Potential zero charge

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. 

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. 

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. 

The potential E of metal electrodes at which the interfacial charge o is zero (hence, = 0) is the potential of zero charge (the zero charge potential), It foUows from Eqn. 5-12 that the potential, 4>pic, across the compact layer at the potential of zero charge is composed of M dip nd gs,dip as shown in Eqn. 5-13 ... 

Fig. 5-30. Potential profile across a compact layer estimated by calculations at various electrode potentials for a mercury electrode in a 03 M sodium chloride solution electrode potential changes fivm No. 1 (a cathodic potential) to No. 6 (an anodic potential), and contact adsorption of chloride ions takes place at anodic potentials. E = electrode potential = zero charge potential x = distance fix>m the interface. [From (3raham, 1947.]...

For pc-Zn/H20, the value of zero charge potential (Apzc) was found to be equal to about —1.15 V (versus saturated calomel electrode (SCE)) [10,11]. The same values of Epzc were obtained by a scrape method in a neutral NaCl04 aqueous solution [12]. 

The first attempt to determine zero charge potential for single-crystal sp metal was described for zinc [3, 6] from C-E capacitance curves in dilute solutions. Epzc for the face of Zn(OOOl) was about 80 mV more positive than that for Zn(lOlO). Later, it was pointed out [6, 10] that the determination of Epzc directly from C-E dependencies was not possible for zinc because the potential is close to the reversible standard potential of zinc in aqueous solution. 

In recent years, similar studies have been carried out for Cd single crystal electrode. Korotkov et al. [3] showed that the zero charge potential ( pzc) of a Cd(1120) in surface inactive electrolytes, NaF and Na2S04, was shifted slightly in the negative direction in comparison with Epzc of pc-Cd. 

Naneva and Popov et al. [4, 5] have studied Cd(OOOl) grown electrolytically in a Teflon capillary in NaF aqueous solution. A value of fpzc equal to —0.99 V (versus saturated calomel electrode (SCE)) was evaluated from minimum potential (Amin) on the differential capacity C-E curves obtained in dilute electrolyte. The zero charge potential was found to be practically independent of the crystallographic orientation. The Apzc and the irmer layer capacity of Cd(OOOl) single crystals were determined in KF solution as a function of temperature [5]. The positive values of AApzc/AT indicated that the water dipoles in the inner part of the double layer were orientated with their negative part to the electrode surface. It was found that the hydrophilicity of the electrodes was increasing in the order Cd(OOOl) < Ag(100)[Pg.768]

The influence of the single crystal face of the Pb electrode on zero charge potential has been analyzed. The difference between these potentials is within the range of 60 mV [8] and the potential value increases with lowering atomic density [11]. The increase of the inner layer capacitance at zero charge in the sequence Pb(lOO) < Pb(llO) < Pb(112) < polycrystalline Pb < Pb(lll) has been explained by increasing hydrophilicity of the surface [6,11]. 

Adsorption of sulfate species at pc-Au electrode has been studied [35] in HF—KF buffer of pFi = 2.8 applying Fourier transform infrared spectroscopy (FTIR). Adsorption of sulfate starts at 0.4 V versus Pd/H2 (which is about 0.28 V more positive than the zero charge potential). Adsorption reaches a maximum at 1.2 V. At any potential applied, a band between 1165 and 1193 cm was observed. It was ascribed to the adsorbed S04 . Adsorption... 

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