Electrical double layer at ITIES

E. J. W. Verwey and K. F. Niessen described the electric double layer at ITIES using a simplifying assumption that consists of only two diffuse electric layers, each in one of the phases (see Fig. 4.1C). The overall potential difference between the two phases, Ao0, is thus given by the relationship... 

Experimentally, the -> electric double layer at ITIES has been studied mainly by -> surface tension [x, xi] and differential capacity [xii] measurements. Experimental results and theoretical models were reviewed [xiii]. 

The structure of the electric double layer at ITIES has been investigated using such common electrochemical... 

This behaviour was already predicted by Verwey and Niessen [7] as early as in 1939. They described theproperties of the electric double-layer at ITIES by the simple Gouy-Chapman model. For potential differences Ao(p >RT/F the potential difference in the diffuse layer in the aqueous phase... 

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]

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

The rule for the electric double-layer at the metal/electrolyte interface is also valid for ITIES, that the surface charges in the aqueous side and in the non-aqueous side of the electric double-layer must be equal in absolute values and opposite in signs. 

The electrical double layer arising at the ITIES has been studied by measuring the surface tension [4, 7-16, 25] or the impedance [17-26] mainly of water/nitrobenzene [4, 7-15, 17, 19-24] and water/l,2-dichlorethane [12, 16, 18, 25, 26] systems. This contribution reviews the principles and the results of the impedance measurements, in particular those based on the AC impedance or galvanostatic pulse techniques, which have been used most frequently for the study of the double layer at the ITIES. The quantity, which can be inferred from the impedance measurements, and which is related to the double-layer structure, is the interfacial capacitance. We shall discuss first the thermodynamic background for the capacitance of the electrical double layer at the ITIES. 

The most general thermodynamic treatment of the electrical double layer at the ITIES was given by Kakiuchi and Senda [28]. Here we shall follow a simpler but instructive analysis by Girault and Schifrin [16]. 

According to the model proposed by Verwey and Niessen (1939), an electric double layer is formed at an ITIES, which consists of two ionic space charge regions. As a whole the electric double layer is electrically neutral, so for the excess charge density in the part of the double layer in the aqueous phase, and for the part in the organic phase,... 

A theoretical approach based on the electrical double layer correction has been proposed to explain the observed enhancement of the rate of ion transfer across zwitter-ionic phospholipid monolayers at ITIES [17]. If the orientation of the headgroups is such that the phosphonic group remains closer to the ITIES than the ammonium groups, the local concentration of cations is increased at the ITIES and hence the current observed due to cation transfer is larger than in the absence of phospholipids at the interface. This enhancement is evaluated from the solution of the PB equation, and calculations have been carried out for the conditions of the experiments presented in the literature. The theoretical results turn out to be in good agreement with those experimental studies, thus showing the importance of the electrostatic correction on the rate of ion transfer across an ITIES with adsorbed phospholipids. 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

The electrical double layer has also been investigated at the interface between two immiscible electrolyte solutions and at the solid electrolyte-electrolyte solution interface. Under certain conditions, the interface between two immiscible electrolyte solutions (ITIES) has the properties of an ideally polarized interphase. The dissolved electrolyte must have the following properties ... 

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]. 

Photoredox reactions at organized assemblies such as micelles and microemulsions provide a convenient approach for modeling life-sustaining processes. Micelles are spontaneously formed in solutions in the presence of surfactants above a certain critical concentration. In aqueous solutions, the hydrophobic tails of the surfactant form aggregates with the polar head facing toward the aqueous environment, as depicted in Fig. 9. The hydrophobic core in micelles is amorphous and exhibits properties similar to a liquid hydrocarbon. The polar heads are also randomly oriented, generating an electrical double layer around the micelle structure. In this respect, surface properties of micelles can be somewhat correlated with the polarized ITIES. The structure of micelles is in dynamic equilibrium, in which monomers are exchanged between bulk solution and the assembly. 

The first reaction is an electron transfer across the double layer at the electrode-electrolyte interface between redox species in the electrolyte that exchange electrons with a metal electrode, the second one is an ion-transfer reaction across the double layer since the electron lost by the Cu atom remains at the metal. The third one is an ion-transfer process across the water-organic solvent interface or ion transfer at immiscible electrolyte solutions (ITIES) without the transfer of electrons. In all cases the electrochemical reaction takes place at an electrified interface and therefore the rate of these reactions follow similar exponential dependence on the interfacial electrical potential. 

For semiconductor electrodes and also for the interface between two immiscible electrolyte solutions (ITIES), the greatest part of the potential difference between the two phases is represented by the potentials of the diffuse electric layers in the two phases (see Eq. 4.5.18). The rate of the charge transfer across the compact part of the double layer then depends very little on the overall potential difference. The potential dependence of the charge transfer rate is connected with the change in concentration of the transferred species at the boundary resulting from the potentials in the diffuse layers (Eq. 4.3.5), which, of course, depend on the overall potential difference between the two phases. In the case of simple ion transfer across ITIES, the process is very rapid being, in fact, a sort of diffusion accompanied with a resolvation in the recipient phase. 

Eqs. 7-23 and 7-24 demonstrate direct proportionality between particle velocity and electric field. The particle mobility (which equals the ratio of veloc-ity/electric field strength, as in Eq. 5-6) only depends on the total particle charge or average zeta-potential. This remarkable result holds for any particle size and shape, even for polarizable particles with a nonuniform charge in the applied field. Dukhin [1] stated that these assumpHons are valid only for a constant double layer on the particle and predicted that the EP mobility of particles where charges are induced (u p, ) may change as a function of and velocity (v pj ) as V. In an AC field that results in nanodielectrophoresis-type EP effects at frequencies below 100 kHz, and Eq. 7-27 for imiform and Eq. 7-28 for nonuniform fields becomes ... 

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