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Formation of Self-Assembled Monolayers of Alkylphosphonic Acid on the Native Oxide Surface of SS316L Aparna Raman,† Manish Dubey,‡ Irina Gouzman,‡,§ and Ellen S. Gawalt*,† Department of Chemistry and Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282, and Department of Chemistry, Princeton UniVersity, Princeton, New Jersey, 08540 ReceiVed March 8, 2006. In Final Form: May 27, 2006 Phosphonate-steel interactions have been industrially significant for decades, but details of the phosphonate-steel interface have not yet been characterized. Self-assembled monolayers of phosphonic acids were formed on stainless steel 316L by room-temperature solution deposition. The acids are covalently bound to the surface as phosphonates in a bidentate manner, as determined by diffuse reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Complete coverage of the surface is confirmed by contact angle measurement and atomic force microscopic imaging. This method of monolayer formation contrasts the requirement for heating and long reaction times found to be necessary to form phosphonate monolayers on other metal oxide substrates, such as titanium and silicon.
Since the 1960s, steel and phosphonates have been used together for the improvement of many structural applications. Perhaps most importantly, phosphonates have been used as corrosion inhibitors in water purification1-3 and other flowing systems. They are also used as degreasers4 or lubricants in rolling steel5 and in polymer mixes to enhance the adhesion of plastics and rubber to steel.6 As important as these uses of phosphonates are, the nature of the interaction between them and the steel has not yet been fully characterized. Understanding the details of the interaction is important to design new corrosion inhibitors and adhesion promoters or to extend this technology to fields in which strong interfacial interactions are crucial, such as biomaterials. Phosphonate-metal oxide interactions have been studied on surfaces other than steel, such as tantalum,7-9 titanium,10-14 copper,15 zirconium,12,16 iron,14,17 aluminum,14,15,18,19 and sili* Corresponding author. E-mail:
[email protected]. † Duquesne University. ‡ Princeton University. § On sabbatical leave from the Space Environment Section, Soreq NRC, Yavne 81800, Israel. (1) Rajendran, S.; Apparao, B. V.; Palaniswamy, N.; Amalraj, A.; Sundaravadivelu, M. The role of phosphonates as transporters of Zn2+ ions in the inhibition of carbon steel in neutral solutions containing chlorides. Anti-Corros. Methods Mater. 2002, 49 (3), 205-209. (2) Rajendran, S.; Apparao, B. V.; Palaniswamy, N. Corrosion inhibition by phosphonic acid-Zn2+ systems for mild steel in chloride medium. Anti-Corros. Methods Mater. 2000, 47 (6), 359-365. (3) Awad, H. S. The effect of zinc-to-HEDP molar ratio on the effectiveness of zinc-1, hydroxyethylidene-1,1 diphosphonic acid in inhibiting corrosion of carbon steel in neutral solutions. Anti-Corros. Methods Mater. 2005, 52 (1), 22-28. (4) Fang, J. L.; Li, Y.; Ye, X. R.; Wang, Z. W.; Liu, Q. Passive films and corrosion protection due to phosphonic acid inhibitors. Corrosion 1993, 49 (4), 266-271. (5) Yu, T.; Li, L.; Lin, C. T. Chemical affinity of in-situ phosphatizing reagents on cold-rolled steel. J. Phys. Chem. 1995, 99 (19), 7613-7620. (6) Niess, R.; Engler, E.; Weber, K.; Wezorke, K.; Kochmann, W.; Koetz, G.; Steinke, W. Corrosion inhibiting rubber compounds for metal surfaces. German Patent DD238060, 1986-08-06, 1986. (7) Hofer, R.; Textor, M.; Spencer, N. D. Alkyl phosphate monolayers, selfassembled from aqueous solution onto metal oxide surfaces. Langmuir 2001, 17 (13), 4014-4020. (8) Adolphi, B.; Jahne, E.; Busch, G.; Cai, X. D. Characterization of the adsorption of omega-(thiophene-3-yl alkyl) phosphonic acid on metal oxides with AR-XPS. Anal. Bioanal. Chem. 2004, 379 (4), 646-652. (9) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Hahner, G.; Spencer, N. D. Structural chemistry of self-assembled monolayers of octadecylphosphoric acid on tantalum oxide surfaces. Langmuir 2000, 16 (7), 3257-3271.
con.16,20,21 The interaction or bonding mode varies among surfaces, and the interaction has been reported as being monodentate,9,16,19 bidentate,9,10 or tridentate.22,23 Therefore, the bonding mode of phosphonate to steel cannot be assumed on the basis of prior work. Here we report the room-temperature formation and characterization of covalently bound self-assembled monolayers (SAMs) of alkylphosphonates on the native oxide surface of stainless steel (SS316L). Typically, the SS316L (99.99%, 0.25 mm thick) substrate used in our experiments was obtained from Goodfellow, Inc., (10) Schwartz, J.; Avaltroni, M. J.; Danahy, M. P.; Silverman, B. M.; Hanson, E. L.; Schwarzbauer, J. E.; Midwood, K. S.; Gawalt, E. S. Cell attachment and spreading on metal implant materials. Mater. Sci. Eng., C 2003, 23 (3), 395-400. (11) Gawalt, E. S.; Avaltroni, M. J.; Koch, N.; Schwartz, J. Self-assembly and bonding of alkanephosphonic acids on the native oxide surface of titanium. Langmuir 2001, 17 (19), 5736-5738. (12) Marcinko, S.; Fadeev, A. Y. Hydrolytic stability of organic monolayers supported on TiO2 and ZrO2. Langmuir 2004, 20 (6), 2270-2273. (13) Hahner, G.; Hofer, R.; Klingenfuss, I. Order and orientation in selfassembled long chain alkanephosphate monolayers adsorbed on metal oxide surfaces. Langmuir 2001, 17 (22), 7047-7052. (14) Gawalt, E. S.; Lu, G.; Bernasek, S. L.; Schwartz, J. Enhanced bonding of alkanephosphonic acids to oxidized titanium using surface-bound alkoxyzirconium complex interfaces. Langmuir 1999, 15 (26), 8929-8933. (15) Van Alsten, J. G. Self-assembled monolayers on engineering metals: Structure, derivatization, and utility. Langmuir 1999, 15, 7605-7614. (16) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Selfassembled monolayers of alkylphosphonic acids on metal oxides. Langmuir 1996, 12, 6429-6435. (17) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.; Rafailovich, M.; Sokolov, J.; Gedanken, A. Self-assembled monolayers of alkanesulfonic and -phosphonic acids on amorphous iron oxide nanoparticles. Langmuir 1999, 15 (21), 7111-7115. (18) Goetting, L. B.; Deng, T.; Whitesides, G. M. Microcontact printing of alkanephosphonic acids on aluminum: Pattern transfer by wet chemical etching. Langmuir 1999, 15 (4), 1182-1191. (19) Pellerite, M. J.; Dunbar, T. D.; Boardman, L. D.; Wood, E. J. Effects of fluorination on self-assembled monolayer formation from alkanephosphonic acids on aluminum: Kinetics and structure. J. Phys. Chem. B 2003, 107 (42), 1172611736. (20) Hanson, E. L.; Schwartz, J.; Nickel, B.; Koch, N.; Danisman, M. F. Bonding self-assembled, compact organophosphonate monolayers to the native oxide surface of silicon. J. Am. Chem. Soc. 2003, 125 (51), 16074-16080. (21) Fontes, G. N.; Malachias, A.; Magalhaes-Paniago, R.; Neves, B. R. A. Structural investigations of octadecylphosphonic acid multilayers. Langmuir 2003, 19 (8), 3345-3349. (22) Guerrero, G.; Mutin, P. H.; Vioux, A. Anchoring of phosphonate and phosphinate coupling molecules on titania particles. Chem. Mater. 2001, 13 (11), 4367-4373. (23) Gouzman, I.; Dubey, M.; Carolus, M. D.; Schwartz, J.; Bernasek, S. L. Monolayer vs. multilayer self-assembled alkylphosphonate films: X-ray photoelectron spectroscopy studies. Surf. Sci. 2006, 600, 773-781.
10.1021/la060636p CCC: $33.50 © 2006 American Chemical Society Published on Web 06/22/2006
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cut into 1 × 1 cm coupons, and polished with a Buehler Ecomet 4 mechanical polisher using 220, 400, 800, and 1200 grit silicon carbide paper followed by a 1 µm diamond suspension. The polished samples were cleaned by ultrasonication in methanol (15 min) and then by immersion in boiling methanol to remove traces of organics and metallic dust. The substrates were stored in an oven at 120 °C. The cleaned room-temperature substrates were dipped in a 1 mM solution of octadecylphosphonic acid (ODPA) or octylphosphonic acid in dry tetrahydrofuran (THF), and excess solution was removed by evacuation (0.1 Torr). It has been observed that an “ordered” aliphatic monolayer is one with chains in an all-trans configuration characterized by νCH2asymm < 2918 cm-1 and νCH2symm < 2850 cm-1 in the IR spectra.11,24-29 Therefore, it appears that ODPA formed a well-ordered film on SS316L, as indicated by the νCH2asymm centered at 2915 cm-1 (Figure 1). The SAM film was not removed by rinsing with or without sonication in THF, but the rinsing did remove any excess acid from the surface. The monolayers were also stable when rinsed in protic solvents such as water and ethanol. Additionally, the effect of acid and base on the monolayer was tested in 0.1 M HCl and 0.1 M NaOH, respectively. A linear adhesion test was performed with Scotch tape 237, which has an adhesion strength to steel of 37 oz/in. The IR spectrum was essentially unchanged after all types of rinse and adhesion tests, with νCH2 asymm ) 2915 cm-1, indicating that the ODPA was strongly bound to the surface (Figure 1). The phosphonate group stretching showed νP-O ) 1050 cm-1 and νPdO ) 1160 cm-1. These observations, coupled with the disappearance of νP-O-H ) 920 cm-1 (Figure 1), indicate a bidentate coordination mode of the phosphonate to the surface. This bonding mode was present after each rinse and adhesion test. It is noteworthy that this bonding mode contrasts that of phosphonates on other oxides, such as silicon oxide, where binding is tridentate (all three oxygens of the phosphonate headgroup are bound to the surface).20 Static contact angles (VCA Optima Goniometer) were measured to determine the wettability of the surface. Such measurements have been shown to be useful in estimating the monolayer quality of methyl-terminated adsorbates. Densely packed and well-ordered monolayers predominantly expose methyl groups at the surface, decreasing the surface wettability. In contrast, loosely packed monolayers expose a substantial fraction of methylene groups in addition to methyl groups at the surface, increasing the wettability and decreasing the contact angle.30 Measurements were made using the procedure described by Bain.31 About 1 µL of deionized water (Millipore) was formed at the end of the microliter syringe and brought into contact with the surface.29 SS316L substrates that had been reacted with ODPA (24) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. Spontaneously organized molecular assemblies. 4. Structural characterization of n-alkyl thiol monolayers on gold by optical ellipsometry, infrared spectroscopy, and electrochemistry. J. Am. Chem. Soc. 1987, 109 (12), 3559-3568. (25) Snyder, R. G. Vibrational spectra of crystalline n-paraffins. J. Mol. Spectrosc. 1960, 4, 411-434. (26) Clarkson, A. B. J.; Mellow, G. H. Phase transitions and nonplanar conformers in crystalline n-alkanes. Science 1981, 214 (9), 188-190. (27) Allara, D. L.; Nuzzo, R. G. Spontaneously organized molecular assemblies. 1. Formation, dynamics, and physical properties of n-alkanoic acids adsorbed from solution on an oxidized aluminum surface. Langmuir 1985, 1 (1), 45-52. (28) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Orthogonal self-assembled monolayers: Alkanethiols on gold and alkane carboxylic acids on alumina. Science 1989, 245, 845. (29) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Self-assembled monolayer growth of octadecylphosphonic acid on mica. Langmuir 1996, 12 (15), 36263629. (30) Park, J. S.; Smith, A. C.; Lee, T. R. Loosely packed self-assembled monolayers on gold generated from 2-alkyl-2-methylpropane-1,3-dithiols. Langmuir 2004, 20 (14), 5829-5836. (31) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. Formation of monolayer films by the spontaneous assembly of organic thiols from solution onto gold. J. Am. Chem. Soc. 1989, 111, 321-335.
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Figure 1. (A) IR spectra of ODPA on stainless steel substrate after deposition (black) and rinse test (red). (B) P-O region of the spectra of ODPA on 316L. (C) Bidentate binding of phosphonates to the SS316L surface.
had a water contact angle value of 108 ( 3°. This is comparable to values obtained for well-ordered, hydrophobic SAMs of thiols on gold and silanes on silicon.13,32 This value, when compared with a contact angle of 45 ( 3° for the unmodified stainless steel substrate, indicates the formation of an ordered film that is significantly more hydrophobic than the control sample. X-ray photoelectron spectroscopy (XPS) analysis of the substrates was performed using a Phoibos 150 hemispherical energy analyzer (SPECS) and a monochromatized Al (1486.6 eV) source. The compositional results for the SS316L reference (Figure 2) substrate were in reasonable agreement with the nominal SS316L bulk elemental composition33 (Fe 66.01%, Cr 19.19%, Ni 9.17%, Mn 3.22%, and Mo 2.42%.) Deconvolution (32) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991.
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Figure 3. Topography (left) and amplitude (right) images of ODPA on SS316L.
Figure 2. (A) Typical survey scan in the 1000-10 eV range of a stainless steel sample (top), and a stainless steel sample after the deposition of ODPA film (bottom). (B) Detailed XPS scans in the O1s region taken at a grazing angle of incidence. The appearance of the PdO peak after deposition of ODPA indicates bidentate bonding.
of Fe2p, Cr2p, C1s, and O1s peaks were in good agreement with the stoichiometry of surface oxides (Fe2O3 (18%) and Cr2O3 (8%)). These data indicate that our cleaning procedures do not change the native oxide composition of the surface, in contrast to the electrochemical pretreatments previously reported to be necessary for monolayer formation on SS316L.34 However, a small amount of silicon was detected on the surface, most likely originating from the polishing process. XPS survey scans of the ODPA on SS316L sample clearly indicate a significant increase in carbon concentration, concurrent with the appearance of P2s and P2p peaks at 191.64 and 134.56 eV, respectively, compared to that of untreated SS316L. The measured intensity of Fe and other metals decreases after the deposition of ODPA, as can be seen from the comparison in Figure 2. The carbon-to-phosphorus (C/P) ratio calculated from the relative intensity of the elements obtained at a grazing angle (33) Wang, X. Y.; Wu, Y. S.; Zhang, L.; Yu, Z. Y. Atomic force microscopy and X-ray photoelectron spectroscopy study on the passive film for type 316L stainless steel. Corrosion 2001, 57 (6), 540-546. (34) Mahapatro, A.; Johnson, D. M.; Patel, D. N.; Feldman, M. D.; Ayon, A. A.; Agrawal, C. M. Surface modification of functional self-assembled monolayers on 316L stainless steel via lipase catalysis. Langmuir 2006, 22 (3), 901-905.
incidence (C/P ) 77.15/4.25 ) 18.2), is consistent with an ODPA surface-bound film. The agreement between the stoichiometric composition and the measured C/P ratio indicates that the film is uniform and homogeneous. The O1s data support the contention based on IR spectra that the ODPA is covalently bound to the oxide substrate of SS316L in a bidentate manner (Figure 2B); in particular, the persistence of PdO (O1s ) 533.8 eV), which appears after the deposition of ODPA on the surface, is inconsistent with tridentate phosphonate-surface bonding. The O1s peak at 529.8 eV can be attributed to surface oxides, and those at 532 eV are attributed to surface hydroxides and P-O.23 Atomic force microscopic imaging (Molecular Imaging) of the substrates was performed in noncontact mode at ambient conditions using silicon nitride tips. Topography and amplitude images of ODPA-modified SS316L are shown in Figure 3. The roughness analysis on the two-dimensional topography image was based on a calculation of the standard deviation of all the height values within the given imaged area (root-mean-square (rms) roughness). The rms roughness value of the control SS316L substrate before reaction with ODPA was 34 Å, and, after reaction with ODPA, it was 35 Å. This indicates that a single monolayer of molecules covers the oxide surface. There was no evidence of micelle or island formation visible in the film (Figure 3). Chain length can alter the ability of molecules to form a quality monolayer.35 Both eight-carbon (octylphosphonic acid) and fourcarbon (butylphosphonic acid) acids were tested for monolayer formation under the conditions described above. Octylphosphonate formed a well-adhered, ordered monolayer characterized by νCH2 asymm ) 2915 cm -1 after deposition and rinse testing (Figure 4). The contact angle with water for substrates reacted with octylphosphonate was 124.5°, indicating complete monolayer coverage. The eight-carbon chain acid is one of the shortest molecules to form monolayers reported in the literature on silicon and other oxides.36 The short chain length of the butylphosphonic is not long enough to provide the enthalpic gain of the van der Waals interactions between the chains to offset the entropic loss when bonds are held in the all-trans configuration present in ordered films. SAMs of phosphonates were formed on the native oxide surface of stainless steel 316L using a simple room-temperature solution deposition method. ODPA and octylphosphonic acid formed ordered monolayers covalently bonded to the surface as phosphonates in a bidentate mode, as indicated by diffuse reflectance Fourier transform infrared spectroscopy and confirmed by XPS. The bonding mode did not vary with heat, solvent, acid, or base treatments. Therefore, this characterization of the bonding mode (35) Meyers, D. Surfaces, Interfaces, and Colloids: Principles and Applications, 2nd ed.; John Wiley and Sons: New York, 1999. (36) Singh, S.; Wegmann, J.; Albert, K.; Muller, K. Variable temperature FT-IR studies of n-alkyl modified silica gels. J. Phys. Chem. B 2002, 106 (4), 878-888.
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Figure 4. (Left) IR spectra of octylphosphonic acid on stainless steel substrate after deposition (black) and rinse and tape test (red). (Right) IR spectra of butylphosphonic acid on stainless steel substrate after deposition (black) and rinse test (red). The X axis is wavenumbers, and the Y axis is in percent of reflectance.
is important because of the prevalent use of phosphonates on steel in many industrial applications under varying conditions. Acknowledgment. We thank Steven L. Bernasek and Jeffrey Schwartz (Princeton University, Department of Chemistry) for helpful commentary, S.L.B. for assistance with XPS experiments,
and Toby Chapman (University of Pittsburgh) for use of the contact angle meter. Additionally, we thank The Samuel and Emma Winters Foundation, Faculty Development Fund, and the Pennsylvania Department of Health CURE Program for generous funding of the project. LA060636P
