Adsorption of dialkylethers of ethylene glycol (dialkoxyethane)at the free surface of water and at the Hg-water interface

ADSORPTION OF DIALKYLETHERS OF ETHYLENE GLYCOL (DIALKOXYETHANE) AT THE FREE SURFACE WATER AND AT THE Hg-WATER INTERFACE*

OF

A. DAGHETTI and S. TRASATTI Department of Physical Chemistry and Electrochemistry, University of Milan, Via Venezian 21, 20133 Milan, Italy I. ZAGORSKA and Z. KOCZOROWSKI Department of Chemistry, University of Warsaw, ul. L. Pasteura, 02-093 Warsaw, Poland (Receiued

23 February

1988)

Abstract-The adsorption of dimethoxy- and diethoxyethane at the Hg-water interface and the free surface of water has been investigated by means of interfacial and surface tension, and adsorption potential shift measurements. The interfacial capacitance at saturation coverage has also been determined. The experimental results do not give evidence of specific interaction with the metal surface. Adsorption is very similar for the two substances and can be described by a Frumkin isotherm with the interaction parameter corresponding to repulsion which weakens on going from the free surface of water to the metal electrode surface. The experimental data have been interpreted in terms of a common non-extended configuration for the two molecules. Comparative analysis with previous results for methoxy- and ethoxyethanol supporting the earlier conclusions has also been carried out.

EXPERIMENTAL

INTRODUCTION Any thermodynamic approach to the adsorption of neutral compounds at electrode surfaces provides parameters which can hardly be interpreted unambiguously on a physical basis. The main reason for this is that adsorption at solid-liquid interfaces is a solvent replacement reaction so that the parameters of the adsorption isotherm of the investigate substance also contain those pertaining to the solvent[l-51. Therefore, comparative analyses of the adsorption of the same substance at two (or more) interfaces[6,7], or of a number of homologous compounds at the same iriterfaceC8, 91, are expected to be more informative than the study of the behaviour of a single substance at a single interface. Within the frame of a programme concerning the comparison of the adsorption behaviour of organic substances at the Hg-water interface and at the free surface df water we have investigated aliphatic nitriles[lO], dinitriles[l l] and monoethers of ethylene glyco1[12]. This kind of analysis, pioneered by Frumkin[l3], has shown that the similarity of the behaviour at the two interfaces depends crucially on the molecular structure of the adsorbate and is very revealing as to the role played by the metal electrode surface. In this context, we have also investigated the adsorption of some diethers of ethylene glycol (dialkoxyethane), R-O-CH,-CH,-O-R, where R is a linear paraffinic chain.

*Presented at the 38th I.S.E. Meeting, 14-18 September 1987, Maastricht, Netherlands.

The following diethers have been studied: dimethoxyethane (DME) and diethoxyethane (DEE). The pure products as sold by EGA-Chemie were first percolated through a column filled with active charcoal, then distilled under vacuum, treated again with charcoal and finally redistilled. The treatment with active charcoal proved to be essential for DEE whose commercial purity was on19 97%. Coincidence between directly determined electrocapillary data and twice-integrated capacitance values was taken as a criterion of maximum purity to be achieved. The prepurified charcoal (Merck, catal. no. 2515) was activated by overnight calcination at 800 “C in a stream of nitrogen gas. Analytical grade Baker Na,SO, was used as the supporting electrolyte without further purification, 0.1 mol dm- s Na,SO, aqueous solutions containing the desired organic substance were made up volumetrically using doubly distilled water. Densities were measured by a pyknometer in order to convert molarities to mole fractions. In the range of concentrations explored a strictly constant conversion factor was found, whose value is 0.01864 for DME and 0.01799 for DEE. The interfacial tension at the Hg-solution interface was measured by means of a capillary electrometer of conventional design using a vertical uncoated glass capillary. The equipment and the experimental procedure have been described elsewhere[lO, 14, 151. The adsorption potential shift at the uncharged Hg electrode was measured directly using the streaming electrode technique[ 161. The surface tension at the free surface of the solution

1705

1706

A. DAGHETTI

was determined

by means of the maximum pressure in taken in the two laboratories were found to agree to within f 0.3 mN m- I, and generally better. Surface potential variations were measured by the dynamic condenser method[l8]. Details of the equipment and of the experimental procedure have been given previously[lO]. The average accuracy was estimated to be ca & 5 mV. Account was taken of the possible wear of the glass capillary[19] by recording a calibration curve with the blank solution every three to four runs with the organic substances. In the light of previous discussions[20, 211, no attempt was made to determine the activity coefficient of the adsorbing substance and of the supporting electrolyte which were assumed to be constant with the explored concentration range. All experiments were carried out at 25 k 0.1 “C in water thermostats. a bubble[17].

Data

et al.

provide some basis for a general qualitative discussion of the given system. Figure 2 shows the plot of the directly determined adsorption potential shifts as a function of the surface pressure. It is remarkable that the two substances follow a common line at the air-solution interface. This is a clear indication of surface thermodynamic and structural similarity. The result is at first surprising in view of the different molecul&r sizes, but in fact such an outcome has been observed with the monoethers[12] and in part with the nitriles[lO]. Some difference (although minor) is observed at the Hg-solution interface since the adsorption potential shift increases with surface pressure less for DME than for DEE. The surface pressure curve of Fig. 1 were differentiated using a computer programme first deriving the surface excess JY at the given mole fractions and then fitting the derived set of I- - Cpdata by non-linear analysis using Frumkin’s equation of regression

RESULTS The electrocapillary data were differentiated using a computer programme providing at integral values of the charge density (0) the electrode potential (E), the interfacial tension(r), Parsons’ function < = y + aE[22] and the surface pressure @ = to - 5, where &, is the value for the base solution. An Apple He personal computer was used for all calculations. The analysis of the effect of the electric field on the adsorption of the organic substances will be reported in future papers. Here, the behaviour at the free surface of water will be confronted with that at the uncharged surface of the Hg electrode (potential of the electrocapillary maximum where w = 0). The decrease in interfacial and surface tension upon adsorption of the diethers is shown in Fig. 1. The behaviour at the two interfaces does not differ very much for both substances. More specifically, the curves for the two interfaces tend to intersect each other. Since the limiting slope does not appear to depend on the interface, the above observation (ie different curvatures) is an indication of the different nature of the intermolecular interactions at the two interfaces. The difference in adsorbability can be quantified in terms of Traube’s rulc[17]. If the ratio of concentration is calculated at Q, = 3 mN m- 1 (to be comparable with other data in the literature), the Traube coefficient is found to be ca 2.5 at both interfaces. Table 1 reports the above value together with those for a number of other related organic compounds. Table 1 clearly shows that the so-called Traube rule is in fact almost an exception so great is the variety of observed coefficients. However, such an approach can always

-

0

0 A .

-

DEE

,,O

alcohols nitriles dinitriles alkoxyethanols

dialkoxyethanes

Hg-solution 3.0 2.7 2.4 3.9 2.5

A

0

.

f 4

.-

due to Fig. 1. surface pressure adsorption of dimethoxyethane (DME) and diethoxyethane (DEE) at the free surface of the solution (0, A) and at the uncharged Hg electrode-solution interface (0, A). c is the concentration in solution.

Table 1. Traube’s coefficient for various organic compounds Substance

a

.

Air-solution 2.8 2.6 1.7 3.1 2.5

Ref. C61 Cl01 iIll1

L121

this work

Adsorption

of dialkytethers of ethylene glycol

-

.

.

.

nhr-4 rn+ Fig. 2. Adsorption potential shift as a function of surface pressure for DME(A. A) and DEE(0, 0)at the uncharged Hg electrode-solution interface ( A, 0) and at the free surface of the solution (A, 0). state[23]: @=

-RTT,@n(l -0)

+a@].

(1)

This procedure provided the values of Tm, the saturation surface excess with adsorbate, and of a, the intermolecular interaction parameter. These parameters were then used to recalculate the theoretical @-log x curve and to test the Frumkin isotherm in the form: [0/( 1 - f?] exp( - 2aB) = /?x,

(2)

by plotting log [e/(1 -0)x] vs 8. The extrapolation to 0=0 of the straight line obtained by least square fitting gave the value of AGz* the standard Gibbs energy of adsorption at zero coverage. The question of the standard states for this quantity has been recently discussed[24]. Fitting of the calculated surface pressure curves to the experimental points also provided the values of AG:,. The adsorption parameters are summarized in Table 2. It is interesting that AG:, has the same value

1707

for DEE and butoxyethanol (BET)[12], whereas DME is somewhat more strongly adsorbed than ethoxyethanol (EET). The two pairs of substances have the same structural entities differently distributed in the molecule. The close behaviour of the symmetrical and unsymmetrical homolog is remarkable. This is true especially at the air-solution interface, while at the Hg-solution interface some appreciable difference can be detected. In all cases a negative value of a has been determined which formally indicates intermolecular repulsion. However, almost the same value is observed for the two substances at the air-solution interface, whereas lower different values can be seen at the Hg-solution interface. The experimental value for the area covered per molecule (as deduced from I-,,,) at surface saturation is compared in Table 2 with the values calculated by means of molecular models of the area projected by molecules in the extended configuration with the major axis either perpendicular (A,) or parallel (A,,) to the electrode surface. The comparison shows that- the actual configuration of both molecules is the same at both interfaces and corresponds neither to the perpendicular orientation nor to the completely flat one. The values for A,, are average between the minimum and the maximum values obtained considering the different thickness of the molecules as they are rotated by 90” around the major axis, a consequence of the structural “zigzag” of the aliphatic chains. Figures 3 and 4 show the fit of the calculated adsorption isotherms to the data points. In both cases adsorpiion increases faster at the Hg-solution interface clearly as a consequence of a lower intermolecular repulsion. Figure 5 shows the fit of the calculated surface pressure curves to the experimental data. Even though the thermodynamic analysis is somewhat flexible, the-fit is remarkably good especially considering the extension of the surface pressure range. Electrical effects are examined in Fig. 6 where the adsorption potential shifts are plotted as a function of the surface concentration (actually surface excess, but the bulk concentration is low enough to be neglected at the surface) of adsorbate. The first interesting observation is that the electrical contribution per molecule is strictly the same for DME and DEE at the air-solution interface, where the adsorption potential shift can be seen to vary non-linearly with coverage. It is thus proved once more that the linear behaviour is more the exception than the rule. Almost the same adsorption potential shifts are observed also at the Hg-solution interface where AE,=, appears to increase with different curvatures for DME and DEE. The adsorption potential shift at the air-solution

Table 2. Adsorption parameters AG:,/kJ mol Dimethoxyethane air*ln Hg-sln

1

a

r,/mo1

cm - 2

&,fnm2

All

A,

(DME) 16.1 IS.7

-0.89 -OS1

4.2 x lo-“’ 4.0x 10-10

0.40 0.42

0.58 * 0.05

0.21

Diethoxyethane (DEE) air-sin 20.7 Hg-sln 20.7

-0.91 -0.45

4.0x 10-10 4,0x lo-‘0

0.42 0.42

0.76 to.07

0.2 1

1708

A. DAGHETTI

et al.

Fig. 3. Adsorption isotherms for DME at the uncharged Hg electrodePsolution interface(W) and at the free surface of the solution (0). x is the mole fraction in solution. The solid lines have been calculated using the parameters in Table 2

DEE

2

Fig. 4. Adsorption isotherms for DEE at the uncharged Hg electrode-solution interface (I) and at the free surface of the solution (0). x is the mole fraction in solution. The solid lines have been calculated using the parameters in Table 2.

interface is fairly high being around 0.5 V. The value of AE, (ie AE,=, at saturation on Hg) was evaluated by using the method of Frumkin and Damaskin[25] knowing C, and omaxrthe value of the capacitance at saturation coverage and the charge of maximum adsorption, respectively. This estimation is based on the assumption that C, is potential independent. The values obtained for AE 1 are reported in Table 3. It is seen that the maximum adsorption potential shifts for DEE and DME differ by only 10 mV. The variation with coverage was calculated by means of the equation[25]: AE,=AE,elC(C,lC,)

(1

-@+Ql,

(3)

derived from the Frumkin-Damaskin approach[23]. Figure 6 shows that the fit to the experimental points is satisfactory.

Table 4 shows a comparison of the adsorption potential shift with those observed with the monoethers[12]. While the general behaviour is the same (in particular, the curves of the monoethers overlap quite nicely at the air-solution interface) the values are systematically higher for the diethers at both interfaces. The difference is presumably related to the lower values of C, and the higher values of I-,,, for the monoethers, both depressing the value of AE at intermediate coverages.

DISCUSSION The experimental picture seems reasonably clear to be interpreted on the basis of a well definite mode of adsorption of the molecules of diethers. Two par-

1709

Adsorption of dialkylethersof ethylene glycol

0.

0.

0. :

7 % 0.

0.

lo’“x r/In01 dm-3 Fig. 5. Adsorption of DME and DEE at the free surface of the solution. Examples of fit of theoretical curves (-), calculated using the parameters in Table 2, to the experimental points.

Fig. 6. Adsorption potential shift as a function of surface excess for DME (A, A) and DEE (0, 0) at the uncharged Hg electrode-solution interface (A, 0) and at the free surface of the solution (A, 0). The solid lines have been calculated by means of Equation (3) in the text with C, =20 ~Fcm-’ using the parameters in Table 3 and I-,,, from Table 2.

ameters are especially indicative: (i) the projected molecular area corresponds neither to perpendicular nor to paiallel orientation for the molecules in the extended configuration and its value is the same for the two substances at both interfaces; (ii) the adsorption potential shift is fairly high and the value per molecule is the same at the two interfaces for both molecules. Since a sloping orientation is unlikely in view of the symmetry of the molecular structure, the experimental data suggest that the ethers are in a rather “folded” configuration with the alkyl groups turned towards the exterior of the liquid phase. The major axis of the molecules is parallel to the interface and the normal component of the molecular dipole is essentially related to the partly ionic C-O bonds. Since two C-O groups are present, their effect is expected to be relatively strong as observed. The interfacial structure of the molecules is probably determined by the hydrophilic-hydrophobic interaction with the solvent. Thus, the hydrophobic alkyl groups are squeezed out of the solution while the CO groups which can interact

with the solvent by hydrogen bonds are immersed in the liquid phase. A sketch of the orientation is given in Fig. 7. The projected area of the molecules calculated using molecular models in the assumed orientation is ca 0.40-0.50 nm*, quite compatible with the experimental value. Such orientation entails some internal stress but results with dinitrilescl l] have shown that c&configurations of symmetrical molecules are favoured at interfaces. Since the functional groups are kept far from the interface by the paraffinic chains no appreciable effect of the metal surface is expected as in the case of aliphatic alcohols. Table 2 shows in fact that the Gibbs free energy of adsorption is the same at the two interfaces. It is remarkable that AGZ, at the free surface is almost the same as for the monoethers with the same number of carbon atoms[12]. This suggests that the interactions with the solvent are not very different for the two classes of molecules. However, the lower Traube coefficient for the diethers indicate that the addition of two methylene groups to a single paraffinic

Table 3. Adsorption parameters on Hg

DME EET DEE BET

8 6.5 7 5.5

- 5.9 -5.0 -5.0 -3.5

0.47 0.54 0.48 0.48

this work

Cl21

this work WI

A.

1710

DACHEITI

et al.

Fig. 7. Sketch of the proposed molecular conformation

Table 4. Dependence of adsorption potential on coverage AEIV 1-=1x10-‘0 air-sin DME EET

2x10-‘0

3x10-‘~molcn-~

0.30 0.25

0.43 0.36

DEE BET

0.17 0.14 0.17 0.15

0.31 0.25

0.42 0.35

Hgsln DME EET

0.05 0.04

0.11 0.09

0.21 0.18

DEE BET

0.05 0.05

0.13 0.12

0.24 0.22

chain has a different effect from the addition of one methylene group separately to two equal paraffinic chains. More specifically, DME is slightly more adsorbed at the air-solution interface than EET[12] and this may be related to the presence of the strongly hydrophilic OH group in the latter substance which keeps the molecule in the solution. However, DEE and BET[l2] show the same value of AGpOd.The effect of the OH group in BET may be compensated by the fact that the two ethyl substituents in DEE are folded and do not stretch out of the solution as in the case of a unsymmetrical molecular structure thus rendering the squeezing Out effect less efficient. The sign of the adsorption potential shift is positive and appreciably large. This is in agreement with a molecular orientation with the positive end of the dipoles towards the exterior of the solution. The fairly high value rules out that the solvent dipole may have any appreciable effect in determining sign and magnitude of AE. The interface appears to affect the dependence of the adsorption potential shift on coverage, but not the saturation coverage. The value of Ax1 at the free surface is 4&50 mV higher than AE,. This difference has been observed before with other substances[9, lo] and has been taken to indicate the difference in water orientation at the two interfacesC26-J. In this context, the data for DME and DEE suggest that the contribution by the molecular dipole is the same for the two interfaces, ie the orientation is the same.

of DME and DEE in the adsorbed state.

The charge of maximum adsorption is more negative for DME. If sign and magnitude of o,,, were determined only by orientation and magnitude of the molecular dipole as claimed[27], the more negative value for DME would indicate a stronger orientation. However, C, is higher for DME and the value of u,,, is determined not only by the polarity but also by the polarizability[28]. Thus, the more negative value of Q,,, for DME is due to the higher apparent “permittivity” of DME as measured by the value of C,. The intermolecular interaction parameter is usually of difficult physical interpretation for two reasons: (i) it is the results of several pair-wise interactions adsorbate-solvent, solvent(adsorbate-adsorbate, solvent interactions)[29]; (ii) especially at the metalsolution interface the adsorption isotherm at a=0 implies constancy in the electrical field as the charge is kept constant and this is not necessarily achieved with all substances[30]. Nevertheless, some quantitative comparison can be attempted. The negative value of a is usually taken to mean “particle-particle” repulsion. However, if all the pairwise interactions are taken into account, a negative value can also mean that the adsorbate tends to be sorrounded by solvent molecules at the interface rather than to cluster[29]. In the case of DME and DEE there may be some hydration of the molecule through the oxygen atoms but this alone cannot explain the large negative values -of a at the air-solution interface. Thus, most of the repulsion can be attributed to dipole-dipole lateral interaction. The same value of Ax,, and of a at the air-solution interface for DME and DEE is consistent with the above molecular picture. At the Hg-solution interface the value of a is still negative but lower. This is consistent with previous observations[lO-121 and can be understood in terms of different adsorbate-solvent and solvent-solvent interactions due to the different structure of the interfacial water at the Hg surface with respect to the free liquid surface. If the permittivity of the adsorption layer at sat&ation is assumed to be the same for DEE as for the DME (which is reliable in view of the close behaviour and structure) the ratio of C, should give an approximate value of the thickness ratio. According to molecular models, the thickness of a DME molecule oriented as shown in Fig. 7 is ca 0.6 nm. Accordingly,

Adsorption

of dialkylethers

that for a DEE molecule is ca 0.75 nm. The thickness ratio is thus 0.80 compared with 0.78 for the capacitance ratio at saturation thus corroborating the assumed molecular orientation. The rather high thickness of the adsorption layer is also inferred by comparing the value of C, for DME and DEE with that of substances known to adsorb flat on the electrode surface. Thus, the value of C, for glutaronitrile[ll] has been found to be 12.3 ~Fcm-’ and that for butanediol[31] 11.7 ~Fcm-*. Since the thickness of a flat paraffinic chain is about 0.40.5 nm, the ratios of C, are consistent with the difference in thickness if the permittivity does not vary substantially. Comparison with the behaviour of water on a more quantitative basis is less ambiguous because definite assumptions have to be made on double layer thickness. Thus, if the value of Co = 28 PF cm-’ at 0 = 0 is taken for water, a molecular thickness of about 0.3 nm would result in a molecular permittivity of about 10. In the case of DME and DEE the values of C, with the calculated molecular thickness converge to give a value of about 6 for the adsorbate permittivity. The ratio of interfacial permittivities is not expected to be far from that of the bulk dielectric constants, although a stronger decrease is expected for interfacial water in view of its associated nature leading to a more marked loss of dipole correlation effects at interfacesC26, 321. Thus also the analysis of capacitance leads to a consistent molecular picture.

of ethylene

glycol

1711

Acknowledgements-This work has been supported in part by the Italian Ministry of Education (40% Funds) and in part has been carried out within the frame of the project C.P.B.P. No. 01.15 (University of Warsaw).

REFERENCES 1. S. Trasatti, J. elecrroanaf. Chem. 53, 335 (1974). and K. Miller, 2. J. O’M. Bockris, M. A. V. Devanathan Proc. R. Sot. A274, 55 (1963). 3. D. H. Everett, Trans. Faraday Sot. 60, 1803 (1964); 61, 2478 (1965). 4. R. Guidelli, J. electroanal. Chem. 110, 205 (1980). 5. P. Nikitas, Electrochim. Acta 32, 205 (1987). 6. A. Frumkin and B. Damaskin, Pure Appl. Chem. 15, 263 (1967). and B. B. Damaskin, EIektrokhim7. N. S. Polyanovskaya iya 16, 531 (1980). 8. U. V. Palm, V. E. Past, Yu. I. Erlikh and T. E. Erlikh, Elektrokhimiya 9, 1399 (1973). A. A. Survila and L. E. Rybalka, 9. B. B. Damaskin, Elektrokhimiya 3, 146 (1967). Daghetti, S. Trasatti, I. Zagorska and Z. 10. A. Koczorowski, J. electroanal. Chem. 129, 253 (1981). and 11. S. Mingozzi, A. Daghetti, S. Trasatti, I. Zagorska Z. Koczorowski, J. them. Sot. Faraday Trans. I79,2801

(1983). Daghetti, 12. A. Koczorowski.

S.

Trasatti,

I.

Zagorska

J. them. Sot. Faraday

and

Z.

Trans. I 79. 2801

(1983). CONCLUSIONS DME and DEE adsorb at the Hg-solution interface and at the free surface of water in a non-extended configuration independent of the kind of interface and very similar to each other. The common behaviour is assumed to be due to the -OXH,-CH,-Omoiety lying parallel to the interface while the terminal alkyl substituents are turned towards the exterior of the solution phase. The substantially oriented C-O bonds contribute a quite appreciable normal dipole moment resulting in high values of adsorption potential shifts at both interfaces. The intermolecular interaction parameter is consistent with dipole-dipole lateral interactions equal for the two substances. The different values at the Hg-solution interface can be understood in terms of a different structure for interfacial water but the interpretation is hampered by the old problem of the constancy in the electrical variable. The orientation postulated for DME and DEE is consistent with the experimental values of C,, the capacitance at saturation coverage, showing that the adsorption layer thickness is higher than that for a flat molecule in the planar configuration. Comparison with previously obtained data for the monoethers of ethylene glycol shows that the substances with the same number of carbon atoms adsorb with comparable thermodynamic parameters although definite differences in the electrical parameters are observed. The comparative analysis ultimately confirms the conclusions drawn in the previous studies on the adsorption of monoethers.

13. A. N. Frumkin,

Ergebn. exakt Naturw. 7, 235 (1928); Colloid Symp. A. 7, 89 (1930). 14. A. De Battisti and S. Trasatti, J. electroanaL Chem. 48,

213 (1973). 15. S. Trasatti, J. electroanab Chem. 28, 257 (1970). 16. D. C. Graharne, E. M. Coffin, J. I. Cummings and M. A. Poth, J. Am. them. Sot. 74, 1207 (1952). Physical Chemistry of Surfaces Inter17. A. W. Adamson, science, New York (1967). 18. S. Mint, I. Zagorska and Z. Koczorowski, Rocz. Chem. 41, 1983 (1967). 19. A. De Battisti, R. Adamelli and S. Trasatti, J. Colloid Interface Sci. 63, 61 (1978). 20. A. De Battisti and S. Trasatti. J. electroanal. Chem. 54. 1 (1974). A. De Battisti and S. Trasatti, 21. B. A. Abd-El-Nabey, J. electroanal. Chem. 56, 101 (1974). 22. R. Parsons, Trans. Faraday Sot. 51, 1518 (1955). 23. A. N. Frumkin and B. B. Damaskin, in Modern Aspects of Hectrochemistry (Edited by J. O’M. Bockris and B. E. Conway), Vol. 3, p. 149, Butterworths, London (1964). M. Jurkiewin-Herbich and S. Trasatti. 24. J. Jastrzebska.

J. electroanal..C!hem.

216, 21 (1987).

25. B. B. Damaskin and A. N. Frumkin, 1. eIec@oanal. Chem. 34, 191 (1972). 26. S. Trasatti, in Modern Aspects of Electrochemistry (Edited by B. E. Conway and J. O’M. Bockris), Vol. 13, p. 81. Plenum Press, New York (1980). D. Schuhmann, E. Tronel-Peyroz and 27. H. Raous, P. Vane], J. electronnnl. Chem. 137, 393 (1982). S. Trasatti, J. electroanaL Chem. 91, 1 (1978). 22;: S. Trasatti, J. electroanal, Chem. 123, 121 (1981). C. Gatti and S. Trasatti, J. electroonal. 30. A. Daghetti, Chem. 196, 179 (1985). 31. F. Pulidori, G. Borghesani, R. Pedriali, A. De Battisti and S. Trasatti, J. them. Sot. Faraday Trans. I 74, 79 (1978). 32. L. I. Krishtalik, Elektrokhimiya 6, 1980 (1977).