J. Phys. Chem. C 2007, 111, 13177-13184
13177
Influence of the Iron Oxide Acid-Base Properties on the Chemisorption of Model Epoxy Compounds Studied by XPS Jan Wielant,*,† Tom Hauffman,† Orlin Blajiev,† Rene´ Hausbrand,‡ and Herman Terryn† Department of Metallurgy, Electrochemistry and Materials Science, Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, and Arcelor Research Industry Gent-OCAS nV, John Kennedylaan 3, B-9060 Zelzate, Belgium ReceiVed: March 25, 2007; In Final Form: June 26, 2007
In this work, the influence of the iron oxide acid-base properties on the adsorption of model epoxy compounds was examined. To study this, iron oxide layers with a different surface hydroxyl fractions were prepared in controllable and reproducible conditions. The surfaces were characterized by X-ray photoelectron spectroscopy (XPS). An expression for the hydroxyl fraction of the oxide films was deduced, starting from the measured XPS intensities. Two model epoxy compounds characteristic of an epoxy/amide system were adsorbed on the oxides: N,N′-dimethylsuccinamide and N-methyldiethanolamine. Additionally, an amine molecule without alcohol groups, N,N′-diethylmethylamine, was adsorbed to investigate the role of alcohol functionalities on the amine adsorption mechanism. The interaction between the oxide layers and the nitrogenous model compounds were studied by examination of the O 1s and N 1s XPS photopeaks. The data showed that the amine and amide nitrogen adsorbed via two different bonding modes: via Lewis-like acid-base interactions and via Bronsted-like interactions or protonation. A direct correlation was found between the protonation level of the adsorbed nitrogenous molecules and the hydroxyl fraction in the outer oxide layer. Additionally, it was noted that the protonation level depended on the mobility and flexibility of the adsorbed molecules. It was also observed that the presence of alcohol groups in the amine molecular chain had a beneficial effect on the number of adsorbed amine molecules.
1. Introduction For many years, metals have been covered by multifunctional organic coatings. Generally, in corrosive environments, the life span of these systems is determined by the adhesion and deadhesion characteristics of the organic coating.1-3 During the past decade, research studies have focused on the adhesion of organic coatings and molecules on aluminum and galvanized steel substrates,4-9 nowadays researchers concentrate more and more on the direct application of organic substances on steel surfaces (without an interfacial zinc layer).10-12 The steel surfaces are treated in order to create thin (oxide) films resulting in better adhesion properties and reduced delamination rate of organic coatings.13-15 The major group of protective coatings used are epoxy-based coatings, which provide high chemical, water, solvent and abrasion resistance for industrial, marine, and packaging applications.16 For steel protection, epoxy/amide systems are very popular as primers because of their good adhesive properties. The most widely used epoxy resins are DGEBA-based (diglycidyl ether of bisphenol A). These epoxy molecules are linked to nitrogen atoms of polyamide-based curing agents. The characteristic or functional groups of epoxy resins largely disappear on crosslinking or curing, although fully cured epoxy networks still contain functionalities, such as alcohol and amide/ amine groups, which are able to interact with the interfacial oxide layer.7,17,18 From this point of view, the understanding of the nature of the acid-base interfacial interactions between the * Corresponding author. E-mail:
[email protected]. † Vrije Universiteit Brussel. ‡ Arcelor Research Industry Gent-OCAS nv.
oxide surface and organic functionalities are of critical importance in determining the subsequent adhesional and mechanical properties. At the oxide surface, oxygen anions can act as electrondonating Lewis base sites, incompletely coordinated metal cations can act as electron accepting Lewis sites, and hydroxyl anions can act as either a Lewis acid or base, but also as protonexchanging Bro¨nsted acid-base sites.19,20 It is widely recognized that the acid-base properties of oxide surfaces alter the adhesion of organic compounds and coatings,20-23 but until now it is still ambiguous how the chemistry of the epoxy/oxide interface is influenced by these acid-base properties. X-ray photoelectron spectroscopy (XPS) has been employed successfully to characterize the oxide acid-base properties and to obtain quantitative information on the amount of hydroxyls in the outer oxide layer.22,24 The surface hydroxyl fraction can be obtained by deconvolution of the O 1s peak into single OH-, O-2 and possible H2O components.24-26 Furthermore, XPS has already proven its usefulness in the study of amine/amide nitrogen bonding with metallic substrates, when ultrathin films (no more than a few nanometers) of model compounds are adsorbed on the surface.4,7,27,28 By using model compounds with the same composition and organic functionalities as the organic coating studied, analysis of the interface existing between the two materials is possible. In this work, the influence of the iron oxide acid-base properties on the adsorption of epoxy model compounds is examined. From XPS data, the hydroxyl fraction of several oxide films is calculated. Subsequently, the interaction between the oxide films and nitrogenous model epoxy compounds is characterized.
10.1021/jp072354j CCC: $37.00 © 2007 American Chemical Society Published on Web 08/11/2007
13178 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Figure 1. Epoxy model compounds and their derivation from epoxy/ amide polymer components for N-methyldiethanolamine, N,N′-dimethylsuccinamide, and N,N′-diethylmethylamine.
2. Experimental Conditions 2.1. Sample Preparation. The substrate used was an interstitial free DC06 steel according to the EN10130(98) norm. The samples were cut into a circular shape, with a diameter of 3 cm and a thickness of 1 mm. 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 mirrorlike finish (rms roughness ≈ 2.5 nm). To form the various iron oxide layers, we treated the samples by one of the four different procedures described below. First, thermal oxide films were formed by putting the steel samples in an oven at 250 °C for 8 min under an oxygen-rich atmosphere, air. After the thermal treatment, the samples were cooled down to ambient temperature in open air. Second, “alkaline” oxides were formed by immersing the polished steel substrates in a NaOH containing Gardoclean 390 solution (Chemetall) at 70 °C for 8 min. The concentration used was 80 g/L. Third, steel samples were passivated in a borate buffer solution (0.075 M Na2B4O7‚10 H2O + 0.3 M H3BO3; pH ) 8.2) at 0.8 V 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 the passivation, the steel sample was reduced at -1 V for 30 min to remove the native oxide film. The fourth 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.15 After oxide application, the samples were ultrasonically cleaned sequencially in acetone and chloroform for 5 min to remove as much organic contamination as possible. 2.2. Adsorption of Organic Molecules. The model compounds representing the functional groups of a cured epoxy/ amide polymer were N-methyldiethanolamine and N,N′dimethylsuccinamide. 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.29 Both components and their derivation from the chosen epoxy/amide system are shown in Figure 1 Immediately after cleaning, the iron oxide films were immersed for 1 h in chloroform solutions containing either N-methyldiethanolamine or N,N′-dimethylsuccinamide with a concentration of 0.1 wt %.
Wielant et al. Additionally, the adsorption of a third compound, N,N′diethylmethylamine, was studied (see also Figure 1). This molecule has the same chemical structure as N-methyldiethanolamine but without the alcohol groups. A chloroform solution with a concentration of 0.1 wt % was used for adsorption. All compounds were purchased form Sigma-Aldrich as a >97.5% purity and were used without further purification. Substrates were withdrawn from the chloroform solutions after treatment, and the unbound molecules were rinsed from the surface using clean solvent. The samples were then transferred directly into the XPS ultravacuum chamber. 2.3. X-ray Photoelectron Spectroscopy (XPS). XPS characterization was performed with a PHI 1600/3057 instrument using an incident X-ray radiation (Mg KR1,2 ) 1253.6 eV). The vacuum pressure was approximately 5 × 10-9 Torr. Narrow multiplex scans were recorded with 29.35 eV pass energy and 0.1 eV step size. Prior to adsorption, measurements were performed on the prepared iron oxide surfaces at takeoff angles of 15°, 30°, and 45° with respect to the sample surface to study the oxide hydroxyl fraction and a possible enrichment of hydroxyls toward the outermost oxide surface. Calculation of the hydroxyl fraction starting from the XPS data involves the procedure described in next paragraph. For each oxide and each adsorbed organic molecule, at least four samples were investigated. To correct for sample charging, the spectra obtained on the untreated oxide layers were shifted to set the C-C/C-H components of the C 1s peak at a binding energy of 284.8 eV.30 The XPS spectra obtained after the molecular treatment were shifted with the same energy as the oxide layers before adsorption. Evaluation of the C 1s, O 1s, and N 1s peaks was carried out using the PHI Multipak V8.0 software. For curve fitting and decomposition, a simple Shirley-type background removal was performed on the data. A constrained fitting procedure was used in which the mixed Gauss-Lorentz shapes for the different fit components in the peaks were allowed to change in the 80-100% region. Only small variations in peak position and fwhm’s (full widths at half-maximum) were permitted. The χ2 value of the evaluated peaks after fitting was lower than 1.5 for all of the measurements. 3. Determination of Hydroxyl Fraction In this section, an expression for the hydroxyl fraction in an oxide layer, obtained by XPS, is deduced by starting from the basic XPS principles. The photoelectron intensity, IX, ejected with kinetic energy E from the nth core level of an atom (or ion) X in the studied substrate at depth z below the solid surface can be written as31
IX ) KσX
(
∞ z CX(z) exp -∫z)0 ∫z)0 λ
)
dz dz X(z) sin θ
(1)
where K is an instrumental factor, σX is the cross section for photoionisation of atom (or ion) X, CX is the atomic concentration for atom (or ion) X, λX(z) is the depth z-dependent inelastic mean free path (IMFP) of the photoelectrons with kinetic energy E originating from atom (or ion) X, and θ is the angle between the sample surface and the analyzer. If the atomic concentrations do not change significantly within the analysis depth and analyzer angle range, then CX and λX can be assumed to be depth-independent. Under these conditions, eq 1 is rewritten as follows:
IX ) KσXCX
(
)
∞ z exp dz ∫z)0 λX sin θ
(2)
Model Epoxy Compounds Studied by XPS
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13179 C 1s and O 1s levels, respectively. λC, λO are the IMFPs for C and O and cCO, cO-CdO, and cOH- are the atomic concentrations of C-O, O-CdO, and OH-. The denominator corresponds with the intensity of the O 1s subpeak at 532 eV, which contains contributions from OH-, C-O, and O-CdO. A similar equation can be formulated for the O2- component
IC(C-O) + IC(O-CdO) IO(O2-)
)
( (
σCλC(cC-O + cO-CdO) 1 - exp
( )
- tcont λC sin θ
- tcont σOλOcO2- exp λO sin θ Figure 2. Schematic representation of a multilayer system representing an oxide-covered steel substrate.
In the case of a multilayer system, the measured intensity InX of photoelectrons originating from atom (or ion) X in layer n can be expressed as follows:21 )
(∏ (
exp -
KσXCnX
i)1
λiX
)) ∫ ( tn
di
n-1
InX
exp -
sin θ
z)0
z λnX
sin θ
)
dz
(3)
The first product term takes into account the attenuation of the photoelectrons in the (n - 1) layers on top of layer n. CnX and λnX are, respectively, the atomic concentration and IMFP for atom (or ion) X in layer n, and tn is the thickness of layer n. For an oxide-covered steel substrate, the multilayer system is presented schematically in Figure 2. It consists of a metallic substrate of semi-infinite thickness (as compared to the IMFP of the photoelectrons), an oxide layer with thickness tox, and a contamination layer with thickness tcont. It is assumed that the contamination and oxide layers are distributed homogeneously across the XPS analysis area and have a constant composition and thickness.21,32 The measured O 1s signal contains contributions from the oxide matrix and the outermost layer of oxygencontaining organic carbon contamination, like C-O and O-Cd O species. Most oxygen functional groups in polymers give O 1s binding energies of ∼532 eV,30 or approximately at the same location as the OH- peak in the O 1s photopeak. Carbonyl oxygen in O-CdO species give an O 1s peak at ∼533 eV,30 or close to the position of the chemisorbed H2O peak in the O 1s peak. To obtain a reliable hydroxyl content, it is necessary to correct the O 1s photopeak for contributions from the C-O and O-CdO species. By using eq 3, the correction for O-Cd O and C-O contributions in the measured OH- peak can be expressed for thick oxide layers (i.e., tox . λO) assuming λcont O 21,24 then ≈ λox O ≈ λO,
IC(C-O) + IC(O-CdO) IO(C-O) + IO(O-CdO) + IO(OH-)
(
)
(
)) ))
- tcont λC sin θ ) I1 (4) - tcont σOλO(cC-O + cO-CdO) 1 - exp + λO sin θ - tcont σOλOcOH- exp λO sin θ σCλC(cC-O + cO-CdO) 1 - exp
(
(
(
)
The terms IX(Y) indicate the intensity in peak X for the group Y between the brackets. σC and σO are the cross sections of the
))
) I2 (5)
where cO2- is the atomic concentration of O2-. I1 and I2 are known from O 1s and C 1s peak deconvolution. After combining and rearranging eqs 4 and 5, the following expression for cO2-/ cOH- is obtained:
cO2- I1 ) cOH- I2
(
(
)) ( ( )) ( ( ))
- tcont λO sin θ - tcont σCλC 1 - exp λO sin θ - tcont I1σOλO 1 - exp λO sin θ σCλC 1 - exp
(6)
From this equation, the hydroxyl fraction can be calculated easily:
OH- fraction (%) )
1 × 100 cO21+ cOH-
( )
(7)
For thin oxide layers (i.e., when tox e λO), the same expression for cO2-/cOH- as in eq 6 can be found. 4. Results and Discussion 4.1. Hydroxyl Fraction of Oxide Layers. For all of the studied oxide samples, the XPS survey spectra showed Fe, O, and C to be the main elements. In Figure 3, typical highresolution C 1s and O 1s responses from the oxide surfaces are shown. Because no carbon contribution from the oxide layer was expected, the entire C 1s signal (Figure 3a) originated from adventitious carbon contamination. The C 1s peak was resolved into three different components, characteristic of C-O, O-Cd O, and C-C/C-H species.19,24,33 The observed binding energy separations were comparable with published values: 1.5 eV for C-O and 3.6 eV for O-CdO with respect to the C-C/C-H peak.19,21,24,33 The O 1s peak (Figure 3b) could be described accurately using three individual photopeaks, corresponding to O2-, OH-, and chemisorbed H2O. A detailed overview of the fitting results for the O 1s peak for the different oxide layers obtained at several takeoff angles are listed in Table 2 and comprise mean values with standard deviations of at least four different samples of each type of oxide. The O2-, OH-, and H2O peak positions were found to be nearly independent of the takeoff angle, and only relatively small variations in binding energy for the different iron oxides were observed. The peak separations between the oxygen peaks due to O2- and OH- were in the 1.5-1.7 eV range, whereas the separations between OHand chemisorbed water were between 1.3 and 1.5 eV. Similar peak intervals have been recorded by Simmons et al.19 and
13180 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Wielant et al. TABLE 2: Binding Energies and Peakwidth (fwhm) in eV Corresponding to O2-, OH-, and H2O Components of the Iron Oxide Layers as Obtained from Peak Deconvolution of the O 1s Peaksa OH-
O2type of oxide thermal oxide
alkaline oxide
borate buffer oxide
plasma oxide
STDEV
takeoff angle
BE (eV)
fwhm (eV)
BE (eV)
fwhm (eV)
H2O BE (eV)
fwhm tcont (eV) (nm)
15
529.85 1.50
531.55 1.70
532.80 1.35
30 45
529.80 1.50 529.85 1.50
531.50 1.75 531.55 1.70
532.75 1.25 532.80 1.25
15
530.00 1.55
531.60 1.80
533.00 1.55
30 45
530.00 1.55 529.95 1.55
531.60 1.80 531.55 1.80
533.00 1.55 532.90 1.55
15
529.80 1.55
531.35 1.95
532.85 1.65
30 45
529.80 1.60 529.85 1.60
531.35 1.85 531.35 1.80
532.85 1.50 532.85 1.35
15
530.00 1.50
531.65 1.90
533.00 1.70
30 45
529.95 1.55 529.95 1.50
531.60 1.80 531.55 1.75
533.00 1.60 532.95 1.45
0.10
0.10
0.10
0.05
0.05
0.05
0.44
0.27
0.34
0.41
0.06
a
The last column shows the averaged contamination layer thickness in nanometers as determined from the C 1s and O 1s photoelectron intensities.
Figure 3. Typical XPS C 1s (a) and O 1s (b) peaks for iron oxide layers. Deconvolution after subtraction of a Shirley-type background. The components have mixed Gauss-Lorentz shapes.
TABLE 1: Inelastic Mean Free Paths (λ) and Cross Sections (σ) for C and O Used in the Calculation of the Hydroxyl Fraction. element symbol
C
O
λ (nm) σa
2.3 24,48 0.157 49
1.1 19,34 0.463 49
a Relative to K 2p photoionisation cross section measured using a Mg K R source
Kurbatov et al.34 on iron oxide films. In addition, the intensity contribution of the H2O peak to the O 1s peak never reached values higher than 10%. The H2O intensity remained constant with changing takeoff angles (not shown in the table), meaning that the H2O signal originated from chemisorbed water vapor on top of the oxide surface. The hydroxyl fraction of the studied oxide layers was determined using the procedure outlined previously. In Table 1, the values for the cross sections σ and IMFPs λ used in the calculation are shown. An estimation for the contamination layer thickness, tcont, was made by minimizing the error between the calculated intensity ratio eq 4/eq 5 and the experimental data.35 The unknown concentrations for C-O, O-CdO, O2-, and the hydroxyl fraction were assumed to be, respectively, in the 0-10%, 0-10%, 40-90%, and 5-60% range, whereas contamination layer thickness was estimated to be in the 0.1-0.6 nm range.24 The iteration was stopped when the error was less than 1.10-8. It was calcuated that the contamination layer thickess altered between 0.25 and 0.45 nm, depending on the studied oxide layer (see Table 2). To determine the hydroxyl fraction of the studied oxide layers, we solved eqs 6 and 7 by using the corresponding contamination thicknesses and experimental XPS intensities. In Figure 4, the hydroxyl fractions for the studied iron oxides as a function of takeoff angle are shown. All oxide films contained hydroxyls to different extents with the surface layers (at low takeoff angles)
Figure 4. Hydroxyl fraction for the studied oxide layers as a function of takeoff angle. Four different measurements were performed on each oxide layer to calculate the average (columns) and standard deviations (error bars).
being more enriched with hydroxyls than the bulk. At the 15° takeoff angle, more of the outer surface scale of the oxide film is being probed when compared to the 45° takeoff angle, and this decrease in hydroxyl fraction with depth correlates with results found previously by other authors.21,24 The thermal oxide layers exhibited the lowest hydroxyl fraction at each studied angle, whereas the highest fraction was obtained on the plasma oxides. Oxide layers formed in aqueous solutions (alkaline and borate buffer solution) contained an intermediate level of hydroxyl fractions. The amount of hydroxyls can be explained by looking at the oxide structure or composition. Thermally formed oxide films are duplex oxide films composed of an inner core of magnetite (Fe3O4) and an outer scale of hematite (R Fe2O3), which only contains some surface hydroxyls due to the reaction of atmospheric water with the oxide surface.36,37 In the case of alkaline oxides, the surface hydroxyls are formed during the alkaline treatment.38 Alternatively, borate buffer and plasma oxide films have totally different hydroxyl localizations. Borate buffer oxides are expected to have an outer (oxy)hydroxide scale,39 whereas the outer scale of plasma oxides consists of oxyhydroxides.15 4.2. Adsorption of Epoxy Model Compounds. This section presents the XPS results obtained after adsorption of the
Model Epoxy Compounds Studied by XPS
Figure 5. Comparison of the normalized xPs O 1s signal obtained for untreated “borate buffer” oxide (solid line) and after one hour of immersion in chloroform solutions containing 0.1 wt % N,N′-dimethylsuccinamide (dash-dot line), N-methyldiethanolamine (dash line), and N,N′-diethylmethylamine (dot line).
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13181
Figure 7. Integrated N 1s peak intensities after adsorption of 0.1 wt % N,N′-dimethylsuccinamide, N-methyldiethanolamine, and N,N′diethylmethylamine on the iron oxide layers as a function of the hydroxyl fraction of the oxide films at a takeoff angle of 15°.
Figure 6. Comparison of the XPS N 1s spectrum obtained for untreated borate buffer oxide (solid line) and after 1 h of immersion in chloroform solutions containing 0.1 wt % N,N′-dimethylsuccinamide (dash-dot line), N-methyldiethanolamine (dash line), and N,N′-diethylmethylamine (dot line).
Figure 8. Deconvolution of the XPS N 1s peak after subtraction of a Shirley-type background. The components have mixed Gauss-Lorentz shapes.
previously described epoxy model compounds on the iron oxide layers. As a result of the molecular interactions, interesting features concerning the bonding mechanism were detected in the XPS peaks corresponding to oxygen and nitrogen. All of the O 1s peaks are normalized for comparison because the intensity changes are too complex to be analyzed quantitatively as such. In Figure 5, the normalized XPS O 1s response obtained after adsorption of the amine/amide molecules on the borate buffer oxide is presented. It can be clearly seen that the adsorbed molecules altered the O 1s signal in the 531-533 eV energy region, with the exception of N,N′-diethylmethylamine adsorption, which does not contain any oxygen bonds (see Figure 1). The O 1s binding energies of the adsorbed functionalities, oxide hydroxyls and oxygen containing contamination are all located in the 531-533 eV region.30 Because no peak change was noted after N,N′-diethylmethylamine adsorption, the oxide layer could be considered to be stable during treatment, so the O 1s change after N,N′-dimethylsuccinamide and N-methyldiethanolamine adsorption can only originate from the interaction of the nitrogenous molecules with the oxide surface. In the case of N-methyldiethanolamine adsorption, the O 1s signal enhancement around 532 eV is due to the contribution of alcohol groups originating from the adsorbed molecules (see Figure 5). These results indicate that no hydroxyl-consuming alkoxide bonds are formed as previously suggested by Fauquet et al. 40 on aluminum. Adsorption experiments of similar compounds on aluminum, titanium, and copper suggest that the alcohol groups interact with the oxide surface by forming hydrogen bonds.27-29 For the N,N′-dimethylsuccinamide case,
it was seen that the amide oxygens contributed to the highenergy part of the O 1s peak and it is generally accepted that these amide oxygens attach on the oxide surface through hydrogen bonding.29,41,42 As can be seen in Figure 6, the N 1s peak intensity after adsorption of the amine/amide molecules clearly increases with respect to the untreated borate oxide sample. The nitrogen peak consists of a main peak located at a binding energy of ∼400 eV and exhibits a small shoulder at the high-energy side of the peak. This energy shoulder becomes more pronounced after N,N′-dimethylsuccinamide adsorption than after adsorption of both amine types. The N 1s peak can be used for further (quantitative and qualitative) interpretations because the observed nitrogen peak originates almost entirely from the adsorbed nitrogenous molecules, and its shape is therefore a direct observation of the interfacial chemistry occurring between the nitrogen atoms and the oxide surface. The N 1s peak intensity and its peak shape were very stable under X-ray exposure. In Figure 7, the integrated N 1s intensity is shown as a function of the hydroxyl fraction obtained on the different oxide layers at a takeoff angle of 15°. The reported peak areas are averaged values; the error bars are the standard deviation values calculated from at the least four different measurements and comprise a maximum of 15% of the calculated average. From the results, a limited effect of the hydroxyl fraction on the amount of adsorbed molecules was observed with only N,N′-diethylmethylamine displaying a lower amount of molecules at high hydroxyl fraction. Currently, no explanation for this phenomenon can be given.
13182 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Wielant et al.
TABLE 3: Binding Energy (eV) of Main C 1s and Deconvoluted N 1s Peaks for the Adsorbed Nitrogenous Molecules. type of oxide
OH- fraction (%)
N,N′-dimethylsuccinamide
N-methyldiethanolamine
N,N′-diethylmethylamine
C 1s
N 1s
C 1s
N 1s
C 1s
N 1s
400.2 402.1 400.1 402.0 400.3 402.2 400.0 401.9
285.4
399.9 401.8 399.9 401.8 399.9 401.8 399.9 401.8
285.2
399.9 401.7 399.8 401.7 399.7 401.6 399.8 401.7
thermal oxide
13.8
285.1
alkaline oxide
17.4
285.1
borate buffer oxide
30.0
285.1
plasma oxide
49.1
285.1
For equal numbers of molecules, equivalent N 1s peak intensities are expected for N-methyldiethanolamine and N,N′diethylethylamine. It is observed that the integrated N 1s peak intensities are significantly lower for N,N′-diethylmethylamine than for N-methyldiethanolamine. Thus, less-adsorbed N,N′diethylmethylamine molecules are to be found on the oxide surface compared to N-methyldiethanolamine. The only difference in chemical composition between both amine molecules is the presence of alcohol groups in the N-methyldiethanolamine molecule. Therefore, it can be concluded that the presence of alcohol groups promotes the adsorption of the latter amine by forming hydrogen bonds with the oxide surface and fixes the molecules in the vicinity of the surface.29 The thickness of the amine/amide organic layers can be calculated by using the XPS intensity of the substrate before and after adsorption.31 The thicknesses were between 0.5 and 1.2 nm depending on the type of adsorbing molecule and oxide film. The N 1s peaks, as displayed in Figure 6, can be deconvoluted into two individual components: N1 and N2 (see Figure 8). The binding energies of the deconvoluted N 1s peaks obtained on the different iron oxides after treating with the epoxy model compounds are summarized in Table 3. These results show the energy shift to be constant throughout all measurements and equal to 1.9 eV. With this peak shift, the error between the experimental data and simulated data is minimal. After adsorption of comparable nitrogeneous compounds, Marsh et al.27,28 observed on aluminum and titanium peak separations of 1.7 and 2.2 eV, respectively. On steel or iron, peak separations from 1.5 to 1.9 eV for nitrogenous inhibitors have been published by several workers.12,43,44 It is presumed that the two N 1s subpeaks correspond to the two different bonding modes as illustrated in Figure 9. The component at low binding energy, N1, is assigned to Lewis-like acid-base bonding, with direct donation of nitrogen lone-pair electron density to metal cationic sites (Figure 9b).19,27,28,43 The second peak at higher binding energy can be attributed to protonation of the nitrogen atom, where protonic Bronsted-like bonding of the nitrogen lone pair with the hydrogen of a hydroxyl present on the oxide surface takes place (Figure 9c). In literature, two further possible interactions of the protonated nitrogen with the oxide surface are suggested. First, it is proposed that the molecule is fixed electrostatically to the oxide surface due to the positive charge of the protonated nitrogen atom.45-47 Alternatively, it is put forward that the protonated nitrogen interacts via hydrogen bonding of the protonated nitrogen proton to an oxygen atom in close proximity to the binding site.27,28 One complicating factor in the data interpretation is that the XPS response of unbound amine/amides is generally considered to be very similar to that of Lewis bonded amine/amide groups. The N 1s response of unbound amine/amide groups is located at approximately 400 eV.30 N,N′-dimethylsuccinamide consists of two amide groups; thus, some unbound amide groups could partially contribute to the N1 component. In the case of N-methyldiethanolamine adsorption, it cannot be stated unequivocally that the entire N1
285.4 285.4 285.3
285.1 285.2 285.2
intensity originates form Lewis interactions. The latter molecule is not necessarily bound to the surface through the amine functional group because the molecule could be attached via hydrogen bonds between the alcohol groups and the oxide surface. The fact that a clear N 1s peak appears in the XPS spectrum after treating the oxide layers with N,N′-diethylmethylamine (Figure 6), reveals that adsorption can also occur through Lewis acid-base interactions between basic nitrogen atoms and acidic iron cations. Thus, for the following it is assumed that the fraction of unbound amine/amide groups is very small compared to the Lewis bound ones. In Figure 10, the relative peak area of the N1 and N2 subpeaks after N 1s peak deconvolution for the oxide samples treated with the amide and amine molecules is shown as a function of the hydroxyl fraction of the oxide surfaces at the 15° takeoff angle. For all molecule studies, it is immediately obvious that the relative peak area of N2 increases with the hydroxyl fraction of the oxide layer. Up to 30% of the amide nitrogens adsorbed on the highly hydroxylated oxides via the nitrogen protonation mechanism. After treating the oxide surfaces with N-methyldiethanolamine, it is observed that less than 20% of the amines bound to the surface via bronsted-like interactions at high hydroxyl fractions, whereas after N,N′-diethylmethylamine adsorption a level of 15% is never exceeded. Combining these results with the integrated N1s peak intensities in Figure 7, it can be concluded that, under the experimental conditions utilized, the primary bonding mechanism between the oxide surface and the amine or amide moecules is via Lewis-like acid-base bonding completed with Bronsted-like interactions. The relative contributions of Lewis and Bronsted interactions depend on the available protonic sites; thus, strictly speaking, a higher amount of available protons will result in a higher protonation level. A second factor influencing the mode of bonding is the chemical structure and consequently the mobility of the adsorbing molecules. Molecules with several functional groups are expected to be protonated to a higher degree than molecules with one functionality because of the structural strain arising from attempting to adsorb all groups simultaneously.43 Buckling of the molecule during adsorption does not permit all nitrogens to coordinate with metal cations. Each N,N′-dimethylsuccinamide molecule contained two amide oxygens and amide nitrogens that are available to bind with the oxide surface. It is well known that amide oxygens like to interact with oxide surfaces by hydrogen-bond formation.27,41,42 The amide nitrogens are very close to the amide oxygens in the molecular chain, resulting in a strongly reduced mobility of the amide nitrogen. In this situation, a relatively high number of nitrogens interacts with the surface by protonic Bronsted bonding. Another explanation for the relatively high fraction of Bronsted interactions could be due to bonding through nitrogenous protons and oxygen lone pairs on the oxide surface. This is disproved as a possibility because the bonding mechanism of amine/amides is hardly influenced by the number of nitrogenous protons.28 Moreover, N-methyldiethanolamine and N,N′-diethylmethy-
Model Epoxy Compounds Studied by XPS
Figure 9. Illustration of bonding of amine/amide compounds with the iron oxide surface: the oxide surface approximated by amine/amide compounds (a), Lewis-like acid-base bonding (b), Bronsted-like acidbase bonding, or nitrogen proteonation (c).
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13183 the bulk. For the adsorption modeling, organic molecules with the same functionalities as those in the epoxy/amide system were applied: N,N′-dimethylsuccinamide, N-methyldiethanolamine, and N,N′-diethylmethylamine. The adsorption mechanism was studied mainly by examination of the O 1s and N 1s XPS peaks. From N 1s peak integration, it could be concluded that the amount of adsorbed molecules was independent of the hydroxyl fraction of the oxide layer. Two different bonding modes between the amine/amide nitrogens and the oxide surface were observed: via Lewis-like acid-base interactions between basic nitrogenous lone pair electrons and metal acidic cation sites and via Bronsted-like interactions between hydroxyl hydrogens and nitrogenous lone-pair electrons (nitrogen protonation). It was noted that adsorbed N,N′-dimethylsuccinamide molecules had the highest and N,N′-diethylmethylamine the lowest protonation level of the studied molecules with Lewis acid-base interactions being the principal bonding mode for all studied molecules and oxide layers. It was shown that the protonation level depended on the adsorbing functional groups and mobility of the adsorbed molecules. The oxide surface also altered the protonation level. A high number of hydroxyls on the oxide surface promoted bonding of the amine/amide nitrogens via protonation. Acknowledgment. J.W. is financed by the Flemish Institute for the promotion of scientific and technological research in industry (IWT). His work is realised with the financial support of Arcelor Research Industry Gent-OCAS nv. This work is part of the project entitled “Nanometer scale electron spectroscopy of organic layers on oxide surfaces”, financed by the Research Foundation - Flanders (FWO). We are grateful to M. Giza from the Max-Planck-Institute for Iron Research (MPIE) for his work in preparing the iron oxide layers formed by plasma treatment. References and Notes
Figure 10. Relative peak area of N1 and N2 after N 1s peak deconvolution for the oxide samples treated for 1 h in a chloroform solution containing 0.1 wt % N,N′-dimethylsuccinamide, N-methyldiethanolamine, or N,N′-diethylmethylamine (as a function of the hydroxyl fraction of the oxide films at a takeoff angle of 15°).
lamine have no nitrogenous protons, but a small amount of nitrogen is still adsorbed via Bronsted interactions. In the N-methyldiethanolamine molecule, the distance between adsorbing alcohol groups and amine nitrogens is higher, giving the amine nitrogen higher mobility and resulting in a lower fraction of protonated nitrogens. The N,N′-diethylmethylamine molecules contain only one adsorbing functionality, the amine nitrogen, which makes these molecules unsusceptible to protonation. Only on highly hydroxylated surfaces is a small amount of protonated nitrogens observed. 5. Conclusions The aim of this work was to examine the influence of the iron oxide acid-base properties on the adsorption of epoxy model compounds. To study this, iron oxide layers with different surface hydroxyl fractions were prepared in controllable and reproducible conditions. An expression for the hydroxyl fraction of the oxide films was deduced, starting from the measured XPS intensities. The hydroxyl fraction of the studied oxide layers increased in the order of thermal oxide < alkaline oxide < borate buffer oxide < plasma oxide, with the surface of the oxide layers being enriched with hydroxyls when compared to
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