Zero-Charge Potential Difference

From the surface tension measurements of the interface between the aqueous solution of LiCl and the nitrobenzene solution of TBATPB, the zero-charge potential difference was estimated as A cpp c 0.020 V [14] on the basis of the standard potential difference Ao tba+ = —0.248 V [37] for the reference tetrabutylammonium cation. If the corrected value Ptba+ = —0.275 V [21] is used instead, the zero-charge potential difference becomes Ao Pp2c= 0.007 V, which is obviously very close to zero. The surface tension data for the system of NaBr in water and tetraalkylammonium tetraphenylborate in nitrobenzene also indicate that A cpp c is zero [11,13]. 

The value of A cpp c can also be inferred from the capacitance data. For the MVN model [4] the Galvani potential difference can be written as  

The potential difference 0 corresponding to the minimum of the double-layer capacitance was found by a least square fit of the capacitance plot around the minimum to a third order polynomial [32]. The values of Po are summarized in Table 2, and it is seen that they are rather close to zero. A shght shift of A Po towards the more positive potential differences, which is pronounced most for the aqueous LiCl solutions, can be ascribed to the variation of the inner-layer capacitance Cj with the surface charge density. On this basis the conclusion was made, that for all the systems studied, the zero-charge potential difference is [32]. 

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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... 

FIG. 3 Inner-layer potential difference A excess surface charge Q for the interface between LiCl in water and tetrabutylammonium tetraphenylborate in nitrobenzene Ref. 38 (full line) and Ref. 36 (dotted line). 

Close to the zero-charge potential difference the effect of the ion penetration on the interfacial capacitance can be estimated by solving the linearized Poisson-Boltzmann equations in all three regions of the MVN model [41]. For the sake of simplicity, it was assumed [32] that only the ions from the organic solvent phase enter the inner layer, so that their concentrations differ from zero at X2organic solvent phase. In that case, the double-layer capacitance C is [32] ... 

Many studies at single-crystal electrodes of xp-metals were directed at the special features of double-layer stracture and the potential of zero charge at the various single-crystal faces. It was shown that rather large differences could exist between the potentials of zero charge of different faces (see Table 10.1). 

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]. 

At the potential of zero charge, the difference in electrostatic inner potential,. d c, across the electrode interface is related to the difference in outer potential, /lippK, between the free surface of metal electrode and the fi surface of aqueous solution as formulated in Eqn. 5-14 and shown in Fig. 5-14 ... 

A representative potential distribution across the interface is shown in Fig. 3.9(c), taking the potential of the bulk solution as zero. The potential difference across the space charge region (psd occurs over a larger distance than that of the Helmholtz layer (pn). For an n-type semiconductor, (psc results from the excess positive charge of ionized donors in the bulk of the space charge region within the... 

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]. 

The rapid flow of mercury from a capillary will assume the potential of zero charge, which is then simply measured. This method was proposed at the end of the 19th century by Helmholtz, and the first experiments were carried out by Ostwald [34]. Later, a number of investigators adapted this approach [35-39]. This method is in wide use today in the determination of the zero charge potentials of mercury electrodes in different solutions (see for instance [40]). 

Potentials of Zero Charge of Different Electrode Materials... 

In the determination of R by the Valette-Hamelin method [59] the inner layer capacity, Cinner, is calculated for the different values of the charge densities, cr, at constant electrolyte concentration, postulating that the Cinner vs. a curve has to be monotonically close to a zero charge potential value. This condition is obtained by adjusting the R value. 

The maximum PI in the current vs. potential curve correlates with the substrate surface transition. The difference in zero charge potentials between the reconstructed (px. 3) and the unreconstructed (1x1) gold surfaces gives rise to an additional current contribution due to double layer charging. Figure ID shows the ideal terminated (1x1) surface with atomic resolution. Around 0.80 V a disorder/order transition takes place within the layer of adsorbed (hydrogen-)sulfate ions [27-30]. The forma-tion/dissolution of the so-called /3 x. 7) overlayer correlates with the... 

The concepts of electrochemical and inner potential can be used to classify interfaces, as show in Table 3.4. If charged species cannot traverse an interface freely, the interface is called polarizable and the condition for equilibrium across the interface is that zero Galvani potential difference exists across it. If charged species can traverse the interface freely, it is called reversible (or nonpolarizable) and the condition for equilibrium across the interface is that zero electrochemical potential difference exists across it. Thus a polarizable interface is analogous to a capacitor and a... 

Koczorowski Z, Zagorska I (1983) Investigations on volta potentials in water-nitrobenzene systems and the surface potentials of these solvents. J Electroanal Chem 159 183-193 Koczorowski Z, Zagorska I (1985) On the surface and zero charge potentials at the wa-ter/nitrobenzene interface. J Electroanal Chem 190 257-260 Koczorowski Z, Zagorska I, Kalinska A (1989) Differences between surface potentials of water and some organic solvents. Electrochim Acta 34 1857-1862 Kreuter J (1983) Physicochemical characterization of polyacrilic nanoparticles. Int J Pharmaceut 14 43-58... 

Theoretical concepts of specific (per unit true electrode surface area) EDL capacitance are based on the known classical theories of EDL developed by Helmholtz, Stern, Gouy-Chapman, Grahame, and so on. One of the directions of modem smdies of EDL is elucidation of ratios between different surface layer characteristics. These include specific capacitances in zero charge points, electronic work functions of metals, their liophilicity, zero charge potentials. Correlations are established between many of these characteristics in a number of metals and solvents. At the same time, there are significant deviations from main trends. Zero charge points were first determined for different carbon materials in the works of Frumkin et al. 

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