Grafted Self-Assembled Monolayers Derived from Naturally Occurring

Duy Hai Dinh , Luc Vellutini , Bernard Bennetau , Corinne Dejous , Dominique Rebière , Émilie Pascal , Daniel Moynet , Colette Belin , Bernard Desba...
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Langmuir 2005, 21, 3338-3343

Grafted Self-Assembled Monolayers Derived from Naturally Occurring Phenolic Lipids J.-P. Pillot,* M. Birot, and T. T. T. Tran Laboratoire de Chimie Organique et Organome´ tallique, UMR 5802 CNRS-Universite´ Bordeaux 1, 351, cours de la Libe´ ration, 33405 Talence CEDEX, France

T. M. Dao Institute for Tropical Technology, CNST Vietnam, Hoang Quoc Viet Road, Cau Giay, Hanoi, Vietnam

C. Belin, B. Desbat, and S. Lazare Laboratoire de Physico-Chimie The´ orique, UMR 5803 CNRS-Universite´ Bordeaux 1, 351, Cours de la Libe´ ration, 33405 Talence CEDEX, France Received October 27, 2004. In Final Form: January 28, 2005 Self-assembled monolayers grafted onto silicon surfaces were obtained from the hydrosilylation products by trialcoxysilanes of naturally occurring phenolic lipid allyl ethers. The as-obtained materials were characterized by various physical and physicochemical methods. Thus, contact angles of water drops showed that they possess very high hydrophobicity. Their excellent regularity was corroborated by AFM microscopy. The frequencies of the stretching CH2 infrared modes indicate the presence of alkyl chains mainly in the trans/trans conformation. Additionally, optical ellipsometry and quartz microbalance measurements enabled us to estimate the thickness of the films. The results, as a whole, are in good agreement with the formation of densely packed monolayers.

Introduction Materials based on the engineering of ordered structures at a nanoscale level have recently received a great deal of attention.1-5 The self-assembly of low-molecular- weight building blocks has emerged as one of the most promising approaches in this field.6-9 Particularly, the preparation of thin films and coatings based on coupling agents that spontaneously give cohesive interactions has become an important route for tailoring surface properties, leading to a wide range of applications such as moleculebased microdevices, biotechnologies, and corrosion protection.6,10-13 In this context, organosilicon compounds * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (2) Kotera, M.; Lehn, J.-M.; Vigneron, J.-P. J. Chem. Soc., Chem. Commun. 1994, 197. (3) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Fomer, B. J. B.; K. Hirschberg, J. H. K.; Lange, R. F.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601. (4) Muthukumar, M. ; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225. (5) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1988, 37, 550. (6) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (7) Ulman, A. Chem. Rev. 1996, 96, 1533. (8) Ruokolainen, J.; Tanner, J.; Ikkala, O.; ten Brinke, G.; Thomas, E. L. Macromolecules 1998, 31, 3532. (9) Maki-Ontto, R.; de Moel, K.; de Odorico, W. Adv. Mater. 2001, 13, 117. (10) Plueddemann, E. P.; Silane coupling agents, 2nd ed.; Plenum Press: New York, 1991; Chapter 2. (11) Navarre, S.; Choplin, F.; Bousbaa, J.; Bennetau, B.; Nony, L.; Aime´, J.-P. Langmuir 2001, 16, 4844. (12) Bennetau, B.; Bousbaa, J.; Choplin, F.; Cloarec, J.-P.; Martin, J.-R.; Souteyrand, E. WO0153303, 2001.

containing both hydrolyzable functions and long hydrocarbon chains possess unique properties, enabling the synthesis of self-assembled molecular monolayers (SAMs) grafted onto various surfaces.7-10 The number of carbon atoms is a key parameter in obtaining highly ordered structures, and it has been shown that materials with a marked crystalline character could result from the presence of linear C18 chains.14,15 From a synthetic point of view, the control of chain lengths usually involves the coupling of halogenated derivatives via organometallic intermediates. However, it should be recognized that the routes that lead to long-chain alkyl groups are inconvenient and give relatively poor yields.10,16 Moreover, most of the precursors are petroleum derivatives, a fossil resource which is being progressively exhausted. Therefore, it seemed advisable to develop alternative approaches to nanostructured materials by using abundant, renewable biomass derivatives. Thus, we anticipated that naturally occurring phenolic lipids from the Anacardiaceae, which possess linear C15 and C17 hydrocarbon chains in the 3-position of the benzene ring, might be convenient raw materials in the synthesis of new silane-based SAMs. The only reports describing the use of these natural phenolic compounds in the synthesis of supramolecular edifices concern applications of 3-pentadecylphenol in liquid crystal polymers and Langmuir-Blodgett multilayers.17-21 Recently, self-assembled nanotubes and gels based on (13) Subramanian, V.; Van Ooij, W. J. In Silanes and other couplings agents; Mittal, K. L., Ed.; VSP: Zeist, The Netherlands, 2000; Vol. 2, p 159. (14) Bierbaum, K.; Kinzler, M.; Wo¨ll, C.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (15) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135. (16) Choplin, F.; Navarre, S.; Bousbaa, J.; Babin, P.; Bennetau B.; Bruneel, J.-L.; Desbat, B. J. Raman Spectrosc. 2003, 34, 902.

10.1021/la047357r CCC: $30.25 © 2005 American Chemical Society Published on Web 03/10/2005

Grafted Self-Assembled Monolayers

cardanol glycolipids have also been obtained.22,23 We report herein the unprecedented use of organosilicon coupling agents derived from cardanol and laccol in the preparation of new SAMs. Experimental Section Materials. Cardanol 1 was obtained from the treatment of cashew nut-shell liquid of Vietnam according to a previously reported procedure.24 It was purified by distillation (Eb0.5 ) 180185 °C) and stored in the dark under an inert gas atmosphere. Its structural characterization (GC/MS, IR, 1H and 13C NMR spectroscopies) was in good agreement with the data previously reported in the literature.25 Laccol 2 was obtained from the sap of the Northern Vietnam lacquer tree, Rhus succedanea. The organic part of the crude sap was separated, dissolved in acetone, filtered, and the solvent removed under vacuum. After being washed with distilled water and being extracted three times with toluene, the organic fractions were combined, dried over sodium sulfate, and the solvent distilled off under vacuum. Pure laccol was obtained as a brown liquid and stored under inert gas in the dark.26 General Procedures. Triethoxysilane and Karstedt’s catalyst used in the hydrosilylation reactions were obtained from Gelest. Pd/C (5% Pd) was purchased from Aldrich. All the solvents were distilled under an inert atmosphere: diethyl ether was dried over sodium-benzophenone, toluene was distilled over sodium, and acetone was dried over CaCl2. Water used to wash the surfaces was distilled and further purified with a Millipore Milli-Q system. IR. Infrared spectra of the compounds (wavenumbers in cm-1) were recorded on a Perkin-Elmer Paragon 1000 FTIR spectrometer (resolution ) 2 cm-1) on films between KBr plates. In the case of the monolayers, the spectra were obtained in transmission mode on a Nicolet Nexus 670 spectrometer (resolution ) 4 cm-1). Double-side polished silicon wafers (1 × 1 cm2, thickness < 0.5 mm) were available from A. C. M. (78640 Villiers Saint Fre´de´ric, France). They enabled us to graft the monolayers onto both faces (to increase the signal) and to obtain spectra with suitable baselines. NMR. 1H and 13C NMR spectra were recorded on a Bruker AC 250 or a Bruker DPX 400; 29Si NMR spectra (INEPT) were recorded on a Bruker DPX 200 (solvent: deuteriochloroform, chemical shifts in ppm, δTMS ) 0 ppm). Microanalysis. Elemental analyses were performed by the Service Central d’Analyse du CNRS, Vernaison, France. Mass Spectrometry. Mass spectra (MS) were obtained on a hybrid Fisons-Instruments spectrometer VG Micromass Autospec type EBQq (EI+ ) 70 eV). HRMS were obtained according to the LSIMS+ (liquid secondary ion mass spectrometry) technique (Cs+, 35 keV, 3-nitrobenzylic alcohol matrix). GC/MS. An Intersmat IGC 121 M chromatograph equipped with a CPSIL 5 CB W column (25 m, i.d. 0.22 mm, carrier gas: helium) was used. This apparatus was coupled to a mass spectrometer VG Micromass type AG-F (70 eV, ion current ) 200 µA, data station: VG 2040). 3-Pentadecylphenol (C21H36O, 1d).27,28 Cardanol 1 (20 g), absolute ethanol (25 mL), and Pd/C 5% (2 g) were introduced (17) De Moel, K.; Alberda van Ekenstein, G. O. R.; Nijland, H.; Polushkin, E.; ten Brinke, G. Chem. Mater. 2001, 13, 4580. (18) Pillai, C. K. S.; Sherrington, D. C.; Sneddon, A. Polymer 1992, 33, 3968. (19) Manjula, S.; Suddha, J. D.; Bera, S. C.; Pillai, C. K. S. J. Appl. Polym. Sci. 1985, 30, 1767. (20) Saminathan, M.; Pillai, C. K. S. Polymer 2000, 41, 3103. (21) Hanabusa, K.; Oumi, T.; Koyama, T.; Shirai, H.; Hayakawa, T.; Kurose, A. J. Macromol. Sci. 1989, A26, 1397. (22) John, G.; Jung, J. H.; Masuda, M.; Shimizu, T. Langmuir 2004, 20, 2060. (23) John, G.; Masuda, M.; Okada, Y. Langmuir 2004, 13, 715. (24) Tyman, J. H. P. J. Chem. Soc., Perkin Trans. 1973, 1, 1639. (25) Strocchi, A.; Lercker, G. J. Am. Oil Chem. Soc. 1979, 56, 616. (26) These phenolic lipids must be handled with gloves because they cause skin dermatitis. (27) Mhaske, S. B.; Bhingarkar, R. V.; Sabne, M. B.; Mercier, R.; Vernekar, S. P. J. Appl. Polym. Sci. 2000, 77, 627. (28) Rupavani, J. N.; Vijayalakshmi, V.; Sitaramam, B. S.; Krishnamurti, N. Eur. Polym. J. 1993, 29, 863.

Langmuir, Vol. 21, No. 8, 2005 3339 into an autoclave. The mixture was put under hydrogen pressure (5 × 106 Pa) and shaken at room temperature for 20 h. After filtration, evaporation of the solvent, and distillation under vacuum, 1d was obtained as a white solid (12.9 g, 65%, bp ) 176 °C/0.014 Pa; mp ) 49.7 °C). IR: 3333 (νOH); 1619 (νCdCaromatic). 1H NMR: 0.88 (t, 3 H, CH3); 1.26 (m, 24 H, 12 CH2); 1.56 (m, 2 H, -CH2-CH2benzylic); 2.55 (t, 2 H, CH2benzylic); 4.85 (s, 1 H, OH); 6.66 (m, 3 H, Haromatic); 7.14 (t, 1 H, Haromatic). 13C NMR: 14.8 (CH3); 22.8, 29.4, 29.7, 31.4, 32.0, 35.9 (chain CH2); 112.5, 115.4, 121.0, 129.4 (CHaromatic); 145.7 (Caromatic-chain); 156.0 (C-OH). Anal. Calcd: C: 82.83; H: 11.92; O: 5.25. Found: C: 82.77; H: 11.87; O: 5.28. 1-Allyloxy-3-pentadecylbenzene (C24H40O, 3). In a 100 mL, three-necked flask were placed 1d (2 g, 0.0065 mol), K2CO3 (1.82 g, 0.013 mol), and acetone (20 mL). This mixture was stirred until the solids completely dissolved. Then, allyl bromide (2.32 mL, 0.0268 mol) was added and the solution heated to reflux for 15 h. After being cooled to room temperature, the mixture was filtered off. The organic phase was washed with water (5 mL) and decanted. After extraction by diethyl ether (2 × 20 mL), drying over Na2SO4, and filtration, the solvent was removed by evaporation under vacuum. After passing through a short chromatography column (silica, petroleum ether/diethyl ether ) 100:0-95:5, v/v), 3 was obtained as a pale yellow liquid (1.62 g, yield ) 72%). IR: 1649 (νCdCallylic); 1610 (νCdCaromatic); 1259 (νAr-O-C). 1H NMR: 0.89 (t, 3 H, CH ); 1.26 (m, 24 H, 12 CH ); 1.58 (m, 2 3 2 H, CH2-CH2benzylic); 2.57 (t, 2 H, CH2benzylic); 4.54 (m, 2 H, CH2O); 5.30 (m, 2 H, CH2dCH); 6.03 (m, 1 H, CH2dCH); 6.75 (m, 3 H, CHaromatic); 7.18 (t, 1 H, CHaromatic). Hydrosilylation of 3 (C30H56O4Si, 4). In a 100 mL, threenecked flask filled with argon were placed 3 (1.6 g, 0.0046 mol) and distilled toluene (5.75 mL). Then, Karstedt’s catalyst (5 drops) and triethoxysilane (1.34 mL, 0.0073 M) (excess), were added. After being heated at 40 °C for 4 h under stirring, the resulting mixture was cooled at room temperature and the solvent removed by evaporation, yielding 4 as a brown liquid (2.35 g, yield ) 99%). IR: 2925 (νasCH2); 2854 (νsCH2); 1081 (νSi-O-C). 1H NMR: 0.77 (t, 2 H, CH2Si); 0.88 (t, 3 H, CH3); 1.25 (m, 33 H, 12 CH2 and 3 O-CH2-CH3); 1.58 (m, 2H, CH2-CH2benzylic); 1.89 (m, 2 H, O-CH2-CH2,); 2.56 (t, 2 H, CH2benzylic); 3.85 (m, 8 H, 4 O-CH2); 6.72 (m, 3 H, CHaromatic); 7.16 (t, 1 H, CHaromatic). 13C NMR: 6.4 (CH2-Si); 14.0 (CH3); 18.2 (O-CH2-CH3 and O-CH2-CH2); 22.6-36.0 (CH2); 58.3 (O-CH2-CH3); 69.6 (O-CH2-CH2); 111.6, 114.7, 120.6, 129.1, 144.4 (Caromatic-chain); 159.0 (Caromatic-O). 29Si NMR: -44.98. 3-Heptadecylcatechol (C23H40O2, 2g). The hydrogenation of long-chain, unsaturated catechols such as urushiol has previously been reported in the literature.29-34 Laccol 2 (7 g), absolute ethanol (8.9 mL), and Pd/C 5% (0.7 g) were put into an autoclave under hydrogen pressure (5 × 106 Pa). This mixture was shaken for 20 h at room temperature. After filtration, evaporation of the solvent, and distillation under high vacuum, 2g was obtained as a white solid (6.0 g, 86% yield, bp ) 190-200 °C/0.014 Pa; mp ) 62.5 °C). 1H NMR: 0.88 (t, 3 H, CH3); 1.26 (m, 28 H, CH2chain); 1.62 (m, 2 H, CH2-CH2benzylic); 2.60 (t, 2 H, CH2benzylic); 5.12 (s, 2 H, 2 OH); 6.71 (m, 3 H, Haromatic). 13C NMR: 14.5 (CH3); 24.8, 29.4, 29.5, 29.7, 29.8, 31.9 (CH2chain); 112.9, 122.1, 122.3 (CHaromatic); 128.9 (Caromatic-chain); 142.1, 143.2 (C-OH). IR: 3333 (νOH); 1619 (νCdCaromatic). MS (EI+): m/z ) 348.2 (calcd), 348.6 (found), (M+). Anal. Calcd: C: 79.25; H: 11.57; O: 9.18. Found: C: 79.21; H: 11.62; O: 9.10. 1,2-Diallyloxy-3-heptadecylbenzene (C29H48O2, 5). In a 50 mL, three-necked flask were placed under an inert atmosphere 2g (4.30 g, 0.0125 mol), allyl bromide (7.55 g, 0.0624 mol) (excess), K2CO3 (3.42 g, 0.025 mol), and freshly distilled acetone (80 mL). The mixture was stirred and heated at reflux for 48 h. After being cooled to room temperature, the resulting mixture was filtered off. The organic phase was washed with (29) Majima, R.; Tahara, J. Chem. Ber. 1915, 48, 1606. (30) Mason, H. S. J. Am. Chem. Soc. 1945, 67, 1538. (31) Dawson, C. R.; Wasserman, D.; Keil, H. J. Am. Chem. Soc. 1946, 68, 534. (32) Boehme, W. R. J. Am. Chem. Soc. 1960, 82, 498. (33) Backer, H. J.; Haack, N. H. Rec. Trav. Chim. 1938, 57, 225. (34) De A. Lima, O. O.; Braz-Filho, R. J. Braz. Chem. Soc. 1997, 8, 235.

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water and decanted. After extraction by diethyl ether, drying over Na2SO4, and filtration, the solvent was removed under vacuum, yielding 5.12 g of crude product. After purification by using a silica chromatography column (petroleum ether/diethyl ether ) 100:0-95:5, v/v), 5 was obtained as a yellow liquid (4.36 g, 82.5% yield). 1H NMR: 0.88 (t, 3 H, CH3); 1.25 (m, 28 H, CH2chain); 1.56 (m, 2 H, -CH2-CH2benzylic); 2.62 (t, 2H, CH2benzylic); 4.51, 4.56 (m, 4 H, 2 CH2O); 5.35 (m, 4 H, 2 CH2dCH); 6.09 (m, 2 H, 2 CHdCH2); 6.73-6.81 (m, 2 H, 2 CHaromatic); 6.94 (t, 1 H, CHaromatic). 13C NMR: 14.0 (CH ); 22.6, 29.5, 29.6, 29.3, 29.5, 30.1, 30.7, 31.9 3 (CH2chain); 69.3, 73.6 (CH2O); 111.5 (CHaromatic); 116.8, 117.1 (CH2d CH); 122.1, 123.4 (CHaromatic); 133.4, 134.6 (CH)CH2); 137.1 (Caromatic-chain); 146.2 (C-O); 151.6 (C-O). IR: 1644, 1601 (νCd + Callylic); 1269, 1201 (νC-O-C). HRMS (LSIMS , NBA): m/z (calcd): 428.3654, (found): 428.1141 (M+•). Anal. Calcd: C: 81.25; H: 11.29; O: 7.46. Found: C: 81.22; H: 11.25; O: 7.49. Hydrosilylation of 5 (C41H80O8Si2, 6). In a 100 mL, threenecked flask were placed 5 (3.135 g, 0.0073 mol) and toluene (16 mL). Karstedt’s catalyst (3 drops) and triethoxysilane (3.06 mL, 0.017 mol) were successively added and stirred. After being heated at 60 °C for 3 h under stirring, the resulting solution was allowed to cool to room temperature. The solvent and the excess of triethoxysilane were removed under high vacuum, yielding 6 as a brown liquid (4.30 g, 99% yield). 1H NMR: 0.78 (m, 4 H, CH2Si); 0.88 (t, 3 H, CH3); 1.24 (m, 46 H, CH2chain and 6 CH3); 1.58 (m, 2 H, -CH2-CH2benzylic); 1.90 (m, 4 H, CH2); 2.60 (t, 3 H, CH2benzylic); 3.87 (m, 16 H, 8 OCH2); 6.75-6.92 (m, 3 Haromatic). 29Si NMR: -44.71, -45.10. IR: 1104, 1083 (νSi-O-C). Octadecyltriethoxysilane (OTES) (C24H52O3Si). OTES was obtained from 1-octadecene (2.52 g, 0.01 mol) by using the same procedure as that for 3 (yield ) 99%). Silane Layers. The silicon wafers (double side polished) were available from A. C. M. (78640 Villiers Saint Fre´de´ric, France). The plates (1×b01 cm2) were cleaned and dried just before the grafting. They were treated successively with hexane, acetone, methanol, and a solution of Hellmanex 1% for 7 min and were then rinsed three times with Milli-Q water before being dried at 120 °C for 3 h in an oven. These supports were dipped into solutions of 4, 6, or OTES (10-4 or 5 × 10-4 M in distilled toluene) at 25 °C for 16 h (24 h in the case of OTES). Two drops of acetic acid were added in the solutions to obtain a pH ) 4. Then, the samples were washed with distilled toluene and dried at 100 °C for 2 h. Contact Angle Measurements. The contact angles were determined using a Kru¨ss goniometer (drop shape analysis system DSA 10 Mk2) at 25 °C. One drop of Milli-Q water was laid down onto the surface by a microsyringe with a curved needle. The average value of contact angle was taken from at least 10 measurements. Atomic Force Microscopy (AFM). The AFM analyses were realized on a ThermoMicroscope Autoprobe CP Research (Park Scientific Instrument) at constant temperature (25-26 °C). All experiments were done in contact mode with a Microlever cantilever using a normal working force in the range of 2.2-2.4 nN. A 5 µm scanner was used at 0.5 Hz, and the data were acquired on 2 × 2 µm2 and 1 × 1 µm2 frames having 512 × 512 data points. Ellipsometric Measurements. The analyses were realized on a I-elli2000 NFT ellipsometer equipped with a 532 nm wavelength laser. The angle of incidence was fixed at 70°. A refractive index of 1.45 was introduced in the instrument software to determine the thickness of the silane layers. Quartz Microbalance. A microbalance constructed in our laboratory was used.35 After grafting on the surface of piezoelectric quartz crystals, the mass of the layers were obtained via measuring the change in the resonance frequency of the crystals.

Results and Discussion Raw Materials. Various members of the Anacardiaceae generate valuable phenolic compounds containing saturated and unsaturated linear long chains linked to different positions of the benzene ring. Thus, cardanol 1 (35) Lazare, S.; Soulignac, J.-C.; Fragnaud, P. Appl. Phys. Lett. 1987, 50, 624.

Pillot et al. Scheme 1. Structure of Cardanol

Scheme 2. Structure of Laccol

results from the distillation of cashew nutshell liquid, an abundant raw material issued from Anacardium occidentale.36-38 It is a mixture of four compounds possessing nearly the same structure with linear C15 hydrocarbon chains in the meta position, which differ from one another by the number and position of the double bonds (Scheme 1, compounds 1a-1d). Laccol 2 corresponds to the main organic fraction of the resinous sap exuded by Rhus succedanea.39-42 Its catechollike structure resembles urushiol, another widespread natural substance obtained from Rhus vernicifera, with a C17 hydrocarbon linear chain in position 3 instead of a C15 chain. Both substances contain ethylenic bonds in their side-chains (Scheme 2).43-45 The numerous industrial applications of cardanol, urushiol, and laccol in the domain of coatings and varnishes, including Oriental lacquer, have already been reviewed.36-38,43-47 Organosilicon Precursors. The precursors used in this study are based on 3-pentadecylphenol 1d and 3-heptadecylcatechol 2g, the hydrogenated products of cardanol and laccol, respectively.48,49 Thus, the organosilicon coupling molecules 4 and 6 were synthesized in good yields via a Williamson reaction followed by hydrosilylation, in the presence of Karstedt’s platinum catalyst, of the allyl ethers 3 and 5, respectively (Schemes 3 and 4). Additionally, OTES was used as a reference precursor without an aromatic group. With the aim of synthesizing coupling agents that could be grafted onto a large variety of surfaces, including metallic substrates, we used the (36) Aggarwal, J. S. J. Colour Soc. 1975, 14, 1. (37) Desai, T. B.; Potnis, S. P.; Aggarwal, J. S. Paintindia 1976, 26, 11. (38) Tyman, J. H. P. Chem. Soc. Rev. 1979, 8, 499. (39) Yoshida, H. J. Chem. Soc. 1883, 43, 472. (40) Bertrand, G. C. R. Acad. Sci. 1894, 118, 1215. (41) Kumanotani, J. Kagaku To Kogyo 1983, 36, 151. (42) Kamiya, Y.; Saito, W.; Miyakoshi, T. J. Oleo Sci. 2002, 51, 473. (43) Vogl, O. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4327. (44) Snyder, D. M. J. Chem. Educ. 1989, 66, 977. (45) Kobayashi, S.; Uyama, H.; Ikeda, R. Chem. Eur. J. 2001, 7, 4754. (46) Lubi, M. C.; Thachil, E. T. Des. Mon. Polym. 2000, 3, 123. (47) Nagase, K.; Miyakoshi, T. Toso Kogaku 1997, 32, 422. (48) Organosilicon coupling agents derived from cardanol had previously been reported in the literature, but they had never been used in the preparation of nanostructured materials. (49) Ghatge, N. D.; Khisti, R. S. J. Polym. Mater. 1989, 6, 145.

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Scheme 3. Synthesis of Coupling Agent 4 from 3-pentadecylphenol

Figure 1. IR spectra of the monolayers obtained from 4, 6, and OTES on silicon wafers. Scheme 4. Synthesis of Coupling Agent 6 from 3-heptadecylcatechol

Table 1. Contact Angles of Water Drops at 25 °C concentration of the solution (c) surfaces silicon surface (neat) OTES 4 6

(10-4 M)

-

contact angles (°) 10 101 102 105

(5 × 10-4 M) 108 108 110

Table 2. IR Wavenumbers of Methylene Groups (cm-1) 4 6

triethoxysilyl compounds instead of the trichlorosilyl ones to avoid any release of hydrochloric acid at the interface between the substrate and the layer. From a general point of view, the presence of long alkyl chains is necessary to form self-assembled monolayers, but densely packed assemblies can also be obtained when additional structural motifs, such as aromatic groups, are present.7,15 Thus, it has been argued that the incorporation of 1,4-substituted benzene rings in the long alkyl chains, independently of their position, influences the chain molecular orientation to the surface but does not prevent dense packing.50,51 However, the precursors investigated in this work are less symmetrical than in the case of the former example, as the benzene ring is substituted in the 3-position instead of the 4-position. In addition to these geometrical parameters, experimental factors, such as the temperature, the amount of water, and the pH, play an important role on the self-assembly pathway. Therefore, it was necessary to ascertain experimentally if the hydrocarbon chains of these coupling agents were able to take an extended conformation and yield grafted oriented monolayers. Monolayers. Silicon surfaces having two different average roughness values (Ra ) 0.14 and 0.45 nm, respectively) were used, while toluene was the most convenient organic solvent for compounds 4 and 6. Wettability of the Surfaces. First, the water repellence of the surfaces was determined by measuring the contact angle of water drops at 25 °C (Table 1). The best results were obtained for concentrations of about 5 × 10-4 M. With respect to the starting silicon surface (contact angle value ≈ 10°), very high contact angles (above 100°) were found in all cases. This leads us to infer that the coupling agents afforded very regular (50) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (51) Buckel, F.; Effenberger, F.; Yan, C.; Go¨lzha¨user, M. Grunze, Adv. Mater. 2000, 12, 901.

OTES

vibration

solution

neat

monolayer

νas(CH2) νs(CH2) νas(CH2) νs(CH2) νas(CH2) νs(CH2)

2927 2856 2927 2854 2927 2856

2926 2855 2926 2853 2926 2855

2922 2851 2921 2851 2921 2852

films. Owing to its unique structure containing two adjacent silyl groups and hydrolyzable functions, compound 6 was expected to graft on the surfaces very efficiently. This was demonstrated by the fact that it has led to the highest contact angle value, which was nearly the same as that reported in the literature for the highly oriented monolayers derived from octadecyltrichlorosilane (OTS, 111°).50 These encouraging preliminary results suggest that the coupling agents 4 and 6 can lead to films characterized by a high homogeneity and stability. It is worth noting that these layers exhibited a quite different behavior than the Langmuir-Blodgett monolayers based on 3-pentadecylphenyl esters of amino acids, which are somewhat unstable.21 IR. The infrared CH2 stretching bands νa and νs of the layers of 4, 6 and OTES are shown in Figure 1. Qualitatively, the relatively weak but reproducible intensities of the bands are consistent with the occurrence of monolayers. The reference film obtained from OTES nearly exhibits the same IR bands. The wavenumbers of the CH2 modes of the layers and the starting coupling agents are shown in Table 2. The wavenumber values are significantly lower than those of the starting compounds, suggesting that the long alkyl chains mainly retain a close-packed, ordered assembly. They reach values close to those reported by Ulman et al. for SAMs containing 1,4-disubstituted benzenes.50 Interestingly, they are almost the same as those reported for Langmuir-Blodgett multilayers obtained from 3-pentadecylphenol esters, in which the chains are oriented nearly perpendicular to the surfaces.21 Nevertheless, these values are slightly higher than those reported for highly compact self-assembled monolayers issued from OTS, e.g., νas(CH2) ≈ 2917-2918 cm-1, leading us to infer that the chain conformations contain some

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Table 3. AFM Average Roughness Values Ra (nm) 4 Si surface (neat) 0.45 (a) 0.14 (b)

(C ) 5 ×

6 10-4 M)

(C )

0.15 0.14

defects. This disorder is probably localized at the end of the chains and modifies the average orientation of the carbon bonds.15 It may also arise from the C3 chains between the silicon atom and the aryl group that could be responsible for a tilt angle resulting in appreciable broadening of the IR bands. AFM Microscopy. In a preliminary approach, layers of 6 on silicon surfaces having a relatively high average roughness value (Ra ) 0.45 nm) were examined. Two

Figure 2. AFM surfaces: starting silicon surface (a); monolayers of 6: C ) 10-4 M (b); C ) 5 × 10-4 M (c).

10-4 M)

0.16 -

OTES (C ) 5 ×

10-4 M)

0.14 0.13

(C ) 5 × 10-4 M) 0.15 0.13

concentrations were used C ) 10-4 M (b) and C ) 5 × 10-4 M (c) (Figure 2). The Ra values are presented in Table 3. For C ) 10-4 M, the presence of small holes randomly distributed at the surface show that the layer contains defects (Figure 2b). Conversely, a homogeneous surface was obtained for C ) 5 × 10-4 M (Figure 2c). Figure 2c shows that homogeneous films with no defects are obtained for C ) 5 × 10-4 M. Interestingly, the layer of 6 exhibits a marked Ra decrease in both cases (Table 3). These results lead us to conclusion that, despite the relatively high roughness of the substrate, the coupling agents coat the whole surface very efficiently; the surface valleys and peaks are substantially leveled due to the bidimensional homogeneous film polymer. Then, films of 4, 6, and OTES on very smooth silicon surfaces (Ra ) 0.14 nm) were studied (C ) 5 × 10-4 M, Figure 3). Unlike the former case, only minor changes were observed in the Ra values (Table 3). This accounts for the very low Ra value of the starting silicon surface, demonstrating that the chemical modification proceeds with this substrate homogeneously, while no molecular aggregate was formed. Interestingly, films of 4, 6, and OTES exhibit the same AFM behavior. Since the surfaces appeared to be uniform and contain no defects, cracks, or holes at this nanoscopic level of characterization, attempts to directly determine the thickness of the layers by AFM remained unsuccessful.11 Fortunately, this could be accomplished by the ellipsometry technique. Visible Optical Ellipsometry. Optical imaging ellipsometry provides a convenient method of verifying the monomolecular nature of layers. In the case of selfassembled monolayers of molecules containing aryl groups, the use of suitable nf values, in the range of 1.45-1.50 has previously been determined.50 So, we have employed the value of 1.45 for the film real refractive index, nf, in the determination of the film thickness, e (Table 4). Moreover, the molecule lengths were determined via a calculation to estimate the theoretical thickness of the layers.52 On the assumption that the molecules take on a fully extended conformation and that the C1-C3 axis of the benzenic ring is not canted, calculated values of 2.9 and 3.2 nm were obtained for the monolayers derived from 4 and 6, respectively (Scheme 5). In the case of OTES, the e theoretical value (2.6 nm) was very close to the experimental values reported in the literature for monolayers derived from OTS (2.5 ( 2 nm), showing that the calculation method is pertinent. Therefore, it was applied to compounds 4 and 6.52 The monolayer derived from 6, which contains C17 chains, corresponds to an experimental e value of 2.7 ( 0.1 nm (C ) 5 × 10-4 M). As expected, this value is appreciably higher than that of 4 (2.2 ( 0.1 nm) with a C15 chain. However, these values are slightly smaller than the calculated ones, leading us to assume that the molecules are tilted. An estimation of the tilt angles for 4 and 6 in their extended forms led to values of 40° and 32°, respectively. The marked decrease in the tilt angle from 4 to 6 parallels the decrease in disorder from the C15 to the C17 chain. (52) ChemOffice 3D. The molecular structures were calculated for the fully extended form, from the silicon atom of the surface to the furthest hydrogen of the terminal group.

Grafted Self-Assembled Monolayers

Langmuir, Vol. 21, No. 8, 2005 3343

Table 4. Thickness (nm) of Monolayers Corresponding to Molecules 4 and 6 Prepared at Two Different Concentrations 4 (C ) 5 × calculated thickness (extended conformation) ellipsometry (nf ) 1.45) quartz microbalance

Quartz Microbalance. Additional experiments based on grafting the organosilicon molecules 4 and 6 onto the plane surfaces of piezoelectric quartz crystals were carried out under the usual conditions (C ) 5 × 10-4 M). Thus,

6 10-4 M)

(C )

2.9 2.2 ( 0.1 -

10-4 M)

3.2 2.3 ( 0.1 2.2 (d ) 0.9) 2.9 (d ) 1.2)

(C ) 5 × 10-4 M) 3.2 2.7 ( 0.1 2.8 (d ) 0.9) 3.5 (d ) 1.2)

Scheme 5. Coupling Agent 4 (extended conformation)

it was expected that the determination of the mass of the films via the change in the quartz resonance frequency might lead to the thickness of the layers. Owing to the accurate knowledge of the grafted surface areas and assuming film densities, d, between 0.9 and 1.2, thickness values in the range 2.2-2.9 nm (4), and 2.8-3.5 nm (6) were obtained, respectively. These values are in good agreement with those expected for well-ordered selfassembled monolayers (Table 4). Conclusion Organosilicon coupling agents derived from cardanol and laccol readily afforded self-assembled monolayers grafted onto silicon and quartz surfaces. The physical and physicochemical characterization of these materials (contact angle measurements, IR, AFM) gave convergent results showing that homogeneous materials containing organized chains were obtained. Moreover, the thickness values of the layers measured by visible optical ellipsometry were in good agreement with the data obtained with a piezoelectric quartz crystal microbalance. They established the monolayer nature of the films and suggest that the molecules are tilted. These results offer new perspectives for the valorization of abundant renewable resources in the field of nanomaterials. Indeed, various phenolic lipids that are found in the Anacardiaceae also possess the capability to sustain further industrial developments via green chemistry routes.

Figure 3. AFM surfaces of layers obtained from 4 (b), 6 (c), and OTES (d); C ) 5 × 10-4 M, Ra value of the substrate (a) ) 0.14 nm.

Acknowledgment. We are grateful to the CNRS (DRI) and the Ministe`re des Affaires E Ä trange`res for grants (T.T.T.T. and T.M.D.). We also thank the Conseil Re´gional d’Aquitaine for financial support. LA047357R