Polarity/polarization interface

One important advantage of the polarized interface is that one can determine the relative surface excess of an ionic species whose counterions are reversible to a reference electrode. The adsorption properties of an ionic component, e.g., ionic surfactant, can thus be studied independently, i.e., without being disturbed by the presence of counterionic species, unlike the case of ionic surfactant adsorption at nonpolar oil-water and air-water interfaces [25]. The merits of the polarized interface are not available at nonpolarized liquid-liquid interfaces, because of the dependency of the phase-boundary potential on the solution composition. 

The case of polarized interfaces is usually described within the context of the metal-electrolyte interface where the metal charge dependence of the SH intensity is dramatic because of the strong interfacial electric field present at the interface [16]. It has long been a real challenge at the polarized liquid-liquid interface but has, however, been observed at charged air-water interfaces [48]. 

At polarized interfaces, the static DC electric field established across the interface couples to the electromagnetic field impinging onto the surface. This process is described with the following nonlinear polarization ... 

A. Electron-Transfer Reactions at Externally Polarized interfaces... 

In the majority of methods described thus far, the interfacial kinetics are deduced by measuring concentration changes in the bulk of the solution rather than at the interface, where the reaction occurs. This introduces a time lag, limiting the resolution of the measurement in the determination of interfacial kinetics. A more direct approach is to identify the interfacial flux. This can be achieved in the electrolyte dropping electrode, via the current flow, but this method is only applicable to net charge-transfer processes at externally polarized interfaces. 

Aguilella, V., Belaya, M. and Levadny, V. (1996). Ion permeability of a membrane with soft polar interfaces. 1. The hydrophobic layer as the rate-determining step, Langmuir, 12, 4817 -827. 

A. P. Demchenko and N. V. Shcherbatska, Nanosecond dynamics of the charged fluorescent probes at the polar interface of the membrane phospholipid bilayer, Biophys. Chem. 22, 131-143 (1985). 

When LaFa is in direct contact with metals, such as Ag, Hg, Pb, Ga, Cd, ZnHg or La, the contact attains the properties of an ideally polarized interface [96] with poorly defined E ise values. 

The characteristics of the diffuse electric double layer at a completely polarized interface, such as at a mercury/aqueous electrolyte solution interface are essentially identical with those found at the reversible interface. With the polarizable interface the potential difference is applied by the experimenter, and, together with the electrolyte, specifically adsorbed as well as located in the diffuse double layer, results in a measurable change in interfacial tension and a measurable capacity. 

Let us now look at the conversion of Curve A to Curve B. What has happened there is that a small charge-transfer resistor has been added in parallel to the doublelayer capacitor, through which electrons can shuttle between Pt and the redox couple (5.6). In other words, the addition of the redox couple has converted the polarized interface (Fig. 5.2a) to a nonpolarized interface (Fig. 5.2b). 

On the other hand, equilibrium at the polarized interface is described by the Gibbs-Lippmann equation (5.9). Here, the equilibrium potential eq, surface concentration Xj Fj of all adsorbing species, their bulk electrochemical potential fa, and the resulting interfacial charge Qi are linked rather less explicitly to surface tension y. 

A notable difference between these two relationships is that the Gibbs-Lippmann equation contains one more independent variable parameter, the interfacial charge. It cannot be determined directly. Several unsuccessful attempts to design chemical sensors (e.g., the immunosensor) based on the measurement of adsorbed surface charge have been made. There are no ideally polarized interfaces that are sufficiently ideal to allow such direct measurement of interfacial charge. 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

Potentiometric measurements are done under the condition of zero current. Therefore, the domain of this group of sensors lies at the zero-current axis (see Fig. 5.1). From the viewpoint of charge transfer, there are two types of electrochemical interfaces ideally polarized (purely capacitive) and nonpolarized. As the name implies, the ideally polarized interface is only hypothetical. Although possible in principle, there are no chemical sensors based on a polarized interface at present and we consider only the nonpolarized interface at which at least one charged species partitions between the two phases. The Thought Experiments constructed in Chapter 5, around Fig. 5.1, involved a redox couple, for the sake of simplicity. Thus, an electron was the charged species that communicated between the two phases. In this section and in the area of potentiometric sensors, we consider any charged species electrons, ions, or both. 

Many of the systems used for electrochemical studies of ion transfer processes taking place at the ITIES are systems of a single polarized interface. In these kinds of systems, the polarization phenomenon is only effective at the sample solution/... 

Note finally that in the case of two-polarized interface systems, the plots of the membrane potential EM versus In (2(1 )2/(1 -7n)) are linear with a slope equal to RT/F and an intercept E 2. 

As for single polarized interface systems, an explicit analytical equation for the CV response for systems with two L/L polarizable interfaces is derived from that corresponding to CSCV when the pulse amplitude AE approaches zero (see also Appendix H). For the case corresponding to the transfer of a cationX+, one obtains... 

In order to show the distribution of the applied potential between the outer and the inner interface in the case of systems with two polarized interfaces, the potential time waveform used in SWV is depicted in Scheme 7.5. The applied potential, E (red line), and the outer ( out, blue line) and inner potentials ( "", green line) have been plotted. 

For comparison of the SWV responses provided for systems of one and two polarized interfaces, Fig. 7.23 shows the/sw — E curves corresponding to the direct and to the reverse scans (solid line and empty circles, respectively) for both kinds of membrane systems, calculated for sw = 50 mV by using Eq. (7.44). The peaks obtained when two polarized interfaces are considered are shifted 8 mV with respect to those obtained for a system with a single polarized one, which implies that the half-wave potential for the system with two polarized interfaces can be easily determined from the peak potential by... 

Regarding the influence of the target ion concentration on the SWV curves, the major difference found between systems with one or two polarized interfaces is that this variable causes a shift of the peak potentials toward more anodic values through an increase of E 2 in the latter case (see Eqs. (7.41)—(7.43)), whereas only an increase in the peak current is observed for systems with one polarized interface [36]. Therefore, SWV is a very good analytical tool for the determination of ion concentrations in both kinds of membrane systems. 

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