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Interaction of Epoxy Analogue Molecules with Organosilane-Treated Aluminum: A Study by XPS and ToF-SIMS Marie-Laure Abel, Acharawan Rattana, and John F. Watts* School of Mechanical and Materials Engineering, University of Surrey, Guildford Surrey GU2 7XH, U.K. Received December 1, 1999. In Final Form: May 22, 2000 Specimens treated with solutions of an epoxy analogue molecule, diethanolamine (DEA), have been studied by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The initial substrates were of two different types: grit-blasted aluminum and aluminum coated with an organosilane adhesion promoter, γ-glycidoxy propyl trimethoxy silane (GPS). Specific interactions between the aluminum substrate and/or GPS treated aluminum and DEA are proposed on the basis of XPS and SIMS data. On oxidized aluminum, DEA forms either a single alkoxy or two alkoxy bonds (bridge). The formation of the so-called bridge brings the nitrogen atom close to the substrate. The nitrogen atom of DEA interacts with the surface via formation of a hydrogen bond or via donor-acceptor interaction with an aluminum atom. When deposited onto GPS-coated aluminum, DEA undergoes two types of interactions: formation of covalent bond with nucleophilic addition or Bro¨nsted type of interaction between nitrogen of DEA and silanol functionality of hydrolyzed GPS.
Introduction The epoxy resins are widely used as adhesives; their excellent adhesion to many substrates, good cohesive strength, low shrinkage on cure, absence of volatile solvents, and low creep have led to structural applications as adhesives, particularly for metal to metal and metal to plastics bonding. For example, they are one of the most widely used adhesives in aerospace. In this same industry, interest has increasingly grown in replacing harmful metallic pretreatments by safer alternatives. An organosilane primer has been used for aircraft repairs on aluminum1 as a replacement for chromium rinse (CrVI). To understand and explain the durability of adhesive joints prepared in this way, it important to know which interactions occur within the joint at various interfaces, in particular the manner in which epoxy resins and the silane primer interact. XPS and ToF-SIMS are valuable tools for the study of interfacial chemistry of adhesion. The usual manner in which specific interactions at the interface are studied by surface analysis techniques is as follows. A very thin film of material is deposited on the substrate or material of choice by spin casting or simple adsorption followed by rinsing. It must be thin enough (no more than few nanometers) to allow analysis of the interface existing between the two materials.2,3 However, this is not readily achievable when using cured thermosetting resins such as amine cured epoxy resins, as they are not readily soluble in any solvent. The alternative is to use an analogue molecule of similar size, shape, and organic functionalities in order to mimic the interaction of an epoxy resin with a silane primer. Amine curing agents are often used in conjunction with epoxy systems.4 The amine used is generally of primary or secondary type, the whole process of cross-linking being catalyzed by the presence of tertiary * Corresponding author. (1) Digby, R. P.; Shaw, S. J. Int. J. Adhes. Adhes. 1998, 18, 261. (2) Leadley, S. R.; Watts, J. F. J. of Adhes. 1997, 60, 175. (3) Leadley, S. R.; Watts, J. F. J. Electron. Spectrosc. 1997, 85, 175. (4) Brydson, J. A., Ed.; Plastics Materials; Butterworth/Linacre House: Oxford, 1995.
(a)
(b) Figure 1. (a) Main reaction involved in the cross-linking of an epoxy resin using a primary amine curing agent. (b) Structure of diethanolamine or DEA, mimic of an epoxy resin cured with a primary amine.
amines. Figure 1a shows an example of curing an epoxy resin with a primary amine. It is seen that in the β position from the amine an alcohol functionality is present after opening of the epoxy ring during cross-linking. Therefore, diethanolamine (Figure 1b) was chosen to mimic an epoxy resin cross-linked with a primary amine because it is readily soluble in absolute ethanol and presents a similar structure to that which is expected in the cross-linked resin. This work presents the results and interpretation on DEA adsorption on grit-blasted aluminum and gritblasted aluminum treated with GPS, using a similar procedure to that described by Murase et al.5 Experimental Section 1. Materials and Methods. Aluminum sheets of commercial purity (>98%) were supplied by Goodfellows Ltd Cambridge. Samples were prepared according to the standard industrial procedure.1 This consisted in degreasing the aluminum using detergent and tap water, then grit blasting after drying with fresh alumina grit (50 µm). Samples were punched to disks of 1 cm diameter and subsequently degreased in flowing acetone or 2-propanol. They were then brushed for 2 min with a 1% aqueous solution of GPS (OSI Silquest, product designation A187) (5) Murase, M.; Brown, A. M.; Watts, J. F. J. Mater. Chem. 1999, 9, 1211.
10.1021/la9915724 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
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Langmuir, Vol. 16, No. 16, 2000 6511 Table 1. Surface Composition (at %) for the Various Samples
Figure 2. Hydrolysis reaction of γ-GPS. that had been hydrolyzed for 60 min (full hydrolysis) in reverse osmosis, deionized and carbon filtered water. The samples were dried in an oven at 93 °C for 60 min. Samples were then treated with 0.01 M or 0.5 M solutions of DEA (BDH, 99.5%, used without further purification, main impurities triethanolamine and monoethanolamine) in absolute ethanol (Haymann Ltd.) for 30 s. They were subsequently rinsed in absolute ethanol for 5 s and left to dry in air at least half an hour before analysis. Some control samples were prepared by dipping grit-blasted and degreased aluminum disks in DEA solutions using the same procedure. In addition, a platinum sample degreased in acetone and coated with several drops of the most concentrated solution of DEA (0.5 M), was used as a DEA film reference sample. A schematic of GPS hydrolysis reaction is given in Figure 2. 2. XPS Analysis. XPS analyses were carried out using a VG Scientific ESCALAB MkII system operated in the constant analyzer energy mode. Mg KR radiation, with an energy of 1253.6 eV, was used in all analyses. Digital acquisition was achieved with a VGS5000 data system based on a DEC PDP11/53 computer utilizing DEC µRSX software interfaced to the spectrometer. Survey spectra were acquired with a pass energy of 100 eV, and the high resolution spectra with a pass energy of 50 eV for the core levels of interest. To prevent sample degradation during analyses, the source position was set slightly away from the sample. It is well-known that organic materials can be partly damaged from either the X-ray irradiation, heating of the sample or inelastic scattering of the electrons within the sample during the analysis. The use of a 50 eV analyzer energy allowed good quality spectra to be acquired despite the reduction in X-ray flux brought about by moving the gun away from the sample during analysis. The takeoff angle was set at 45° and the area of analysis was 5 × 2 mm2. Quantification and peak fitting, based on the high-resolution spectra, were obtained using the manufacter’s software. Wagner sensitivity factors were used for the quantification procedures. The C 1s and N 1s spectra were peak fitted using the manufacturer’s software. 3. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Analysis. 3.1. ToF-SIMS Analysis. ToF-SIMS spectra were acquired using on a VG Scientific type 23 system. This instrument is equipped with a double stage reflectron timeof-flight analyzer and a Gallium pulsed liquid-metal ion source. Static SIMS conditions (i.e., total ion dose less than 1 × 1013 ions/cm2/analysis) were employed using a pulsed (10 kHz and ca. 30 ns) 20 keV 69Ga+ primary ion beam rastered over a frame area of 1 mm2 at 50 frame s-1. SIMS spectra were acquired over a mass range of 5 to 600 Da in both the positive and negative modes (mass veto ) 0-5 Da). 3.2. ToF-SIMS Analysis at High Resolution. ToF-SIMS spectra were also acquired using a Physical Electronics PHI 7200 TOF/SALI system. This instrument is equipped with a reflectron time-of-flight analyzer and a Cesium pulsed liquid-metal ion source. Static SIMS conditions were employed using a pulsed (5 kHz and ca. 1 ns) 8 keV 133Cs+ primary ion beam rastered over an area of 200 × 200 µm2. SIMS spectra were acquired over a mass range of 5 to 1000 Da in both the positive and negative modes. The resolution (m/∆m) achieved on the grit-blasted samples was 3800 at mass 41 and 4300 at mass 247. Calibration was achieved using exact masses for the fragments C2H3+ (27), C3H5+ (41), and C5H11+ (71). To assess whether the correlation between the assigned structure and the mass of experimentally determined ions is correct we used the concept of mass accuracy, delta, defined as:
∆)
|Mexptl - Mr| Mr
(1)
specimen treatment
C
O
Al
Si
N
Ca
Na
Al grit blast degreased Al/GPS 1% 93 °C dry Al/DEA 0.01 M Al/DEA 0.5 M Al/GPS/DEA 0.01 M Al/GPS/DEA 0.5 M
16.1 31.5 24.4 27.2 23.1 29.4
59.9 51.3 49.5 49.4 50.2 45.7
20.7 9.2 19.5 15.6 18.0 15.0
2.3 5.3 1.5 2.0 4.4 4.7
0.7 1.1 2.3 1.6 2.7
3.7 2.3 3.3 3.3 2.5 2.3
0.6 0.4 0.5 0.2 0.2 0.2
Table 2. Composition of the Alumina Grit Used for Grit Blasting materials
concentration (wt %)
Al2O3 SiO2 Fe2O3 TiO2 CaO Na2O K2O
balance 0.02 0.03 0.01 maximum 0.01 0.2 0.01 maximum
where Mexptl is the experimental mass obtained from the high mass resolution spectrum and Mr is the mass calculated from exact masses. When the high-resolution mass is given (Table 4), the correlation is striking with a delta parameter always below 20 ppm which indicates that the ion assignment is correct.
Results 1. XPS Results. Survey spectra are shown in Figure 3 for aluminum first grit-blasted only then (a) treated with DEA solution 0.01 M, (b) DEA solution 0.5 M, (c) aluminum coated with GPS, and (d) then treated with DEA solution (e) 0.01 M and (f) 0.5 M, respectively. The following signals are clearly visible: C 1s, O 1s with their respective Auger counterparts as well as N 1s nitrogen signal indicative of DEA adsorption (ca. 400 eV) for the specimens treated with DEA. Other unexpected signals are visible such as Ca2p, Na1s, and Si2p (present on gritblasted only sample). Another signal is visible around 439 eV and is assigned to Ca2s. The shape of the background, indicating more energy loss via inelastic scattering of electrons, also shows that the sample coated by GPS bears a thicker layer of organic material than any of the other specimens.6 Table 1 presents the surface composition for all samples examined. Grit-blasted aluminum exhibits a low carbon concentration together with high concentration of oxygen and aluminum. This shows the surface is very clean as the carbon level from adventitious contamination is below 20 at %. The high proportion of oxygen is arising from the native oxide of aluminum formed in ambient atmosphere following grit-blasting. Other elements showing the presence of impurities were detected on the surface. Because pure water was used in the washing/hydrating and hydrolysis steps of the process, calcium and sodium probably originate from the grit used for grit blasting. This is the most likely explanation for their presence on the surface as indicated by the actual composition of the grit given in Table 2, although the concentration of calcium is relatively small for the bulk alumina grit compared to the calcium concentration obtained on the surface. This is reinforced by the fact that their intensity is decreasing with increasing coverage of the substrate, i.e., addition of GPS and/or DEA coating. In all cases, the adsorption of organic material is shown by the increase of carbon and nitrogen or silicon concentrations, and a decrease in oxygen and aluminum concentration. DEA was adsorbed either on grit-blasted (6) Watts, J. F. An Introduction to Surface Analysis by Electron Spectroscopy; Oxford University Press: New York, 1990; Chapter 1.
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Figure 3. XPS survey scane of (a) grit-blasted aluminum only, (b) grit-blasted aluminum treated with DEA 0.01 M, (c) grit-blasted aluminum treated with DEA 0.5 M, (d) grit-blasted aluminum treated with GPS, (e) grit-blasted aluminum treated with GPS then DEA 0.01 M, (f) grit-blasted aluminum treated with GPS then DEA 0.5 M. Table 3. Binding Energies (eV) and Assignments of Nitrogen Signal with Their Respective Proportions specimen treatment
peak 1 (%)
peak 2 (%)
Al/DEA 399.5 (49) 401.3 (51) 0.01 M Al/DEA 399.5 (50) 400.8 (50) 0.5 M Al/GPS/DEA 400.4 (76) 402.4 (24) 0.01 M Al/GPS/DEA 400.2 (69) 401.8 (31) 0.5 M assignment -C-NH-C- -NHδ+-a -NHδ+- -C-NH2+-Ca
Figure 4. DEA uptake (nitrogen at %) versus type of substrate and concentration.
aluminum or on GPS-coated aluminum. Table 1 and Figure 4 show that two factors lead to the highest concentration of nitrogen or highest uptake of DEA, increase in DEA concentration and presence of GPS on the aluminum surface. The concentration of nitrogen, indicative of DEA adsorption, increases with DEA concentration and is significantly improved if GPS-coated aluminum is used versus grit-blasted aluminum only. Similarly, the aluminum concentration is the lowest for samples treated by both GPS and DEA. This suggests a more favorable adsorption of DEA on GPS-coated aluminum compared to untreated aluminum. The intensity of the silicon signal decreases for the samples treated with DEA compared to the grit-blasted sample but increases for the samples treated with GPS prior to DEA adsorption. The concentration of silicon on samples treated GPS and by both GPS and DEA is significantly intense and allows discrimination between the adsorbate and the contamination on the initial substrate.
δ in the second column smaller than δ in the third column.
High-resolution spectra of nitrogen and carbon were peak fitted to identify functionalities present at the surface of the samples. The results are presented for nitrogen in Table 3 and Figure 5 and are corrected from charging using hydrocarbon as an internal reference. Although not presented here, the carbon signal was peak fitted with two components. The first one, set at 285 eV for internal reference, was assigned to adventitious and unfunctionalized carbon. The second component exhibited a chemical shift ranging from 1.6 to 2.0 eV corresponding to oxidized carbon species from alcohol to epoxy functionality. It was not possible to include an additional C-N type component in the peak fitting, and it is thought this component is weak in intensity as indicated by the nitrogen signal. It should be noted that the chemical shift for carbon bound to partially charged nitrogen is slightly higher than reported in the literature.7,8 From peak-fitting the nitrogen signal, two components are obtained for all samples treated with DEA. When DEA is adsorbed onto aluminum, they can be assigned to neutral (7) Liang, W.; Lei, J.; Martin, C. R. Synth. Met. 1992, 46, 53. (8) Malitesta, C.; Losito, I.; Sabattini, L.; Zambonin, P. G. J. Electron Spectrosc. Relat. Phenom. 1995, 76, 629.
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Figure 5. High resolution XPS scans of peak-fitted N 1s signal for (a) grit-blasted aluminum treated with DEA 0.01 M, (b) grit-blasted aluminum treated with DEA 0.5 M, (c) grit-blasted aluminum treated with GPS then DEA 0.01 M, (d) grit-blasted aluminum treated with GPS then DEA 0.5 M.
nitrogen9,10 and partially charged nitrogen suggesting that two types of interaction occur between DEA and aluminum. For samples treated by both GPS and DEA two types of nitrogen are also to be found and can be assigned to partially charged nitrogen and/or neutral nitrogen, and quaternary nitrogen, suggesting in this case a strong interaction of the nitrogen from DEA with the GPS coated surface. 2. SIMS Results. Figure 6 shows the positive SIMS spectrum (m/z ) 0-200 Da) in the positive mode of gritblasted aluminum treated with GPS. The assignment of the fragments is discussed elsewhere,11,12,13 but for reference the most intense can be recalled. Fragments obtained at m/z ) 23, 27, and 40 are assigned to Na+, Al+, and Ca+ respectively. The m/z ) 28 ion is assigned partly to silicon Si+. Mass 57 is assigned to a fragment obtained from the epoxy end of GPS, C3H5O+, but also to calcium hydroxide CaOH+. Fragments at m/z ) 149, 91, and 77 arise from a phthalate type of molecule present in the water used for GPS hydrolysis resulting from partial dissolution of the container plasticizer. Few hydrocarbon fragments consecutive to breaking within the GPS chain are also present and can be assigned as follows: 29 (C2H5+), 41 (C3H5+), and 43 (C3H7+). Please note that there is no intense even mass fragment present in the spectrum, which should be diagnostic of a nitrogen-containing species. Figure 7a to d show the positive SIMS spectra (m/z ) 0-200 Da) for aluminum treated with DEA at 0.01 and 0.5 M concentrations, and aluminum treated by GPS then DEA at 0.01 and 0.5 M concentrations, respectively. It is obvious that most fragments are of even masses and can therefore be assigned to the presence of DEA on the surface according to the nitrogen rule.14 All fragments are listed in Table 4 with their assignments, some of them have (9) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Molder, J. F.; Mu¨llenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Mu¨llenberg, G. E., Ed.; Perkin-Elmer Corporation: Eden Prairie, 1979. (10) Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, 1990; Vol. 1. (11) Abel, M-L.; Watts, J. F.; Digby, R. P. Int. J. Adhes. Adhes. 1998, 18, 179. (12) Rattana, A. MSc Thesis Dissertation, University of Surrey, 1999. (13) Abel, M-L.; Digby, R. P.; Fletcher, I. W.; Watts, J. F. Surface Int. Anal. 2000, 29, 115.
been confirmed with high-resolution ToF-SIMS and are then given with delta parameter. Exact masses were used for the calculation of theoretical structures masses.15 3. Fragmentation of DEA Dimer and DEA Molecule. Several of the fragments are obtained at masses above the molecular mass of DEA, such as m/z ) 116, 118, 130, 177, 184 186, or 190. Although not all structures have been identified, it is thought that m/z ) 116 and 118 as well as 100 and 98 arise from the fragmentation of DEA dimer of mass 192. The fragmentation pattern of the DEA dimer is illustrated in Figure 8. The presence of a dimer is somewhat unexpected although nitrogen containing molecules are used as catalysts in many adhesion science applications. Furthermore, and as described in following sections, the fragmentation pattern of this dimer is confirmed by exact mass determination of the subsequent fragments. The presence of dimer in the DEA solution can partly be explained by the presence of triethanolamine. It is indeed well-known that tertiary amines are used as catalysts, particularly in the presence of primary amine curing agent, to cross-linked epoxy resins.4 The presence of the amino group in the DEA itself may also contribute to a catalysis effect. This is a recognized effect in various molecules such as aminosilanes where the presence of an amino group contributes the accelerate the reaction of hydrolysis and autocondensation.16 Masses below 105 are more likely to occur from the fragmentation of the DEA molecule itself, although they can obviously result from the fragmentation of the DEA dimer as well. Several fragments have been identified such as m/z ) 88, 86, 74, 72, 70, 62, 58, 56, 30, and 28. Their structures can be found in Table 4 with their respective mass accuracies. The fragment 88 is of specific interest as it may result from (M-OH)+ consecutively to dehydration of the DEA molecule. 4. Specific Interactions. Figure 9 shows a range of (14) MacLafferty, F. W.; Turecˇek F. Interpretation of Mass Spectra, 4th ed.; University Science Books: Mill Valley, 1993. (15) IUPAC Commission on Atomic Weights. Pure Appl. Chem. 1984, 56, 653. (16) Blum, F. D.; Meesiri, W.; Kang, H-J.; Cambogi Silanes and Other Coupling Agents; Mittal, K. L., Ed.; VSP: Utrecht, 1992.
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Figure 6. Survey SIMS spectrum (m/z ) 0 to 200 Da) in the positive mode of grit-blasted aluminum treated with GPS. Table 4. Fragments and Their Respective Assignments nominal mass 28 30 40 42 44 56 58 62 70 72 74 86 88
structure CHtNH+
CH2dNH2+ +CHdCdNH
CH2dCdNH2+ CH3-CHdNH2+ CH2dCH-NH-CH2+, CH2dCH-N+H CH2 CH3-CH2-N+HdCH2, OdCH-CHdNH2+ NH3+-CH2-CH2-OH, NH2-CH2-CH2-OH2+ CH2dCH-NH-CH2-CH2+, CH2dCH-N+HdCH-CH3 CH3-CH2-NH-CH2-CH2+, CH3-CH2-N+HdCH-CH3 HO-CH2-CH2-N+HdCH2, CH3-CH2-N+H2-CH2-CH3 HO-CHdCH-N+HdCH-CH3 HO-CH2-CH2-N+HdCH-CH3, HO-CH2-CH2-N+H2CHdCH2
nominal mass
experimental mass (Daltons)
exact mass (Daltons) from ref 12
|∆| (ppm)
formula
98 100 106 116 118 130 142 144 247
98.0601 100.0760 106.0876 116.0717 118.0877 130.0858 142.0883 144.1049 247.1633
98.0606 100.0762 106.0868 116.0712 118.0868 130.0868 142.0868 144.1025 247.1658
5 3 8 4 8 8 11 17 10
C5H8NO+ C5H10NO+ C4H12NO2+ C5H10NO2+ C5H12NO2+ C6H12NO2+ C7H12NO2+ C7H14NO2+ C11H23N2O4+
positive SIMS spectra encompassing GPS treatment and DEA adsorption on aluminum. Figure 9b,d shows a
striking feature which is absent of Figure 9a,c at nominal mass m/z ) 247. It is only present when aluminum has been coated with GPS prior to DEA adsorption. This fragment could therefore be characteristic of a specific interaction between DEA and hydrolyzed GPS. Another important feature can be noticed in the SIMS survey spectra (Figures 7a to d). A fragment present at nominal mass 106 has been assigned to protonated DEA. It is very intense compared to the molecular fragment of DEA expected at m/z ) 105 which is hardly noticeable in these spectra. This feature exhibits a higher intensity when samples are treated with the least concentrated solution of DEA. The presence of fragments at masses higher than the molecular mass of DEA together with the presence of protonated DEA shows that the epoxy-like molecule has adsorbed intact on the surface. To confirm whether the fragment obtained at m/z ) 106 shows a genuine interaction between DEA and/or DEA dimer and aluminum or GPS, a platinum sample coated with several drops of the most concentrated solution of DEA (0.5 M) was also examined by ToF-SIMS analysis. Figure 10a-c show a narrow mass window between m/z ) 102 and 110 for an (a) aluminum sample treated with DEA 0.01 M, an (b) aluminum sample treated with GPS then with DEA 0.01 M, and finally a platinum sample coated with (c) DEA solution, respectively. Platinum was chosen as a substrate as it is a noble material and it is thought that this metal will not then interact with DEA. The melting point of DEA is close to room temperature (25 °C) and to investigate possible evaporation, a thick film was deposited
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Figure 7. Survey SIMS spectra (m/z ) 0 to 200 Da) in the positive mode of (a) grit-blasted aluminum treated with DEA 0.01 M, (b) grit-blasted aluminum treated with DEA 0.5 M, (c) grit-blasted aluminum treated with GPS then DEA 0.01 M, (d) grit-blasted aluminum treated with GPS than DEA 0.5 M.
Figure 8. Fragmentation pattern of DEA dimer leading to fragments at m/z ) 118, 116, 100, and 98.
and the aspect of the sample assessed visually before and after exposure to ultrahigh vacuum for many hours. No change of the sample or degradation of the ultrahigh vacuum was observed. The main fragment in Figures 10a and b shows the fragment of protonated DEA (M+1). These spectra are in contrast with the spectrum of pure DEA, for which this fragment is visible but not as intense being merely one of several seen within this portion of the spectrum. For aluminum and aluminum coated with GPS spectra, the 106 ion is clearly the dominant one. This is an indication that DEA interacts with aluminum and GPS with a similar type of interaction and also that it does not happen on platinum. The peak intensities of m/z ) 247 and m/z ) 106 ions were evaluated, when applicable, and are presented in Table 5. It can be seen that, although m/z ) 106 is present in the spectrum of pure DEA on platinum,
Figure 9. SIMS positive spectra in the mass window of m/z ) 240 to 255 Da of (a) grit-blasted aluminum treated with DEA 0.01 M, (b) grit-blasted aluminum treated with GPS then DEA 0.01 M, (c) grit-blasted aluminum treated with DEA 0.5 M, (d) grit-blasted aluminum treated with GPS then DEA 0.5 M. Table 5. Relative Peak Intensity of m/z ) 247 and 106 versus Concentration of DEA sample treatment
RPI (247+) × 1000
RPI (106+) × 1000
Pt/thick DEA film Al/0.01M DEA Al/0.5 M DEA Al/GPS/0.01M DEA Al/GPS/0.5M DEA
N/Aa
2.0 5.6 2.7 7.6 3.0
a
N/A N/A 0.7 3.9
N/A ) not available.
it is more intense for the other samples and thus is indicative of a specific interaction occurring between DEA and aluminum and GPS coated aluminum independently
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from any protonation of the DEA molecule due to other causes. Fragment m/z ) 106 is also more intense for the least concentrated solutions of DEA and may be indicative of the conformation adopted by the DEA molecule. The intensity of fragment m/z ) 247 increases with the concentration of DEA which indicates that either this interaction is favored by a higher concentration of DEA because DEA occupies more available sites or because the presence of DEA dimer is increased. Discussion As a result of the grit blasting and exposure to the atmosphere, the oxidized aluminum substrate will be at least partly hydroxylated. Most of the literature on this topic accounts for the formation of alkoxy species via condensation of a hydroxyl on the aluminum surface and an alcohol functionality from the DEA molecule. This was shown by Affrosman and MacDonald17,18 using monoethanolamine and Fauquet et al.19 on very thin (∼1 nm) hydroxylated (in vacuo) aluminum oxide films with the same molecule, Murase and Watts showed a similar interaction on chromate steel samples5 using DEA. It is well-known that the thickness of an aluminum oxide formed in the atmosphere conforms to the Mott limit is of the order of two nanometers and therefore would satisfy the condition of “thin oxide film”. Also, the binding energy of the first component of the nitrogen signal (Figure 5a and b, Table 3) is consistent with a neutral nitrogen9 and would therefore indicate that the DEA molecule is partly interacting on only one side of the molecule. This assumption is reinforced by the presence of an ion in the positive mode at m/z ) 88 or (M-OH)+, indicative of single alkoxy formation.20 The second component of the nitrogen signal indicates that part of the adsorbed DEA bears a partially charged nitrogen. In the case of DEA molecule, three functionalities can interact with aluminum oxide. It is also known that on chromic anodized aluminum or phosphoric anodized aluminum the preferential interaction occurs via the alcohol functionality forming an alkoxy species on the hydroxyls presents at the aluminum surface.17,21 Moreover, the adsorption of DEA is preferential when DEA is used compared to ethanolamine21 and also when methylaminethanol is used rather than diethylamine on phosphated alumina.17 Therefore, it is very likely that DEA is interacting with the aluminum surface via the formation of two alkoxy bonds. This has also been postulated by Kelber and Brow22 and by Evans and Weinberg.23 A bridge is formed that brings the nitrogen close to the surface and an interaction is possible via hydrogen bond between the nitrogen lone pair of electron and a hydrogen from aluminum hydroxyl. When the concentration of DEA is increased, the conformation of DEA at the surface may change as less adsorption sites per molecule are available to allow the DEA molecule to form a bridge. This results in the nitrogen atom being more remote from the surface and therefore less charge is transferred from the nitrogen lone pair of electrons. This assumption is reinforced by the binding energy of the second component of nitrogen being lower when aluminum is treated with a DEA solution at 0.5 M (17) Affrosman, S.; MacDonald, S. Langmuir 1996, 11, 2090. (18) MacDonald, S., Ph.D. Thesis, University of Strathclyde , 1998. (19) Fauquet, C.; Dubot, P.; Minel, L.; Barthe´s-Labrousse, M-G.; Rei Vilar, M.; Villatte, M. Appl. Surf. Sci. 1994, 81, 435. (20) Affrosman, S.; Comrie, R. F.; MacDonald, S. J. Chem. Soc., Faraday Trans. 1998, 94, 289. (21) Ross, G. Final year project dissertation, University of Surrey, 1996. (22) Kelber, J. A.; Brow, R. K. Appl. Surf. Sci. 1992, 59, 273. (23) Evans, H. E.; Weinberg, W. H. J. Chem. Phys. 1979, 71, 1537.
Figure 10. SIMS positive spectra in the mass window of m/z ) 102 to 110 Da of (a) grit-blasted aluminum treated with DEA 0.01 M, (b) grit-blasted aluminum treated with GPS then DEA 0.01 M, and (c) clean platinum coated with DEA solution at 0.5 M.
Figure 11. Interaction of DEA with an oxidized aluminum surface, via the hydrogen of the hydroxylated aluminum substrate. A similar reaction can also occur directly between the nitrogen atom of the DEA molecule and the Al3+ ion, which is omitted for clarity.
concentration rather than 0.01 M. This also explains why the relative peak intensity of fragment m/z ) 106 decreases in the same manner (Table 5) versus DEA concentration as the ion of m/z ) 106 will not be formed as easily. It has also been claimed in the literature17,24,25 that the epoxy analogue molecule can form an adduct on the aluminum surface in which both alcohol and amine functionalities can interact with the surface. One type of interaction proposed by Affrossman et al.24,25 shows the alcohol side of the molecule interacting forming an alkoxy whereas the amine side is interacting with an aluminum atom via a donor-acceptor interaction or Lewis acid-base interaction. The amine can therefore act as an electron donor and aluminum as an electron acceptor. This accounts for a shift in the nitrogen binding energy as the nitrogen is then left with a slight positive charge.26 The argument developed above for the formation of an alkoxy bridge also applies if the amine interacts via Lewis acid-base interaction. The various conformations are shown and reviewed in Figure 11. As shown in the results section, one particular feature observed when GPS coated aluminum is treated with DEA is a fragment at m/z ) 247. This illustrates the interaction of DEA dimer rather than DEA with GPS, as shown schematically in Figure 12. This type of interaction is supported by the chemistry of an epoxy resin reacting with an amine curing agent.4 Moreover, the DEA dimer is present on the surface, there is a possibility for an interaction of the DEA dimer with GPS rather than DEA only especially in the case of the more concentrated solution. The free doublet on the nitrogen undergoes a (24) Affrosman, S.; MacDonald, S. Langmuir 1994, 10, 2257. (25) Affrossman, S.; Brown, N. M. D.; Pethrick, R. A.; Sharma, V. K.; Turner, R. J. Appl. Surf. Sci. 1983, 16, 469. (26) Chehimi, M. M.; Watts, J. F.; Jenkins, S. N.; Castle, J. E. J. Mater. Chem. 1992, 2, 209.
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Figure 14. Fragmentation pattern of fragment m/z ) 247 leading to fragments 142 and 144 (+Ve).
Figure 12. Formation of m/z ) 247 fragment from a covalent bond between (a) GPS and DEA dimer. (b) Final structure of m/z ) 247 cation.
Figure 15. High mass resolution of m/z ) 144 and 142 for grit-blasted aluminum treated with GPS then DEA 0.5 M.
Figure 13. High mass resolution of m/z ) 247 for grit-blasted aluminum treated with GPS then DEA 0.5 M.
nucleophilic attack via its lone pair of electrons. The doublet is attracted by a carbon site with a slight deficiency in charge, or a small positive charge as it is expected for that type of carbon involved in an epoxy ring. There is then transfer of a hydrogen and finally fragmentation which is stabilized by electron delocalization. This structure was confirmed by the high mass resolution ToFSIMS analysis, presented in Figure 13, which gives a delta parameter of 10 ppm for this particular fragment. This assignment can also be confirmed by the fragmentation shown in Figure 14 giving rise to the fragments at masses m/z ) 142 and 144. The high-resolution spectra of these two fragments are also given in Figure 15 and it is clearly visible that at least one other fragment is present at both masses of m/z ) 142 or 144. The mass accuracy is given for the most intense fragment. Alternative assignments have been considered to allow for a possible reaction between two adjacent GPS molecules. Assuming DEA may catalyze the reaction, a structure resulting from the coreaction of two epoxy groups was examined. Similarly,
fragments containing only carbon, oxygen, hydrogen, and silicon have been investigated. The possible fragments exhibit mass accuracies within the range of 26-47 ppm which means they are further from the experimental mass than the fragment proposed formerly. Also, it was not possible to obtain good mass accuracy for any of these fragments from the structures that would result from the reaction of two GPS molecules. Impurities present in the initial DEA were also considered. A similar reaction to that presented in Figure 12 does not lead to a fragment of nominal mass 247 but to nominal mass 160 and 116 for a reaction with triethanolamine and monoethanolamine, respectively. It is also to note that a similar reaction with triethanolamine is less likely as this molecule does not have any proton to transfer. Surprisingly, for these samples, the binding energy of the first nitrogen component is slightly higher than expected for a neutral. This indicates that although the ToF-SIMS analysis indicates the formation of a covalent bond, there must also be a slight transfer of charge from the nitrogen atom once the bond is formed. The second component of the nitrogen shows unequivocally that the nitrogen is of a quaternary type and that a proton has been totally transferred.9,10 This is showing that a Bro¨nsted acid-base interaction is taking place. This type of interaction is only possible with a silanol, indicating that the GPS film is randomly orientated. This is also illustrated by the presence of the ion of protonated DEA at
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Conclusions In this study XPS and ToF-SIMS were used in order to investigate the interaction of both grit-blasted aluminum and GPS coated aluminum with DEA which was employed as a model of an amine cured epoxy resin. Several types of interaction were shown and they are as follows: Formation of an alkoxy or two alkoxy bonds of DEA on oxidized aluminum. When two alkoxy are formed in a bridge, the nitrogen is brought close to the surface and interactions such as hydrogen bonding or donor-acceptor are induced. The bridge structure is favored by least concentrated solutions of DEA. Covalent bond formed between the epoxy ring of GPS and nitrogen of DEA dimer and Bro¨nsted acid-base interaction between GPS and DEA via interaction of the silanol and nitrogen lone pair of electrons. The type of bond formed depends on the nature of the substrate but also on the number of adsorption sites available and/or on the steric hindrance between the molecules when adsorbing. Figure 16. High mass resolution of m/z ) 106 for grit-blasted aluminum treated with GPS then DEA 0.5 M.
m/z ) 106 of which structure was confirmed via highresolution ToF-SIMS analysis. This is shown in Figure 16. For these samples as the transfer of the proton is total the relative peak intensity of 106 by ToF-SIMS is higher respectively than for aluminum treated with DEA only. The decrease in intensity of ion m/z ) 106 versus DEA concentration occurs in favor of the interaction described above where a covalent bond is formed.
Acknowledgment. A.R. thanks the Thai Government for financial support, M.L.A. is a DERA/SMC Fellow, the high resolution ToF-SIMS experiments were made possible by EPSRC Grant GR/M36670. The authors thank Mr Nigel Porritt (DERA/SMC Farnborough) for his help with the preparation of samples and Dr Ian Fletcher and Mr Andrew Brown for assistance with high resolution ToF-SIMS and ToF-SIMS analyses, respectively. LA9915724
