Inner layer, ITIES

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

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. 

The description of the ion transfer process is closely related to the structure of the electrical double layer at the ITIES [50]. The most widely used approach is the combination of the BV equation and the modified Verwey-Niessen (MVN) model. In the MVN model, the electrical double layer at the ITIES is composed of two diffuse layers and one ion-free or inner layer (Fig. 8). The positions delimiting the inner layer are denoted by X2 and X2, and represent the positions of closest approach of the transferring ion to the ITIES from the organic and aqueous side, respectively. The total Galvani potential drop across the interfacial region, AgCp = cj) — [Pg.545]

From a practical point of view, the potential drop across the inner layer A 02 must be determined by fitting the experimental data to the equations derived from this theoretical approach, which led to some controversy about its value [53,54,56,57]. For the sake of simplicity, and also because recent studies of the ITIES structure do not confirm the presence of an inner layer [51,58], we neglect the finite size of the transferring ion and take X2 = X2 = 0 and A 02 = 0. This is equivalent to accepting that the potential difference Afl02 — A 0 is not modified by the presence of the phospholipids. 

Inner Layer and Potential Profile Across the ITIES. 311... 

Charge Transfer Kinetics at Water-Organic Solvent Phase Boundaries 2.3.2 Inner Layer and Potential Profile Across the ITIES... 

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. 

Kakiuchi [130] integrated the Nernst-Planck equation by assuming a constant gradient of the electrochemical potential in the inner layer at the ITIES. This layer was not supposed to be necessarily the same entity as the ion-free inner layer at the interface (Sec. 2.3.2). In the absence of an activation barrier at the interface, the... 

The electrical potential distribution has been extensively studied in the 1980s and 1990s by capacitance measurements, as excellently reviewed in 1998 by Samec [48], The first model of polarized ITIES is that of Verwey-Niessen [49], dating back to 1939, of two back-to-back diffuse layers, and then adapted by Gavach et al. [50,51] by considering the presence of an inner layer. The key problem with... 

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]

The inner potential drop across the ITIES, Aq2 0, is related to the rational potential [57], Aq 0r = E-Ep f., and the two diffuse layer potentials, each in the aqueous phase and in the organic phase, and 0°through... 

Because of the relatively small outer diameter of an NMR autoclave, the insulation separating the heating elements from the inner wall of the pressure vessel is of critical importance. This insulation must meet the conflicting requirements of low thermal conductivity and low permeabU-ity to convection in the pressure medium. In one scheme, the insulation consists of a layered structure formed by a roll of 0.025 mm molybdenum foil. The layers were separated with small spacing by 0.05 mm pimples embossed on the foil. With this insulation, a sample temperature of 1500 =C at 90 bar can be obtained with 450 W of DC power. A more efficient, but slightly more complex variation uses layers of molybdenum foil separated by thin sheets of alumina cloth. 

Fig. 17 Schematic of (a) the array of nano-ITIES and the time-dependent growth of diffusion layers in the inner solution from (b) linear to (c) radial and back to (d) linear in form. Adapted with permission from ref 84. Copyright 2003 American Chemical Society.

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