Charging potential

Hydrolysis. The surfaces of metal oxides and hydroxides can take up or release or OH ions and become charged. Potentials as high as 100 mV may be sustained ia aqueous solutions. For aqueous solutions this is a function of the pH the zeta potential for the particle is positive if the solution pH is below the particle s isoelectric pH (pH ), and negative if the pH is above pH Isoelectric poiats for metal oxides are presented ia several pubheations (22,23). Reactions of hydroxyl groups at a surface, Q, with acid and base may be written as follows ... 

Stmctuie-activity studies in the seties cleady demonstrate the need for a polar, preferably positively charged potential... 

Poor distribution Implement appropriate procedures and training of solids or liquid charge. Potential for excessive reaction rates due to localized over-concentrations of reactants. CCPS G-22... 

The experiment is performed at various voltages and the barrier height values are corrected for the image charge potential by extrapolating to zero voltage. 

The end-of-charge potential of non-graphitic carbons, either hydrogen-containing carbons (Fig. 12) or cokes... 

The results of experimental capacitance studies at two plane model pc-Bi electrodes were in agreement with these conclusions.2 266 Thus it has been shown that the potential of the diffuse-layer capacitance minimum for a pc electrode does not correspond to the zero charge potential of the whole surface, i.e., Zfipj Oat E n-... 

A. Frumkin, Zero Charge Potentials, Nauka, Moscow, 1979. 

Partial Least Squares (PLS) regression (Section 35.7) is one of the more recent advances in QSAR which has led to the now widely accepted method of Comparative Molecular Field Analysis (CoMFA). This method makes use of local physicochemical properties such as charge, potential and steric fields that can be determined on a three-dimensional grid that is laid over the chemical stmctures. The determination of steric conformation, by means of X-ray crystallography or NMR spectroscopy, and the quantum mechanical calculation of charge and potential fields are now performed routinely on medium-sized molecules [10]. Modem optimization and prediction techniques such as neural networks (Chapter 44) also have found their way into QSAR. 

A detailed analysis of this behavior, as well as its analogy to the mercury-KF solution system, can be found in several papers [1-3,8,14]. The ions of both electrolytes, existing in the system of Scheme 13, are practically present only in one of the phases, respectively. This allows them to function as supporting electrolytes in both solvents. Hence, the above system is necessary to study electrical double layer structure, zero-charge potentials and the kinetics of ion and electron reactions at interface between immiscible electrolyte solutions. 

Alternatively, it has been found that the Galvani potential of zero charge, in the absence of specific adsorption, equals zero. This means that there is no specific orientation of the molecules of both solvents, and the dipolar part of the Galvani potential, Eq. (12), is zero [8,22,41]. The observed discrepancies between the results of various measurements in different ITIES systems have been mainly caused by the specific adsorption [8]. Recently, the analysis of thermodynamic and free charge potentials at ITIES was performed by Volkov [42]. 

The possibility of determination of the difference of surface potentials of solvents, see Scheme 18, among others, has been used for the investigation of Ajx between mutually saturated water and organic solvent namely nitrobenzene [57,58], nitroethane and 1,2-dichloroethane (DCE) [59], and isobutyl methyl ketone (IB) [69]. The results show a very strong influence of the added organic solvent on the surface potential of water, while the presence of water in the nonaqueous phase has practically no effect on its x potential. The information resulting from the surface potential measurements may also be used in the analysis of the interfacial structure of liquid-liquid interfaces and their dipole and zero-charge potentials [3,15,22]. 

Recently, Samec et al. [38] have investigated the same system by the video-image pendant drop method. Surface tension data from the two studies are compared in Fig. 2, where the potential scale from the study [36] was shifted so that the positions of the electrocapillary maxima coincide. The systematic difference in the surface tension data of ca. 3%, cf. the dotted line in Fig. 2, was ascribed to the inaccurate determination of the drop volume, which was calculated from the shape of the drop image and used further in the evaluation of the surface tension [38]. A point of interest is the inner-layer potential difference A (pj, which can be evaluated relative to the zero-charge potential difference A cpp c by using Eq. 

Girault and Schiffrin [6] and Samec et al. [39] used the pendant drop video-image method to measure the surface tension of the ideally polarized water-1,2-dichloroethane interface in the presence of KCl [6] or LiCl [39] in water and tetrabutylammonium tetraphenylborate in 1,2-dichloroethane. Electrocapillary curves of a shape resembling that for the water-nitrobenzene interface were obtained, but a detailed analysis of the surface tension data was not undertaken. An independent measurement of the zero-charge potential difference by the streaming-jet electrode technique [40] in the same system provided the value identical with the potential of the electrocapillary maximum. On the basis of the standard potential difference of —0.225 V for the tetrabutylammonium ion transfer, the zero-charge potential difference was estimated as equal to 8 10 mV [41]. 

FIG. 3 Inner-layer potential difference A"y), relative to the zero-charge potential difference... 

This potential is termed the zero-charge potential and is denoted as Epxc. In earlier usage, this potential was also called the potential of the electrocapillary zero this designation is not suitable, as Epzc is connected with the zero charge a(m) rather than the zero potential. 

The zero-charge potential is determined by a number of methods (see Section 4.4). A general procedure is the determination of the differential capacity minimum which, at low electrolyte concentration, coincides with Epzc (Section 4.3.1). With liquid metals (Hg, Ga, amalgams, metals in melts) Epzc is directly found from the electrocapillary curve. 

Table 4.1 Zero-charge potentials (vs. SCE). (According to A. N. Frumkin)...

Double integration with respect to EA yields the surface excess rB+ however, the calculation requires that the value of this excess be known, along with the value of the first differential 3TB+/3EA for a definite potential. This value can be found, for example, by measuring the interfacial tension, especially at the potential of the electrocapillary maximum. The surface excess is often found for solutions of the alkali metals on the basis of the assumption that, at potentials sufficiently more negative than the zero-charge potential, the electrode double layer has a diffuse character without specific adsorption of any component of the electrolyte. The theory of diffuse electrical double layer is then used to determine TB+ and dTB+/3EA (see Section 4.3.1). 

When the surface charge decreases to zero, the energy bands become horizontal. The corresponding flat-band potential A 0 is an analogy of the zero-charge potential Epzc (Fig. 4.12B). 

It is interesting that the experimentally measured zero-charge potential is practically identical with the value of A = 0, calculated using the TATB assumption (3.2.64). This fact helps to justify the use of this assumption. 

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