Polarized interface potentials

After polarization to more anodic potentials than E the subsequent polymeric oxidation is not yet controlled by the conformational relaxa-tion-nucleation, and a uniform and flat oxidation front, under diffusion control, advances from the polymer/solution interface to the polymer/metal interface by polarization at potentials more anodic than o-A polarization to any more cathodic potential than Es promotes a closing and compaction of the polymeric structure in such a magnitude that extra energy is now required to open the structure (AHe is the energy needed to relax 1 mol of segments), before the oxidation can be completed by penetration of counter-ions from the solution the electrochemical reaction starts under conformational relaxation control. So AHC is the energy required to compact 1 mol of the polymeric structure by cathodic polarization. Taking... 

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 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. 

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. 

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. 

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. 

A persistent question regarding carbon capacitance is related to the relative contributions of Faradaic ( pseudocapacitance ) and non-Faradaic (i.e., double-layer) processes [85,87,95,187], A practical issue that may help resolve the uncertainties regarding DL- and pseudo-capacitance is the relationship between the PZC (or the point of zero potential) [150] and the point of zero charge (or isoelectric point) of carbons [4], The former corresponds to the electrode potential at which the surface charge density is zero. The latter is the pH value for which the zeta potential (or electrophoretic mobility) and the net surface charge is zero. At a more fundamental level (see Figure 5.6), the discussion here focuses on the coupling of an externally imposed double layer (an electrically polarized interface) and a double layer formed spontaneously by preferential adsorp-tion/desorption of ions (an electrically relaxed interface). This issue has been discussed extensively (and authoritatively ) by Lyklema and coworkers [188-191] for amphifunctionally electrified... 

In the SECM measurements, an ultramicroelectrode tip is used as a probe for ET occurring at an O/W interface. The electrode potential is controlled by a three-electrode potentiostat, whereas the potential drop across the O/W interface is usually determined by adding a common ion to both phases (except for the recent study [23] using an externally polarized interface). This sets the SECM measurements free from the restriction of the potential window. It should also be noted that in ordinary SECM measurements, all electrodes are in a single phase, so that it is possible to avoid the problems of IR drop and charging current. These advantages of SECM have been realized in kinetic studies of ET at O/W interfaces. 

For polarized Interfaces, capacitances can be readily measured directly with great precision. For mercury, capacitance-applied potential curves constitute the basic information for double layer analyses, outweighing that from electro-capillary curves. 

For polarized interfaces (mercury) the point of zero charge is not defined by the composition of the solution rather, it is a certain applied potential with respect to a reference electrode. It is the potential of the electrocapillary maximum, also called the potential of zero charge, see [1.5.6.17]. In this case, the two charging processes are supply and withdrawal of electrons, the einalogues of desorption and adsorption of protons on oxides. 

Interfaclal polarization see potential difference, x Interfaclal potential Jump xY, see potential difference, x interfaclal potentials 1.5.5, 3.9 interfaclal pressure see surface pressure interfaclal science (first review) 1.1.2, 1.1.3 interfaclal tension, surface tension I.1.4(intr.), 1.1.25, fig. 1.1.16 measurement 1.1.11,1.2.5,1.2.96, 3.139 of curved interfaces 1.2.23. 1.6d of films 1.95ff of solid surfaces 1.2.24... 

The location of Aq < pzc is usually in the middle of the potential window for a polarized interface between the immiscible electrolyte solutions [21]. It is expected therefore... 

A reference electrode scanned along the metal surface will measure the series of (E"x)n and (E"M)n interface potentials. From these values, solution potentials (t))s) at the metal/solution interface may be calculated (< )s = -E") and presented as in Fig. 4.6. When the anodic and cathodic sites are microscopic relative to the size and position of the reference electrode, identity of the anodic and cathodic sites on a macroscale is lost, and a single mixed or corrosion potential, Ecorr, is measured as discussed previously. There is essentially a uniform flux of metal ions from the surface, and cathodic reactants to the surface, which constitute anodic and cathodic currents. Since the relative areas to which these currents apply usually are not known, the total area is taken as the effective area for each reaction. It is these currents, however, that mutually polarize the anodic reaction potential from E M up to Ecorr and the cathodic reaction potential from E x down to Ecorr. 

The book is organized into five parts. Part I consists of seven chapters and deals with fundamental aspects of interfacial phenomena such as catalytic properties of liquid interfaces, electrochemistry at polarized interfaces, ion solvation and resolvation, interfacial potentials, separations, and interfacial catalysis in metal complexation and in enhanced oil recovery. 

As shown in Fig. 3 the curve Q versus potential has a wide plateau (from —0.125 to -I- 0.2 V) whereas the Q values are close to zero. With this change in potential, only a negligible amount of charge (0.00076 F) is transferred through the interface, which behaves like an ideal polarization interface. The potential window for voltammetric measurement is wide. On the other hand, the equilibrium potential is sensitive to the presence of ions that have standard transfer potentials within the window. Therefore, system II cannot be used as a reference electrode. 

Before closing this section, the case of polarized interfaces has to be introduced since SSHG at interfaces between two immiscible electrolyte solutions (ITIES) constitues one of the main trends of nonlinear optical applications at liquid/liquid interfaces. It has been shown for metals, that upon polarization by an externally applied electric potential, a specific SH response was generated from the coupling between the static dc-field established across the interface and the fundamental electromagnetic wave [20]. The main property of this contribution is that it evolves... 

To allow for such possibilities, it is highly useful to study a polarized interface charging process over at least several decades of frequency or time. It is felt that the best technique for doing this is potential-step chronocoulometry. In this, a small voltage step is applied to a polarization cell and the integrated charging current Q(t) may be directly displayed over many decades... 

Oil/water interfaces are classified into the ideal-polarized interface and the nonpolarized interface. The interface between a nitrobenzene solution of tetrabutylam-monium tetraphenylborate and an aqueous solution of lithium chloride behaves as an ideal-polarized interface in a certain potential range. Electrocapillary curves of the interface were measured. The results are analyzed using the electrocapillary equation of the ideal-polarized interface and the Gouy-Chapman theory of diffuse double layers. The electric double layer structure consisting of the inner layer and the two diffuse double layers on each side of the interface is discussed. Electrocapillary curves of the nonpolarized oil/water interface are discussed for two cases of a nonpolarized nitrobenzene/water interface. 

The interface between TBATPB(NB) and LiCl(W) behaves as an ideal-polarized interface in a certain range of the potential difference across the interface [17]. Thus the electrocapillary equation of this system at constant temperature and pressure is given by [18,46] ... 

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