Three-Phase Junctions

FIGURE 1.34 Electrode assembly with a droplet of an aprotic solvent containing a neutral redox probe fRedj attached to a graphite electrode, which is immersed in an aqueous 

Another interesting aspect of three-phase junctions is electrowetting. Indeed, Monroe et al. have shown that electrowetting phenomena at ITIES can be controlled by small voltages [317,318] compared to the higher voltages used in commercial electrowetting-based devices such as the zoom lens in some mobile phones. 

In summary, many novel systems have been developed over the past two decades to study charge-transfer reactions. Micro-ITIES and microdroplet electrodes have circumvented most of the drawbacks of classical flat liquid-liquid interfaces, namely, the large iR drop and the large capacitance. Also, both systems allow a reduction in the amount of supporting electrolyte needed. 

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Hasse et al. [366] have used in situ AFM for the detection of silver nucleation at the three-phase junction of the type metal-silver halide-electrolyte solution. At this phase boundary, electrochemical reduction of submicrometer size silver halide crystals immobilized on the surface of gold and platinum electrodes took place. Following nucleation, the reaction advanced until the entire surface of the silver hahde crystals was covered with 20 atomic layers of silver. Then, reduction was terminated. The obtained silver layer could be oxidized and the next layer of silver halide crystals became accessible for further reduction. 

The three interfacial surface energies, as shown at the three-phase junction in Figure 2.29, can be used to perform a simple force balance. The liquid-solid interfacial energy plus the component of the liquid-vapor interfacial energy that lies in the same direction must exactly balance the solid-vapor interfacial energy at equilibrium ... 

Such electrochemical processes can be described on the basis of the model developed by Lovric and Scholz [115, 116] and Oldham [117] for the redox reactivity of nonconducting solids able to be permeated by cations or anions (so-called ion-insertion solids). As described in the most recent version of Schroder et al. [118], the electrochemical process is initiated at the three-phase junction between the electrode, the electrolyte solution, and the solid particle, as schematized in Fig. 2.6. From this point, the reaction expands via charge diffusion across the solid particle. It is assumed that, for a reduction process, there is a flux of electrons through the... 

Oldham KB (1998) Voltammetry at a three-phase junction. J Solid State Electrochem 2 367-377. 

Schroder U, Oldham KB, Myland JC, Mahon PJ, Scholz F (2000) Modelhng of solid state voltammetry of immobilized microcrystals assuming an initiation of the electrochemical reaction at a three-phase junction. J Solid State Electrochem 4 314-324. 

In this equation, A represents the number of crystals, p is the length of the three-phase junction (i.e., the perimeter of the electrode/crystal interface), and Ax, Az denote the size of the discrete boxes in which the crystal is divided for numerical simulation procedures. 

Because there can be degrees of wetting of particles at an interface, another quantity is needed. The contact angle, 6, in an oil—water—solid system is defined as the angle, measured through the aqueous phase, that is formed at the junction of the three phases. Whereas interfacial tension is defined for the boundary between two phases, the contact angle is defined for a three-phase junction. If the interfacial forces that act along the perimeter of the drop are represented by the interfacial tensions, then an equilibrium force balance can be written as... 

The cyclic voltammograms and the changes that occur to them during repetitive cycling are similar to those of 3-methylthiophene oxidation in acetonitrile. When a platinum electrode is used, the color change (red-blue) due to the redox transformation of poly (3-methylthiophene) is easily visible. A visual inspection also reveals that the electropolymerization reaction starts at the three-phase junction, as theoret-... 

Line Tension. Where three phases meet there may exist a line tension (force) along the three-phase junction. For a lens of material at the interface between two oflier immiscible phases, the three-phase contact junction takes the form of a circle along which the line tension acts. 

Unlike the situation at a solid/electrolyte interface where a three-electrode system is used, four- and two-electrode systems have been widely employed for large and small liquid/ liquid interfaces. Most of the four-electrode potentiostats are homemade and only a few instruments with such functions have been commercialized (98). This is probably one of the reasons why this field has not been very popular since most electrochemical laboratories are equipped with a three-electrode potentiostat. In 1998, Anson et al. reported that charge transfer reactions at a liquid/liquid interface could also be studied by a three-electrode system with a thin-layer cell (99,1(X)). Later, Scholz et al. reported a three-phase junction setup (101, 102). Shao et al. supported a small droplet of aqueous solution (pL) containing a certain concentration ratio of redox couples on a Pt surface and demonstrated that charge transfer could be studied by a three-electrode setup (103). Girault et al. extended this to a supported small droplet of aqueous (organic) phase on the surface of... 

Electrochemistry at liquid/liquid interfaces has progressed markedly in the past 30 years. Excellent work on modified liquid/liquid interface with lipids and nanoparticles have been reported (75, 78, 124-127). Droplet electrodes and three-phase jnnctions have made this field more popular and versatile (102, 103, 128). The ET induced IT reactions at three-phase junction have been employed to obtain the log P of different drugs at W/n-octanol interfaces (102, 129, 130). New and less toxic solvents, such as room tanperature ionic liquids (RTILs) have replaced organic solvents to form W/RTIL interfaces (131, 132). However, from a theoretical point of view, the key aspects of potential distribution remain the major challenge. Only a few biological applications have been so far reported based on the techniques developed from this field. 

Here Agi is the driving force of the reaction L+R—>i per atom of i. The first two terms in Equation 4.60 represent the classical model of heterogeneous nucleation ( classic ( i jout, however, taking into account Young s equihbrium conditions at three-phase junctions - otherwise a nonsymmetrical cap would be obtained with much less transparent mathematics for the gradient effect). The gradient effect is represented both by linear in Vc and by quadratic (Vc) terms, providing the fourth and fifth power size dependence, respectively. 

An alternative method is to deposit an aqueous drop on a solid electrode such as a silver-silver chloride electrode or a platinum electrode and immerse it in an organic solvent [291]. This method provides a fast and convenient method to measure standard transfer potentials, for example, of ionized drug molecules. It differs from the three-phase junction discussed in the following text, in the sense that the electrode is in contact with one phase only, namely, the aqueous one. In this case, we have two interfaces in series. 

Scholz, R, S. Komorsky-Lovric, and M. Lovric, A new access to Gibbs energies of transfer of ions across liquid I liquid interfaces and a new method to study electrochemical processes at well-defined three-phase junctions, Electmchem Commun, Vol. 2, (2000) p. 112. 

Komorsky-Lovric, S., M. Lovric, and R Scholz, Cyclic voltammetry of decameth-ylferrocene at the organic liquid I aqueous solution I graphite three-phase junction, J Electmanal Chem, Wo. 508, (2001) p. 129. 

Figure 12.3 ElectrcxJeposition of silane at the electrode/nitrobenzene/aqueous electrolyte three-phase junction. (Reproduced with permission from Ref. [15].)...

NiedzioUsi, J. and Opallo, M. (2008) Electrochemically assisted sol gel process at a three phase junction. Electrochem. Commun., 10,1445 1447. 

Niedziolka-Jonsson, J, Jonsson-Niedziolka, M., Nogala, W., and Palys, B. (2011) Electrosynthesis of thin sol-gel films at a three-phase junction. Electrochim. Acta,... 

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