Aqueous Electrolyte

Dec 1, 1996 - excess and the small shift of the zero charge potential due to 1,3-DMU adsorption, ENI, indicate that the organic molecules are oriented...
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Langmuir 1996, 12, 6597-6603

6597

Adsorption of 1,3-Dimethyluracil at the Au(111)/Aqueous Electrolyte Interface. A Chronocoulometric Study Th. Wandlowski* and M. H. Ho¨lzle† Department of Electrochemistry, University of Ulm, D-89069 Ulm, Germany Received August 1, 1996. In Final Form: September 19, 1996X The interfacial behavior of 1,3-dimethyluracil (1,3-DMU) on Au(111) has been investigated quantitatively using chronocoulometry. The adsorption parameters such as film pressure, relative Gibbs surface excess, Gibbs energy of adsorption, and electrosorption valency were determined as a function of electrode potential and charge density for concentrations of 1,3-DMU up to 50 mM. The values of the relative Gibbs surface excess and the small shift of the zero charge potential due to 1,3-DMU adsorption, ENI, indicate that the organic molecules are oriented parallel to the electrode surface within the entire region of an ideal polarizable interface. The Gibbs energy at maximum adsorption is equal to -35.4 kJ mol-1 and suggests weak chemisorption. The strategy of analyzing chronocoulometric data, as shown in the present communication, provides the tools for a subsequent project aimed to address the role of the adsorbate structure on the thermodynamics of adsorption and phase-formation of N- and C-alkylated uracil derivatives on single crystal electrodes.

1. Introduction The interaction of organic molecules with metal interfaces is an interesting topic for a variety of technological and fundamental applications. Organic molecules are often used as corrosion inhibitors1 or they are employed as additives in plating baths in order to facilitate the formation of uniform overlayers of high brightness.2,3 Thin films of organic molecules on metal electrodes bear also a substantial potential in modifying the chemical nature of surfaces.4 This strategy focuses on the design of “tailormade” properties, which are the basis for a better understanding of electrode reactions in electrosynthesis and electrocatalysis5 as well as for the design of highly selective electrochemical sensors.6 Besides these more technological applications, organic molecules adsorbed on well-defined single crystal electrodes seem to become a model system for studies of phase transitions in adlayers and metal substrates.7,8 Since the pioneering work of Frumkin9 many “equilibrium” and “dynamic” adsorption studies at the metal/electrolyte interfaces have been performed with mercury or lowmelting-point electrodes of sp-metals, such as Bi, Pb, Sn, and Zn, onto which organic molecules are usually weakly adsorbed.10,11 A second class of studies is focussed on d-metals of the platinum group, where strong adsorbatesubstrate interactions often cause irreversible and/or †

Present address: ZAK/I-M301, BASF AG, 67056 Ludwigshafen, Germany. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Parkins, R. N. In Comprehensive Treatise of Electrochemistry; Bockris, J. O. M., Conway, B. E., Yeager, E., White, R. E., Eds.; Plenum Press: New York, 1980; Vol. 4, p 307. (2) Fischer, H. Elektrolytische Abscheidung und Elektrokristallisation von Metallen; Springer Verlag: Berlin, 1954. (3) Plieth, W. Electrochim. Acta 1992, 37, 2115. (4) Murray, R. W. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; J. Wiley: New York, 1992; p 1. (5) Baizer, M. M. Organic Electrochemistry; Plenum Press: New York, 1991. (6) Janata, J. Principles of Chemical Sensors; Plenum Press: New York, 1989. (7) Stolberg, L.; Lipkowski, J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992. (8) Buess-Herman, C. Prog. Surf. Sci. 1994, 46, 335. (9) Frumkin, A. N. Z. Phys. Chem. 1925, 116, 466; Z. Phys. 1926, 35, 972. (10) Damaskin, B. B.; Petrii, O. A.; Batrakov, V. V. Adsorption of Organic Compounds on Electrodes; Plenum Press: New York, 1971. (11) Parsons, R. Chem. Rev. 1990, 90, 813.

S0743-7463(96)00760-3 CCC: $12.00

dissociative adsorption.12,13 The character of molecular adsorption onto group IB elements, such as gold, silver, and copper, is just in between these two extreme classes.14 Here, thermodynamic and kinetic methods, as for instance capacitance measurements, cyclic voltammetry, or chronocoulometry, are still applicable to quantify the energetics and dynamics of adsorption. These classical electrochemical studies provide a macroscopic description of adsorption and/or phase-formation processes, but lack direct structural insight.7 This missing information may be obtained from spectroscopic techniques like infrared or Raman spectroscopy,15 electroreflectance,16 secondharmonic generation,17 or the use of ultrahigh vacuum (UHV) techniques after emersion of the electrode.18 As it was shown more recently, in-situ scanning probe microscopies19,20 as well as surface X-ray scattering methods21 even further increase our possibilities to monitor and understand the properties of organic adlayers at a molecular level. We have started a project aimed to explore the nature of structural transitions in ordered organic adlayers of 2,4-dihydroxypyrimidine (uracil) derivatives on gold and silver single-crystal electrodes.21-25 On the basis of capacitance measurements we have already shown that up to four potential regions may be distinguished for the (12) Hubbard, A. T. Chem. Rev. 1988, 90, 633. (13) Soriaga, M. P. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1993; Chapter 4. (14) Lipkowski, J.; Stolberg, L.; Yang, D. F.; Pettinger, B.; Mirwald, S.; Henglein, F.; Kolb, D. M. Electrochim. Acta 1994, 39, 1045. (15) Stole, S. M.; Popenoe, D. D.; Porter, M. D. In Electrochemical Interfaces; Abruna, H., Ed.; VCH: New York, 1991; p 341. (16) Henglein, F.; Kolb, D. M.; Stolberg, L.; Lipkowski, J. Surf. Sci. 1993, 291, 325. (17) Pettinger, B.; Lipkowski, J.; Mirwald, S.; Friedrich, A. J. Electroanal. Chem. 1993, 329, 289. (18) Kolb, D. M. Z. Phys. Chem. 1987, 154, 179. (19) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir 1996, 12, 2849. (20) Wandlowski, Th.; Lampner, D.; Lindsay, D. M. J. Electroanal. Chem. 1996, 404, 215. (21) Wandlowski, Th.; Ocko, B. M.; Magnussen, O. M.; Wu, S.; Lipkowski, J. J. Electroanal. Chem. 1996, 409, 155. (22) Ho¨lzle, M. H.; Wandlowski, Th.; Kolb, D. M. Surf. Sci. 1995, 335, 281. Dretschkow, Th.; Dakkouri, A. S.; Wandlowski, Th. In preparation. (23) Ho¨lzle, M. H.; Wandlowski, Th.; Kolb, D. M. J. Electroanal. Chem. 1995, 394, 271. (24) Wandlowski, Th. J. Electroanal. Chem. 1995, 395, 83. (25) Ho¨lzle, M. H.; Krznaric, D. M.; Kolb, D. M. J. Electroanal. Chem. 1995, 386, 235.

© 1996 American Chemical Society

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adsorption of uracil on the low-index planes of gold and silver. Two distinctly different organized (long-range order) uracil layers were found. The first one has typical properties of a two-dimensional condensed physisorbed phase, as it is also known for the mercury/electrolyte interface,26 while the second type of adlayer was assigned to a chemisorbed phase. In-situ scanning tunneling microscopy (STM) measurements revealed for the latter a highly ordered, hexagonal densely packed monolayer with a nearest-neighbor distance of 4.7 ( 0.3 Å between adjacent molecules.22 Substrate effects on the properties of these uracil adlayers on gold, silver, and mercury electrodes were discussed in ref 24. The present communication is part 1 of two papers, which will address the effect of the adsorbate structure on the phase behavior of uracil derivatives on Au(111)(p×x3) electrodes. On the basis of a comprehensive chronocoulometric study, the energetics of the adsorption and phase formation as well as the role of the electrical variable and of the bulk concentration on the interfacial orientation of various methyl-substituted derivatives of uracil will be analyzed. The objective of part 1 is to describe the chronocoulometric technique and data treatment, as applied in our laboratory, for the rather simple adsorption of 1,3-dimethyluracil (1,3-DMU) onto Au(111). This system exhibits only one adsorption region and does not show any indications of a two-dimensional condensed phase. Significant changes occur as soon as one or both methyl groups, which block the ring nitrogen atoms in the N(1)- or N(3)-position, are substituted by hydrogen atoms. The energetics and orientation properties of these systems will be addressed in part 2 of these thermodynamic studies. 2. Experimental Section The Au(111) electrode was a single-crystal cylinder about 4 mm in diameter and 4 mm thick with a gold wire attached to the rear for mounting purposes (MaTecK, Ju¨lich). The sample was oriented to better than (1° and polished down to 0.03 µm. Before each experiment the electrode was annealed in the flame of a Bunsen burner and, after a short cooling period in air, quenched in ultrapure water. The crystal was then transferred to the electrochemical cell with a droplet of water adhering to the polished surface to prevent contamination. Contact with the electrolyte was established under potential control, typically at E ) -0.60 V, employing the so-called “dipping” technique.22,27 All solutions were prepared with ultrapure water from a MilliQ-system (Millipore). The supporting electrolyte consists of 50 mM KClO4 (Fluka, puriss. p.a.) to which various concentrations of 1,3-dimethyluracil (Sigma, p.a.) have been added.

Wandlowski and Ho¨ lzle

Figure 1. (A) Cyclic voltammogram for Au(111) in 50 mM KClO4 + 12 mM 1,3-DMU. (B) Differental capacitance curves for various concentrations of 1,3-DMU in 50 mM KClO4. The filled and open rectangles represent the initial (t f 0) and final (t f ∞) values of the interfacial capacitance obtained after a potential step with Ei ) -0.800 V/Ef variable for 12 mM 1,3DMU. The dotted curves symbolize the pure base electrolyte. The electrochemical setup consists of a home-made potentiostat and a Stanford Research 830 digital lock-in amplifier, the analog outputs of which were digitized and transferred to a personal computer by a Digidata 1200 data acquisition board (Axon Instruments). The lock-in amplifier measured the ac-voltage across an RC-reference circuit connected in series with the counter electrode (for details see ref 28). The interfacial impedance was usually measured at -0.7 V, e.g., at a potential with practically no adsorption of the organic material, in the frequency range between 0.3 Hz and 5 kHz to determine the solution resistance in the hanging meniscus configuration. This procedure ensured the reposition of the electrode, even after flame annealing or addition of aliquot amounts of organic stock solution, with high accuracy. Simple capacitance measurements were performed using a 18 Hz sine wave of 10 mV peak-to-peak amplitude and a sweep rate of 10 mV s-1. The cyclic voltammograms were recorded with the same scan rate. The data aquisition in the potential step experiments was controlled with custom-made software (Pclamp6, Axon Instruments) in combination with a fast and low noise A/D//D/A-unit (Digidata 1200). The basic strategy of the chronoamperometric measurements was rather similar to that described by Lipkowski et al.7,29 Some details will be addressed in the following sections.

3. Results and Discussion The overall volume in the electrochemical cell was approximately 80 mL. KClO4 was twice recrystallized before use. The solutions were carefully deaerated with 5 N purity nitrogen before and during the measurements. The counter electrode was a platinum wire, and a saturated calomel electrode (SCE) in a side compartment of the cell served as a reference. All potentials are quoted with respect to the SCE, and all experiments have been performed under temperature control, usually at 10 ( 1 °C.

3.1. Qualitative Capacitance and Voltammetric Measurements. Cyclic voltammograms and capacitance curves have been recorded for Au(111)/50 mM KClO4 in the absence and in the presence of various concentrations of 1,3-DMU (Figure 1). All capacitance curves, even for the highest bulk concentrations of 1,3-DMU, merge at potential E e -0.80 V, which indicates complete desorption of the organic molecules. The adsorption of 1,3-DMU is characterized by a substantial decrease of the interfacial

(26) Wandlowski, Th.; Heyrovsky, M.; Novotny, L. Electrochim. Acta 1992, 37, 2663. (27) Dickertmann, D.; Koppitz, J.; Schultze, J. W. Electrochim. Acta 1976, 21, 1439.

(28) Pajkossy, T.; Wandlowski, Th.; Kolb, D. M. J. Electroanal. Chem. 1996, 414, 209. (29) Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1987, 238, 333.

Adsorption of 1,3-DMU on Au(111)

capacitance, most pronounced around E ) 0.20 V, and typical, rather broad negative capacitance maxima. The latter increase in height and shift toward more negative potentials with increasing adsorbate concentration. The saturation capacitance, CmI, which corresponds to maximum adsorption, was estimated by extrapolation 1/C vs 1/c with 1/c f 0 as 12.5 µF cm-2. The peaks at positive potentials and the corresponding voltammetric response indicate that the adsorption of 1,3-DMU at E > 0.50 V is no longer simple capacitive. The increasing current points to a faradaic process, which may involve the partial oxidation of 1,3-DMU.30 Consequently, we restrict the potential range of subsequent thermodynamic studies to -0.80 V < E < 0.50 V. We also note that our electrochemical measurements provided evidence neither for dynamic changes of the adlayer (as for instance in twodimensional condensation processes) nor of the substrate surface. The latter is also supported by preliminary insitu STM experiments, which revealed that the Au(111)(p×x3) reconstruction is stabilized by 1,3-DMU up to 0.50 V.31 Therefore, we do not need to consider time-dependent changes of the substrate surface structure in the following chronocoulometric measurements. 3.2. Data Aquisition and Treatment. Electrode Charge Density. Potential-step experiments were performed to determine the absolute electrode charge-density σM.29 The potential was initially held at a value Ei between -0.775 and 0.500 V for 100 s and then stepped to the final potential Ef ) -0.800 V, where 1,3-DMU is totally desorbed from the electrode surface. The very first initial potential was Ei ) -0.775 V. The waiting time at Ei was proven to be always long enough to establish adsorption equilibrium in a gently stirred solution of 50 mM KClO4 containing 10-5 M up to 0.03 M 1,3-DMU. The currenttransient following a single potential step Ei ) -0.775V/ Ef ) -0.800 V was subsequently recorded. The time window was usually 80 ms. Then the potential was stepped back to a new initial value Ei, 25 mV more positive than the previous one, and the cycle started once again until the last transient of each concentration (Ei ) 0.500 V/Ef ) -0.800 V) was obtained. The current transients were integrated digitally in order to obtain the corresponding charge transients. Some typical data, here for 0.39 mM 1,3-DMU in 50 mM KClO4, are plotted in Figure 2. The chronocoulometric curves display an initial, fast rising section, which represents the charging of the double layer and the desorption of 1,3-DMU, followed by a plateau, which shows just a slight positive slope. The latter indicates only minor contributions of a faradaic process (incipient hydrogen discharge) at Ef ) -0.800 V. In order to be exact throughout, we calculated the relative charge density ∆σM(t)0) ) σM(Ei) - σM(E)-0.800V), consumed exclusively during the transition Ei/Ef, by linear extrapolation of the plateau region to zero time. One set of transients, as obtained with a constant composition of the solution and variable initial potential Ei, yields a complete relative charge-density vs potential curve, e.g., ∆σM(t)0) vs Ei (if not stated otherwise, we will replace Ei by E in all subsequent parts of the paper). This procedure was performed for the supporting electrolyte and 16 concentrations of 1,3-DMU. The adsorbate concentration was usually changed by adding aliquot amounts of a 10 mM stock solution or solid substance. The position of the meniscus was always checked and, if necessary, readjusted via monitoring the solution resistance as described in the Experimental Section. We also monitored the cyclic voltammogram and the corresponding capacitance curves (30) Dryhurst, G. Electrochemistry of Biological Molecules; Academic Press: New York, 1977. (31) Dretschkow, Th.; Wandlowski, Th. Unpublished.

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Figure 2. Enlarged initial section of selected current transients Ei variable(t(wait) ) 100 s)/Ef ) -0.80 V recorded for Au(111) in 50 mM KClO4 + 0.39 mM 1,3-DMU within the potential region -0.75 < Ei < 0.50 V. First initial potential -0.70 V, ∆Ei ) 0.1 V. The inset shows some charge density vs. time curves as obtained after numerical integration of the i vs t transients over the entire time range measured.

Figure 3. Electrode charge density vs potential curves determined by chronocoulometry for Au(111) in 50 mM KClO4 solution with variable concentrations of 1,3-DMU.

before each new run of current transients, in order to characterize the interface and to check its stability. With the independently determined value of the potential of zero charge Epzc ) 0.275 V of Au(111)-(p×x3)/50 mM KClO428 and the set of relative charge-density vs potential curves, the absolute charge-densities σM can now be calculated according to the procedure outlined in ref 29. The charge density curves of the supporting electrolyte and of selected concentrations of 1,3-DMU are plotted in Figure 3. The shape of these curves is typical for onestate adsorption. They intersect within a rather narrow interval of potentials (0.15 V < E < 0.21 V) and charge densities (-6.0 µC cm-2 < σM < -3.0 µC cm-2). This region corresponds also to the minimum in the capacitance curves (Figure 1B) and represents the range of maximum adsorption (EmI, σmIM).

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Figure 4. (A) Film pressure vs electrode potential and (B) surface pressure vs charge density determined on a Au(111) electrode in 50 mM KClO4 with different concentrations (in mol dm-3) of 1,3-DMU.

Film Pressure and Surface Pressure The film pressure π, at constant potential, was determined by back-integration of the charge density data

π(E) ) γΘ)0 - γΘ )

∫EEσΘM dE - ∫EEσΘ)0M dE f

f

(1)

where γΘ)0 and γΘ are the values of the specific surface work (cf discussion in ref 7) in the absence and in the presence of the adsorbate and σΘM and σΘ)0M are the corresponding charge densities, respectively. A family of film pressure vs potential curves is plotted in Figure 4A. The curves are bell-shaped and their maximum corresponds to EmI. The values of the film pressure, even for the highest 1,3-DMU concentrations studied, are relatively low. They are comparable with the data obtained previously for the adsorption of diethyl ether32 and butanol33 on gold surfaces and indicate rather weak adsorption. The film pressure at constant charge, also known as the surface pressure, is defined via the Parsons function ξ ) γ + σME34 as

Φ(σM) ) ξΘ)0 - ξΘ ) γΘ)0 - γΘ + σM(EΘ)0 - EΘ) (2) and was calculated with the experimental values of σM and the corresponding values of the (relative) surface work γΘ)0(σM), γΘ(σM).7 The data are plotted in Figure 4B, and they will enable us to carry out the thermodynamic analysis of 1,3-DMU adsorption onto Au(111) using the charge as the independent variable as well. All Φ vs σM curves exhibit a common maximum at σM ≈ -5 µC cm-2, and their general shape is rather similar to that of the π vs E plots. Adsorption Isotherms and Gibbs Energy of Adsorption. The maximum bulk concentration of 1,3-DMU investigated corresponds to a mole fraction of 3.6 × 10-4, which is still small enough to assume that the activity of 1,3-DMU depends linearly on its concentration according (32) Lipkowski, J.; Nguyen van Huong, C.; Hinnen, C.; Parsons, R. J. Electroanal. Chem. 1983, 143, 375. (33) Beltowska-Brzezinska, M.; Dutkiewicz, E.; Skoluda, P. J. Electroanal. Chem. 1984, 181, 235. (34) Parsons, R. Trans. Faraday Soc. 1955, 51, 1518.

to Henry’s law.35 Thus concentrations instead of activities may be used in the determination of the relative Gibbs surface excess Γ. The latter was calculated by numerical differentiation of π or Φ vs the logarithm of the bulk 1,3DMU concentration at constant E or σM (eq 3a and 3b), respectively

Γ ) (1/RT)(δπ/δ ln c)T,p,E

(3a)

Γ ) (1/RT)(δΦ/δ ln c)T,p,σM

(3b)

The calculated Gibbs isotherms are plotted in threedimensional coordinates in Figure 5. The plateau region of the isotherm yields a maximum surface excess ΓmI ) (2.2 ( 0.2) × 10-10 mol cm-2 rather independent of the electrical variable. This value is identical to ΓmI as obtained on mercury36 and corresponds to an area per molecule of A ) 75 ( 7 Å2. The Gibbs energy of adsorption is often calculated from a fit of the experimental (Γ vs c)E or σMsdata to the equation of a particular isotherm.37 But this approach is modeldependent and not universal.38 However, in the limit of zero coverage almost all isotherms reduce to the Henry isotherm39

π(Φ) ) RTΓmIβc/55.5

(4)

where β is the adsorption equilibrium constant, which is related to the Gibbs energy of adsorption through eq 5

∆GI° ) -RT ln β

(5)

With this procedure, the standard state is represented by the pure solvent, unit mole fraction of the organic species in the bulk of the solution and a monolayer of “ideal” noninteracting adsorbate molecules at the surface.38 (35) Mohilner, D. M.; Nakadomari, N. J. Electroanal. Chem. 1975, 65, 843. (36) Brabec, V.; Christian, S. D.; Dryhurst, G. Biophys. Chem. 1978, 7, 253. (37) Trasatti, S. J. Electroanal. Chem. 1974, 53, 335. (38) Nikitas, P. J. Electroanal. Chem. 1984, 170, 333. (39) Parsons, R. Proc. R. Soc. London, Ser. A 1961, 261, 79.

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Figure 5. Three-dimensional graphs representing the adsorption isotherms for 1,3-DMU on an Au(111) electrode surface employing either the electrode potential (A) or the charge density (B) as the independent electrical variable.

We computed values of β using the initial segment of the experimental π (or Φ) vs c-curves (c f 0) after interpolation with a third-order polynom and subsequent differentiation. The Gibbs energy of adsorption is finally obtained with eq 5. ∆GI° is plotted in Figure 6 as a function of the electrode potential as well as of the charge density. Both curves show characteristic parabolic dependencies with maxima at 0.15 V and -3.5 µC cm-2, respectively. The corresponding values of ∆GmI are practically identical and amount to -36.0 kJ mol-1. This value is more negative than the data found on mercury36 and may be understood in terms of weak chemisorption. With ΓmI ) 2.2 × 10-10 mol cm-2 we also fitted the Frumkin isotherm to the data presented in Figure 5

βFc/55.5 )

Θ exp(-2aFΘ) 1-Θ

(6)

where Θ ) Γ/ΓmI and aF and βF are the lateral interaction parameter and the Frumkin adsorption constant. The latter yields the Gibbs energy of adsorption. (Note also that we employed the same standard states in the analysis according to the Henry and the Frumkin isotherms, respectively.) The values of the lateral interaction constant aF are small and vary only slightly with potential or charge (-0.2 < aF < 0.2) within the potential range investigated. Both methods of calculating ∆GI° agree rather satisfactorily (Figure 6). Proof of Self-Consistency. The self-consistency of the results presented above can be verified by testing several useful thermodynamic correlations provided that the following two assumptions are fulfilled: (i) The potential difference accross the diffuse layer is negligibly small and (ii) ∆GI° is determined within the limit Γ f 0 (Henry isotherm). Under these conditions the first derivative of ∆GI° vs E is equal to the electrosorption valency γ′. On the other hand, γ′ can be determined independently from a plot of the charge density vs surface

Figure 6. Gibbs energy of adsorption of 1,3-DMU on Au(111) plotted vs the electrode potential (A) and the charge density (B). The filled (open) symbols express the results of the Henry (Frumkin) isotherm fit. The solid lines are fits of a secondorder polynom to the individual sets of experimental data. The squares represent data as obtained for the adsorption of 1,3DMU at the mercury/electrolye interface.36

excess at constant potential40

1 1 γ′ ) (δ∆GI°/δE)Γ ) - (δσM/δΓ)E F F

(7)

Figure 7 shows a series of σM vs Γ data for selected electrode potentials. The plots are fairly linear, except for the region which corresponds to the highest concentrations studied. The initial slopes yield γ′. These results (filled circles in Figure 8) are in good agreement with γ′ as obtained from the first derivative of ∆GI° with respect to E (open circles in Figure 8). The shape of the γ′ vs E curve is characterized by a broad plateau region in -0.5 V < E < 0.2 V with γ′ ≈ -0.15 ( 0.08 and a subsequent increase at more positive potentials. The order of magnitude of the computed values of γ′ is comparable with literature data reported for benzonitrile,41 and pyrazine42 on Au(111) in their horizontal orientation. In addition, the first derivative of ∆GI° with respect to charge should be equal to the derivative of the electrode potential with respect to the Gibbs surface excess43

(∆GI°/δσM)Γ ) (δE/δΓ)σM

(8)

The fairly linear plots of E vs Γ, as presented in Figure 9, enable us to compute the initial slopes (δE/δΓ)qM in the limit of low coverage, e.g., Γ f 0. The data are plotted in Figure 10 (filled circles), and they correspond nicely with (40) Schultze, J. W.; Vetter, K. J. Electroanal. Chem. 1973, 44, 63. (41) Richer, J.; Ianelli, A.; Lipkowski, J. J. Electroanal. Chem. 1992, 324, 339. (42) Ianelli, A.; Merza, J.; Lipkowski, J. J. Electroanal. Chem. 1991, 307, 241. (43) Delahay, P. Double Layer and Electrode Kinetics; Wiley Interscience: New York, 1965.

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Figure 7. Charge density σM vs Gibbs surface excess of 1,3DMU at constant electrode potential. The straight lines have been computed on the basis of a linear regression analysis within the low-coverage region.

Figure 9. Plots of the electrode potential as a function of the Gibbs surface excess of 1,3-DMU at different values of the charge density (in µC cm-2). The straight lines were computed based on the low-coverage data.

Figure 8. Dependence of the electrosorption valency on the electrode potential: (b) data obtained from (δσM/δΓ)E (Figure 7); (O) first derivative of ∆GI° vs E (Figure 6) according to eq 7.

Figure 10. Comparison of the dependencies (δE/δΓ)σM (b) and (δ∆GI°/δσM)Γ (O) on the electrode charge density.

those obtained from the analysis of ∆GI° (open circles). Both correlations are rather linear, as actually expected for one-state adsorption. The data sets referring to the cross-relations expressed by eq 7 and eq 8 illustrate convincingly that the thermodynamic analysis of our chronocoulometric data is throughout self-consistent. No indications were found to use either the potential or the charge density as preferential electrical variable for the analysis of the 1,3-DMU adsorption on Au(111). Orientation of the Adsorbate Molecule. The experimental data indicate one-state adsorption of 1,3-DMU on Au(111)-(p×x3) in -0.80 V < E < 0.40 V. According to a simple electrostatic model proposed by Parsons, the potential difference across the inner region of the double layer due to replacement of water molecules by 1,3-DMU is given by44 (44) Parsons, R.; Peat, R.; Reeves, R. M. J. Electroanal. Chem. 1975, 62, 151.

∆φM-2 ) 4π/(σMx2 + Γµ)

(9)

where  and x2 are the permittivity and the thickness change of the inner layer, respectively. µ is the effective dipole moment expressed by

µ ) µad - nµw

(10)

with µad and µw as the components of the dipole moment of the adsorbate molecule and the water normal to the surface and n representing the number of solvent molecules displaced by one adsorbed organic molecule. With the potential drop across the diffuse layer, φ2, we may approximate φM-2 by42

φM-2 ) E - Epzc - φ2

(11)

At constant charge density σM, φ2 is independent of Γ; hence φM-2 ) E + const. Consequently, the dependence of E vs Γ (Figure 9) is almost exclusively caused by the

Adsorption of 1,3-DMU on Au(111)

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Table 1. Adsorption Parameters for 1,3-DMU on Au(111) and Hg36 parameter

Au(111)

Hg

electrolyte Epzc/V Em/V σm/µC cm-2 ENI/V CmI/µF cm-2 ΓmI/10-10 mol cm-2 -∆GmI°/kJ mol-1 γpzc

0.05 M KClO4 0.275 0.15 < EmI < 0.21 -6.0 < σM < -3.0 0.080 12.5 2.2 35.4 0.12

0.5 M NaF/phosphate -0.433 ∼-0.55 0.124 11.1 2.2 27.9

potential drop across the inner layer, which allows the test of appropriate molecular models. The shift of the zero charge potential due to adsorption of 1,3-DMU from Γ ) 0 to ΓmI is obtained as a special case of eq 9 with σM )0

∆φM-2 ) ENI ) (4π/)ΓmIµ

(12)

The corresponding plot in Figure 9 yields ENI ) 0.075 V, while ENI ≈ 0.080 V was estimated from the σM vs E curves shown in Figure 3. The small shift of Epzc with increasing adsorption of 1,3-DMU may be explained in view of eq 8 and eq 9 by a small positive dipole component of this molecule oriented perpendicular to the electrode surface or by the replacement of water molecules having a negative value of µw. The latter seems to be more probable in view of two independent results on the adsorption behavior of water: Hamelin et al. reported a positive temperature coefficient of the Epzc on gold electrodes, which is consistent with the oxygen atom of the water molecules facing the metal.45 A small, but significant negative dipole contribution due to adsorption of water on metal electrodes around Epzc was also obtained by Trasatti in a theoretical study.46 This discussion implies a rather negligible dipole contribution of 1,3-DMU in the direction normal to the surface, which clearly points to a horizontal orientation of the molecule at the Au(111)/electrolyte interface. This conclusion is supported by the values of ΓmI ) 2.2 × 10-10 mol cm-2 and A ) 75 Å2, which compare with the experimental data obtained for the planar orientation of other rigid heterocyclic molecules having rather similar molecular dimensions: Lipkowski et al. reported ΓmI ) 1.4 × 10-10 mol cm-2, ΓmI ) 4.5 × 10-10 mol cm-2, and ΓmI ) 3.5 × 10-10 mol cm-2 for the adsorption of pyridine,47 benzonitrile,41 and pyrazine molecules,42 respectively, at Au(111). The one-state orientation of 1,3-DMU also suggests that the variation of the effective dipole moment with charge is entirely determined by changes of the average dipole moment of the solvent µw. Two-state and multistate models of solvent adsorption actually predict a quasi-linear dependence of µw on σM,48 which is equivalent to a linear correlation between (δE/δΓ)qM and σM within the present model. Figure 10 illustrates that this dependence exists indeed for σM < 0 µC cm-2. Deviations at higher charge densities might be related to the increasing interaction between the electronic states of the adsorbate and the substrate, which are not explicitly considered in the above electrostatic model. (45) Hamelin, A.; Stoicoviciu, L.; Silva, F. J. Electroanal. Chem. 1987, 229, 107. (46) Trasatti, S. Electrochim. Acta 1983, 28, 1083. (47) Stolberg, L.; Morin, S.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1991, 307, 241. (48) Guidelli, R. In Trends in Interfacial Electrochemistry, Silva, F., Ed.; NATO ASI Series; Reidel: Dordrecht, 1987; Vol. 179, p 49.

Comparison with Related Substances and Other Substrates. The above thermodynamic analysis revealed that the characteristic adsorption properties of 1,3-DMU on Au(111)-(p×x3) are rather similar to the previously reported data on mercury.36 The important parameters are compiled in Table 1. Significant differences occur only in the magnitude of the Gibbs energy of adsorption, ∆GI° (Figure 6). In order to explain this fact we consider the 1,3-DMU adsorption as an interfacial water substitution process, which gives rise to the following incremental expression of ∆GI° at Epzc

-∆GI° ) (GA°,b - GA°,s) - (Gw°,b - Gw°,s)

(13)

The first term depends on the magnitude of the hydrophobic repulsion of 1,3-DMU from the bulk of the solution (GA°,b) and on the magnitude of the interactions with the metal surface (GA°,s), b and s represent bulk and surface, respectively. The second term stands for the metal/water interactions.37 With the same adsorbate studied, the same interfacial orientation, and rather similar hydrophobic properties of Au(111) and mercury,32 the main difference in ∆GI° for 1,3-DMU adsorbed on both metals might be attributed to the substrate interactions GA°,s. The interaction of 1,3-DMU with Au(111) (∆GI° ) -35.4 kJ mol-1) is obviously stronger than with mercury (∆GI° ) -27.9 kJ mol-1). The Gibbs energy of classical physisorbed systems on Au(111), like alcohols or ethers, amount to values between -10 kJ mol-1 and -20 kJ mol-1 (cf. refs 32 and 49). Higher values of the Gibbs energy on Au(111) are often characteristic of a weak chemisorption, as for instance in the cases of benzonitrile (∆GA° ) -27 kJ mol-1 41), pyrazine (∆GA° ) -27 kJ mol-1 42), or pyridine (∆GA° ) -34 kJ mol-1 47). The structure-determining role of the methyl groups in N1 and N3 position on the interfacial behavior of uracil derivatives on Au(111) will be addressed in part 2. 4. Summary We have investigated the interfacial behavior of 1,3DMU on Au(111), which shows typical properties of onestate adsorption. The system was chosen to develop and test the experimental setup and the numerical programs for adsorption studies based on chronocoulometric measurements. The self-consistency of our data was carefully discussed. Additional support for our treatment was obtained in several correlations with literature data, including the 1,3-DMU/mercury system. The strategy of analyzing chronocoulometric data, as shown in the present communication, provides the tools for a subsequent project aimed to address the thermodynamic aspects of adsorbate-structure effects on the adsorption and phaseformation of N- and C-alkylated derivatives of uracil on single crystal electrodes. Acknowledgment. Th. Wandlowski thanks the Deutsche Forschungsgemeinschaft for support through a Heisenberg-Fellowship. Stimulating discussions with Professor J. Lipkowski and Professor R. Guidelli are gratefully acknowledged. It is also a pleasure for the authors to acknowledge the continuous support and encouragement of Professor D. M. Kolb. LA960760G (49) Richer, J.; Lipkowski, J. J. Electroanal. Chem. 1988, 251, 217.