2647
Langmuir 1994,10, 2647-2653
Adsorption of Isoquinoline at the Au(111)-Solution Interface Dong-fang Yang and Jacek Lipkowski* Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N l G 2Wl Received December 14, 1993. In Final Form: May 17, 1994@ Adsorption of isoquinoline at a Au(ll1) single-crystal electrode has been investigated using chronocoulometry, cyclic voltammetry, and phase-sensitive ac voltammetry. The experiments were performed in pH 10.7 solutions in order to extend the double layer region to more negative potentials. The adsorption parameters, such as the film pressure, Gibbs surface excess, Gibbs energy of adsorption, and electrosorption valency, were determined as functionsofthe electrode potential. Adsorption ofisoquinolinehas the character of a weak chemisorption. Two limiting surface coordinations of the isoquinoline molecule to the Au(ll1) surface were identified: the flat x-bonded coordination at negative potentials and the vertical N-bonded orientation at positive polarizations. The transition from the flat to the vertical orientation has a gradual character and proceeds through a series of intermediate states.
Introduction This work constitutes part of a systematic study of adsorption of organic molecules at gold electrodes. It extends our previous investigations of coordination of azabenzenes to Au(hkZ)surfaces1-’ to a class of azanaphthalenes. Adsorption of isoquinoline a t the aqueous solution-mercury interface has been investigated by both electrochemical methods8-14 and spectroscopic t e ~ h n i q u e s . l ~The - ~ ~isoquinoline molecule, whose structure is shown in Figure 1,has a dipole moment of 2.6 D in vacuum which is oriented in the plane of the molecule a t a 70” angle with respect to the bond between the two median carbons (C4a-C8a). The molecule can assume four orientations a t the mercury surface. At positive potentials and low bulk concentrations it has a tendency to adsorb flat with the aromatic rings oriented parallel to the metal surface. By increasing the bulk concentration andlor by Abstract published in Advance A C S Abstracts, July 15, 1994. (1)Stolberg, L.; Richer, J.; Lipkowski, J.:Irish, D. E. J . Electroanal. Chem. 1986.207.213. (2)Stolberg, L.;Irish, D. E.; Lipkowski, J. J . Electroanal. Chem. 1987,238,333. (3)Stolberg, L.; Lipkowski, J.; Irish, D. E. J . Electroanal. Chem. 1990,296,17-1. (4)Stolbera. L.: Liukowski. J.; Irish, D. E. J. Electroanal. Chem. 1991,300,563. (5)Stolberg, L.; Morin, S.; Lipkowski, J.;Irish, D. E. J . Electroanal. Chem. 1991,307,241. (6)Iannelli, A.;Richer, J.; Lipkowski, J. Langmuir 1989,5 , 466. (7)Yang, D. F.;Stolberg, L.; Lipkowski, J.;Irish, D. E. J.Electroana1. Chem. 1992,329,259. (8) Buess-Herman, C. Contribution a l’Btude du phenomene de coadsorption a l’interface mercure-solution aqueuse: cas des melanges quinoleine-perchlorate. Ph.D. Thesis, Universis Libre de Bruxelles, 1978. (9)Buess-Herman, C.; VanLaethem-Meuree,N.; Quarin, G.; Gierst, L. J. Electroanal. Chem. 1981,123,21. (10)Jenard, A.; Hurwitz, H. D. J . Electroanal. Chem. 1976,70,27. (11)Lipkowski, J.;Quarin, G.; Buess-Herman, C. Electrochim. Acta 1981,26,357. (12)Quarin, G.; Buess-Herman, C.; Gierst, L. J . Electroanal. Chem. 1981,123,35. (13)Lipkowski, L.; Buess-Herman, C.; Lambert, J. P.; Gierst, L. J . Electroanal. Chem. 1986,202,169. (14)Buess-Herman, C.; Franck, C.; Gierst, L. J . Electroanal. Chem. 1992,329,91. (15)Humphreys, M. W.; Parsons, R. J . Electroanal. Chem. 1977,82, 369. (16)Fleischmann, M.; Hill, I. R.; Sundholm, G . J . Electroanal. Chem. 1983,158,153. (17)(a) Blackwood, D. J.; Pons, S. J . Electroanal. Chem. 1988,247, 277. (b) Blackwood, D.; Korzeniewski, C.; Makenna, W.; Li, J.; Pons, S. In Electrochemical Surface Science; Soriaga, M . P., Ed.; ACS Symposium Series 378;American Chemical Society: Washington, DC, 1988;p 339. @
5
4
a 1 Figure 1. Structure of the isoquinoline molecule. applying negative potentials, the molecule stands up to assume a perpendicular position. Initially, the molecule is believed to contact the metal surface through the 4,5 or 5,6 carbons. At higher bulk concentrations and at more negative potentials, isoquinoline assumes another upright orientation with the 6,7 carbons in contact with the metal ~ u r f a c e . ~In J ~this position the molecules form a twodimensional solidlike condensed monolayerg-14 which effectivelyblocks ion transfer processes.13 The interaction between isoquinoline molecules and mercury is weak, and the adsorption has the character of weak physisorption. The adsorbed molecules are oriented with the nitrogen atoms directed toward the solution and hydrocarbon skeleton toward the metal. The driving force for adsorption is the hydrophobic repulsion of the hydrocarbon tail of the molecule out of the solution and into the interface and intermolecular interactions between adsorbed molecules. Isoquinoline is expected to adsorb differently a t the gold-solution interface. We have already shown that azabenzenes may coordinate to the gold surface either through the n-bond system of the aromatic ring or by mixing of the nonbonding orbital at the nitrogen heteroatom with the electronic states in the metall-’ Similar surface coordination is expected for the azanaphthalanes as well. The adsorption of isoquinoline onto the gold surface should have the character of weak chemisorption, and in the vertical position the molecule should be oriented with the nitrogen toward the metal and with the hydrocarbon tail toward the solution, which is the opposite direction to that observed on mercury. In this work we will address two issues. In relation to adsorption of azabenzenes a t gold electrodes we will examine the effect of the molecular structure of the adsorbate on the relative strengths of the x-bonded and N-bonded surface coordination. With a reference to
0743-746319412410-2647$04.50/0 0 1994 American Chemical Society
Yang and Lipkowski
2648 Langmuir, Vol. 10, No. 8, 1994 150
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Figure 3. Differential capacity curves for (dashed line) M KOH) and (solid supporting electrolyte(0.1M Kclo4 lines) solutions that contain the following concentrations of M; (0) isoquinolinein the supporting electrolyte: (*) 1.7 x 4.5 x M; (A)1.2 x M; ( 0 )5.5 x M (sweep rate 5 mV s-l, ac amplitude 5 mV s-l, ac modulation frequency 25 Hz, 5-mV rms ac amplitude). The inset shows the differential capacitycurve for a pure solution of the supporting electrolyte.
+
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E I V vs SCE
Figure 2. Cyclic voltammograms recorded for (a)0.1 M KC104 M KOH and (b) 0.1 M Kclo4 M KOH 5.5 x M isoquinolinesolutions (sweeprate 20 mV s-l). Dashed lines correspond t o curves recorded at 10 times expanded sensitivity.
+
+
+
adsorption of the isoquinoline at the mercury electrode we will assess the effect of the nature of the metal on the molecule orientation (N toward the metal or N toward the solution) at the interface.
Experimental Section Experimentalproceduresand instrumentation were described in our previous publi~ations.~aJ9 Solutions were prepared from Milli-Q water. The supporting electrolyte was 0.1 M KC104 + 0.001M KOH. By addition of KOH t o KC104 solution we were able to suppress the hydrogen evolution reaction and to extend the double layer region of the gold electrode by about 100 mV toward the negative direction. The KC104 (ACScertified,Fisher) was purified usingthe procedure describedprevi0usly.~J8Avery pure KOH solution (99.995%,ASAR) was used t o adjust the pH of the solution. The isoquinoline (97%,Aldrich) was purified by distilling twice under vacuum. The purified isoquinoline was stored in the dark in a refrigerator. The Au(ll1) single crystal (99.99%from Johnson Matthey) was grown, cut, and polished in our laboratory. It was flame annealed and quenched with Milli-Q water before it was transferred to the electrochemical cell. The solutions were deaerated with argon before measurements and kept under an argon atmosphere during measurements. A saturated calomel electrode (WE)was used as the reference electrode and a gold coil as the counter electrode. The isoquinoline concentrationin to 5.5 x M. All measuresolution ranged from 6.1 x ments were carried out at room temperature, 20 i 2 "C. Results and Discussion Cyclic Voltammetry and Differential Capacity. Figure 2a shows cyclic voltammograms (CVs) recorded for the alkaline supporting electrolyte (pH 10.7). The section of the curve corresponding to the potentials ranging between -0.9 and +0.3 V was recorded at a 10-fold expanded sensitivity. The voltammograms presented in (18)Richer, J.;Lipkowski, J. J . Electrochem. SOC.1986, 133, 121. (19)Lipkowski, J.;Stolberg, L. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J.,Ross, P. N., Eds.;VCH Publisher: NewYork, 1992; Chapter 4.
Figure 2a agree well with that reported in the literature.20 The double layer region extends from -0.9 V (SCE) to +0.15 V (SCE). At potentials more negative than -0.9 V (SCE),hydrogen evolution takes place; at potentials higher than +0.15 V (SCE), oxide formation is observed. The symmetric shape of the CV in the double layer region indicates that the solutions were free of oxygen and that no creeping of solution on the wall of the electrode occurred. Figure 2b shows the voltammograms recorded in the presence of isoquinoline in the solution. Two pairs of peaks are present in the double layer region. The negative peaks seen at E = 0.7 V (SCE) correspond to the adsorptiondesorption of isoquinoline. The peaks located between -0.2 V (SCE) and 0.0 V (SCE) correspond to the reorientation of the adsorbed molecules. The CVs also show that adsorption of isoquinoline shifts the oxide formation toward more positive potentials. Figure 3 shows the differential capacity curves for the Au(ll1) electrode in the pure supporting electrolyte and in solutions with various amounts of isoquinoline. In the alkaline solution, adsorption of OH- and/or gold oxide formation starts at quite negative potentials and is responsible for the appearance of three broad peaks seen between -0.5 V (SCE) and +0.1 V (SCE) and of the high pseudocapacity peak observed a t E = 0.2 V (SCE). These features are consistent with the results reported in the literature.20 Multiple peaks are seen on the differential capacity curves in the presence of isoquinoline in the solution. The adsorption-desorption peaks seen at the negative limit of potentials grow taller and shift toward the negative direction as the concentration of isoquinoline increases. The adsorption of isoquinoline apparently suppresses adsorption of OH- and/or gold oxide formation a t the positive limit of potentials. This could be seen as a general decrease of the electrode capacity at positive potentials when isoquinoline is present in the solution. The peaks seen between E = 0.1 V (SCE) and E = 0.3 V (SCE) were assigned earlier to the reorientation of the adsorbed molecules. They move in the negative direction with the bulk isoquinoline concentration. Their height and shape change in a n irregular fashion. This may reflect a complex nature of the reorientation kinetics and/or coadsorption of OH- or c104-, particularly in solutions of ~
~~
(20) Hamelin, A.;Sottomayor,M. J.;Silva, F.; Chang, S.-C.;Weaver,
M. J. J . Electroanal. Chem. 1990, 295, 291.
Adsorption of Isoquinoline
Langmuir, Vol. 10, No. 8, 1994 2649
low isoquinoline concentration. The middle section of the differential capacity curve is flat in 1.7 x 10-5 M isoquinoline solution. One or two broad peaks appear in this range of potentials for higher bulk concentrations of this molecule. This behavior suggests that isoquinoline assumes multiple adsorption states at the surface of the Au(ll1) electrode. Before closing this section, we would like to stress that both the cyclic voltammetry and the differential capacity curves do not represent the adsorption equilibrium. They are used chiefly to characterize the adsorption of isoquinoline qualitatively and to choose the experimental conditions for the potential step experiments. Electrode Charge Density. Potential step experiments were performed to determine the charge density a t the electrode surface (uM). Initially, the potential E was held at a value within the double layer region (between -0.85 V (SCE) and +0.25 V (SCE)) and then stepped to the final potential Ef= -0.9 V (SCE) where isoquinoline completelydesorbs from the electrode surface. The current flowing to recharge the interface was measured and integrated to give the difference between the charge densities at potentials E and Ef(AuM). In order to achieve the adsorption equilibrium,the solutions with isoquinoline concentration lower than M were stirred vigorously over a period of about 4 min when the electrode was held a t the initial potential. The stirring was interrupted and the solution was allowed to calm down during an additional period of 1 min, before the potential step was applied. Similar experiments were performed for the pure supporting electrolyte (0.1M Kclo4 0.001 M KOH) and for a neutral 0.1 M KC104 solution. For the 0.1 M KC104 solution the absolute charge densities UM were calculated from the measured AUM,with the help ofthe independently determined value of the potential of zero charge, according to the procedure described in refs 18 and 19. The charge density UM a t E = -0.8 V (SCE) has the same value in 0.1 M Kc104 and in 0.1 M KC104 M KOH solutions. Hence, this value of UM was used to calculate the absolute charge densities a t all other potentials from the experimental AUM’Sfor the alkaline solution of the supporting electrolyte. Finally, the charge densities for the pure supporting electrolyte and for the isoquinoline solutions have to be equal at E = -0.9 V (SCE) (isoquinoline is completely desorbed from the electrode surface a t this potential). The value of UM a t E = -0.9 V (SCE) was therefore used to convert the measured AUM’Sfor the isoquinoline-containing solutions into the absolute charge densities. Figure 4 shows the absolute charge density-potential curves for the supporting electrolyte and for the solutions with various amounts of isoquinoline. The section of the curves corresponding to -0.9 5 E < -0.8 V (SCE) for isoquinoline solutions merges with the curve for the supporting electrolyte. This behavior is consistent with a total desorption of the organic molecules a t these negative polarizations and justifies our choice ofE = -0.9 V (SCE) as the final potential for the potential step experiments. The charge density curves for isoquinoline solutions display multiple steps and inflections, characteristic of a multiple-state adsorption of a n organic molecule. The first adsorption state may be identified as that corresponding to the negative end of potentials. Starting a t E = -0.9 V (SCE) and moving in the positive direction, the charge densities rise above the value for the supporting electrolyte and then level off to run almost parallel to the charges for the supporting electrolyte. Here the charge densities become independent of the bulk concentration M. of isoquinoline if its concentration is higher than
+
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E I V vs SCE Figure 4. Charge density-potential curves for solutions containing the following concentrations of isoquinoline: ( 0 )0 M; (*) 1.0 x 10-5M; (0) 1.7 x 10-5 M; (+I 2.8 x 10-5M; (VI4.5 x 10-6 M; (VI7.5 x 10-5 M; (A) 1.2 10-4 M; (AI2.0 10-4 M; (0)3.4 x 10-4 M; (m) 5.5 10-4 M.
This feature suggests that a limiting surface concentration for the molecules adsorbed in state 1was attained. The rising sections of the charge density plots correspond to the position of the adsorptioddesorption peaks in Figure 3. The less steep segment of the curves corresponds to the region where the differential capacities for the isoquinoline covered and the film free surface do not differ substantially. This behavior for state 1is consistent with a flat (aromatic rings parallel to the surface) orientation of the adsorbed molecules. State 2 of the adsorption may be identified as that corresponding to the positive end of the applied potentials and positive surface charge. The charge potential curves rise here in a steplike fashion and then level off, The charge densities become independent of the bulk concentration of the organic compound (for solutions with the isoquinoline concentration higher than M) in the upper section of the curve. This behavior suggests that the surface concentration of molecules adsorbed in state 2 attains a limiting value. Perchlorate ions are known to coadsorb with isoquinoline molecules a t the positively charged mercury surface.6 Coadsorption of the anion and the isoquinoline molecule is therefore expected for the gold electrode as well. Figure 5 shows the differential capacity and the charge density plots for the M isoquinoline solution, determined using 0.1 M KC104and 0.1 M NaF electrolytes. The differential capacity and the charge density curves for the two electrolytes overlap at E < 0 V (SCE);however, they diverge a t positive potentials (or positive charge densities). This behavior suggests that anions may coadsorb with the isoquinoline molecules a t the positive end ofpotentials and that state 2 corresponds to a mixed monolayer. The inflections and steps seen on the charge density plots a t intermediate potentials indicate that the transition from state 1to state 2 is gradual and goes through one or more intermediary states. Adsorption Isotherms. The charge density curves were integrated to give the film pressure n = y(f3 = 0) y ( 0 ) ,where y ( 0 = 0)and y ( 0 ) are the interfacial tensions in the absence and presence of isoquinoline adsorption, r e s p e c t i ~ e l y . ~Figure ~ J ~ 6 shows a family of film pressure versus potential plots for various isoquinoline concentrations. The curves display two regions. The first corresponds to a negatively charged metal surface and is characterized by a gradual change of the film pressure with potential. The second corresponds to a positively charged surface and is distinguished by a steep change of n with E . Consequently, the film pressure curves can be seen as arising from the superimposition of two bell-
Yang and Lipkowski 210 N
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E I V v s SCE Figure 7. Gibbs surface excess-potential curves obtained for M;(*) the followingisoquinoline concentrations: ( 0 )6.1 x 1.0x 10-5 M; (0)1.7 x 10-5 M; (+I 2.8 x 10-5 M; (v)4.5 10-5 M; (VI7.5 x 10-5 M; (A) 1.2 x 10-4 M;(A)2.0 x 10-4 M; (0)3.4 x M; (m) 5.5 x M. The inset shows the adsorption isotherms obtained for the following electrode potentials: (0) -0.6V,(A)-0.4V;(V)-0.2V,(e)-0.1V,(O)0.0V,(*)+0.10 v; ( 0 )f0.2 v.
0.4
E I V vs SCE Figure 5. (a) Differential capacity curves for 5.5 x M isoquinoline solution in 0.1 M KC104 (solidline) and 0.1 M NaF (dotted line). (b) Charge density-potential curves for (solid line) 0.1 M KC104 and (dotted line) 0.1 M NaF solutions. The two lower curves are for solution without isoquinoline;the two M isoquinoline. upper curves are for solution with 5.5 x
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E I V v s SCE Figure 6. Film pressure-electrode potential curves for solutions containing the following concentrations of isoquinoline: ( 0 )6.1 x 10-6 M; (*I 1.0x 10-5 M;(0)1.7 10-5 M; (e)2.8 x 10-5 M; (v) 4.5 x 10-5 M; (VI7.5 x 10-5 M; (A) 1.2 x 10-4 M; (A)2.0 x 10-4 M; (01 3.4 x 10-4 M; (m) 5.5 10-4 M. shaped curves corresponding to each of the negatively and positively charged interfaces. Such a shape is typical for a multistate adsorption of a n organic molecule and was observed previouslyfor the case ofpyridine adsorption at the Au(ll1) electrode.5 The Gibbs excess of adsorbed moleculeswas determined by differentiating the film pressure versus logarithm of isoquinoline concentration plots a t constant E.lSJ9Figure 7 shows versus E curves for various bulk isoquinoline M. The concentrations ranging from 1 x lom5to 5 x inset to Figure 7 shows the dependence of r on the bulk isoquinoline concentration plotted for selected values of the electrode potential. We can easily distinguish two regions of E where r depends strongly on the bulk concentration. The first is seen a t potentials that are smaller than -0.5 V (SCE), the second a t E values that are larger than -0.05 V (SCE), but smaller than $0.2 V (SCE). Outside these regions r attains a maximum, bulk concentration independent value (rmm).
Figure 8 shows the dependence of rmax on the electrode potential. The plot consists of two sections separated by a discontinuity a t about 0 V (SCE). (Note that the r versus In c plot for E = 0 V (SCE), shown in the inset to Figure 7, displays a continuous increase of the surface concentration while the plots for E either higher than +0.1 V (SCE) or lower than -0.1 V (SCE) display a well-defined plateau). r,,varies between 1.2 x 10-loand 2.8 x 10-lo mol cm-2 a t the negative potentials and between 5.8 x 10-lo and 6.4 10-lo mol cm-2 a t the positive potentials. These values can be compared with the maximum packing densities equal to 2.7 x mol cm-2 for the flat ( m bonded) and to 6.2 x mol cm-2 for the vertical (5,6) orientations of the isoquinoline estimated by BuessHerman et aL8s9 The packing densities for the vertical N-bonded orientation of the chemisorbed molecule and the 5,6 orientation of the physisorbed isoquinoline should be approximately equal. Hence, the agreement between shown in Figure 8 and the estimated the values of rmax packing densities for the flat and the vertical orientations is very good. The Gibbs excess data are therefore consistent with the model which assumes flat orientation of isoquinoline molecules a t the negatively and a vertical (either N-bonded or 5,6) orientation a t the positively charged surface. Gibbs Energies of Adsorption. The zero coverage Gibbs energies of adsorption AG"e=o can be determined either from the initial slopes of the film pressure versus
Adsorption of Isoquinoline
Langmuir, Vol. 10, No. 8, 1994 2651 -14
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E I V v s SCE Figure 9. Zero coverage Gibbs energy of adsorption versus E determined from (A)the Henry isotherm and ( 0 )the Frumkin isothermp" full coverage Gibbs energy of adsorption versus E for (13) the flat adsorbed or tilted adsorbed molecules and (M) the vertically adsorbed molecules.
bulk isoquinoline concentration plot (Henry isotherm) or from a fit of the Gibbs excess data to a n equation of a n adsorption i s ~ t h e r m . ' ~ JThe ~ ~ first ~ ~ method uses low values of the film pressure (where Henry's law can be applied); the second requires that the data for a complete adsorption isotherm (rvalues ranging from 0 to rmm) are available. Figure 6 shows that the low values for the film pressure are known for E < -0.4 V (SCE) only. Likewise, the inset to Figure 7 shows that the complete adsorption isotherms were determined for a few electrode potentials (for -0.6 V (SCE) E < -0.4 V (SCE)). Therefore, the zero Gibbs energies of adsorption can be determined for potentials more negative than -0.4 V (SCE) only. The black triangles in Figure 9 show the zero coverage Gibbs energies of adsorption determined from the initial slopes of t h e n versus bulk isoquinoline concentration plots using the procedure described in refs 19 and 21. Open circles in this figure show the zero coverage Gibbs energies of adsorption determined from the fit of the Gibbs excess data to the equation of the Frumkin isotherm. The standard state corresponds to a unit mole fraction of isoquinoline in the bulk and monolayer coverage by noninteracting adsorbates, where the monolayer coverage corresponds to the rmm values given in Figure 8 (asymmetric choice of the standard state).21s22The agreement between the two sets of data is very good; however, they are determined for a very narrow range of the electrode potentials. The energies of isoquinoline adsorption may be described in terms of the full coverage Gibbs energies of adsorption (AGo+l) in a much broader range of potentials. The AGO61 values can be determined by plotting the film pressure as a function of the logarithm of the bulk pyridine concentration and extrapolating the linear segment of this plot to zero film pressure. The intercept of the In c axis multiplied by RT gives the full coverage Gibbs energy of a d s o r p t i ~ n . ~The , ~ ~difference ,~~ between the magnitudes of the zero coverage and full coverage Gibbs energies of adsorption illustrates how the properties of the surface phase deviate from the properties of a perfect solution (a perfect two-dimensional mixture of solvent and the adsorbate molecules). Open squares in Figure 9 show the full coverage Gibbs energies of adsorption determined for the n-bonded isoquinoline molecules,and black squares show the Gibbs energies for the vertically adsorbed (21)Richer, J.;Lipkowski, J. J. Electroanal. Chem. 1988,251,217. (22)Torrent, J.;Sanz,F. J . Electroanal. Chem. 1990,286,207. (23)Torrent, J.; Sanz,F. J . Electroanal. Chem. 1993,359,273.
0
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Figure 10. UM versus r obtained at the following electrode potentials: (M) -0.7 V; ( 0 )-0.65 V; (A)-0.6 V; (A) -0.55 V; (v)-0.50 V; (v)-0.45 V; ( 6 )-0.40 V; (0)-0.35 V. The inset shows the dependence of the electrosorption valencies on the electrode potential determined ( 0 )from the slope of the OM versus r plots and (M) by numerical differential of the AGO ' versus E curves.
isoquinoline molecules. At the negative end ofthe applied potentials the zero coverage and full coverage Gibbs energies of adsorption can be compared. The differences are less than 2 kJ mol-' and suggest that the x-bonded molecules form a two-dimensional phase whose properties deviate little from the properties of a perfect solution. Overall the Gibbs energies of adsorption attain fairly large absolute values which indicate that the interaction of the isoquinoline molecule with gold has the character of weak chemisorption. The first derivative of AGO versus E is equal to the electrosorption valency, y'. Independently, the electrosorption valency can be determined from the slope of the charge density versus surface excess plots:
Thus, the slope of the UM vesus r plots can be compared to the first derivative of the AGO versus E plot to check the consistency of our results. Figure 10 shows a plot of UM versus r for various electrode potentials in the region -0.7 to -0.35 V (SCE). The electrosorption valencies were determined from the slope of the initial sections of the UM versus r plots. The calculated values of y' are plotted as a function of the electrode potential in the inset to Figure 10. The AGO versus E plot was fitted by a polynomial of degree 3 and differentiated numerically to determine the electrosorption valencies. The electrosorption valencies determined in this way are also plotted in the inset to Figure 10. The agreement between the two sets of data is very good, indicating that our data are self-consistent.
Discussion To conclude this work, we will first contrast the adsorption of isoquinoline a t the Au(ll1) surface and that a t a Hg electrode, and in this way we will assess the effect of the nature of the metal on the character of isoquinoline adsorption a t electrode surfaces. Second,we will compare the adsorption of isoquinoline with previously studied pyridine adsorption5 a t the Au(ll1) surface and will analyze the relation between the molecular structure and the character of the surface coordination of these compounds. Figure 11illustrates the effect of the nature ofthe metal on the character of isoquinoline adsorption. Open points
Yang and Lipkowski
2652 Langmuir, Vol. 10, No. 8, 1994 N
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Figure 11. (a)Gibbs surface excesses for isoquinoline adsorpisoquinoline solution at the tion from 0.1 M Kclo4 + 5.5 x Au(111)electrode surface (opensquares)and from 1.OMNaC104 6.5x M isoquinoline solution at the Hg electrode (black plotted against the rational potential. (b) Dependence of the full coverage Gibbs energies of adsorption on the rational potential for isoquinoline adsorption at the Au(ll1) (open points) and Hg electrodes (black point^).^^^
+
show the data for the Au(ll1) electrode; the black points are the results for a mercury electrode, taken from BuessHerman’s s t u d i e ~ . ~ Isoquinoline ,~ displays a multiplestate adsorption a t both Hg and Au electrodes. However, there are significant differences between the adsorption a t the two metals. At mercury, the isoquinoline molecule assumes a vertical orientation a t the negatively charged surface and a flat orientation a t the positively charged interface. In contrast, the flat orientation is observed a t the negatively charged and the vertical orientation a t the positively charged surface a t the Au(111)electrode. Figure l l b shows the Gibbs energy data determined for the two metals. The difference between the energies of isoquinoline adsorptionat the surface ofAu(ll1) andHgis striking. In the case of the full coverage Gibbs energy of adsorption the observed difference may be caused either by the change of the character of the metal-adsorbate interaction or by a change of the energy of the adsorbate-adsorbate interaction. The full coverage and zero coverage Gibbs energies differ by less than 2 kJ mol-’ for isoquinoline adsorbed a t the Au(ll1) electrode in state 1(open square in Figure Ilb). The energy of interaction between adsorbed molecules in state 1a t gold is therefore small. The Gibbs energy data for mercury correspond to the upright orientation of the isoquinoline molecule with the 6,7 carbons in contact with the metal and nitrogen heteroatom directed toward the s o l ~ t i o n . ~In, ~ this orientation isoquinoline molecules interact through stacking forces and form a condensed, solidlike monolayer. The energy of adsorbate-adsorbate interaction may constitute a significant fraction of the measured Gibbs energies (black squares in Figure l l b ) . Consequently, the difference between the Gibbs energies of adsorption shown in Figure 1l b gives the lower estimate of the difference between the strength of the metal-adsorbate interaction. For state 2 ofisoquinoline adsorption a t the Au(ll1)electrode (open triangle in Figure l l b ) , the Gibbs energy of adsorption may be lowered by the coadsorbed perchlorate ions.
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-0.2
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0.2
0.4
E I V vs SCE Figure 12. Comparison of the Gibbs excess versus potential M)and pyridine (1x plots for isoquinoline (1x MI5 adsorption at the Au(ll1)electrode from 0.1 M Kc104 solution.
However, studies of isoquinoline and perchlorate adsorption a t the Hg suggest that the energy change caused by the coadsorption is too small to explain the difference between the Gibbs energies of adsorption a t gold and mercury. In conclusion, the interaction of isoquinoline with the Au(ll1) electrode in states 1and 2 must be much stronger than the interaction of this molecule with Hg. The differences between isoquinoline adsorption a t Au and Hg may be explained by considering the electronic structure of these metals. In the case of gold the d band is located only 2 eV below the Fermi level, while in mercury it lies 8 eV below the Fermi Therefore, d electronic states in Au may mix with the molecular orbitals of isoquinoline while they may not participate in the surface coordination of isoquinoline at Hg. Adsorption of isoquinoline at Au is likely to have the character of weak chemisorption,n-bonded for state 1and N-bondedfor state 2. In contrast adsorption of this molecule a t Hg is an example of a weak physisorption. The metal-adsorbate interaction for the flat orientation involves chiefly dispersive forces between polarizable n electrons in the molecule and free electrons in the metal. In the vertical surface geometry the adsorbed molecule is oriented with the nitrogen atom directed toward the solution and hydrocarbon skeleton toward the metal. The differences between the character of surface coordination explain why the vertical geometry is observed in a different range of rational potentials a t the surface of the two metals (see Figure l l a ) . For the vertically adsorbed, N-bonded molecule a t the Au(ll1)electrode, the negative pole ofthe dipole faces the metal and the positive pole the solution. The dipole-field interaction stabilizes this coordination a t the positively charged metal surface. In contrast, the vertically oriented, physisorbed molecule turns with the positive pole of its permanent dipole toward the metal and with the negative pole toward the solution. The dipole-field interaction stabilizes this orientation at the negatively charged surface. The differences between isoquinoline adsorption a t Au(ll1) and Hg surfaces have many similarities to the recently discussed case of pyridine adsorption at the surfaces of the two metals.25 In fact, as Figure 12 shows, the adsorption of the azanaphthalene and the azabenzene a t the Au(ll1) surface is quite similar so that the adsorption isotherms for the two compounds overlap to a large extent. The major difference is that the adsorption (24) Buess-Herman, C. J.Electroanal. Chem. 1993,349, 93. (25) Lipkowski, J.;Stolberg, L.; Yang, D-F.; Pettinger, B.; Minvald, S.; Henglein, F.;Kolb, D.M . Electrochim. Acta, in press.
Adsorption of IsoquinoZine of pyridine takes place in two well-defined states corresponding to the flat and the vertical N-bonded orientations, respectively. The transition between the two states is steep. In contrast, the isotherm for isoquinoline displays a whole series of adsorption states corresponding to a gradual transition between the flat bonding orientation a t the negative end to the vertical N-bonded coordination a t the positive end of potentials. The two adsorption isotherms correspond to different bulk isoquinoline and pyridine concentrations. This behavior displays a difference between the Gibbs energies of adsorption of the two compounds. However, the difference between the Gibbs energies may be chiefly caused by the solute-solvent
Langmuir, Vol. 10, No. 8, 1994 2653 rather than by the metal-adsorbate interactions. In fact pyridine is fully miscible with water while isoquinoline has a limited solubility in a n aqueous electrolyte which is on the order of 5 x M.8 In conclusion, our studies indicate remarkable similarities between the coordination of isoquinoline and pyridine to the Au( 111)surface. The presence of the second aromatic ring in the molecule apparently does not affect the character of its surface coordination.
Acknowledgment. This work was supported by a grant from the Natural Science and Engineering Research Council of Canada.
