Interfacial potential differences

An electric current flowing through an ITIFS splits into nonfaradaic (charging or capacity) and faradic current contributions. The latter contribution comprises the effects of both the transport of reactants to or from the interface, and the interfacial charge transfer, the rate of which is a function of the interfacial potential difference. By applying a transient electrochemical technique, these two effects can be resolved... 

Samec et al. [15] used the AC polarographic method to study the potential dependence of the differential capacity of the ideally polarized water-nitrobenzene interface at various concentrations of the aqueous (LiCl) and the organic solvent (tetrabutylammonium tetra-phenylborate) electrolytes. The capacity showed a single minimum at an interfacial potential difference, which is close to that for the electrocapillary maximum. The experimental capacity was found to agree well with the capacity calculated from Eq. (28) for 1 /C,- = 0 and for the capacities of the space charge regions calculated using the GC theory,... 

FIG. 9 Differential capacity C of the interface between 0.1 M LiCl in water and 0.02 M tetrabu-tylammonium tetraphenylborate ( ) or tetrapentylammonium tetrakis[3,5-bis(trifluoromethyl)phe-nyl]borate ( ) in o-nitrophenyl octyl ether as a function of the interfacial potential difference (From Ref 73.)... 

The dependence of the interfacial tension at the W/NB interface on the interfacial potential difference [29,30] was investigated by using an aqueous solution dropping electrode [26,31]. In this investigation, the aqueous solution was forced upward dropwise in NB and the drop time of W was measured as a function of potential difference applied at the W/ NB interface. When W contained 1 MMgS04 and NB contained 4 x 10 " M Cs" TPhB ... 

When ionizabie compounds are concerned, the situation is complicated by the fact that all the species present may transfer from one phase to the other depending on the interfacial potential difference, so that the transfer process of the solute is entirely determined by the thermodynamic cycle described in Fig. 4. 

The description of the sorption of charged molecules at a charged interface includes an electrostatic term, which is dependent upon the interfacial potential difference, Ai//(V). This term is in turn related to the surface charge density, electric double layer model. The surface charge density is calculated from the concentrations of charged molecules at the interface under the assumption that the membrane itself has a net zero charge, as is the case, for example, for membranes constructed from the zwitterionic lecithin. Moreover,... 

Direct measurement of the change in interfacial potential difference at the oxide-electrolyte interface with change in pH of solution can be measured with semiconductor or semiconductor-oxide electrodes. These measurements have shown d V g/d log a + approaching 59 mV for TiC (36, 37). These values are inconsistent with the highly sub-Nernstian values predicted from the models with small values of K. (Similar studies 138.391 have been performed with other oxides of geochemical interest. Oxides of aluminum have yielded a value of d t)>q/A log aH+ greater than 50 mV, while some oxides of silicon have yielded lower values.)... 

Modified electrodes. Where relevant, we have followed the recent lUPAC directive on the recommended list of terms for chemically modified electrodes (CMEs) [1]. A CME is thus an electrode made up of a conducting or semiconducting material that is coated with a selected monomolecular, multimolecular, ionic or polymeric film of a chemical modifier and that, by means of faradaic reactions or interfacial potential differences exhibits chemical, electrochemical and/or optical properties of the film . 

We can contrast this interface with the C/Ag4Rbl5 interface where no charged species start to equilibrate once the bulk phases have been brought into contact. For a range of interfacial potential differences extending to 0.7 V there is an electrostatic equilibrium whereby the charge on the surface of the carbon is balanced by an equal and opposite charge... 

The interface structure for non-blocking interfaces is similar to that for related blocking interfaces. Thus the distribution of charge at the C/ Ag4Rbl5 interface will be similar to that at the Ag/Ag4Rbl5 interface. The major difference is that there is one particular interfacial potential difference at which the silver electrode is in equilibrium with Ag ions in the bulk electrolyte phase. At this value of A, there is a particular charge on the electrolyte balanced by an equal and opposite charge — on the metal. At any potential different from value of q different... 

As in the previous section there is a single charged species Li which tends to equilibrate between the metal and the electrolyte, so that there is an equilibrium interfacial potential difference A(j> at which Li is in equilibrium and this equilibrium is characterised by an exchange current io. The difference between this system and the systems considered in the... 

In general the interfacial potential difference will be finite and equal to... 

One can have more complicated cells (Fig. 6.30), and in all of them it can be seen that the attempted measurement of a metal-solution potential difference will conclude with the measurement of the sum of at least two interfacial potential differences, i.e., the desired PDM /s and as many extra potential differences as there are new phase boundaries created in the measurement. In symbolic form, therefore, the potential difference V indicated by the measuring instrument can be expressed as... 

Thus, suppose that when one wishes to measure an interfacial potential difference PDm /s, one connects the electrode with another electrode and measures the potential of this cell. Suppose one always keeps this second electrode constant in nature (i.e, the algebraic sum of the potentials associated with it is kept constant). Then, measurements of the potential of a cell in which the one (same) electrode and its associated solution were always present and the other [i.e., the first electrode mentioned here (Mt)] and its solution were changed would clearly reflect the changing interfacial potential difference FDM /s. This is in fact what is done to measure the relative values of PDM /sas Mjor S (or both) are varied. 

Here v is the number of ions per unit time and area transferring from the solution layer next to the electrode, to the electrode. It will at once be noticed that as long as the interfacial potential difference d< > is negative, the electrostatic work term Pde0 increases the velocity of the ion transfer reaction in an exponential way (Fig. 7.10). 

It is now necessary to take a more unified view by considering situations in which the rate of the electrodic process at the interface is subject both to activation and to transport limitations. One refers to a combined activation-transport control of the electrodic reaction. Under such conditions, there will be, in addition to the overpotential T)c produced by the concentration change (from c° to c ) at the interface, an activation overpotential because the charge-transfer reaction is not at equilibrium. The total overpotential rj is the difference between the interfacial-potential difference... 

In this cell there are three significant interfacial potential differences (neglecting any small liquid junction potential difference) ... 

Use of a reference electrode to measure the electrode—electrolyte potential difference also introduces a new reference—solution interface, but this is designed so that the potential gradient at the new interface is constant regardless of whatever electrode process occurs isothermally at the working electrode. Changes in electrode potential are thus proportional to changes in the interfacial potential difference... 

The distinctive feature of electrochemical kinetics is the strong dependence of reaction rates on the interfacial potential difference. 

This current is conducted by electrons in a metal electrode, electrons and other charge carriers in a semiconductor, and by ions in the electrolyte. The conduction process provides an additional impediment, represented by the ohmic resistance Rn. Its effect is added to the interfacial potential difference, E, so that the total voltage will be... 

With any electrochemical technique to study kinetics, the electrode-solution interface is perturbed from its initial situation. The initial conditions may be such that the system is in a chemical equilibrium and this usually means that the interfacial potential difference is determined by Nernst s law holding for the two components O and R of a redox couple being present... 

As a result of the availability of charge carriers, all the potential difference between two electrodes is dropped across the two interphases for an electrolyte solution and not across the bulk solution phase. When a current passes across the solution, there is a possibility that a potential difference will develop due to the finite conductivity of the solution. In most electroanalytical experiments this is very small compared to the interfacial potential difference and always results in a comparatively weak electric field (small potential dropped across a large distance). This matter will be dealt with beginning in Chapter 6. 

The counter electrode in the two-electrode system serves two functions. First, it completes the circuit, allowing charge to flow through the cell, second, it is assumed to maintain a constant interfacial potential difference regardless of the current. These two functions are mutually exclusive only under very restricted circumstances. Both needs are better served by two separate electrodes ... 

Although the equilibrium principle was available (equality of electrochemical potential of each ion that reversibly equilibrates across an immiscible liquid/liquid interface), the elementary theory and consequences were not explored until recently (6). To develop an interfacial potential difference (pd) at a liquid interface, two ions M, X that partition are required. However,... 

The interfacial potential difference (pd) for the partition equilibrium interface is given by the equality of electrochemical potential in terms of all ions in equilibrium, equation (4). 

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