Article pubs.acs.org/Langmuir
Structure and Long-Term Stability of Alkylphosphonic Acid Monolayers on SS316L Stainless Steel Medea Kosian,† Maarten M. J. Smulders,† and Han Zuilhof*,†,‡ †
Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Department of Chemical and Materials Engineering, King Abdulaziz University, 21589 Jeddah, Saudi Arabia
‡
S Supporting Information *
ABSTRACT: Surface modification of stainless steel (SS316L) to improve surface properties or durability is an important avenue of research, as SS316L is widely used in industry and science. We studied, therefore, the formation and stability of a series of organic monolayers on SS316L under industrially relevant conditions. These included acidic (pH 3), basic (pH 11), neutral (Milli-Q water), and physiological conditions [10 mM phosphate-buffered saline (PBS)], as well as dry heating (120 °C). SS316L was modified with alkylphosphonic acids of chain length (CH2)n with n varying between 3 and 18. While alkylphosphonic acids of all chain lengths formed self-assembled monolayers with hydrophobic properties, only monolayers of chain lengths 12−18 formed ordered monolayers, as evidenced by static water contact angle (SCA), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and infrared reflection absorption spectroscopy (IRRAS). A long-term stability study revealed the excellent stability of monolayers with chain lengths 12−18 for up to 30 days in acid, neutral, and physiological solutions, and for up to 7 days under dry heating. Under strong basic conditions a partial breakdown of the monolayer was observed, especially for the shorter chain lengths. Finally, the effect of multivalent surface attachment on monolayer stability was explored by means of a series of divalent bisphosphonic acids.
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INTRODUCTION Surface modification of inorganic substrates with organic monolayers is a widely used technique for the tuning and improvement of surface properties such as resistance to wear and corrosion.1−3 Though many surface modification techniques have been reported using a range of oxidic substrates and also silicon-derived materials such as Si(111),4 silicon nitride,5 and SiC,6 the tuning of the outer layers of metallic substrates has great potential and industrial relevance as well. Construction metals, such as stainless steel, are used in a wide variety of applications including power plants,7 medical prostheses,8 and food processing.9 Many applications involve repeated or long-term exposure to harsh conditions such as acidic or alkaline7 solutions and heat,10 causing corrosion of the steel. Surface modification of stainless steel has been reported using various reagents and modes of attachment. Examples include organosilanes,11 catechols,12 primary amines,13 alkenes,5 and diazonium salts.14 However, all of these methods are problematic when applied under typical stainless steel use conditions, either due to hydrolytic instability of the monolayers,5 polymerization of remaining reactive groups,15 or difficult and unscalable modification methods. A more promising strategy of surface modification of stainless steel for industrial use is based on the use of phosphonic acids.16 Alkylphosphonic acids have been found to © XXXX American Chemical Society
exhibit corrosion-inhibiting effects on metals such as mild (carbon) steel, copper, and aluminum.17 Corrosion inhibition is thought to take place via the formation of a thin layer of densely packed phosphonic acid on a surface, which effectively prevents diffusion of water and oxygen toward the base metal.18 Monolayer formation and stability on metal oxide surfaces are promoted by the strong binding of the phosphonic acid group, which coordinates to the metal in at least bidentate mode. Thermal annealing of the layers can further increase stability.19 Phosphonic acids and their esters have been used as corrosion inhibitors in water purification,20−22 as lubricants in production processes,23,24 and as adhesives in polymer processing.25 An important prerequisite to the use of phosphonic acid derived layers on stainless steel is their stability under industrial conditions. However, stability studies have so far been limited to studies under physiological conditions8 or to short immersion (rinsing) in water, acidic and basic conditions.26 Stability studies involving long-term exposure of phosphonic acid monolayers on stainless steel to the harsher conditions that are relevant for industrial use have not been reported, yet are needed to assess, as well as to extend, the industrial applicability of stainless steel. Received: November 16, 2015 Revised: January 5, 2016
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Figure 1. Top: schematic representation of the monolayer formation on steel based on alkylphosphonic acids of various chain lengths (n = 3, 4, 8, 10, 12, 16, 18). Bottom: monoalkyl bisphosphonic acids 1−3 under current study.
characterization of monolayers of various alkylphosphonic acids and dendritic monoalkyl bisphosphonic acid analogues on SS316L (Figure 1) and investigate the effects of chain length, monovalent versus divalent attachment, and packing density on their structure and stability. We monitor the stability of the monolayers over a period of 30 days under a standardized set of neutral, physiological, acidic, and basic conditions,5 as well as under conditions of dry heating. To this end, alkylphosphonic acid monolayers are characterized in detail using water contact angle goniometry, Xray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and infrared reflection absorption spectroscopy (IRRAS).
Stability in water and physiological buffer is important in biological and biomedical applications, which prompted Kaufmann et al. to carry out their study in physiological solution.8 Stabilities in high- and low-pH solutions are relevant mainly to the food industry, a large-scale industrial user of stainless steel worldwide. Many fruit and vegetable juices (apple, orange, tomato) are stored and transported in stainless steel vessels and have pH values between 3.0 and 3.3, making pH 3 an important study condition.27 Basic solutions at pH 11 are mandated in the dairy industry for the cleaning of milk pipelines,28 so stability of any modification under these harsh conditions is of industrial interest. Finally, since steel machine parts may become hot during operation, the ability of a modification to still function at high temperature (120 °C) is of importance if surface modification is to be used for, e.g., selflubrication purposes. Insight is also needed into the effect of the phosphonic acid chain length on the structure and stability of the monolayers; however, this relation has so far also not been systematically investigated. To further investigate factors contributing to layer stability on steel, and to find new avenues toward greater performance, we also describe a short series of bisphosphonic acids (Figure 1) and their behavior on the surface. The concept of multivalent attachment on metal oxides has been alluded to in the literature, starting with the attachment of phosphonylated poly(ethylenimine) to aluminum oxide,29 as studied by Fourier transform infrared (FTIR) spectroscopy. Subsequently, Villemin et al. studied anticorrosion applications of phosphonylated poly(ethylenimine) PEIP.30 This work does combine steel with a bisphosphonic acid, but while the anticorrosive properties were studied, no detailed surface characterization of the surface or evidence for formation of a covalently bound layer was provided. More detailed studies of the binding and stability of bisphosphonic acids include the work of Ide et al. on titanium zirconium oxides31 and the work of Lecollinet et al. on (fluoro)alkylated 1-hydroxy-1,1-bisphosphonic acids on titanium, stainless steel, and silicon.32 The current work presents a systematic study of the formation, structure, and long-term stability of a range of alkylphosphonic acids on marine grade stainless steel (SS316L) under a wide variety of conditions commonly encountered in industrial applications. We describe the formation and
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EXPERIMENTAL PROCEDURES
Materials. Ethanol (UPLC grade Ultra-Pure, Sigma-Aldrich), isopropyl alcohol (99.9+%, for analysis, Sigma-Aldrich), methanol (HPLC grade, Ultra-Pure, Biosolve), and dichloromethane (for analysis, Biosolve) were used as analytical grade. SS316L substrates were obtained from Goodfellow Inc. and further cut into 1 × 1 cm2 pieces using an industrial metal cutter. Propyl-, butyl-, and hexadecylphosphonic acid (from Sigma-Aldrich) and octyl-, decyl-, dodecyl-, and octadecylphosphonic acid (from SiKEMIA) all had purities >98% and were used as received. Butyl-, decyl-, and hexadecylamine were obtained from Sigma-Aldrich in purities >98%. Formaldehyde was used as a 37% aqueous solution. Phosphorous acid was obtained from Sigma-Aldrich as a white crystalline solid (99%). A solution of 6 M HCl was obtained by dilution of commercial 37% HCl with deionized water. All NMR measurements were carried out on a Bruker 400 MHz instrument. Plasma treatment of the bare surfaces was carried out using air plasma in a PDC-002 plasma cleaner (Harrick Scientific Products, Inc.) operating at 0.3 SCFH air flow, 29.6 W power, and 300 mTorr pressure. Formation of Alkylphosphonic Acid Self-Assembled Monolayers. SS316L substrates (1 × 1 cm2) were removed from their plastic casing. The bare surface was initially cleaned by 15 min of sonication in methanol, followed by 15 min of sonication in dichloromethane. Final removal of adsorbed carbon impurities was effected by a 3 min plasma cleaning. A solution of the appropriate alkylphosphonic acid (1 mM in ethanol)26 was prepared at room temperature, after which the cleaned substrates were immersed in this solution at room temperature overnight. The substrates were then rinsed with ethanol and placed in a curing oven at 120 °C for 4 h. The modified surfaces were allowed to cool to room temperature, rinsed twice with isopropyl alcohol and twice with dichloromethane, and dried under a stream of N2. B
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Figure 2. Left: static water contact angles of monolayers of alkylphosphonic acids on SS316L as a function of alkyl chain length. Right: static water contact angles of monolayers of bisphosphonic acids on SS316L as a function of alkyl chain length. was stirred at 125 °C for 6 h. The mixture was cooled to room temperature, and ethanol was added to precipitate the product. The resulting solid was filtered and dried in vacuo. The bisphosphonic acids were recrystallized from water. N,N-Di(phosphonomethyl)butylamine (1). This compound was prepared via the general procedure, using 2 mL of 6 M HCl, and was obtained in 56% yield (2.0 g). 1H NMR (DMSO-d6) δ (ppm): 3.34 (d, 4H, JH−P = 12.7 Hz), 3.27 (m, 2H), 1.63 (m, 2H), 1.27 (q, 2H, JH−H = 7.4 Hz), 0.88 (t, 3H, JH−H = 7.3 Hz). 13C NMR (DMSO-d6) δ (ppm): 56.59, 52.71, 51.11, 25.28, 19.29, 13.48. ESI-MS exptl, 260.15 (M− H+); calcd, 260.05. N,N-Di(phosphonomethyl)decylamine (2). Following the general procedure, this compound was obtained in 54% yield (1.2 g). 1H NMR (DMSO-d6) δ (ppm): 3.49 (d, 4H, JH−P = 12.9 Hz), 3.31 (m, 2H), 1.63 (m, 2H), 1.25 (m, 14H), 0.86 (t, 3H, JH−H = 6.8 Hz). 13C NMR (DMSO-d6) δ (ppm): 56.78, 50.93, 49.56, 31.10, 28.88, 28.59, 28.52, 25.92, 21.87, 13.97. ESI-MS exptl, 344.31 (M−H+); calcd, 344.14. N,N-Di(phosphonomethyl)hexadecylamine (3). Following the general procedure, this compound was obtained in 56% yield (1.0 g). 1 H NMR (DMSO-d6) δ (ppm): 3.32 (d, 4H, JH−P = 12.3 Hz), 3.26 (m, 2H), 1.63 (m, 2H), 1.24 (m, 26H), 0.85 (t, 3H, JH−H = 6.9 Hz). 13C NMR (DMSO-d6) δ (ppm): 56.93, 51.03, 49.66, 31.24, 28.87, 28.64, 28.52, 25.98, 23.32, 22.05, 13.90. ESI-MS exptl, 428.47 (M−H+); calcd, 428.23.
Surface Characterization. Water contact angle measurements were performed using the sessile drop method on a DSA100 optical contact angle meter (Krüss instruments), with a bare, plasma-cleaned SS316L substrate as reference. The elemental composition of both the bare and modified substrates was determined using XPS. XPS spectra were recorded on a JPS-9200 photoelectron spectrometer (JEOL, Japan) under UHV conditions using monochromatic Al Kα X-ray radiation at 12 kV and 20 mA with an analyzer pass energy of 10 eV. In all XPS spectra, on the y-axis the unit is “counts per second” (cps). Peak-fitting was performed using 70−30 mixed Gaussian−Lorentzian curves. In all XPS spectra, Fe and Cr signals were corrected with a Shirley background, and all others with a linear background. All XPS spectra were evaluated using the Casa XPS software (version 2.3.15). The C 1s hydrocarbon peak was calibrated at a binding energy of 285.0 eV. IRRAS spectra were recorded using a IR-ATR Bruker TENSOR 27 instrument. A Harrick AutoSeagull grid polarizer was installed in front of the detector and used to record spectra with ppolarized (parallel) radiation with respect to the plane of incidence at the sample surface with a liquid nitrogen-cooled MCT (mercury, cadmium, telluride) detector and a grazing angle (83°) attachment. Typically, 2048 scans were taken at a resolution of 4 cm−1 for each spectrum. A bare, plasma-cleaned SS316L surface was used as background. AFM measurements were performed on an Asylum MFP-3D AFM instrument operating in tapping mode. Long-Term Stability Tests. The stability of these modified substrates was investigated under previously described, standardized conditions.5 SS316L substrates modified with alkylphosphonic acids of chain lengths varying between 3 and 18 carbon atoms were immersed in solutions simulating industrial use conditions and placed in a shaking bath. All measurements were taken in triplicate for each chain length, with each surface sample placed in a separate solution. Solutions included Milli-Q water, phosphate-buffered saline (PBS, 10 mM; pH 7.4), and aqueous solutions at pH 3 (1 mM HCl) and pH 11 (1 mM NaOH). Furthermore, three samples of each chain length were placed in a drying oven at 120 °C. Static contact angles and IRRAS spectra were measured for all samples prior to treatment as a reference. Measurements were repeated on all samples after 1, 7, and 30 days. Derivatization of Heat-Exposed C10 Monolayer. A decylphosphonic acid modified steel substrate that had been exposed to 120 °C for 30 days was washed with ethanol and CH2Cl2 and subsequently dried. This surface was then immersed in a 5 mM solution of dicyclohexylcarbodiimide (DCC) and N-hydroxysuccinimide (NHS) in dry CH2Cl2 and placed in a shaking bath at room temperature for 3 h. Afterward, it was washed with dry CH2Cl2 and transferred to a 5 mM solution of 4-(trifluoromethyl)benzylamine in dry CH2Cl2. The solution was placed in a shaking bath at room temperature for 3 h, after which the steel substrate was washed with dry CH2Cl2 and dried under a stream of N2. General Procedure for Synthesis of Bisphosphonic Acids via the Moedritzer−Irani Reaction.33 The appropriate amine (1 equiv) was dissolved in 2−8 mL of 6 M HCl. Phosphorous acid (3 equiv) was added, and the mixture was heated while stirring to 125 °C. Formaldehyde (37%; 4 equiv) was added dropwise, and the mixture
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RESULTS AND DISCUSSION Monolayer Formation and Characterization. Selfassembled monolayers of alkylphosphonic acids on stainless steel (SS316L) were prepared by overnight immersion of the plasma-cleaned substrates in a 1 mM solution of the appropriate alkylphosphonic acid in ethanol26 and subsequent curing for 4 h at 120 °C. The formation of a monolayer was first of all probed by determination of the static water contact angle (CA). Modified SS316L substrates had a CA between 90 ± 1° and 108 ± 1°, increasing with the phosphonic acid chain length (Figure 2). When compared with the CA values between 0° and 10° observed for unmodified plasma-cleaned SS316L, this indicates the successful attachment of the alkylphosphonic acids to the steel surface. In particular, the values for the higher chain length phosphonic acids of 106−108° correspond to literature values reported for well-ordered self-assembled monolayers of alkylphosphonic acids on stainless steel,26 alkylsilanes on silicon,34 and alkanethiols on gold,35 indicating that the long-chain phosphonic acids had formed densely packed layers on the substrate surface. Bisphosphonic Acids on SS316L. To further understand the stability and improve performance, the effect of the number of attachments to steel on the stability of the monolayer was also investigated. To this end, SS316L was also modified with C
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Figure 3. Left: XPS survey scans of unmodified SS316L (bottom) and SS316L modified with octadecylphosphonic acid (top). Right: XPS survey scan of SS316L modified with dendritic hexadecyldiphosphonic acid 3. Note the difference in P signal intensity: for the monophosphonic acid the P/ C ratio is 0.062, while for the bisphosphonic acid the ratio is 0.11. For reference, a fully assigned XPS spectrum for stainless steel is given in the Supporting Information (Figure S73).
divalent bisphosphonic acids bearing one alkyl chain (1−3; Figure 1). These compounds were synthesized from the corresponding primary amines using the procedure developed by Moedritzer and Irani,33 which allows the direct one-pot transformation of amines into bisphosphonic acids. The effect of chain length was mitigated by choosing a series of divalent acids varying from short to long alkyl chains. As shown in Figure 2, the dendritic alkyl diphosphonic acids have significantly lower contact angles than their monovalent analogues with identical alkyl chain lengths. Two components likely contribute to this: (a) the polar group connecting the two phosphonic acid moieties generated in the Moedritzer−Irani reaction, which is less masked in shorter chain lengths and increases the polarity of the molecule; (b) the footprint of the alkyl chain is for 1−3 significantly smaller than that of the double attachment group, yielding space in between adjacent alkyl chains and an increased tilt angle of that chain, both of which allow water to access the surface more readily (analogous to what was observed for monoalkyl 1-hydroxy-1,1-biphosphonic acids by Lecollinet et al.32). Successful formation of monolayers was further corroborated by XPS, which clearly showed a significant increase in the carbon C 1s signal intensity on the modified substrate relative to the unmodified substrate, with the concomitant appearance of a characteristic P 2p signal at 134 eV (Figure 3). A concurrent marked decrease of intensity of the Fe and Cr 2p signals due to overlayer attenuation was also observed as can be seen in Figure 3 too. Unfortunately, due to the variable elemental composition of the outer layer of SS316L steel and the absence of a reliable, corresponding attenuation factor, the layer thickness could not be determined from XPS. In addition, the nonflat nature of the samples (due to cutting) rendered conventional methods such as ellipsometry and X-ray diffraction unusable. The full set of XPS data for the whole series of monolayers, including the divalent analogues, is given in the Supporting Information (Figures S1−S7, S52−S54).
No changes in the XPS spectra were observed upon repeated rinsing with methanol, isopropyl alcohol, or dichloromethane, suggesting that the monolayer remained intact during these treatments. These observations are in accordance with literature reports of alkylated monolayers on stainless steel,22 indicating the formation of covalently bound monolayers of the alkylphosphonic acids on the surface of SS316L. Several literature reports about alkylphosphonic acids on steel26 and other construction metals16 confirm the formation of only monolayers, rather than multilayered structures. In the case of bisphosphonic acid 3, formation of a covalently bound layer was confirmed by appearance of the N 1s signal in combination with the P signals. Note the higher intensity of the P signal showing the difference between monovalent and bisphosphonic acid−based monolayers. Atomic force microscopy imaging displayed the intrinsic roughness of the steel surface used, which is attributed to the cold-rolling process, yielding an rms roughness of the unmodified SS316L substrate of 3.5 nm. This was basically unchanged upon formation of the monolayers (3.6 nm, Figure S16). No evidence of granules or islands could be observed in the AFM image of octadecylphosphonic acid modified steel (Figure S17), in accordance with the presence of a fully formed monolayer on the surface.22 The structure and short-range order of the phosphonic acid monolayers were further investigated using IRRAS. IR spectra for decyl-, dodecyl-, hexadecyl-, and octadecylphosphonic acid showed a high extent of short-range order (which can be characterized by an antisymmetric C−H stretch vibration around or below 2920 cm−1 and a symmetric C−H stretch vibration around or below 2852 cm−1, as reported previously36). The high extent of order was indicated by the presence of sharp signals for the antisymmetric C−H stretch vibration at 2915 cm−1 (C18), 2917 cm−1 (C16), 2923 cm−1 (C12), and 2925 cm−1 (C10) and the symmetric C−H stretch vibration at 2848 cm−1 (C18), 2849 cm−1 (C16), 2852 cm−1 D
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Figure 4. (A) IR spectra (C−H stretch region) of propylphosphonic acid (black), octadecylphosphonic acid (red), and bisphosphonic acid 3 (green) on stainless steel. (B) The effect of the alkyl chain length on antisymmetric (squares) and symmetric (open triangles) C−H stretch vibration frequencies for monovalent phosphonic acids. (C) Variation of tilt angle with chain length for monovalent phosphonic acids. (D) The effect of the alkyl chain length on antisymmetric (squares) and symmetric (open triangles) C−H stretch vibration frequencies for bisphosphonic acids.
Figure 5. Long-term stability study of alkylphosphonic acids of various chain lengths as measured by their static water angle (SCA) under the following conditions: (A) neutral solution (Milli-Q water), (B) physiological solution (10 mM PBS buffer), (C) acid solution (pH 3), (D) basic solution (pH 11), and (E) dry heating (120 °C). Blank steel samples under the same conditions are listed in panel F. The SCA was measured after formation (blue), and after 1 day (red), 7 days (green), and 30 days (purple) in the respective solutions.
E
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Figure 6. XPS survey scans of (A) C10 monolayer before exposure, (B) C10 monolayer after 30 days of exposure to alkaline (pH 11) solution, and (C) C10 monolayer after 30 days of exposure to heat (120 °C). The C/P ratio was found to be 10.4 (A), 43.6 (B), and 6.45 (C).
(C12), and 2853 cm−1 (C10), respectively (Figure 4). As expected, monolayers of propyl-, butyl-, and octylphosphonic acid lacked order, as evidenced by position of the antisymmetric and symmetric C−H stretch vibration, which were as high as 2937 and 2873 cm−1, respectively, for the propyl layer (Figure 4A). As the chain length increases both the antisymmetric and symmetric C−H stretch vibration keep shifting to lower wavenumbers, in line with a continuously increasing order in the monolayer (Figure 4B). Monolayer ordering was further corroborated by the tilt angle (with respect to the surface normal) as calculated from the relative areas of the antisymmetric methylene stretch and the antisymmetric methyl stretch obtained from the IR C−H stretch regions (Figure 4C).36 A marked, continued decrease in tilt angle was observed upon increasing the alkylphosphonic acid chain length, which points toward a well-ordered packing of the alkyl chains in the all-trans configuration for alkylphosphonic acids with chain lengths of 12−18. Bisphosphonic acids 1−3 showed a shift to lower wavenumbers with increasing chain length similar to that of the monovalent phosphonic acids (Figure 4D). However, the decrease is less sharp and the wavenumbers do not reach the low values obtained for the monovalent layers. These observations indicate that these monoalkyl diphosphonic acids form less ordered monolayers, as expected for their lower packing density Long-Term Stability. Having established the conditions under which stable alkylphosphonic acid monolayers on steel are formed, their time-dependent stability under a range of demanding and standardized conditions5 was investigated. Substrates modified with the alkylphosphonic acid of variable chain lengths were screened for their stability in neutral solution (Milli-Q water), physiological solution (10 mM PBS buffer), acid solution (HCl, pH 3), basic solution (NaOH, pH 11), and dry heating at 120 °C, during a period of up to 30 days. Modified stainless steel substrates were immersed in the appropriate solutions which were swirled at room temperature to achieve the effect of continuous flow across the steel surface. Substrates for testing under dry heating conditions were placed on a glass dish in a conventional oven at 120 °C. Contact angles were measured in triplicate after 1, 7, and 30 days (Figure 5). XPS survey and P narrow scans (see Supporting Information Figures S16−S37) were recorded before exposure and after 30 days, showing the change in P signal before and after exposure. Due to variable amounts of carbon adsorption from the
atmosphere on steel (i.e., adventitious carbon, as also reported by Kerber and Tverberg37), we were not able to gain fully quantitative information from time-dependent C/P ratios. In water (Figure 5A) and physiological solution (Figure 5B), the water contact angles remained high even for the relatively short-chain C8 monolayer. This indicates a stable monolayer hardly suffering from any hydrolysis under these conditions. The importance of chain length in monolayer stability becomes clear already under these relatively mild conditions, as evidenced by the sharp decrease in the contact angle of the C3 and C4 monolayers in physiological solution, which was not observed for longer chain lengths. XPS C/P ratios for the long chain monolayers remained almost constant, whereas significant changes were observed in the short chains. These results contrast with those obtained by Kaufmann et al., where the stability of the monolayers in PBS buffer was more limited.8 We attribute these discrepancies to differences in exposure conditions [Kaufmann et al. used a significantly higher temperature (37 °C) of their PBS buffer than the room temperature (20 °C) used in this work] and buffer concentration (20 mM vs 10 mM in this work). Additionally, the 18 h curing time used by Kaufmann et al. versus the 4 h curing in this work might also be a contributing factor. As shown above, prolonged exposure to 120 °C causes some degradation of the monolayers, which is more marked in the C12 layer than for the higher chain lengths. The results in Figure 5 indicate the potential for even further improving the stability from the level already attained by Kaufmann et al., by modification of the conditions of exposure. Also under acidic conditions (pH 3; Figure 5C), contact angles for C8−C18 monolayers showed very little change. Here, even the short-chain monolayers showed remarkably good performance compared with their low stability in physiological solution, though their long-term stability was still lower than that of the long chains. The effect of chain length is more pronounced after 30 days; XPS P narrow scans showed significant reduction of the P signal in the C4 layer after 30 days at pH 3. In contrast to the high stability in acidic medium, under basic conditions all but the C18 monolayer showed limited stability (Figure 5D). Already after 1 day a sharp decrease in contact angle was observed for all chain lengths up to C16, suggesting alkaline hydrolysis of the monolayers had occurred. This decrease in contact angle persisted as the stability study was continued, indicating continued hydrolysis. The total drop in F
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Figure 7. Derivatization of a hypothesized carboxy-terminated oxidized monolayer.
Figure 8. XPS scans of heat-exposed C10 monolayer treated with 4-(trifluoromethyl)benzylamine. (A) Survey scan of derivatized monolayer. (B) C 1s narrow scan of derivatized monolayer.
To further investigate the structure of the monolayer after exposure to heat, the C10 monolayer, of which the XPS spectrum is shown in Figure 6C, was reacted with 4(trifluoromethyl)benzylamine under amide-forming conditions. Reaction of the monolayer surface with amide coupling reagents would indicate the presence of carboxylic acids derived from oxidation at the outer end of the monolayer, as would be expected in case of an oxidative top-down breakdown (Figure 7). Even after several washings, fluorobenzylamide was visible by XPS on the monolayer surface (Figure 8), suggesting an at least a partially carboxy-terminated surface was present after heat exposure. In other words, whereas basic conditions remove the monolayer from the substrate up, prolonged heating oxidizes the alkyl chains from the top down, likely irrespective of the attachment chemistry. Overall, when comparing the monolayer stability of alkylphosphonic acids with varying alkyl chain length under different conditions, it can be concluded that especially the long-chain monolayers are stable under a range of rather stringent conditions (e.g., 30 days at pH 3, or physiological conditions), although all phosphonic acid monolayers are susceptible to hydrolysis under basic conditions. To obtain a more detailed view on a molecular level, the changes in short-range order within the monolayers during the exposure were monitored using IRRAS (Figure 9). Only longchain phosphonic acid monolayers C12−C18 were used in these
contact angle was largest for the shorter alkyl chains: for the shortest C3 and C4 monolayer the value ultimately dropped below 30°, i.e., near-complete removal of the monolayer (as corroborated by XPS, which showed almost complete absence of P signals after 30 days), while for the C18 monolayer this value stayed above 80° after 30 days (cf., CA ≈ 10° for plasmacleaned stainless steel). Prolonged heating (120 °C) under air caused little decrease in contact angles after 1 day, but a considerably larger drop was observed after 7 days of exposure (Figure 5E), with the long chains performing only marginally better than the short chains in long-term trials. This means that irrespective of chain length all monolayers suffer from prolonged heating. Interestingly, final contact angles after 30 days of heating remained for all monolayers above 60°, which is considerably higher than the contact angle of unmodified steel and the values obtained after basic exposure. A large increase was observed in the XPS-based C/P ratio of the layer on the steel after long-term exposure to basic solution (Figure 6, panel B versus A), which points toward basic hydrolysis of the phosphonate bond to the metal, followed by adsorption of phosphorus-free adventitious carbon species from solution or air. In contrast, heating, which also caused (albeit to a lesser degree) degradation of the monolayers after long exposure, showed an increase in C/P ratio. This suggested a top-down breakdown of the hydrocarbon chains, rather than hydrolysis (Figure 6C). G
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Figure 9. Antisymmetric C−H stretch vibrations plotted as a function of exposure time, for chain lengths of 12 (squares), 16 (open triangles), and 18 (half-open circles) carbon atoms: (A) neutral solution (Milli-Q water), (B) physiological solution (10 mM PBS buffer), (C) acid solution (pH 3), (D) basic solution (pH 11), and (E) dry heating (120 °C).
an increase in short-range order of C12 and C16 monolayers. Tentatively, this might be ascribed to a thermal annealing effect in which surface-bound molecules migrate from regions of high concentration toward defects in the monolayer, causing a reorganization of the monolayer into a more regular structure in a mechanism similar to metal annealing.38 The above results indicate good stability of alkylphosphonic acids on stainless steel under a variety of (harsh) conditions. The stability of alkylphosphonic acid monolayers on stainless steel is comparable to, e.g., that found for phosphonic acids on porous aluminum oxide,39 and Co−Cr alloy,40 and exceeds that of phosphonic acids on CrN,5 ITO,5 and HfO2.41 The stability of phosphonic acids on stainless steel far exceeds that of amines and alkenes5 on the same substrate, and higher performances have, to the best of our knowledge, only been obtained through cross-linked polymer depositions, such as derived from dopamine.42,43 Having considered the effects of chain length, time, and medium conditions on the monolayer stability, we also set out to study the effect of multivalent surface attachment. To this end, a stability study was carried out under the same conditions (Figure 10) with the divalent bisphosphonic acids 1−3 (Figure 1). The effect of chain length was mitigated by choosing three
measurements, as the shorter chains did not form ordered monolayers from the start. In all solutions, a shift to higher wavenumbers of the C−H antisymmetric vibration was observed for the monolayers within the first day of exposure, indicating partial loss of shortrange order. In neutral (Figure 9A) and physiological (Figure 9B) solution, relatively large shifts of more than 10 cm−1 were observed for monolayers of C12 and C16, while for C18 the increase in wavenumber remained comparatively low. This indicates more retention of short-range order in the C18 monolayer, a result in accordance with the high stability this monolayer displayed in contact angle measurements. In acidic (Figure 9C) solution the same effect was observed, though the changes under these conditions were smaller. This smaller loss of short-range order corroborates the contact angle results, which showed a surprisingly high stability of the long-chain monolayers under acidic conditions. In basic solution the monolayers quickly degrade, and the resulting disordered structure can be seen in the rise of C−H stretch vibrations (Figure 9D). After exposure to heat (Figure 9E), a lower antisymmetric C−H stretch vibration was seen in case of C12 and C16 monolayers, with C18 showing the same increase as observed in the other trials. These findings point to H
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Figure 10. Stability of bisphosphonic acids. The SCA was measured after formation (blue), and after 1 day (red), 7 days (green), and 30 days (purple) in the respective solutions.
significantly better packed monoalkyl C16 phosphonic acid. Multivalent attachment thus has the potential of improving stability. Further study of multivalent attachment, including higher generations of dendritic structures and multiple long chains, is recommended.
monoalkyl divalent acids varying with different alkyl chain lengths (C4, C10, and C16). As mentioned above, the somewhat higher polarity of the headgroup and the lower packing density of the monoalkyl bisphosphonic acids lowered the initial water contact angles of these monolayers. This does, however, not hamper study of the time-dependent stability, by considering the relative decrease in contact angle. Compared to their monovalent analogues, and in line with their lower packing density, the stability of the bisphosphonic acids displayed an increased dependence on chain length: there is greater variation in both the starting contact angles and end values for the three different chain lengths (Figure 10). The dendritic C4 monolayer performed worse overall than its monovalent analogue, the divalent C10 is almost equal to its monovalent C10 analogue, while a slight improvement of stability was observed for the dendritic C16 layer over the monovalent analogue, particularly in pH 11 solution and under heating (Figure 10, parts D and E). FTIR spectroscopy on the best performing bisphosphonic acid, C16 acid 3, was measured before exposure and after 30 days (see Supporting Information Figures S65−S69). These IR measurements qualitatively confirm the quantitative contact angle measurement. For all media, after 30 days an increase in asymmetric C−H stretch wavenumber was observed. For neutral and PBS buffer solution the increase was relatively small (2322.0−2323.9 cm−1), suggesting little loss in short-range order. For pH 3 and pH 11 solutions larger increases in wavenumber (2922.0−2927.1 cm−1) were found, pointing toward a larger loss of short-range order. This pattern is similar to that observed for the long-chain monovalent monolayers, despite the lower degree of short-range order in the divalent layers at the start of the stability study (as mentioned above in the discussion of FTIR data). In summary, divalent attachment of phosphonic acids seems capable of offering some improvement to the monolayer stability, particularly in the case of basic and hot environments: despite a significantly lower packing density the monoalkyl C16 bisphosphonic acid is already slightly better than the
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CONCLUSIONS Alkylphosphonic acids with alkyl chains ranging from 3 through 18 carbon atoms form highly stable self-assembled monolayers on steel, as evidenced by 30 day long testing in various media. Especially acid (pH 3), neutral, and physiological conditions hardly affected the monolayer. Prolonged heating breaks down the alkyl chain from the top, forming carboxylic acid moieties, but leaves the steel−phosphonate bonds intact. Only strongly basic solution (pH 11) caused degradation of the monolayers, suggesting the need for, e.g., multivalent strategies to further improve the monolayer stability for those conditions. Multivalent attachment of phosphonic acids seems capable of offering improvements to the monolayer stability, although further work on combining multiple anchoring points with a concomitantly increased packing density of attached alkyl chains is needed. Overall, these results indicate the usefulness of phosphonic acid monolayers for many applications, even under rather harsh conditions, particularly in the light of their corrosion-inhibiting effects known from prior work.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04217. Additional XPS, FTIR (IRRAS), AFM, NMR, and contact angle data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. I
DOI: 10.1021/acs.langmuir.5b04217 Langmuir XXXX, XXX, XXX−XXX
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Langmuir Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Barend van Lagen, Sidharam Pujari, and Zhanhua Wang for helpful comments and technical assistance, NanoNextNL (a micro- and nanotechnology consortium of the Government of The Netherlands and 130 partners; program 5D) for generous funding, and several reviewers for constructive suggestions on previous versions of this manuscript.
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DOI: 10.1021/acs.langmuir.5b04217 Langmuir XXXX, XXX, XXX−XXX
