Adsorption of C-5 Nitriles at Liquid Metal Electrodes. A Comparison of

Adsorption of C-5 Nitriles at Liquid Metal Electrodes. A. Comparison of Adsorption Parameters for Isovaleronitrile at Polarized Surfaces of Mercury an...
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Adsorption of C-5 Nitriles at Liquid Metal Electrodes. A Comparison of Adsorption Parameters for Isovaleronitrile at Polarized Surfaces of Mercury and Indium-Gallium Alloy (Eutectic Composition) L. M. Doubova,*,† A. De Battisti,‡ and W. R. Fawcett*,§ IENI-CNR, Corso Stati Uniti 4, 35127 Padua, Italy, Department of Chemistry, University of Ferrara, Via Borsari 46, 44100 Ferrara, Italy, and Department of Chemistry, University of California, Davis, California 95616 Received April 15, 2003. In Final Form: July 28, 2003 In this paper a quantitative study of the adsorption of iso-C4H9CN (isovaleronitrile, IVN) at the interfaces between Hg and In-Ga alloy (eutectic composition) electrodes and aqueous solutions is described. The equilibrium charge densities at Hg electrodes were determined from both electrocapillary and impedance measurements, whereas at the In-Ga electrode only impedance data were available. The adsorption parameters for IVN, including the standard Gibbs energy of adsorption (∆Gads°), the lateral interaction parameter a, and the limiting Gibbs surface excess (Γmax), were calculated and compared with previous data relative to the behavior of aliphatic compounds at different interfaces. The experimental results are discussed in terms of metal-solvent and metal-adsorbate interactions.

Introduction The dependence of adsorption parameters on electrode nature has been discussed1-3 with respect to the structure of the compact layer and the role of the potential of zero charge(pzc).4,5 Theinfluenceofelectrodesurfacestructure6-8 has also been considered. Liquid metal systems, which have renewable surfaces, represent a considerable advantage with respect to solid surfaces.9-11 According to present views of compact layer structure, the state of the solvent, and water in particular, is influenced by the nature of the electrode metal and by its structural features. In fact, literature data show that the nature and atomic structure of the electrode surface influence the adsorption parameters.2,7,8,12 A reason for this can be found in the interaction between the polar groups of the adsorbate and * To whom correspondence should be addressed. E-mails: [email protected]; [email protected]. † IENI-CNR. ‡ University of Ferrara. § University of California, Davis. (1) Damaskin, B. B.; Petrii, O. A.; Batrakov, V. V. Adsorption of Organic Compounds at Electrodes; Plenum Press: New York, London, 1971. (2) (a) Trasatti, S. In Modem Aspects of Electrochemistry; Bockris, J. O’M., Conway, B. E., Eds.; Plenum Press: New York, London, 1979; Vol. 13. (b) Trasatti, S. In Modem Aspects of Electrochemistry; Bockris, J. O’M., Conway, B. E., Eds.; Plenum Press: New York, London, 1999; Vol. 33, pp 1-215. (3) Guidelli, R. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P., Eds.; VCH: Weinheim, Germany, 1992. (4) Frumkin, A. N. Potentials of Zero Charge; Nauka: Moscow, 1979. (5) Frumkin, A. N.; Petrii, O. A.; Damaskin, B. B. In Comprehensive Treatise of Electrochemistry; Bockris, J. O’M., Conway, B. E., Yeager, E., Eds.; Plenum Press: New York, London, 1980; Vol. 1. (6) Hamelin, A.; Vitanov, T.; Sevastianov, E.; Popov, A. J. Electroanal. Chem. 1983, 145, 225. (7) Trasatti, S.; Doubova, L. M. J. Chem. Soc., Faraday Trans. 1995, 91, 3311. (8) Lipkowski, J.; Nguyen Van Huong, C.; Hinnen, C.; Chevalet, J.; Parsons, R. J. Electroanal. Chem. 1983, 143, 375. (9) Poiyanovskaya, N. S.; Damaskin, B. B. Elektrokhimia 1981, 17, 98. (10) Doubova, L. M. Ph.D. Thesis, Soviet Academy of Sciences, Moscow, 1978. (11) van Venrooij, T. G. J.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1999, 462, 111.

the metal surface. The simultaneous occurrence of solvent-metal interactions, implicit in the idea of the hydrophilicity of the metal surface, does not allow clearcut trends to be observed. For example, the lateral interaction parameter a of the Frumkin isotherm11,12 should depend on the electrode metal and its structure, through their influence on the state of the solvent and of the adsorbate. The nature of the electrode metal should also affect the value of the charge and potential of maximum adsorption. The aim of the present work is to find correlations between the parameters of molecular adsorption and the nature of the electrode surface. An aliphatic nitrile was chosen as a model adsorbate for several reasons. First, the degree of interaction between the -CtN functional group and the surface of sp metals is not strong enough to cause significant adsorbate reorientation at low electrode charge densities, as shown by data for acetonitrile solutions.2 On the other hand, an increase in the hydrocarbon chain length (four carbon atoms in the present case) should hinder reorientation of the adsorbate, since the polar CtN groups are likely to be in contact with the solution, away from the electrode surface. The coupling of these two features of the structure of the adsorbate should minimize the effects of changes in its orientation at different metal surfaces, which is observed for the aliphatic alcohols.13-16 Furthermore, the behavior of aliphatic nitriles has been studied and general trends observed at the Hg/solution17-20 and air/solution (12) Lipkowski, J.; Stolberg, L. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P., Eds.; VCH: Weinheim, Germany, 1992. (13) Rybalka, L. E.; Damaskin, B. B.; Leikis, D. I. Elektrokhimia 1973, 9, 414. (14) Rybalka; L. E.; Damaskin, B. B.; Leikis, D. I. Eiektrokhimia 1975, 11, 9. (15) Ipatov, Yu. P.; Batrakov, V. V. Eiektrokhimia 1975, 11, 1282 (16) Ipatov, Yu. P.; Batrakov, V. V. Eiektrokhimia 1975, 11, 1717. (17) De Battisti, A.; Trasatti, S. J. Electroanal. Chem. 1973, 48, 213. (18) Amadelli, R.; Daghetti, A.; Vergano, L.; De Battisti, A.; Trasatti, S. J. Electroanal. Chem. 1979, 100, 379. (19) Doubova, L. M. Doctoral Thesis , University of Ferrara, Italy, 1983.

10.1021/la0346447 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003

C-5 Nitrile Adsoption at Liquid Metal Electrodes

interfaces.21 In addition, the adsorption of benzonitrile, an aromatic nitrile, has been investigated on mercury and gold electrodes, on nickel and palladium films by a variety of methods.23-26 In these studies it was established that adsorbed molecules are coordinated to the metal surface either through the π-bond system of the aromatic ring or through the polar CN group. Finally, a C-5 compound is preferred to the better known lower homologues, because of its higher adsorbability. The latter is important when an electrode like In-Ga is used because standard Gibbs energies of adsorption are expected to be less negative than those at the Hg/solution interface. By restricting the study to liquid metals, the focus is on its chemical properties rather than its structure. In fact, water structure at the surfaces of Hg and In-Ga alloy (eutectic composition (ec)) are significantly different. As a result, the extent of organic adsorption should also be significantly different. The In-Ga electrode (16.5% In atoms) has been studied in both aqueous27,28 and nonaqueous solutions.10,29,30 According to earlier literature,19,20 the surface of the alloy in the range of ideal polarizability shows a behavior quite similar to pure In, due to the very high surface activity of In in the liquid binary system (mp 15.73 °C), which ensures a coverage θ )1 by In, for an In content much lower than that typical of the eutectic composition. Recent studies31 of the kinetics of the Zn(II) reduction from aqueous 1 M NaClO4 solutions at dropping In-Ga electrodes of various compositions have demonstrated a strong dependence on In content. In summary, it is clear from recent work11,32 that the In-Ga alloy electrode used in the present studies may be regarded as essentially an indium electrode because of its surface composition. Experimental Section The analysis that follows is based on capacity data at Hg and In-Ga electrodes. The differential capacitance was obtained with an impedance bridge described elsewhere.36 Both Hg and In-Ga (20) Abd-El-Nabey B. A.; Trasatti, S. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1230. (21) Pulidori, E.; Borghesani, G.; Pedriali, R.; De Battisti, A.; Trasatti, S. J. Chem. Soc., Faraday Trans. 1 1978, 74, 79. (22) Borghesani, G.; De Battisti, A.; Pedriali, R.; Pulidori, E. Ann. Chim. (Rome) 1984, 74, 269. (23) Blomgren, E.; Bockris, J. O.’M.; Jesch, J. J. Phys. Chem. 1961, 65, 2000. (24) Richer, J.; Iannelli, A.; Lipkowski, J. J. Electroanal. Chem. 1992, 324, 339. (25) Kishi, K.; Okimo, Y.; Fujimoto, Y. Surf. Sci. 1986, 176, 23. (26) Nakayama, T.; Inamura, K.; Inone, Y.; Ikeda, S.; Kisci, K. Surf. Sci. 1987, 179, 47. (27) Doubova, L. M.; Bagotskaya, I. A. Elektrokhimia 1978, 14, 580. (28) Grigor’ev, N. B.; Kalyuzhnaya, A. M. Elektrokhimia 1974, 10, 149. (29) Emets, V. V.; Damaskin, B. B.; Kazarinov, V. E. Russ. J. Elektrochem. 1999, 35, 1356. (30) Emets, V. V.; Damaskin, B. B.; Bagotskaya, I. A. Russ. J. Elektrochem. 2001, 37, 1038. (31) van Venrooij, T. G. J.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Electroanal. Chem. 1999, 472, 53-63. (32) Emets, V. V.; Mishchuk, V. Ya.; Damaskin, B. B.; Elkin, V. V.; Grafov, B. M. Russ. J. Electrochem. 2001, 37, 1274. (33) Griqor’ev, N. B.; Kalyuzhnaya, A. M.; Bagotskaya, I. A. Elektrokhimia 1975, 11, 1574. (34) Griqor’ev, N. B.; Kalyuzhnaya, A. M.; Bagotskaya, I. A. Elektrokhimia 1976, 12, 418. (35) Emets, V. V.; Damaskin, B. B.; Kazarinov, V. E. Russ. J. Electrochem. 1999, 35, 505. (36) Borkowska, Z.; Fawcett, W. R. Can. J. Chem. 1981, 59, 710. (37) De Battisti, A.; Doubova, L. M.; Fawcett, W. R. Extended Abstract of the 34th ISE Meeting, Erlangen, Germany, Sept. 18-23, 1983; No. 0902. (38) Damaskin, B. B.; Frumkin, A. N.; Survila, A. A. J. Electroanal. Chem. 1968, 16, 493. (39) Palm, U. V.; Pyarnoya, M. P. Elektrokhimia 1978, 14, 1070. (40) Paltusova, P. A.; Alumaa, A. R.; Pal’m, U. V. Elektrokhimia 1981, 18, 475.

Langmuir, Vol. 19, No. 22, 2003 9277 alloy dropping electrodes were used. Mercury was purified as previously described.36 In (99.997%, Fluka) and Ga (99.9998%, Fluka) were used for the In-Ga eutectic alloy preparation. They were melted under a slightly alkaline aqueous solution with gentle heating. The alloy was also stored under a slightly alkaline solution. The capillaries used for the In-Ca dropping electrode (DIGE) had a bore in the range from 90 to 110 µm. The capillary was fed by a reservoir containing a volume of about 25 mL of liquid alloy. Above the latter, a pressure of 1.5-2.0 kg/cm2 was maintained during the measurements using pure nitrogen in order to ensure the regular behavior of the dropping electrode. The capillaries used were not silanized. Further details regarding the DIGE are available in the literature.18 The capillary used for the DME was also unsilanized. The height of the Hg column above it was 1.80 m. The performance of the capillary was very stable, and the reproducibility of drop times was maintained up to electrode potentials of - 1.85 V (SCE). The potential of zero charge (pzc) was also measured, for both electrodes using the streaming electrode technique. The iso-valeronitrile, (Eastman Kodak), was purified by distillation under an argon atmosphere. Triply distilled water was used for the preparation of the solutions. 0.15 M Na2SO4 (Fisher cert. ACS) was used as supporting electrolyte in all the measurements without further purification.

Results The double-layer capacity measured at a frequency of 1 kHz, is shown in Figure 1a for the DME in contact with solutions containing different IVN concentrations. The potential range explored for the DME was from +0.25 to -1.85 V (against the SCE). The capacity against potential curves merge with the blank, both at negative and positive potentials. The dependence of the differential capacity on frequency (ν) in the region of the adsorption-desorption peaks required extrapolation of plots of the capacity against the square root of the frequency to ν ) 0, to obtain the equilibrium capacity. This extrapolation procedure is well-known in the analysis of capacity data obtained for the adsorption of organic compounds at the mercury/ solution interface which show frequency dispersion.1 Its applicability to systems such as the nitriles was discussed in more detail recently.51 Figure 1b shows that the results for the DIGE are similar to those for the DME shown in Figure 1a. In the presence of IVN the surface of the InGa eutectic alloy undergoes some anodic process at the pzc, and at potentials slightly more negative than it. It has been shown recently11 that this process depends on both the solution pH and In content in the alloys. For this reason further analysis of the capacity curves at the DIGE in the presence of IVN could only be carried out by backintegration using the pzc determined at the DIGE in the absence of IVN as a reference potential at far negative potentials where the adsorption of IVN was absent. A comparison of capacity values for the two electrodes in the region of the minimum shows a substantial similarity, especially for higher values of cIVN for which large coverages are attained on both electrodes. Thus, conditions (41) Vitanov, T.; Popov, A. Elektrokhimia 1974, 10, 1373. (42) Vitanov, T.; Popov, A. Elektrokhimia 1976, 12, 319. (43) Grigor’ev, N. B.; Bagotskaya, I. A. Elektrokhimia 1966, 2, 1449. (44) Damaskin, B. B.; Survila, A. A.; Rybalka, K. Elektrokhimia 1967, 3, 146. (45) Borghesani, G. Electrokhim. Acta 1983, 28, 483. (46) Trasatti, S. J. Electroanal. Chem. 1974, 53, 335. (47) Damaskin, B. B.; Frumkin, A. N. J. Electroanal. Chem. 1972, 34, 191. (48) Doubova, L. M.; De Battisti, A.; Daolio, S.; Trasatti, S. J. Electroanal. Chem. 2001, 500, 134. (49) Monelli, M. R.; Foresti, M. L. J. Electroanal. Chem. 1993, 359, 293. (50) Trasatti, S. J. Electroanal. Chem. 1978, 91, 293. (51) Doubova, L. M.; Trasatti, S. J. Electroanal. Chem. 2003, 550551, 33.

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Figure 1. Capacity against potential curves in 0.15 mol dm-3 Na2SO4 aqueous solutions containing isovaleronitrile at different concentrations: (a, top left) at Hg, (1) 0, (2) 0038, (3) 0.0102, (4) 0.0118, (5) 0.0171, (6) 0.0258, (7) 0.0334, and (8) 0.0596 mol dm-3; (b, top right) at In-Ga, (1) 0, (2) 0.0038, (3) 0.0171, (4) 0.019, (5) 0.0258, (6) 0.0334, (7) 0.0437, (8) 0.0596, and (9) 0.0759 mol dm-3; (c, bottom) Hg and In-Ga electrodes in contact with 0.15 mol dm-3 Na2SO4 solution and containing the same concentration of IVN (0.0759 M). Insert: In-Ga in supporting electrolytes, (1) 0.15 mol dm-3 Na2SO4 (this work) and (2) 1 mol dm-3 NaClO4 solution from ref 11.

are reached for both systems for which complete coverage of the interface by the adsorbate is obtained. The experimental differential capacity data for the blank and one IVN solution are shown in more detail for the two electrodes in Figure 1c. The agreement between the capacity data at far-negative potentials is excellent. In the absence of IVN, the capacity at indium-gallium is higher than that at Hg in the vicinity of -1 V. The other striking feature is that the adsorption-desorption peak at indium-gallium occurs at more negative potentials. Of course, anodic adsorption-desorption peaks are not observed at the DIGE because of the nonpolarizability of the electrode in this potential region. Capacity data for the DIGE in 0.15 M Na2SO4 and 1 M NaClO4 are also

compared in Figure 1c (see insert). The small differences are attributed to differences in electrolyte activity. Similar results were obtained earlier for the adsorption of aliphatic alcohols at indium-gallium.28,32 The peak potentials observed at the two electrodes at negative potentials are plotted against the logarithm of the concentration of IVN in Figure 2. The data can be fitted to straight lines which are separated by 0.170 V for cIVN ) 0.05 M. Data for the pzc at the two electrodes are plotted in a similar way in Figure 3. The difference between the pzc’s for the two electrodes is much greater, being equal to 0.682 V for the same IVN concentration (0.05 M). The pzc data obtained in the absence of IVN allow one to estimate the peak potentials on the rational potential scale,

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Figure 2. Dependence of peak potential on the logarithm of the concentration of IVN: (1) Hg (b); (2) In-Ga ([).

Figure 3. Plot of the potential of zero charge against the logarithm of the IVN concentration: (1) Hg (b); (2) In-Ga ([).

which is defined as

φm peak ) Epeak - Epzc(cIVN ) 0)

(1)

m are -0.749 V at Hg and The corresponding values of φpeak -0.395 V at In-Ga for cIVN ) 0.05 M. A comparison of capacity values for the two systems in the region of the capacity minimum shows that they are substantially the same. This demonstrates that one of the prerequisites for IVN, namely, that full coverage be reached at higher IVN concentrations, has been fulfilled. The results of integrating the capacity curves are reported in Figure 4a,b. The charge density against potential curves for the Hg electrode in the presence of different IVN concentrations (Figure 4a) exhibit a common point of intersection. According to the theory of organic adsorption, the coordinates of the intersection point correspond to the charge σmax and potential Emax of maximum adsorption of the given adsorbate, repectively. For the Hg/solution interface, σmax occurs at -3.50 µC/ cm2 and Emax at -0.560 V. It is also apparent that the potential-charge density curves do not coincide at farnegative potentials. This is attributed to a corresponding change in the electrolyte activity with an increase in IVN concentration. The electrode charge density for the indium-gallium electrode was obtained by back-integration of the capacity data from a far-negative potential using the corresponding

Figure 4. Charge-potential curves obtained by integration of the capacitance curves in Figure 1a,b for two liquid electrodes in 0.15 mol dm-3 Na2SO4 + different concentrations of IVN: (a, top) Hg, (1) 0, (2) 0038, (3) 0.0102, (4) 0.0118, (5) 0.0171, (6) 0.0258, (7) 0.0334, and (8) 0.0596 mol dm-3; (b, bottom) In-Ga, (1) 0, (2) 0.0258, (3) 0.0334, (4) 0.0437, (5) 0.0596, and (6) 0.0759 mol dm-3.

charge density from the In-Ga data obtained in the absence of IVN as the integration constant. The charge density-potential plots also reach an intersection point for this system (see Figure 4b), but at more negative values of the charge density and potential. For the In-Ga interface, the value for σmax ) -5.75 µC/cm2 and that for Emax ) -1.088 V. The potential of maximum adsorption of IVN deserves some comment. On the rational scale, the value for the Hg system is -0.124 V and for the In-Ga system -0.128 V. In fact, if one estimates the potential drop across the inner layer at the point of maximum adsorption, it is the same in the two systems to within experimental error (-0.108 V). This is a surprising result because the extent of interaction of the two electrodes with water molecules is quite different. Thus, this result is attributed to a fortuitous cancellation of effects when IVN replaces water in the inner layer. Analysis of the data to estimate the extent of surface coverage was based directly on the capacity results at both electrodes. Initially, the reciprocal of the differential capacity 1/C at the potential E ) Emax was plotted against 1/cIVN, the reciprocal of the IVN concentration (Figure 5). Good linearity of these plots was observed at the higher

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Figure 5. Reciprocal of the experimental capacitance for InGa at -1.088 V/E against the reciprocal of the IVN concentration in 0.15 mol dm-3 Na2SO4.

values of cIVN. The capacity values obtained from extrapolation to 1/cIVN ) 0 gives the value of the capacity at full surface coverage, that is, the value when θA ) 1, where θA represents the coverage of the electrode surface by an adsorbate A. The above-mentioned extrapolation procedure has been frequently used in studies of organic adsorption1 and was applied successfully in the specific case of the nitriles.20 The results for the minimum values of C, obtained by this method, were C1 ) 7.5 µF cm-2 for the Hg/solution interface and C1 ) 8.0 µF cm-2 for the In-Ga/solution interface. Thus, the nature of the electrode surface does not significantly influence the structure of the adsorbate layer. Both in the case of the DME and the DIGE, satisfactory linear portions of the charge density against potential plots can be found around the point of maximum adsorption for the highest IVN concentrations (Figure 4a,b). This allowed a determination of EN, the adsorption potential shift at the pzc for the two interfaces. The two values are EN ) 0.360 V for the Hg/solution interface and EN ) 0.530 V for the In-Ga/solution interface. The uncertainty of these results amounts to (10 mV and is due to the features of the extrapolation procedure. The difference between the results at the two surfaces is much higher than that observed for the aliphatic alcohols at the same metals.28 If one assumes that the interfacial behavior of IVN should be similar to that of butyronitrile (BN),18 and pivalonitrile (PN),19,20 then the parallel capacitors model should hold for IVN, provided (E - Emax) is not too negative. Moreover, the fact that C0/C1 for IVN at the Hg/solution interface is larger than that for PN and BN further supports the applicability of the above-mentioned model37 to the present case. In this light, the surface coverage by IVN, θ, can be estimated using the formula1

θ ) (C0 - C)/(C0 - C1)

(2)

Here C0 represents the differential capacity of the electrode in the absence of specific adsorption. The θ values obtained by means of eq 2 are plotted in Figure 6 at the potential E ) Emax. The concentration coordinate has been normalized to the concentration of IVN required to reach halfcoverage for these conditions, that is, (cIVN)θ)0.5. The characteristic sigmoidal shape is a feature of both isotherms shown in Figure 6. To assess better the nature of the isotherm appropriate to the present system and to calculate the standard free energies of adsorption at the two interfaces for maximum adsorption, and the lateral

Figure 6. Surface coverage θ plotted against the concentration of IVN relative to that at half-coverage for IVN adsorption on liquid electrodes: (1) In-Ga (O); (2) Hg (4).

Figure 7. Fitting of the experimental data to the Frumkin isotherm for the two interfaces investigated at Emax: (1) Hg this work (O); (2) data on Hg from refs 19 and 22 (4); (3) In-Ga (b).

interaction parameter a, the function log[θ/(1 - θ)cIVN] was plotted against θ, in Figure 7. According to the Frumkin isotherm, such a plot should be linear with the interaction parameter obtained from the slope and the standard Gibbs energy of adsorption, from the intercept. Good linearity of the plots was observed; the values of the lateral interaction parameter and adsorption Gibbs energy for the Hg electrode were a ) 1.38 and ∆Gads° ) - 17.30 kJ mol-1; in the case of In-Ga the results were a )1.75 and ∆Gads° ) -14.84 kJ mol-1. The lateral interaction parameter a has also been evaluated from the tangent drawn at the point where the abscissa cIVN /(cIVN)θ)0.5 ) 1 on the plots of θ against cIVN. The values of a obtained were 1.40 (Hg) and 1.75 (In-Ga). According to the Frumkin model, further information regarding the adsorption parameters can be obtained from an analysis of the adsorption-desorption peaks. The variation of Cmax with the logarithm of the IVN concentration is shown in Figure 8. The slope of these plots is given by44

dCmax/d(ln c) ) (C0 - C1)(2 - a)

(3)

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Langmuir, Vol. 19, No. 22, 2003 9281 Table 1. Parameters of IVN Adsorption at Electrodes electrolyte c/(mol dm-3) cmax(IVN) C0/(µF cm-2) C1/(µF cm-2)a C1/(µF cm-2)b σmax/(µC cm-2) Eσmax/V ∆Eσ)0/mV ∆EN/mV Emax - Eσ)0 isotherm (at σmax) -∆Gads°/(kJ mol-1) a ref.

In-Ga

Hg

Hg

Na2SO4 0.15 0.114 29.2 8 8.4 -5.75 -1.088 50 530 128 Frumkin 14.84 1.75 this work

Na2SO4 0.15 0.114 21.3 7.5 7.45 -3.5 -0.56 280 360 124 Frumkin 17.3 1.38 this work

Na2SO4 0.15 0.137 20.6 7.2

Ag(111)

NaClO4 0.15 0.114 48.5 12.95 13.7 -3.6 -4.4 -0.59 -0.802 300 242 350 283 154 77 Frumkin Frumkin 17.1 16.1 1.3 1.48 19, 22 51

a By extrapolation to c f 0. b From the tangent to the σ/E curve at σmax.

Figure 8. Plot of the equilibrium capacity values at the negative adsorption-desorption peaks against the logarithm of the IVN concentration on a log scale for the (1) Hg and (2) In-Ga electrodes.

difference in the chemical nature of the two elecrodes used. The electrical variable also does not seem to influence C1, at least in the region where maximum adsorption is observed. Using the present data for the mercury electrode as a reference, extrapolated EN values provide information on the water dipole contribution to the potential drop across the inner layer for the uncharged In-Ga surface. Assuming that the orientation of the adsorbate molecules at the Hg and In-Ga surfaces is the same, one may write to a good approximation that

∆(EN)In-Ga ) ∆gH2O(dip)0 Hg

Figure 9. Plot of the square of the potential difference between negative (1, 2) and positive (3) peak potentials and potential of maximum adsorption against the logarithm of the IVN concentration: (1) In-Ga; (2, 3) Hg.

Since C0 and C1 are known independently, the value of a can be obtained from eq 3. From the present results, a )1.38 for Hg and a )1.8 for In-Ga, which are in excellent agreement with the values obtained directly from the Frumkin isotherms. In addition, acccording to the Frumkin model44

d(Epeak - Emax)2/d(ln c) ) 2RT Γmax/(C0 - C1) (4) The plots of the data according to eq 4 are satisfactorily linear (Figure 9). The slope of the plot is 0.579 for the Hg data and 0.326 for the In-Ga data. On the basis of eq 4, Γmax ) 7.14 × 10-10 mol cm-2 in the case of Hg and Γmax ) 6.56 × 10-10 mol cm-2 in the case of In-Ga (see Table 1). Discussion The parameters for IVN adsorption at DME and DIGE are summarized in Table 1. The above-mentioned similarity of C1 values at the two electrodes for θIVN seems to confirm that the overall polarizability of an aliphatic compound like IVN is not strongly influenced by the

(5)

Using this expression the estimate of ∆gH2O(dip)0 is 0.170 ( 0.02 V. Trasatti estimated the same quantity to be 0.12 ( 0.11 V following a procedure not involving organic adsorption.2 The result obtained in the present work gives qualitative support to the hypothesis that a larger component of the interfacial water dipole moment is perpendicular to the uncharged electrode surface in the case of In-Ga than in the case of Hg. On the basis of capacity data obtained in the absence of specific adsorption, it has been shown2 that the charge of minimum water dipole orientation at the In-Ga surface is around -6 µC cm-2, while for Hg it occurs at -3.5 µC cm-2. Accordingly, the adsorption of IVN takes place more favorably for field conditions at which there is a minimum in water orientation.Thissituationisnotgenerallyobservedexperimentally.45-47 The charge of maximum adsorption σmax for IVN at the two interfaces utilizing capacity data can be estimated using the formula4

σmax ) ENC0C1/(C0 - C1)

(6)

For the In-Ga/solution interface, C0 ) 29.20 µF cm-2 (average value), C1 ) 8.0 µF cm-2, and EN ) 0.530 ( 0.010 V. Thus, σmax falls within the range from -5.26 to -5.46 µC cm-2.. Similarly, for the Hg/solution interface, the corresponding value falls between -3.61 and -3.81 µC cm-2, using C0 ) 21.30 µF cm-2, C1 ) 7.5 µF cm-2, and EN ) 0.360 ( 0.010 V. The agreement between calculated and experimental values of σmax is qualitatively satisfactory (see Table 1). It is noted that C0C1/(C0 - C1), a quantity which is comparable with the reciprocal of the difference between solvent and solute polarizabilities at the interface,33 decreases with increasing C0. Of course, an increase in C1 has the opposite effect. Variations of C1 are commonly

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met in studies involving different adsorbates for constant electrode material (Hg). The lateral interaction parameter “a” of the Frumkin isotherm depends on the nature of the metal for the present system. Several factors are involved in determining this parameter.18,21 It is clear that the adsorbed IVN molecules in the compact layer should not be sensitive to the nature of the metal electrode. If the aliphatic part of the adsorbate faces the electrode surface, while the polar heads mainly interact with the solution side, it is reasonable to assume that the interactions between adsorbate and electrode are negligible, compared with water-electrode interactions. According to this view, the difference in hydrophilicity should play a role in determining the lateral interactions in the compact layer, by acting on the state of the water molecules only. According to the expression for a given earlier,21 the higher value of this parameter for IVN at the In-Ga surface, compared with Hg, implies a higher degree of solvent organization at the former electrode surface. Thus, the present results indicate that the higher hydrophilicity of the In-Ga electrode results in an increase in a. A similar trend has been found for other systems (see Table 1). Thus, the results obtained earlier at single-crystal Ag electrodes7,51 follow the same trends observed here. The value of -∆Gads˚ is smaller than at Hg because of the higher hydrophilicity of the Ag(111) electrode. At the same time, the interaction parameter a is higher on Ag(111) than on Hg. This is also attributed to higher hydrophilicity of the Ag(111) surface. A comparison with results for the adsorption of aliphatic alcohols at the two interfaces shows that Epeak - Eσ)0 ) - 0.463 V at In-Ga, and that Epeak - Eσ)0 ) -0.740 V at Hg for the alcohols.4 The difference in the Gibbs energies of adsorption at the pzc is 1.43 kJ mol-1.28 In the present study this diffeerence is 2.46 kJ mol-1 for IVN. The oftenmentioned stronger interaction of the hydroxylic polar heads with hydrophilic electrode surfaces, compared with the interaction of the -CN group, could explain this result. In the case of aliphatic alcohols, changes in the chemical nature and surface structure of the electrode surface affect C1 rather strongly. Evidence for this fact can be found from the interfacial behavior of n-hexanol,41 isobutanol,42 and n-pentanol48 at silver single-crystal faces, of several aliphatic alcohols on gallium,43 and of cyclohexanol at the basal15 and prismatic16 faces of zinc single crystals. For liquid Ga and most of the solid metals, the picture is less simple. In earlier work,2 the correlation between the nature of the electrode metal and C1 has been dealt with in detail, and a correlation between gH2O(dip)0 and the latter has been established, together with a dependence on the nature of the metal at negative charges. The charge corresponding to the adsorption-desorption extrema on the capacity curves for the DIGE with IVN is close to that observed for n-amyl alcohol.28 This seems to rule out significant interactions between the surface and the adsorbate polar heads, even when these are hydroxyl groups. Adsorption isotherms for aliphatic compounds at Ag48,51 and Hg exhibit higher a values at the former, at which less negative values of ∆Gads˚ have been measured. The same is observed for adsorption of cyclohexanol39 and aniline at Bi single-crystal faces.40 Both, for aniline and cyclohexanol, the adsorption takes place with stronger lateral interactions (attractive) at the crystal faces at which ∆Gads˚ is less negative. Also, the data for the adsorption of n-hexanol and isobutanol at different Ag single-crystal faces41,42,49 and for diethyl ether adsorption at gold faces8 give the same indication. The same holds for the adsorption of aliphatic alcohols at a Cd polycrystal13,14 and Zn monocrystal faces.15,16 In other cases,

Doubova et al.

however, the overall interfacial behavior of the species is rather complicated, as already observed. EN often decreases as ∆Gads˚ becomes less negative, while in some cases saturation surface excesses increase, thus indicating that the orientation of the adsorbate molecules is perpendicular to the electrode surface. These observations involve changes in the state of the solute and the solvent at the electrode surface, which are absent in the case of IVN in the present work. Considering now the general case of an adsorbate A, and a set of metal/solution interfaces, the above observations can be conveniently represented graphically. A set of curves of the type shown in Figure 4a,b can be drawn for the interfaces taking into account the two limiting conditions θA ) 0 and θA ) 1. The following procedure was used.50 Superimposition of the negative branches of the σ against E curves was obtained in the absence of contact adsorption. Then, the curves relative to the condition θA )1 were superimposed, making the assumption that C1 does not change significantly with a change in the electrode metal. The simpler case of C1 dependence on the latter has been taken into account. From this simplified picture, qualitative information on the relative position of the charge of maximum adsorption of the present adsorbate at electrodes of different hydrophilicity is obtained. The value of the charge of maximum adsorption relative to σmax allows one to interpret the respective potentials of maximum adsorption.The quantity ∆0σmax∆φ can be split into two components, one due to the free charge on the electrode and the other due to dipoles in the compact layer. Using the symbols adopted in ref 2, one may write for the condition of maximum adsorption

∆0σmax∆φ ) σmax/Kion - ∆0σmax∆g(dip)

(7)

The previously mentioned equality of the two contributions simply means that the variations of g(ion) and g(dip), which affect the variation of the surface charge density from 0 to σmax, compensate one another. The prediction formerly made, that, for a given set of electrode metals, the charge of maximum adsorption may be more negative, the more hydrophilic the surface at which the adsorption takes place, qualitatively justifies a certain degree of compensation. However, the almost complete elimination of both the free-charge term and the dipole term is clearly fortuitous. Equation 7 contains two contributions, ∆0σmax∆φ and σmax/Kion, which are directly obtainable from the available experimental results; the second term can be estimated provided a value for Kion is assumed using a reasonable model. The third term, g(dip), can be evaluated as a difference and compared with the terms gH2O(dip)0 for Hg and In-Ga, as estimated in ref 2. Once a value for gH2O(dip)0 has been assumed, the actual potential drop across the compact layer can be evaluated. This step is unnecessary when systems with a given electrode are dealt with but may become useful when adsorption processes are considered at electrode surfaces that differ by structure and chemical nature, that is, for which the term gH2O(dip)0 differs substantially. In the latter case it could be difficult to define a suitable quantity for the potential variable. In the present case, following Trasatti, for the resolution of ∆0σmax∆φ, and for the quantities which give the dipolar contribution to the potential drop across the inner layer, when the electrode surface is uncharged, the actual potential drop across the inner layer is given to a good approximation by the term due to the free charges, that is, by σmax/Kion. This applies for both the Hg- and the In-Ga/solution interfaces.

C-5 Nitrile Adsoption at Liquid Metal Electrodes

Conclusions During the past 2 decades the effect of electrode surface structure on the properties of the electrode/solution interface has become an important aspect of research in physical electrochemistry. The important developments reported encourage further studies of the role played by the nature of the electrode surface. This in turn is most easily achieved using liquid electrodes. Among them the indium-gallium alloy (eutectic composition) certainly deserves attention. The results of the present work confirm that In-Ga alloy may serve as an important system for comparison with Hg, with the advantage of easier experiments compared with Ga, the other “model” liquid

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electrode. The extension of the study to the adsorption of model neutral substrates, like the aliphatic nitriles on a comparative basis, has proven to be a powerful tool in the investigation of compact layer structure. Acknowledgment. This research was carried out in Canada with the financial support of the National Science and Engineering Research Council (NSERC). Other research support to L.M.D. came from the Italian Research Council (CNR) and the Science Foundation, Washington, D.C. (Grant No. CHE0133758). LA0346447