Interfacial Potential Differences Galvani Potentials

Interfacial potential (difference) — Synonym for -> Galvani potential difference. The latter term should be preferred. 

From the perspective of two-phase catalysis, the relevance of these concepts can be discussed in similar terms to ion-transfer phenomena. For instance, by introducing ions with the appropriate transfer potential, the Galvani potential difference between two immiscible liquids can be adjusted in order to enhance the driving force for interfacial redox processes. Furthermore, a combination of electrochemical and spectroscopic techniques allows unraveling of the interaction between redox catalysts and substrates in the interfacial region [56]. 

The sum of interfacial potential and distribution potential is termed the iimer or Galvani potential, designated by . It will be discussed below that the observed transmembrane potential difference, Em, is due either to AV, AU, or to AV plus AU, as defined above. It should be remembered that there are two components of (=V + U). They are concerned with only the distribution potential. If compounds, such as phospholipids and interface-active agents, are preferentially adsorbed at the interface, the so-called adsorption potential (V) may also develop. As shown in Fig. 2, both adsorbed fixed charge species and dipoles may contribute to the observed potential. The nature and origin of the adsorption (or interfacial) potential can be discussed in terms of the classical EDL theory of Gouy-Chapman-Stern-Graham (7,15-19]. 

An interfacial potential difference appears as a vertical step in a profile of the Galvani potential, as shown schematically in Fig. 14.4(a). The zero-current cell potential, E ceii.eq is the algebraic sum of the interfacial potential differences within the cell. 

FIGURE 32.4 Potential dependence of the interfacial tension J and the capacity C for the interface between solutions of 5mM tetrabutylammonium tetraphenylborate in 1,2-dichloroethane and lOOmM LiCl in water. The potential scale E represents the Galvani potential difference relative to the standard ion transfer potential for tetraethylammonium ion, cP o EA+ = 0.02 V. 

The presence of an electrical potential drop, i.e., interfacial potential, across the boundary between two dissimilar phases, as well as at their surfaces exposed to a neutral gas phase, is the most characteristic feature of every interface and surface electrified due to the ion separation and dipole orientation. This charge separation is usually described as the formation of the ionic and dipolar double layers. The main interfacial potential is the Galvani potential (termed also by Trasatti the operative potential), which is the difference of inner potentials (p and of both phases. It is a function only of the chemical... 

The previous analysis indicates that although the voltammetiic behavior suggests that the aqueous phase behaves as a metal electrode dipped into the organic phase, the interfacial concentration of the aqueous redox couple does exhibit a dependence on the Galvani potential difference. In this sense, it is not necessary to invoke potential perturbations due to interfacial ion pairing in order to account for deviations from the Butler Volmer behavior [63]. This phenomenon has also been discarded in recent studies of the same system based on SECM [46]. In this work, the authors observed a potential independent ket for the reaction sequence. 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

In initial ET rate measurements, both the NB and aqueous phases contained 0.1 M TEAP, enabling measurements to be made with a constant Galvani potential difference across the liquid junction. In these early studies, the concentration of Fc used in the organic phase (phase 2) was at least 50 times the concentration of the electroactive mediator in the aqueous phase which contained the probe UME (phase 1). This ensured that the interfacial process was not limited by mass transport in the organic phase, and that the simple constant-composition model, described briefly in Section IV, could be used. 

When an aqueous phase (noted w) is brought in contact with a second immiscible phase (noted o), the different species dissolved in one or the two phases spontaneously distribute depending on their hydrophilic-lipophilic balance until the thermodynamic equilibrium is reached. The distribution of the charged species generates an interfacial region, in which the electrical field strength differs from zero, so that an electrical Galvani potential difference, is established across the interface ... 

The inner potential difference between two contacting phases is cafied in electrochemistry the Galvani potential difference, and the outer potential difference is called the Volta potential difference. The outer potential difference corresponds to what is called the contact potential between the two phases. We call, in this test, the inner potential difference across an interface the interfacial potential. 

Besides the Galvani potential, another important interfacial potential is the Volta potential, Aj I, sometimes called the contact potential. A/ T is the difference of the outer potentials of the phases, which are in electrochemical equilibrium with regard to the charged species, i.e., ions or electrons. As for any two-phase electrochemical system, including the w/s system, it may be characterized by the commonly known relation ... 

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