Volta potentials measurement

A relatively new arrangement for the study of the interfacial region is achieved by so-called emersed electrodes. This experimental technique developed by Hansen et al. consists of fully or partially removing the electrode from the solution at a constant electrical potential. This ex situ experiment (Fig. 9), usually called an emersion process, makes possible an analysis of an electrode in an ambient atmosphere or an ultrahigh vacuum (UHV). Research using modem surface analysis such as electron spectroscopy for chemical analysis (ESCA), electroreflectance, as well as surface resistance, electrical current, and in particular Volta potential measurements, have shown that the essential features (e.g., the charge on... 

The starting materisd is really nothing more than k(A) curves at different temperatures. Additional evidence from spectroscopy or Volta potential measurements is of a molecular, rather than thermodynamic nature, and rheological parameters are not characteristic for a system at rest (although it is possible to obtain certain... 

Volta potential measurements Electrical surface polarization. Film homogeneity arfd limited data on the orientation of the molecules or molecular groups. Sequestering of the dipolar contribution is often ambiguous. 

For charged monolayers additional Information can be obtained from Volta potentied measurements. The data In figs. 3.83 and 84 eire not so suitable for that because different buffers were used (phosphate for pH 6-9 and citrate for pH 3-7) which are known to adfect V " see ref. J. 

Besides these specific methods, the general arsenal of techniques described in sec. 3.7 remains available. So, optical and Volta potential measurements cure often invoked to obtain structural information on the monolayers. These techniques do not basically differ from the corresponding ones for Langmuir monolayers. However, surface rheology differs drastically because for Gibbs monolayers transport to and from the bulk is possible. Differences start to appear if the molecules are not very small and therefore diffuse with time scales comparable (or shorter) than those of the measurements. Therefore this theme will be devloped separately before surfactant monolayers are discussed, see sec. 4.5. 

In this section we introduce the development of electric double layers under the most elementary conditions, viz. those caused by simple electrolytes. By simple we understand containing ions without sizeable hydrophobic groups that would drive these ions towards the interface. This matter has already been discussed in secs. II.3.10f and g and in sec. 11.3.9 we looked at the polarization of the surface of pure water, as judged by Volta potential measurements. Here we shall briefly repeat the main findings, extending and updating them. 

Surface tension and Volta potential measurements of aqueous electroljde surfaces can be supplemented by electrophoresis studies. Although the technical problems are the same as for the water surface in its pristine state, the interpretation is slightly easier because of the swamping nature of the electrolyte. We... 

Case and Parsons used the volta potential measurements directly to measure real free energies of transfer of an ion from water to a given solvent, i.e. 

Fig. 5 Volta-potential, measured in the chain (I), as a function of NADH concentration. Medium 10 M 2-N-methylamino-l,4-naphtoquinone -1-10 M tris-HCl + 5 fig/ml chlorophyll [7]...

Volta-Potential Measurements on Polymer-Coated Electrodes... 

For the Volta-potential measurements, the coated electrodes are withdrawn from the electrolyte under potential control. Experimentally this is done in the device shown in Fig. 3.14. The electrode is mounted vertically in a holder that enables it to be moved down (immersion into the electrolyte for electrochemical equilibration) and up (removal from the electrolyte and positioning in front of the Kelvin vibrator). The Atl> measurements are conducted in an atmosphere of... 

FIGURE 3.17. Results of Volta-potential measurements on an emersed Nafion-coated electrode, is the Volta-potential difference (A r) between the Kelvin vibrator... 

Summarizing it is demonstrated that variation in the Donnan potential as a function of the electrolyte concentration is experimentally accessible as long as the electrolyte concentration remains below the ion exchanger s fixed-charge concentration. The Volta-potential measurements give in addition information on a surface dipole layer that forms on the membrane surface when it is removed from the electrolyte. 

The results of Volta-potential measurements on the emersed PMPy-coated electrodes are summarized in Fig. 3.22, which presents the results at various electrode potentials derived from the preceding measurements. The increasingly positive slope indicates that the polymer assumes the state of an anion exchanger as it is oxidized. In Fig. 3.23 the expected Donnan potentials were calculated with Eqn. 18 the polaron/bipolaron density, obtained by integrating the oxidation current in Fig. 3.21, was assumed to constitute the fixed-charge density c. At the electrolyte concentration = 0.1 M, the calculated and experimental values (see Fig. 3.22) were matched, and at lower concentrations, theoretical and experimental values were plotted. The agreement can be considered quite good. Based on this analysis. Fig. 

Fig. 6. Schematic illustration of an experimental realisation of the Volta potential measurement by a dynamic condenser method a - cell (XVII), b -cell (XVIII)...

In the absence of specific adsorption at the interface between immiscible electrolytes the potential drop occurs mainly in two diffuse layers contained in different phases. True enough, in the octane/water system there are practically no ions in the hydrocarbon phase in a concentration sufficient to screen the adsorption potential at the interface. Due to this fact, in investigating the processes in the electric double layer, it is possible to use the Volta potential measurements for determining the surface potential. The surface potential is known to consist of two terms ... 

If a hydrophobic electron acceptor is added to a nonaqueous phase and a reducing agent to water, then the Volta potential measured in the chain ... 

The adsorption of catalyst at the interface indicates that a reaction must occur there. As shown by the Volta potential measurements in the chain ... 

Fig. 7. Dependence of the Volta potential measured in chain (8) on FeEP concentration. The figure shows the results of 10 experiments obtained on different FeEP preparations. Medium octane/water, pH 6.5 [50]...

A quantity that is known for only a few of the relevant solvents is the surface potential at the vapor/liquid interface, Aj. This has been obtained from Volta potential measurements and reported in the publications of Trasatti and of Krishtalik et al. [15, 16] and more recently in those of Parfenyuk [17, 18]. The values are based on the A/ of water, +0.10V established by Parfenyuk [19], probably known to be no... 

In most of the equations of Sec. 7.4.3, the Volta potential difference A f is related to the electrode or corrosion potential in a linear manner however, a calibration constant is needed in order to calculate the electrode potential from a Volta potential measure-... 

Table 7.5 Compositions and Volta potentials measured by SKPFM (at 40% RH and at 20°C) at points 1-4 in Fig. 7.3.

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