Surface potential method

The Volta potential is defined as the difference between the electrostatic outer potentials of two condensed phases in equilibrium. The measurement of this and related quantities is performed using a system of voltaic cells. This technique, which in some applications is called the surface potential method, is one of the oldest but still frequently used experimental methods for studying phenomena at electrified solid and hquid surfaces and interfaces. The difficulty with the method, which in fact is common to most electrochemical methods, is lack of molecular specificity. However, combined with modem surface-sensitive methods such as spectroscopy, it can provide important physicochemical information. Even without such complementary molecular information, the voltaic cell method is still the source of much basic electrochemical data. 

Using these results, we will see that it is possible to check certain electric behaviors peculiar to this device, with regards to the size of the electrodes and the information recorded by the surface potential method. 

Figure 5.35 Surface potential method for determining corrosion involves electrodes (a) and (b) connected through a voltmeter (V). Potential is measured between the two electrodes...

Fig. IV-6. Vibrating electrode method for measuring surface potentials. (From Ref 1.)...

Yan E C Y, Liu Y and Eisenthal K B 1998 New method for determination of surface potential of microscopic particles by second harmonic generation J. Phys. Chem. B 102 6331-6... 

Delchar T. Eberhagen A and Tompkins F C 1963 A static capacitor method for the measurement of the surface potential of gases on evaporated metal films J. Sci. Instrum. 40 105-7... 

Appearance potential methods all depend on detecting the threshold of ionization of a shallow core level and the fine structure near the threshold they differ only in the way in which detection is performed. In all of these methods the primary electron energy is ramped upward from near zero to whatever is appropriate for the sample material, while the primary current to the sample is kept constant. As the incident energy is increased, it passes through successive thresholds for ionization of core levels of atoms in the surface. An ionized core level, as discussed earlier, can recombine by emission either of a characteristic X-ray photon or of an Auger electron. 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

III. MODELS AND SURFACE POTENTIALS A. Methods for Computing Potentials... 

Fig. 16. Example of a A s.p. = f(t) relation, manifesting surface potential changes in a nickel-hydrogen system as a function of time and amount of hydrogen introduced onto a surface of a nickel film deposited at liquid nitrogen temperature hydrogen-nickel film interactions were studied by Tompkins-Eberhagen static condenser method at liquid nitrogen temperature. After Dus (60). Each dose of H2 — 2.5 X 10 molecules.

Potential methods of measurement for dilatation parameters are the damping of transverse and longitudinal surface waves and the damping of vibrating bubbles. For theory and measuring techniques see Wiistneck and Kretzschmar [47]. 

Amplitude equations and fluctuations during passivation, 279 Analytical formulae for microwave frequency effects, accuracy of, 464 Andersen on the open circuit scrape method for potential of zero charge, 39 Anisotropic surface potential and the potential of zero charge, (Heusler and Lang), 34... 

Guidelli, and methods for the determination of, 63 Heusler and Lang, anisotropic surface potential and, 34 and the impedance method, 35 importance, 5... 

Surface recombination, at semi conductors, 490 Surface reconstruction of gold, 83 and work of Kolb, 86 Surface tension and determination of the potential of zero charge, 32 Surface tension methods, and the potential of zero charge, 32 Surfaces,... 

The primary classical method of study of Langmuir monolayers is clearly that of recording H-A isotherms. Another classical method applied to the study of Langmuir monolayers is the measurement of surface potential [8,9], which is sensitive to changes in the orientation and density of the molecular dipoles of the monolayer. In addition, surface potential fluctuations were clearly observed in the coexistence region of palmitic acid [35]. 

In contrast to the ionizing electrode method, the dynamic condenser method is based on a well-understood theory and fulfills the condition of thermodynamic equilibrium. Its practical precision is limited by noise, stray capacitances, and variation of surface potential of the air-electrode surface, i.e., the vibrating plate. At present, the precision of the dynamic condenser method may be limited severely by the nature of the surfaces of the electrode and investigated system. In common use are adsorption-... 

The measurement of change in the surface potentials of aqueous solutions of electrolytes caused hy adsorption of ionophore (e.g., crown ether) monolayers seems to he a convenient and promising method to ascertain selectivity and the effective dipole moments of the ionophore-ion complexes created at the water surface. 

The impossibility of x being equal to about 1 V, as suggested by Kamieifki, " " has been demonstrated by Frumkin on the basis of a discussion of the real energies of hydration. Estimates from the variation in the solution surface potential with electrolyte molarity have yielded the value of +0.025 0.010 V.21 For methanol, the same method results in a value of -0.09 V.146 Later the authors of that investigation stated that both estimated values should be understood as the lower limits of surface potentials of water and methanol. "... 

The Vacuum Reference The first reference in the double-reference method enables the surface potential of the metal slab to be related to the vacuum scale. This relationship is determined by calculating the workfunction of the model metal/water/adsorbate interface, including a few layers of water molecules. The workfunction, — < ermi. is then used to calibrate the system Fermi level to an electrochemical reference electrode. It is convenient to choose the normal hydrogen electrode (NHE), as it has been experimentally and theoretically determined that the NHE potential is —4.8 V with respect to the free electron in a vacuum [Wagner, 1993]. We therefore apply the relationship... 

The potential that develops in an electrochemical system such as a fuel cell can also act to significantly influence the energies, kinetics, pathways, and reaction mechanisms. The double-reference potential DFT method [Cao et al., 2005] described earlier was used to follow the influence of an external surface potential on the reaction... 

Studies of the adsorption of surface active electrolytes at the oil-water interface provide a convenient method for testing electrical double layer theory and for determining the state of water and ions in the neighborhood of an interface. The change in the surface amount of the large ions modifies the surface charge density. For instance, the surface ionic area of 100 per ion corresponds to 16, /rC/cm. The measurement of the concentration dependence of the changes of surface potential were also applied to find the critical concentration of formation of the micellar solution [18]. 

What was evident in 1950 was that very few surface-sensitive experimental methods had been brought to bear on the question of chemisorption and catalysis at metal surfaces. However, at this meeting, Mignolet reported data for changes in work function, also referred to as surface potential, during gas adsorption with a distinction made between Van der Waals (physical) adsorption and chemisorption. In the former the work function decreased (a positive surface potential) whereas in the latter it increased (a negative surface potential), thus providing direct evidence for the electric double layer associated with the adsorbate. 

As a function of the surface potential the electron work function for a given material depends on the state of the surface of that material (adsorption, the presence of surface compounds, etc.). For crystalline substances (see Table 3.1), various crystal faces have various electron work function values, which can be measured for single crystals. For poly crystalline substances, the final value of the electron work function depends on the contribution of the individual crystal faces to the entire area of the phase and the corresponding electron work functions the final value of the work function, however, is strongly dependent on the experimental method used for the measurement. 

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