Interface Dipoles Observed after Adsorption of Model Compounds on

Jul 30, 2008 - First, the effect of the functionality type on the Volta potential of a thermally formed iron oxide was investigated. It was shown that...
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J. Phys. Chem. C 2008, 112, 12951–12957

12951

Interface Dipoles Observed after Adsorption of Model Compounds on Iron Oxide Films: Effect of Organic Functionality and Oxide Surface Chemistry Jan Wielant,*,† Ralf Posner,‡ Guido Grundmeier,‡,§ and Herman Terryn† Department of Metallurgy, Electrochemistry and Materials Science, Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, Christian Doppler Laboratory for Polymer/Metal Interfaces at the Max-Planck-Institut fu¨r Eisenforschung GmbH, Max-Planck-Strasse 1, 40237 Du¨sseldorf, Germany, and Technical and Macromolecular Chemistry, UniVersity of Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany ReceiVed: March 28, 2008; ReVised Manuscript ReceiVed: June 5, 2008

In this article, the interface dipole formed after adsorption of model compounds on thin iron oxide films is studied by scanning Kelvin probe (SKP). Amine and amide molecules with the same organic functionalities as an epoxy/amide coating system and a carboxylic acid molecule representing a maleic anhydride grafted or copolymerized polyolefin polymer were used as adsorbants. First, the effect of the functionality type on the Volta potential of a thermally formed iron oxide was investigated. It was shown that after carboxylic acid adsorption, the surface potential shifted in positive direction, whereas the amine and amide molecules induced a negative potential shift. Second, the Volta potential shift after adsorption of the amine and amide molecules on oxides with different amounts of surface hydroxyls was studied. A changing overall dipole moment as a function of the hydroxyl amounts was observed. Lewis acid-base interactions and protonation took place between the amine or amide functionality and the oxide surface. It was found that the Volta potential shift was mainly affected by the number of protonated amine or amide molecules. The individual interface dipole moments of the two binding types could be deduced from the Volta potential shift, assuming that the individual dipole moments of the two binding types contributed linearly to the overall dipole moment. For the protonation reaction, an interface dipole of -1.28 and -1.54 D was observed for the respective amine and amide molecule, whereas for the Lewis acid-base interaction, a dipole of +0.10 and +0.14 D was found. 1. Introduction The interaction of organic molecules and coatings with metallic or oxide surfaces has been an important issue for many years in all kind of research fields, like corrosion science,1–3 the aerospace industry,4,5 optoelectronics,6,7 catalysis,8 and lithography.9 In corrosion science, direct application of organic molecules and coatings on steel substrates is nowadays an important research topic to slow down metal degradation.10–12 Steel surfaces are treated in order to create thin oxide films with beneficial properties for coating adhesion.13–16 The surface properties of the interfacial oxide films are crucial and determine the nature of the interfacial chemical interactions and consequently the adhesion strength of metal-coating interfaces. Mostly, iron oxide structures are very complex and are present in several crystallographic configurations depending on the formation conditions.16–19 In atmospheric conditions, the structure of iron oxide surfaces is often different from the bulk structure due to surface hydration.20,21 The presence of singly, doubly, and/or triply coordinated surface hydroxyls is unavoidable.18,22,23 When oxides are in contact with organic molecules, several acid-base interactions can occur. Oxygen anions can act as electron-donating Lewis base sites, incompletely coordinated metal cations as electron-accepting Lewis sites, and hydroxyl anions can act either as a Lewis acid or base but also as proton-exchanging Bro¨nsted acid-base sites.24,25 * Corresponding author. E-mail: [email protected]. † Vrije Universiteit Brussel. ‡ Max-Planck-Institut fu ¨ r Eisenforschung GmbH. § University of Paderborn.

It is widely recognized that the acid-base properties of oxide surfaces alter the adhesion of organic compounds and coatings,8,25–27 but until now it is still ambiguous how the chemistry of the coating/oxide interface really looks. To study the interfacial chemistry, ultrathin films or monolayers are deposited on the oxide or metal substrate. Generally, model compounds with the same functional groups as the investigated coating systems are used for adsorption. Recent publications show that infrared reflection adsorption spectroscopy (IR-RAS) and X-ray photoelectron spectroscopy (XPS) are useful techniques to study the interfacial chemistry.2,5,28,29 In a previous publication,30 it is shown by XPS that the presence of surface hydroxyls affects the bonding mode of epoxy model compounds. In the case of molecular adsorption on metal or oxide surfaces, an interfacial dipole will be formed. There are various possible origins for the dipole. It can result from the reduction of the substrate surface electron density tail upon adsorption, which always decreases the surface dipole potential31 and/or the intrinsic dipole moment of the adsorbed molecule. In the latter case, the induced work function variation can have both positive or negative sign, depending on the molecular orientation on the surface.32,33 The interface dipole can also originate from the chemical dipole potential created by partial electron transfer between the substrate and the adsorbate upon chemisorption.34,35 Changes in the dipole layer as well as the stability of the interfacial bonds can be investigated using a scanning Kelvin probe (SKP). Molecular adsorption on the oxide surface will lead to a positive or negative Volta potential shift depending on the type of interaction. SKP has shown its usefulness in the characterization of dipole interfaces after deposition of self-

10.1021/jp802703v CCC: $40.75  2008 American Chemical Society Published on Web 07/30/2008

12952 J. Phys. Chem. C, Vol. 112, No. 33, 2008 assembled monolayers (SAM) on Au,36 Ru,37 and for all kinds of optoelectronic devices.32,38–41 Furthermore, recent publications from Nazarov and Thierry42,43 described the effect of coating chemistry on the Volta potential after application of several coatings on metallic substrates. But, until now the effect of the oxide surface conditions on the dipole layer formed after adsorption of organic molecules has not been discussed in literature. In this article, the direction of the Volta potential shift is discussed in detail after adsorption of molecules with totally different organic functionalities. To do this, amine and amide molecules with the same organic functionalities as an epoxy/ amide coating system and a carboxylic acid molecule representing a maleic anhydride grafted or copolymerized polyolefin polymer28 are adsorbed on a thermally formed iron oxide film on polished steel. In the second part of the article, three iron oxides with a different amounts of surface hydroxyls are prepared and treated with the same amine and amide molecules. The Volta potential of the treated and untreated substrates is measured, and the interface dipole moment is deduced from the obtained potential shifts. 2. Experimental Conditions 2.1. Sample Preparation. The substrate used was an interstitial-free DC06 steel according to the EN10130(98) norm. The steel surfaces were first mechanically polished with SiC-paper (500-800-1200-4000 grid) and then with 1 µm grade diamond paste to obtain a mirror-like finish (rms roughness ≈ 2.5 nm). To form the various iron oxide layers, samples were treated by one of the three procedures described below. First, thermal oxide films were formed by putting the steel samples for 8 min in an oven at 250 °C under an oxygen-rich atmosphere, air. After the thermal treatment, the samples were cooled down to ambient temperature in open air. The resulting oxide film had a thickness of approximately 20 nm and was composed by an inner core of magnetite (Fe3O4) and an outer scale of hematite (R-Fe2O3), which contains surface hydroxyls due to the reaction of atmospheric water with the oxide surface.18,44 Second, steel samples were passivated in a borate buffer solution (0.075 M Na2B4O7 · 10H2O + 0.3 M H3BO3; pH ) 8.2) at 0.8 V versus the reference electrode for 1 h. The electrochemical setup used for the latter treatment contained a saturated calomel reference electrode (SCE), a large platinum grid as counter electrode, and the steel sample as working electrode. Prior to passivation, the steel sample was reduced at -1 V versus SCE for 30 min to remove the native oxide film. This procedure resulted in a 4.5 nm thick oxide film related to Fe3O4 and γ-Fe2O3.17 The oxide surface was expected being composed by oxy(hydroxides).45 The third method produced an iron oxide film by a plasma treatment. The steel sample was first treated in a H2/Ar plasma chamber to remove the residual surface contamination, followed by a H2O plasma treatment.16 In this case, an oxyhydroxide scale was formed having a thickness of approximately 7 nm. After oxide application, the samples were ultrasonically cleaned sequentially in acetone and chloroform for 5 min to remove organic contamination. 2.2. Adsorption of Organic Molecules. The model compounds representing the functional groups of a cured epoxy/ amide polymer were N,N′-dimethylsuccinamide and N-methyldiethanolamine. These compounds are representative of an epoxy coating based on a DGEBA (diglycidyl ether of bisphenol A) and cured using a poly(amide)-based curing agent.46 Their derivation from the chosen epoxy/amide system was explained in ref 30. The molecular structures are shown in Figure 1, parts

Wielant et al.

Figure 1. Chemical structure of molecules adsorbed on iron oxide covered samples: (a) N,N′-dimethylsuccinamide, (b) N-methyldiethanolamine, and (c) succinic acid.

a and b. The carboxylic acid molecule representing a maleic anhydride grafted or copolymerized polyolefin polymer was succinic acid, see Figure 1c. Immediately after cleaning, onehalf of the iron oxide covered samples was immersed for 1 h in a chloroform solution containing either N-methyldiethanolamine or N,N′-dimethylsuccinamide with a concentration of 0.1 wt %. For the carboxylic acid deposition, a tetrahydrofuran (THF) solution with 0.1 wt % of succinic acid was used. The three compounds were purchased form Sigma-Aldrich as a > 97.5% purity and were used without further purification. Substrates were withdrawn from the chloroform and THF solutions, respectively, after treatment, and the molecular excess was rinsed from the surface using clean solvent. The samples were then directly transferred into the SKP chamber. 2.3. Scanning Kelvin Probe. The local electrode potential of the (un)treated oxidized steel surfaces was measured by means of a custom-made height-regulating SKP. A detailed description of the height regulation and potential measurement can be found in ref 47. The measured electrode potential is directly correlated to the Volta potential difference between the substrate and the reference electrode.48 The reference electrode used was a vibrating Cr/Ni wire with a diameter of 100 µm at the tip. The potential of the reference electrode was calibrated against a Cu/CuSO4 electrode in humid air (RH > 95%). All the potentials were given relative to a standard hydrogen electrode (SHE). Potential mappings were performed on the partly treated samples in dry air (RH < 10%). 3. Results and Discussion 3.1. Adsorption of Amide, Amine, and Carboxylic Acid Functionalities. In this paragraph, the Volta potential shift of thermal oxide films after adsorption of the model compounds described in the previous paragraph is discussed. In Figure 2, the Volta potential distributions of the iron oxide surfaces after adsorption of succinic acid, N-methyldiethanolamine, and N,N′dimethylsuccinamide are presented. The left parts of the mappings correspond to the untreated iron oxide surface (reference), the right ones to the treated iron oxide surface. A relatively constant potential distribution is observed in the treated and untreated surface zones. Only in the transition zone (width about 3-4 mm), a sharp peak in the potential distribution shows up, but this potential peak is treated as an artifact and is not taken into consideration for further interpretation. A Volta potential of 200-250 mV is obtained for the untreated thermal oxide surface. After molecular adsorption, clear potential shifts of +130, -85, and -45 mV are observed for succinic acid, N-methyldiethanolamine, and N,N′-dimethylsuccinamide, respectively. The oxide substrates were also immersed in pure solvent for 1 h, but negligible potential shifts were measured. This indicates that the potential shifts are due to the adsorbed molecules and that the solvent did not change the oxide surface during immersion. The direction of the potential shift is determined by the orientation of the interface dipole moment.33 In order to explain the dipole orientation, all interface dipole contributions should

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Figure 3. FTIR-RAS spectrum of succinic acid on the thermal iron oxide, obtained at 80° with respect to the normal of the surface. The spectrum obtained from clean thermal oxide was used as background spectrum.

natively bonded carboxylate species.49–51 The adsorption of carboxylic acid groups can be described by the following equation:

RCOOH + HOsFed f RCOO-+Fed + H2O

Figure 2. Volta potential mappings after adsorption of (a) succinic acid, (b) N-methyldiethanolamine, and (c) N,N′-dimethylsuccinamide. The left-hand parts of the mappings represent the potential of the uncovered iron oxide surfaces, and the right parts reflect the potential of the oxide surfaces after adsorption of the respective molecules.

be known. It is important to have an idea of the adsorption mechanism of the different functionalities, the dipole layer formed after interaction, and the intrinsic dipole moment of the adsorbing molecules. Carboxylic acid groups are capable of bonding with the oxide surface by consuming surface hydroxyls and forming coordi-

(1)

The formation of carboxylate species on this oxide was confirmed by IR-RAS. A part of the IR spectrum obtained at an incident angle of 80° (with respect to the normal of the surface) is shown in Figure 3. The band at around 1580 cm-1 is attributed to the asymmetric carboxylate stretching vibration νas(COO-). The asymmetrically shaped band in the 1450-1400 cm-1 region is assigned to the symmetric carboxylate stretching vibration νs(COO-). The peak at 1735 cm-1 is due to the presence of undissociated carboxylic acid groups which are involved in intramolecular or intermolecular hydrogen bonding.52 The intrinsic dipole moment of isolated succinic acid is zero.53 Consequently, the positive Volta potential shift observed after adsorption of the succinic acid molecules can be fully assigned to the formation of the carboxylate species at the oxide surface. In this case, the interface dipole is determined by the chemical dipole created by the condensation reaction. A chemical dipole is formed with the positive charge located on the oxide surface and the negative charge on the carboxylate group. This makes the extraction of electrons from the substrate more difficult, which results in a more positive substrate potential. Here, the potential shift was equal to 130 mV, which is in good agreement with shifts observed by Nazarov and Thierry after adsorption of oleic acid on hydroxylated steel.42 The interaction between oxide surface and amine functionalities is more complex. Two possible interactions might occur depending on the hydroxyl availability at the oxide surface.2,30,54 First, Lewis acid-base interactions take place between the electron lone pair of the basic amine nitrogen and the uncompletely coordinated acidic iron cations leading to charge transfer from the nitrogen atom to the iron cation. The charge transfer of this Lewis acid-base interaction is assumed to be small, but it can contribute significantly to the interfacial potential drop.42 This interaction mode is the primary binding mechanism on the thermally formed iron oxide substrates.30 A small fraction of only 13% of the amine nitrogens adsorbs by proton exchange. In the latter case, proton transfer from the surface hydroxyls to the amine nitrogen occurs resulting in positively charged amine nitrogens and a deprotonated oxide surface:

RR′R′′N : + HOsFed f RR′R ′ NH+O-sFed

(2)

Such as for the first binding mechanism charge is transferred between the oxide surface and the amine groups, but the latter mechanism is characterized by a higher charge transfer, which

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Σ)

Figure 4. Typical shape of the XPS N 1s peak after amine or amide adsorption.

Figure 5. Amide molecule and resonance structures after oxygen protonation.

was observed by XPS, see Figure 4.30 The ionic bond formed in these conditions should result in a higher chemical dipole compared to the dipole of the Lewis acid-base interaction. For this interaction as well as for the protonation reaction, the resulting dipole has its negative pole on the oxide surface and its positive pole on the amine nitrogens. Oppositely to succinic acid, the adsorbed amine molecules have a significant intrinsic dipole moment and could contribute considerably to the overall interface dipole moment,55,56 although a negative Volta potential shift is observed, which means that the overall interface dipole moment is oriented in the same direction as the chemical dipole moment. The interaction of amide molecules with oxide surfaces is not investigated in detail in literature. Anyhow, from XPS results it could be shown that a significant amount of the adsorbed amide nitrogen atoms were positively charged.30 However, direct protonation of the amide nitrogen is not expected since proton adsorption on the amide oxygen is energetically more favorable.57–59 In Figure 5 the amide molecule and its resonance structures after oxygen protonation are presented. The structure with lowest energy is the one with the positively charged nitrogen. It is assumed that these amide molecules attach to the oxide surface after oxygen protonation by interacting electrostatically with the deprotonated oxide surface. As for the amine molecules, a positively charged organic layer and a negative oxide surface is formed. Just as the amine, the adsorbed amide molecules have a significant dipole moment, but again a negative Volta potential is observed, see Figure 2. The Volta potential shift ∆Ψ due to the interface dipoles can be estimated by following equation:33,38

∆Ψdipole )

Σµ˜ εε0

(3)

where µ˜ is the component of the interface dipole moment perpendicular to the surface (in units of Cm), Σ is the surface density of dipoles (in m-2), ε is the dielectric constant of the organic layer, and ε0 is the dielectric permittivity of vacuum. The surface dipole density Σ, starting from XPS data, can be calculated as follows:60

IaσsEaNFλs cos R IsσaEsM

(4)

where Ia and Is are the integrated adsorbate and substrate line intensities, σa and σs are the photoionization cross sections, F is the substrate density, Ea and Es are the kinetic energies of the adsorbate and substrate photoelectrons, λs is the mean free path of electrons within the substrate, R is the angle of collection with respect to the sample normal, M is the molecular mass of the substrate, and N is the Avogadro constant. For the studied molecules a surface dipole density of 8-9 × 1018 1/m2 is obtained. From eq 3, the interfacial dipole moment µ˜ is calculated. In general, the dielectric constant for adsorbed monolayers is assumed to be around 3.32,38,61 After succinic acid adsorption, the interface dipole moment is equal to +0.12 D. For the amine and amide molecules, it is impossible to calculate the dipole moment of the individual interactions since two binding mechanisms have taken place in parallel, leading to two different dipole types. In order to estimate the individual dipole moments, measurements on oxide surfaces with different surface properties were performed. 3.2. Adsorption of Amine and Amide Molecules on Different Iron Oxide Surfaces. The amine and amide molecules were adsorbed on three different iron oxides: the thermally formed oxide, the borate buffer oxide, and the plasma oxide. The surface hydroxyl fraction of these oxide layers increased in the order of thermal oxide < borate buffer oxide < plasma oxide, with the surface of the oxide layers being enriched with hydroxyls when compared to the bulk.30 In the outer oxide layer, the hydroxyl fraction was equal to 13%, 30%, and 48%, respectively.30 Just as in the previous paragraph, reference potential measurements were performed in pure chloroform. For none of the oxides were potential variations after immersion in the pure solvent observed. Figures 6 and 7 show the SKP mappings after adsorption of N-methyldiethanolamine and N,N′-dimethylsuccinamide on the three iron oxide films. The Volta potentials for the three untreated oxide surfaces were slightly different. The potential can be affected by several factors as oxide thickness, oxide conductivity, and chemical composition.62,63 The effect of these factors on the observed potentials was not thoroughly investigated here as it was not of particular interest in this study. A negative Volta potential shift was measured for all oxides after molecular adsorption. In the case of amine adsorption, the plasma oxide showed a potential shift of -200 mV, whereas the thermal oxide and borate buffer oxide exhibited a shift of only -85 mV. Similar potential shifts were observed for steel after adsorption of amine-containing inhibitors and aminosilanes.42 After amide adsorption, even higher shifts were observed. The potential of the plasma oxide was lowered by 450 mV, whereas the potential drop for the thermal oxide was around 45 mV. The borate buffer oxide had a potential which was 170 mV lower. Since different potential shifts are observed after adsorption on the iron oxides, it can be concluded that the interface dipole moment is not determined by the intrinsic dipole moment of the adsorbing molecules but by the chemical dipole created by partial charge transfer between the oxide surface and the adsorbate upon chemisorption. Previously it was shown that a higher hydroxyl content at the oxide surface resulted in a higher protonation level of the adsorbed amine and amide molecules. Here, the protonation level means the fraction of amine or amide nitrogens which were protonated during the adsorption process. In Figure 8, the protonation level as a function of the surface hydroxyl fraction is repeated from ref

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Figure 6. Volta potential mappings after adsorption of N-methyldiethanolamine on (a) the thermally formed iron oxide film, (b) the borate buffer oxide, and (c) the plasma oxide. The left-hand parts of the mappings represent the potential of the uncovered iron oxide surfaces, and the right parts reflect the potential of the oxide surfaces after adsorption of the molecules.

Figure 7. Volta potential mappings after adsorption of N,N′-dimethylsuccinamide on (a) the thermally formed iron oxide film, (b) the borate buffer oxide, and (c) the plasma oxide. The left-hand parts of the mappings represent the potential of the uncovered iron oxide surfaces, and the right parts reflect the potential of the oxide surfaces after adsorption of the molecules.

30. If a high hydroxyl concentration is present at the oxide surface, more protons are exchanged between the oxide surface and the amide or amine molecules. It is expected that a stronger chemical dipole is formed after proton exchange than after Lewis acid-base interactions. The increased negative Volta potential shift after adsorption on the highly hydroxylated oxide films

confirms this. A relatively high number of protonated molecules results in a higher chemical dipole moment. In Figure 9, -∆Ψ is shown as a function of the protonation level of the respective molecules. A clear trend exists between the Volta potential shift

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Figure 8. Protonation level of the amine and amide nitrogens after adsorption on the iron oxide surfaces, repeated from ref 30. The hydroxyl fractions of the thermal oxide, borate buffer oxide, and plasma oxide are calculated from the XPS C 1s and O 1s peaks at a take-off angle of 15°.

Figure 9. Relationship between Volta potential shift and protonation level after N-methyldiethanolamine and N,N′-dimethylsuccinamide adsorption on the different iron oxide surfaces.

TABLE 1: Moment of Interface Dipoles (in debye) Formed after Adsorption of N-Methyldiethanolamine and N,N′-Dimethylsuccinamide Molecules on the Iron Oxide Films molecule dipole type

N-methyldiethanolamine

N,N′-dimethylsuccinamide

+0.10 -1.28

+0.14 -1.54

µ˜ Lewis µ˜ proton

and the protonation level of the adsorbed molecules. If two different dipoles are considered, the numerator of eq 3 can be rewritten as

Σµ˜ ) Σ(fprotonµ˜ proton + (1 - fproton)µ˜ Lewis)

(5)

with fproton being the protonation level, µ˜ proton the moment of the dipole formed by the protonation reaction, and µ˜ Lewis the dipole moment after the Lewis acid-base interaction. Then, the Volta potential shift ∆Ψ is equal to

∆Ψdipole )

Σ (µ˜ + (µ˜ proton - µ˜ Lewis)fproton) εε0 Lewis

(6)

Values for µ˜ proton and µ˜ Lewis can be obtained by a linear fit of the data in Figure 9. µ˜ Lewis can be estimated from the extrapolation of the curve to fproton ) 0. µ˜ proton can be calculated from the slope of the curve. The correlation coefficient of the linear fit was higher than 0.97. In Table 1, the values obtained for µ˜ proton and µ˜ Lewis are presented. From these results, it can be concluded that the dipole formed by proton exchange dominates

the oxide-coating interface. This interaction creates a dipole which is 10 times larger than the dipole induced by the Lewis acid-base interaction. Moreover, the dipole of the latter interaction is oriented in the opposite direction, meaning that a positive Volta potential shift should be observed when only Lewis interactions take place. This is not expected if the interface dipole moment is determined by the chemical dipole formed by this interaction. In the case of Lewis interactions, the charge transfer between the amine and amide nitrogen and the iron cation is assumed to be very small; thus, a very small chemical dipole is formed. Since the amide and amine molecules have a significant intrinsic dipole moment compared to the chemical dipole,55,56 it is expected that for these types of interactions, the overall interface dipole is dominated by the intrinsic dipole moment of the adsorbing molecules. Assuming that the amine and amide nitrogen interact with the oxide surface, the dipole moments for the amine and amide molecules are oriented in the opposite direction as the chemical dipole. Because of this, the substrate experiences a dipole with its positive pole pointing to the oxide surface and the negative pole to the organic layer. In the case of proton exchange, the chemical dipole is much larger and orientates the overall interface dipole moment in the other direction. The similar effect has been observed after formation of poly(amidoamine) dendrimers on indium tin oxide.64 It should be kept in mind that the obtained dipole values are only an estimation of the interface dipole moments perpendicular to the oxide surface. Since the orientation of the adsorbed molecules is unknown, it is impossible to know the contribution of the molecular dipole to the overall interface dipole moment and to calculate the exact dipole value for the interfacial bonds. 4. Conclusions The interface dipole formed after adsorption of three different model compounds on a set of differently prepared iron oxide films has been investigated using SKP. Amine and amide molecules with the same organic functionalities as an epoxy/ amide coating system and a carboxylic acid molecule representing a maleic anhydride grafted or copolymerized polyolefin polymer were under study. It was shown that the oxide Volta potential after adsorption is shifted in positive or negative direction depending on the oxide surface chemistry, the orientation of the adsorbants, and the type of interaction taking place between the oxide surface and the organic functionality of the adsorbing molecules. After succinic acid adsorption on thermal oxide, a positive shift was observed which could be attributed to the chemical dipole formed between carboxylate species and the oxide surface. The adsorption of N-methyldiethanolamine and N,N′dimethylsuccinamide on iron oxides turned out to be much more complex. To study this, oxide surfaces with a different amount of surface hydroxyls were used as substrate materials. Two different interactions take place which lead to two dipoles: (i) Lewis acid-base interaction occurring between the electron lone pair of amine and amide nitrogens and the iron cations and (ii) proton transfer between the oxide surface and the adsorbing molecules. By varying the amount of surface hydroxyls, the number of exchangeable protons could be altered. It could be seen that a direct correlation exists between the protonation level of the adsorbed molecules and the negative Volta potential shift of the substrate. The negative potential shift is mainly due to the chemical dipole formed after proton exchange. A small dipole with opposite orientation was found for the Lewis-type

Interface Dipoles on Iron Oxide Films interaction. Here, the substrate experienced a dipole moment whose orientation is not originating from the chemical dipole but from the intrinsic dipole of the adsorbing molecules. Acknowledgment. Jan Wielant is financed by the Flemish Institute for the promotion of scientific and technological research in industry (IWT). His work is realized with the financial support of ArcelorMittal Research and Development Industry Gent-OCAS N.V. The authors thank the Christian Doppler Research Association, Vienna, for providing the experimental facilities at the Max-Planck-Institut fu¨r Eisenforschung GmbH. This work is part of the project entitled “Nanometer scale electron spectroscopy of organic layers on oxide surfaces”, financed by the Research FoundationsFlanders (FWO). References and Notes (1) Welle, A.; Liao, J.; Kaiser, K.; Grunze, M.; Ma¨der, U.; Blank, N. Appl. Surf. Sci. 1997, 119 (3-4), 185–190. (2) Marsh, J.; Minel, L.; Barthe´s-Labrousse, M.; Gorse, D. Appl. Surf. Sci. 1998, 133, 270–286. (3) Ochoa, N.; Moran, F.; Pe´bere, N. J. Appl. Electrochem. 2004, 34 (5), 487–493. (4) Affrossman, S.; MacDonald, S. Langmuir 1996, 12, 2090–2095. (5) Abel, M.; Rattana, A.; Watts, J. Langmuir 2000, 16, 6510–6518. (6) Papageorgiou, N.; Gra¨tzel, M.; Enger, O.; Bonifazi, D.; Diederich, F. J. Phys. Chem. B 2002, 106, 3813–3822. (7) Lindell, L.; de Jong, M.; Osikowicz, W.; Lazzaroni, R.; Berggren, M.; Salaneck, W.; Crispin, X. J. Chem. Phys. 2005, 122, 084712. (8) Farfan-Arribas, E.; Madix, R. J. Phys. Chem. B 2003, 107, 3225– 3233. (9) Alexander, M.; Beamson, G.; Blomfield, C.; Leggett, G.; Duc, T. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 19–32. (10) Tsuji, N.; Nozawa, K.; Aramaki, K. Corros. Sci. 2000, 42 (9), 1523– 1538. (11) Rohwerder, M.; Stratmann, M. Macromol. Symp. 2002, 187, 35– 42. (12) Mikhailova, S.; Povstugar, V. Colloids Surf., A 2004, 239 (1-3), 77–80. (13) Grundmeier, G.; Stratmann, M. Appl. Surf. Sci. 1999, 141, 43–56. (14) Wapner, K.; Stratmann, M.; Grundmeier, G. Silicon Chem. 2003, 2, 235–245. (15) Harun, M.; Marsh, J.; Lyon, S. Prog. Org. Coat. 2005, 54 (4), 317– 321. (16) Raacke, J.; Giza, M.; Grundmeier, G. Surf. Coat. Technol. 2005, 200, 280–283. (17) Davenport, A. J.; Oblonsky, L. J.; Ryan, M. P.; Toney, M. F. J. Electrochem. Soc. 2000, 147 (6), 2162–2173. (18) Cornell, R. M.; Schwertmann, U. The Iron Oxides, 2nd ed.; WileyVCH: Weinheim, Germany, 2003. (19) Goossens, V.; Wielant, J.; Van Gils, S.; Finsy, R.; Terryn, H. Surf. Interface Anal. 2006, 38, 489–493. (20) Rustad, J.; Felmy, A.; Hay, B. Geochim. Cosmochim. Acta 1996, 60 (9), 1553–1562. (21) Hiemstra, T.; Van Riemsdijk, W. J. Colloid Interface Sci. 2006, 301, 1–18. (22) Rustad, J.; Felmy, A.; Hay, B. Geochim. Cosmochim. Acta 1996, 60 (9), 1563–1576. (23) Rustad, J.; Dixon, D.; Felmy, A. Geochim. Cosmochim. Acta 2000, 64 (10), 1675–1680. (24) Simmons, G.; Beard, B. J. Phys. Chem. 1987, 91, 1143–1148. (25) Barthe´s-Labrousse, M. Vacuum 2002, 67, 385–392. (26) McCafferty, E.; Wightman, J. Surface Interface Anal. 1998, 26, 549–564. (27) van den Brand, J.; Snijders, P.; Sloof, W.; Terryn, H.; de Wit, J. J. Phys. Chem. B 2004, 108, 6017–6024.

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