Surface inactive electrolyte

Tucceri R I and Posadas D 1990 The effect of surface charge on the surface conductance of silver in surface inactive electrolytes J. Electroanal. Chem. 283 159-66... 

The contact potential difference between Hg and water (actually a dilute aqueous solution of a surface-inactive electrolyte) has been measured42,43 to be -0.25 V. The negative sign means that the work function of Hg decreases upon contact with water. Since 4.50( 0.02) cV is the currently accepted5 value for 0 of Hg, the value of 0 for the uncharged metal (at the potential of zero charge) is 4.25 eV. 

For an electrochemical cell consisting of a metal at the potential of zero charge in a solution of surface-inactive electrolyte and a reference electrode (let us assume that any liquid junction potential can be neglected), the electrode potential is given by (cf. Eq. (20)]... 

Japaridze et al.m 323 have studied the interface between Hg and a number of vicinal and nonvicinal diols such as 1,2-, 1,3-, 2,3- and 1,4-butanediol (BD), ethanediol (ED), and 1,3-propanediol. KF and LiC104 were used as surface-inactive electrolytes. The potential of zero charge was measured by the capacitance method against an SCE in water without correction for the liquid junction potential at the solvent/H20 contact (such a potential drop is estimated to be in the range of 20 to 30 mV). The potential of the capacitance minimum was found to be independent of the electrolyte concentration while capacitance decreased with dilution. Therefore, Emin was taken to measure E . These values are reported in Table 4. 

The electrical double-layer structure at Ga/DMF, In(Ga)/DMF, and Tl(Ga)/DMF interfaces upon the addition of various amounts of NaC104 as a surface-inactive electrolyte has been investigated by differential capacitance, as well as by the streaming electrode method.358 The capacitance of all the systems was found to be independent of the ac frequency, v. The potential of the diffuse layer minimum was independent of... 

Cu crystallizes in the fee and its melting point is 1356 K. The experimental data for single-crystal Cu/H20 interfaces are also controversial. 567 570,572 57X The first studies with Cu(l 11), Cu(100), and Cu(l 10) in surface-inactive electrolyte solutions (NaF, Na2S04) show a capacitance minimum at E less negative than the positive limit of ideal polarizability of Cu electrodes (Table 11). depends on the method of surface... 

The impedance characteristics of pc-Pb have been obtained in aque-ous 220-221,599-607 an(j n0naqueous (glacial acetic acid, MeOH, EtOH, dimethyl formamide)10,74,608-612 surface-inactive electrolyte solutions. The first attempt to obtain the potential of zero charge of pc-Pb with a mechanically polished and remelted surface was made by Borissova et al.220,221 in 1948 and 1950. Pc-Pb anodically polished in H20 + NaF (0.001 < cNlF < 0.1 M) was studied by Rybalka and Leikis.599 The value of = -0.810 0.02 V(SCE) was found to be independent of c the... 

The electrical double layer at pc-Zn/fyO interfaces has been studied in many works,154 190 613-629 but the situation is somewhat ambiguous and complex. The polycrystalline Zn electrode was found to be ideally polarizable for sufficiently wide negative polarizations.622"627 With pc-Zn/H20, the value of Eg was found at -1.15 V (SCE)615 628 (Table 14). The values of nun are in reasonable agreement with the data of Caswell et al.623,624 Practically the same value of Eff was obtained by the scrape method in NaC104 + HjO solution (pH = 7.0).190 Later it was shown154,259,625,628 that the determination of Eo=0 by direct observation of Emin on C,E curves in dilute surface-inactive electrolyte solutions is not possible in the case of Zn because Zn belongs to the group of metals for which E -o is close to the reversible standard potential in aqueous solution. 

Therefore some indirect methods have been worked out to determine the value of ff=0.154,259 In particular (1) salting out of organic compounds from a surface-inactive electrolyte solution, (2) F"" for 1-pentanol or other organic compounds with a high attractive interaction constant a, and (3) dependence of the capacitance minimum on thiourea concentration. It should be noted that indirect estimates based on TU adsorption give... 

Zinc crystallizes in the hexagonal close-packed system its electronic structure is 4s2 and the melting point is 693 K. Since the zinc dissolution takes place at potentials very close to ffa0 the differential capacitance curves in the region of Ea=c in pure surface-inactive electrolyte solutions (KC1, pH = 3.7) can be determined directly for the Zn(llJO) face only... 

Studies in surface-inactive electrolyte solutions with various organic compounds (cyclohexanol, 1-pentanol, 2-butanol, camphor, tetra-buthyl ammonium ion, TBN+) show that the adsorption-desorption peak shifts to more negative potentials in the order (0001) < (1010) < (1120) this was explained by the increasing negative value of Eff=0 in the same direction.259 629-635... 

Vitanov and Popov et al.156 660-662 have studied Cd(0001) electrolyti-cally grown in a Teflon capillary in an aqueous surface-inactive electrolyte solution. The E is independent of ce) and v. The capacity dispersion is less than 5%, and the electrode resistance dispersion is less than 3%. The adsorption of halides increases in the order Cl" < Br" < I".661 A comparison with other electrodes shows an increase in adsorption in the sequence Cd(0001) < pc-Cd < Ag( 100) < Ag(l 11). A linear Parsons-Zobel plot with /pz = 1.09 has been found at a = 0. A slight dependence has been found for the Cit a curves on ce, ( 5%) in the entire region of a. Theoretical C, E curves have been calculated according to the GCSG model. 

First attempts to study the electrical double layer at A1 electrodes in aqueous and nonaqueous solutions were made in 1962-1965,182,747,748 but the results were not successful.190 The electrical double-layer structure at a renewed Al/nonaqueous solution of surface-inactive electrolytes such as (CH3)4NBF4) (CH3)4NC104, (CH3>4NPF6, and (C4H9)4NBF4, has been investigated by impedance.749-751 y-butyrolactone (y-BL), DMSO, and DMF have been used as solvents. In a wide region of E [-2.5 [Pg.128]

The first studies of the electrical double-layer structure at Sn + Pb and Sn + Cd solid drop electrodes in aqueous surface-inactive electrolyte solutions were carried out by Kukk and Piittsepp.808 Alloys with various contents of Pb (from 0.2 to 98%) were investigated by impedance.615,643,667,816 Small amounts of Pb caused dramatic shifts of toward more negative values. For alloys with Pb bulk content 0.2%, was the same as for pc-Pb. The was independent of Crf and frequency. C xt Cjl plots were linear, with/pz very close to unity. Thus the surface of Sn + Pb alloys behaves as if it were geometrically smooth, and Pb appears to be the surface-active component. 

Specific adsorption of ions changes the value of E, hence, one distinguishes the notion of a point of zero charge, in solutions of surface-inactive electrolytes, which depends on the metal, from that of a point of zero charge, in solutions of surface-active ions, which in addition depends on the nature and concentration of these ions. The difference between these quantities. 

The charge density on the electrode a(m) is mostly found from Eq. (4.2.24) or (4.2.26) or measured directly (see Section 4.4). The differential capacity of the compact layer Cc can be calculated from Eq. (4.3.1) for known values of C and Cd. It follows from experiments that the quantity Cc for surface inactive electrolytes is a function of the potential applied to the electrode, but is not a function of the concentration of the electrolyte. Thus, if the value of Cc is known for a single concentration, it can be used to calculate the total differential capacity C at an arbitrary concentration of the surface-inactive electrolyte and the calculated values can be compared with experiment. This comparison is a test of the validity of the diffuse layer theory. Figure 4.5 provides examples of theoretical and experimental capacity curves for the non-adsorbing electrolyte NaF. Even at a concentration of 0.916 mol dm-3, the Cd value is not sufficient to permit us to set C Cc. 

Fig. 4.4 The dependence of the potential difference in the diffuse layer on the difference E — Epzc (the rational potential) for various concentrations of the surface inactive electrolyte KF. (According to R. Parsons)... 

Fig. 4.7 Schematic dependence of the quantities y, y°, o °o, Ce — C and T on the electrode potential. The quantities with the superscript 0 refer to a surface-inactive electrolyte while those without a superscript refer...

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. 

The experimental data concerning capacitance of edl at the selected faces of Bi, Sb, and Cd single crystals in solutions of surface inactive electrolytes in water and organic solvent were analyzed in terms of various models [11]. From these data, it follows that the interface electrode/electrolyte properties depend hoth on the crystallographic and electronic characteristics of the metal and on the nature of the solvent. 

The surface or interfacial tension of a solution of long-chain surfactants, non-ionics or ionics in the presence of surface-inactive electrolyte with the same counterion, is given by the bulk concentration c via the fundamental Gibbs adsorption equation dy... 

Such information (supplementary to capacitance data) may be obtained from electrocapillary experiments. According to Eq. (3), the interfacial tension, y, passes the maximum at the p.z.c. as a function of the potential (at a fixed concentration). In dilute solutions this maximum value of y for the Hg-water interface is almost independent of the concentration of the surface-inactive electrolyte (see following text). The data for more concentrated solutions recalculated with the use of Eq. (5) show a linear dependence on the bulk concentration, c ... 

Equations (13)-(15) reproduce qualitatively the experimental findings for sufficiently concentrated solutions of surface-inactive electrolytes. In accordance with expectations, the capacitance values and their dependence on the interfacial potential are characteristic for each particular metal/solvent contact. One also observes a noticeable (sometimes, even strong) variation of the capacitance with the electrode charge. Equation (13) predicts a distorted (due to the dependence of A h on a) parabolic shape for y (E), with a maximum at p.z.c., see Eq. (3), also in conformity with experimental observations. 

The version of this theory, which is actually used in interpretation of all data for surface-inactive electrolytes, was proposed by Grahame [32). The total potential drop across the interface is written as a sum of the compact- and diffuse-layer contributions ... 

This distinction can be checked experimentally if the measurements are performed for different surface-inactive electrolytes but for the same electrode and solvent. According to the general theory [42-44] discussed in Sect. 2.1.7, the distance of the closest approach, which should be noticeably different for these electrolytes, has got almost no effect in the compact-layer capacitance if the ions do not penetrate into the region of the reduced dielectric response near the surface. This theoretical prediction turns out to be in conformity with experimental data [35, 37, 45, 46] for three mercury-aqueous solution interfaces for which the PZ plot at the p.z.c. gives practically identical values for the compact-layer capacitance, Ch(0) = 29gFcm-2 (Fig. 6). 

In accordance with the Stern-Grahame model of the EDL structure the values of C are determined by both the diffuse and compact-layer properties, the latter being dependent on the metal properties. However, in very dilute solutions of a surface-inactive electrolyte the dominant contribution to C near the p.z.c. (at the capacitance minimum) is given by the diffuse layer, C = Cgc(0, c). Therefore the ratio of capacitances in these conditions should be close to the RF for the surface of the solid metal M ... 

The difference between a (a) and Za (Fig. 14) leads to KhCc) plots that are in line with the data for simple metals (Figs. 15 and 16), (for review and references, see Ref. [88]) in contact with surface-inactive electrolytes the slope at the p.z.c. is positive, the minimum lies in the cathodic range and a land of a spike, which might be apprehended as a hump, takes place in the anodic range. 

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