Chapman model. The theoretical curve underestimates the experimental trend, and the difference can be associated with the third term in Eq. (14). 

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

Girault and Schiffrin [4] proposed an alternative model, which questioned the concept of the ion-free inner layer at the ITIES. They suggested that the interfacial region is not molecularly sharp, but consist of a mixed solvent region with a continuous change in the solvent properties [Fig. 1(b)]. Interfacial solvent mixing should lead to the mixed solvation of ions at the ITIES, which influences the surface excess of water [4]. Existence of the mixed solvent layer has been supported by theoretical calculations for the lattice-gas model of the liquid-liquid interface [23], which suggest that the thickness of this layer depends on the miscibility of the two solvents [23]. However, for solvents of experimental interest, the interfacial thickness approaches the sum of solvent radii, which is comparable with the inner-layer thickness in the MVN model. 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

Monte Carlo and molecular dynamics calculations of the density profile of model system of benzene-water [70], 1,2-dichloroethane-water [71], and decane-water [72] interfaces show that the thickness of the transition region at the interface is molecu-larly sharp, typically within 0.5 nm, rather than diffuse (Fig. 4). A similar sharp density profile has been reported also at several liquid-vapor interfaces [73, 74]. The sharpness of interfaces thus seems to be a general characteristic of the boundary between two stable phases and it is likely that the presence of supporting electrolytes would not significantly alter the thickness of the transition region at an ITIES. The interfacial mixed solvent layer [54, 55], if any, would probably have a thickness comparable with this thin inner layer. 

The information available about the structure of the ITIES is also sparse. In spite of recent progress in spectroscopic studies (13) and molecular dynamics simulations (le,14), questions remain about the nature and properties of the boundary (mixed-solvent) layer, which is supposed to separate two liquid phases (1). Since the thickness of the interface and physical localization of the ET reaction are largely unknown, the microscopic description of this process remains problematic. 

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. 

A sharp-boundary model (6a) rather than a model assuming a significant penetration of species into a mixed-solvent layer (6b, c) is applicable to the ET at the interface between two very low miscibility solvents,... 

Another way to treat an ion-transfer reaction is to consider the reaction as an elementary jump across a thin interfacial layer, which we will call the reaction plane, but it may also be called inner layer or mixed-solvent layer depending on the interfacial model used. The interface (about 1 nm thick) comprises back-to-back diffuse layers. 

Scanning ion conductance microscopy was applied to investigate the interface of two immiscible electrolyte solutions (ITIES). Two opposing views describe interfacial structure one opinion is that solvent dipoles orient to form an ion-free compact layer contained in a molecularly sharp interface. The sharp interface is proposed to separate two back-to-back double layers. The opposite view suggests the interface is composed of a mixed solvent layer that ions of both phases can penetrate. To adequately examine the interface, a technique with high-resolution and small probe are desired. An SECM study performed by Bard and coworkers of a water/nitrobenzene... 

Back in 1983, the concept of mixed solvent layer [16] resulted from the determination of water surface excess concentrations at different interfaces by interfacial tension measurements that showed that, in the case of the H2O-DCE interface, and unlike the liquid water-vapor or the water-heptane interfaces, the water excess concentration was less than a monolayer as expected for aqueous 1 1 electrolyte. The molecular dynamics results of Wick and Dang seem therefore to corroborate this early concept of interfacial structure in the presence of electrolytes in the aqueous phase. 

Of course, the problem here is that the standard term( )f(x) = - j,f/Z F, which expresses the standard chemical potential, varies in a stepwise manner aaoss the mixed-solvent layer, say, 1 nm thick, whereas the potential drop varies in a monotonic way in the absence of specific adsorption across the two back-to-back diffuse layers, say 10 nm thick. As a result. Equation 1.30 is a very rough approximation, as soon as the Gibbs energy of transfer of the ion is larger than 5 kJ-mol. ... 

The measured rate constants were more than an order of magnitude lower than those obtained previously for tetraalkylammonium transfers at the DCE/water interface. This difference was attributed to higher viscosity of IL as compared that of DCE. Possible origins of the viscosity effect on considered in ref. 17 are lower dilfusivities in the interfacial mixed solvent layer, slower formation of the interfacial protrusions and different ion solvation energies in IL. 

The interface can be described as a mixed solvent layer separating two diffuse layers. 

The potential drop across the interfacial mixed solvent layer is negligible near the PZC. 

The second question one may ask is If the potential drop across the mixed solvent layer is very small, why do we observe a Butler-Volmer relationship for the potential dependence of the apparent rate constant ... 

---