Self-Assembly Is Not the Only Reaction Possible between

Aug 3, 2000 - Silicon-supported alkylsiloxane layers were prepared by reaction of alkylmethyldichlorosilanes and alkyltrichlorosilanes with silicon wa...
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Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon Alexander Y. Fadeev1 and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 Received March 29, 2000. In Final Form: June 10, 2000 Silicon-supported alkylsiloxane layers were prepared by reaction of alkylmethyldichlorosilanes and alkyltrichlorosilanes with silicon wafers under two conditions: (1) in the vapor phase and (2) in toluene in the presence of ethyldiisopropylamine. Covalent attachment of di- and trichlorosilanes to the surface of silicon/silicon oxide through SiS-O-Si bonds occurs for the amine-catalyzed reactions. This sets apart this reaction from the self-assembly process that occurs in the reaction between certain trichlorosilanes and hydrated silica with no amine present. The thickness of the layers formed from dichloro- and trichlorosilanes (as assessed by ellipsometry) is on the order of the single molecule sizes and increases gradually with alkyl chain length. The thickness values are considerably smaller (by a factor of ∼0.75) than the length of the fully stretched alkyl chain, which argues for disordered structures of the monolayers. Dynamic advancing and receding contact angles for water, methylene iodide, and hexadecane argue for interaction between the probe fluids and accessible silanol groups (Si-OH) on the surface. Water contact angles increase with alkyl chain length and level at θA/θR ) ∼103°/∼90° for relatively long alkyl chains (∼C6 and longer), indicating that these surfaces project disordered methyl and methylene groups toward the probe fluid. n-Hexadecane and methylene iodide contact angles show more complex behavior, which is discussed in the paper. The vapor-phase reaction of di- and trichlorosilanes with silicon wafers yields surfaces that depend dramatically on the alkyl chain of the silane. Alkylsilanes with short and medium chains form polymeric grafted layers with thicknesses ranging from a few nanometers for dichlorosilanes up to tens of nanometers for trichlorosilanes. We suggest a mechanism that involves polycondensation of chlorosilanes into 3-D alkylsiloxanes in the presence of adsorbed water. Dynamic advancing and receding contact angles of water, methylene iodide, and hexadecane on these surfaces are consistently higher than for surfaces prepared in the liquid phase. Alkylsilanes with long alkyl moieties yield approximately monomolecular layers that exhibit wettabilities similar to those for surfaces prepared in the liquid phase.

Introduction The use of reactive organosilicon compounds to modify the surface properties of inorganic materials is a process that is widely used in both research and technology. Depending on reaction conditions, chemistry of the organosilane, and surface prehistory, a number of different structures can be produced on the surface. The complexity of this chemistry is illustrated in Figure 1. Monofunctional organosilanes (R3SiX) having only one hydrolyzable group in the molecule (usually X ) Cl, OR, NMe2) are attractive in terms of the reproducibility of surface structures because only one type of grafting is possible in the systemscovalent attachment to the surface by chemical bonds (SiS-O-Si). The structure of these monolayers has been studied extensively, primarily for porous and nonporous highly dispersed silicas, and these results are discussed in a number of publications: reaction conditions;2-8 bonded layer structure2-5,9-12 and (1) Current address: Chemistry Department, Seton Hall University, South Orange, NJ 07079. (2) Kiselev, A. V. Korolev, A. Y.; Petrova, R. S.; Shcherbakova, K. D. Colloid J. 1960, 22, 671. (3) Boksanyi, L.; Liardon, O.; Kovats, E.sz. Adv. Colloid Interface Sci. 1976, 6, 95. (4) Berendsen, G. E.; Pikaart, K. A.; de Galan, L. J. Liq. Chromatogr. 1980, 3, 1437. (5) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86, 5208. (6) Szabo, K.; Ha, N. L.; Schneider, P.; Zeltner, P.; Kovats, E. sz. Helv. Chim. Acta 1984, 67, 2128. (7) Kinkel, J. N.; Unger, K. K. J. Chromatogr. 1984, 316, 193.

dynamics;13-16 phase transitions of bonded layers;17-19 use as chromatographic stationary phases.20,21 The maximum bonding density is limited by the size of the dimethylsilyl group and decreases slightly with larger alkyl groups.18 The cross-sectional area of alkyldimethylsilyl groups in a densely packed monolayer2-8,18 is 32-38 Å2, which is almost twice the area per alkyl chain observed in dense monolayers of fatty acids,22 alkanethiols,23 and self(8) Boushevski, B.; Jurasek, A.; Garaj, J.; Nondek, L.; Novak, I.; Berek, D. J. Liq. Chromatogr. 1987, 10, 2325. (9) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068. (10) Cheng, W.; McCown, M. J. Chromatogr. 1985, 318, 173. (11) Staroverov, S. M.; Fadeev, A. Y. J. Chromatogr. 1991, 544, 77. (12) Fadeev, A. Y.; Eroshenko, V. A. J. Colloid Interface Sci. 1997, 187, 275. (13) Gilpin, R. K.; Gangoda, M. E. Anal. Chem. 1984, 56, 1470. (14) Lochmuller, C. H.; Hunnicutt, M. L. J. Phys. Chem. 1986, 90, 4318. (15) Albert, K.; Bayer, E. J. Chromatogr. 1991, 544, 345. (16) Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1991, 113, 6349. (17) Van Miltenburg, J. C.; Hammers, W. E. J. Chromatogr. 1983, 268, 147. (18) Claudy, P.; Letoffe, J. M.; Gaget, C.; Morell, D.; Serpinet, J. J. J. Chromatogr. 1985, 329, 331. (19) Morel, D.; Tabar, K.; Serpinet, J.; Claudy, P.; Letoffe, J. M. J. Chromatogr. 1987, 395, 73. (20) Unger, K. K. Porous silica, its properties and use as support in column liquid chromatography; Journal of Chromatography Library; Elsevier: Amsterdam, 1979; Vol. 16. (21) Sander, L. C.; Wise, S. A. CRC Anal.Chem. 1987, 18, issue 4, 299. (22) Backer, H. R, Shafrin, E. G.; Zisman, W. A. J. Phys. Chem. 1952, 56, 405. Levine, O.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1062.

10.1021/la000471z CCC: $19.00 © 2000 American Chemical Society Published on Web 08/03/2000

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Figure 1. Possible products of the reaction of alkylchlorosilanes with silicon dioxide surfaces.

assembled alkyltrichlorosilane-derived monolayers24,25 (molecules in these monolayers exhibit cross-sectional areas of ∼20 Å2). A survey of wettability studies of monolayers prepared with monofunctional silanes on single surfaces has been published.26 There are notable inconsistencies in this literature because many research groups did not recognize the importance of reaction (23) Bain, C. D. et al. J. Am. Chem. Soc. 1989, 111, 321. (24) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (25) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (26) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759.

conditions, particularly the kinetics of the reaction; many of the studies involved incomplete monolayers. The reactions of monofunctional silanes at the solution-solid interface are very slow in the later stages of the reaction, and long reaction times (tens of hours/several days) are necessary to achieve maximum bonding density, even for a single surface substrate like a silicon wafer.26 Wettability studies indicate that dense monolayers of monofunctional silanes project disordered alkyl chains toward the probe fluids. Water contact angles (θA/θR ) ∼105°/∼94°) are independent of chain length,26 indicating that these surfaces project disordered methyl groups toward the

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probe fluid and that water does not penetrate the monolayers. Hydrophobization is achieved topologically: the monolayers prevent water from penetrating and interacting with residual silanols. n-Hexadecane and methylene iodide contact angles decrease with increasing chain length for this series, indicating that these probe fluids penetrate the monolayers and interact with methylene groups. Trifunctional organosilanes (RSiX3), compared to their monofunctional analogues, are more reactive and are capable of polymerizing in the presence of water, which gives rise to a number of possible surface structures. Along with covalent attachment, 2-D (self-assembly) and 3-D surface-induced polycondensation are possible (Figure 1). The self-assembly process of long-chain alkyltrichlorosilanes (primarily octadecyltrichlorosilane, OTS) has received the major attention of researchers in this area. The deposition process is now reasonably well understood, and the importance of temperature,27-30 solvent, and surface water has been appreciated.31-34 The structure of these supported films has been investigated using a range of techniques, and the best monolayers are composed of close-packed (∼4.5-5 groups/nm2), close to vertical chains in an extended all-trans conformation.35-39 The strong bonding between molecules in this system (lateral siloxane bonds and van der Waals interactions between alkyl chains) and to a small extent bonding with the surface is a distinctive property of these monolayers. This allows the preparation of monolayers of the same quality on different substrates, whether or not covalent bonds can form with the substrate, for example, silicon, glass, mica, and gold.35,37 The water and hexadecane advancing and receding contact angles that are typically reported for selfassembled monolayers derived from long-chain alkyltrichlorosilanes are on the order θA/θR ) ∼110-115°/∼100° (water) and θA/θR ) ∼42-46°/∼40° (hexadecane), indicating pure methyl-terminated surfaces. The self-assembly mechanism of bonding (horizontal polymerization) has been reported as well for the reaction of several alkyltrichlorosilanes with porous40-42 and high surface area nonporous silica.43 Obviously, self-assembly is not the only reaction possible between alkyltrichlorosilanes and the silica surface. Under certain conditions, alkyltrichlorosilanes react with surface silanols predominately in a covalent attachment manner, which has been referred to as poor self-assembly because (27) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992, 360, 719. (28) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (29) Rye, R. R. Langmuir 1997, 13, 2588. (30) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102 (23), 4441. (31) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (32) Flinn, D. H.; Guzonas, D. A.; Yoon, R.-H. Colloids Surf. 1994, 87, 163. (33) Britt, D. W.; Hlady, V. J. Colloid Interface Sci. 1996, 178, 775. (34) Rye, R. R.; Nelson, G. C.; Dugger, M. T. Langmuir 1997, 13, 2965. (35) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. (36) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367. (37) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (38) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996. (39) Ulman, A. Chem. Rev. 1996, 96, 1533. (40) Fatunmbi, H. O.; Bruch, M. D.; Wirth, M. J. Anal. Chem. 1993, 65, 2048. (41) Fairbank, R. W. P.; Xiang, Y.; Wirth, M. J. Anal. Chem. 1995, 67, 3879. (42) Fairbank, R. W. P.; Wirth, M. J. J. Chromatogr. 1999, 830, 285. (43) Brandriss, S.; Margel, S. Langmuir 1993, 9, 1232.

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of the substantially lower bonding density. Covalent attachment takes place in the reaction of trichlorosilanes with dry silica at elevated temperatures44-46 or in the presence of adsorbed amine at room temperature.4,7,47-49 The stoichiometry of the covalent attachment of trichlorosilanes, that is, the average number of bonds with the silica surface per silane molecule, is not completely clear yet. According to NMR,15 IR,47,50,51 and other methods,52,53 1:1 (one Sis-O-Si bond) as well as 1:2 (two Sis-O-Si bonds) grafted structures are present on the surface. Structures (1:3) with three Sis-O-Si bonds are not formed due to steric reasons.50,52 After exposure to the ambient atmosphere, unreacted Si-Cl bonds undergo hydrolysis to form Si-OH groups. The latter were shown to have a significant impact on adsorption54 and chromatographic properties53 of silicas modified with alkyltrichlorosilanes. The wettability of covalently attached monolayers prepared from alkyltrichlorosilanes has not been studied in any detail. If there is a sufficient amount of water in the system, the polycondensation of trifunctional silanes into 3-D siloxanes grafted to the surface can become the dominant process. Polymeric grafted layers were reported for the reaction of methyltrichlorosilane with glass in the vapor phase55 and the reaction of alkyltrichlorosilanes with high surface area silicas.43,56 Trialkoxysilanes with the general formula Z(CH2)nSi(OR)3, where R ) Me or Et (often used Z are amino, epoxy, acryloyl, vinyl, bromo), which are often referred to as “silane coupling agents” and are used for adhesion improvement, also form relatively thick polymeric layers on the surface when applied from aqueousbased solvents.57 Difunctional organosilanes (R2SiX2) are the least studied reactive silanes in respect to their reaction with hydrated silica. Covalent attachment as well as surface-induced polymerization to form grafted polysiloxane are conceivable reactions with silica (Figure 1). Dimethyldichlorosilane and octadecylmethyldichlorosilanesthe most studied alkyldichlorosilanesswere shown to react with silica surfaces yielding either covalently attached monolayers15,51,53 or polymeric layers55,58,59 depending on the reaction conditions used. The few publications in this area makes comparisons between the data nearly impossible. Some wettability data on surfaces prepared from alkylmethyldichlorosilanes has been reported.58 We report here the study of surfaces prepared by the reaction of di- and trifunctional alkylsilanes with silicon wafers under two different conditions: in solution and (44) Hair, M. L.; Hertl, W. J. Phys. Chem. 1969, 73, 2372. (45) Tiertykh, V. A.; Chuiko, A. A.; Mashchenko, V. M.; Pavlov, V. V. Zh. Phys. Chem. 1973, 47, 158. (46) LeGrange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (47) Hair, M. L.; Tripp, C. P. Colloids Surf., A 1995, 105, 95. (48) Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693. (49) Tiertykh, V. A.; Belyakova, L. A. Chemical Reactions with Participation of Silica Surface; Naukova Dumka: Kyev, 1991. (50) Tiertykh, V. A. et al. Izv. AN SSSR 1968, issue 8, 1739. Tiertykh, V. A.; et al. Teor. Eksp. Khim. 1975, 11, 174. (51) Hair, M. L.; Hertl, W. J. Phys. Chem. 1969, 73, 2372. (52) Roumeliotis, P.; Unger, K. K. J. Chromatogr. 1978, 149, 211. (53) Unger, K. K. Becher, N.; Roumeliotis, P. J. Chromatogr. 1976, 125, 115. (54) Kiselev, A. V.; et al. Zh. Fiz. Khim. 1983, 57, 1829. Shoniya, N. K.; et al. Zh. Fiz. Khim. 1984, 58, 702. Fadeev A. Y.; et al. J. Chromatogr. 1991, 588, 31. (55) Trau, M.; Murray, B. S.; Grant, K.; Griezer, F. J. Colloid Interface Sci. 1992, 148, 182. (56) Verzele, M.; Mussche, P. J. Chromatogr. 1983, 254, 117. (57) Plueddemann, E. P. Silane coupling agents, 2nd ed., Plenum: New York, 1991. (58) Herzberg, W. J.; Marian, J. E., Vermeulen, T. J. Colloid Interface Sci. 1970, 33, 164. (59) Ruhe, J.; Novotny, V. J.; Kanazava, K. K.; Clarke, T.; Street, G. B. Langmuir 1993, 9, 2383.

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in the vapor phase. Ten n-alkylmethyldichlorosilanes (H(CH2)nSi(CH3)Cl2) and nine alkyltrichlorosilanes (H(CH2)nSiCl3), n ) 1-18 have been studied. We report the results of wettability, ellipsometry, and X-ray photoelectron spectroscopy (XPS) characterizations of these surfaces. Experimental Section General Information. Ethanol and hexane (HPLC grade), sulfuric acid, hydrogen peroxide, and sodium dichromate were obtained from Fisher and used as received. Ethyldiisopropylamine and anhydrous toluene were obtained from Aldrich and used as received. All alkylchlorosilanes were obtained from Gelest and used as received. House-purified water (reverse osmosis) was further purified using a Millipore Milli-Q system that involves reverse osmosis, ion-exchange, and filtration steps (1018 Ω/cm). X-ray photoelectron spectra (XPS) were recorded with a Perkin-Elmer Physical Electronics 5100 with Mg KR excitation (400 W). Spectra were obtained at two different takeoff angles, 15° and 75° (between the plane of the surface and the entrance lens of the detector optics). Contact angle measurements were made with a Rame´-Hart telescopic goniometer and a Gilmont syringe with a 24-gauge flat-tipped needle. The probe fluids used were water, purified as described above, methylene iodide (Aldrich, used as received), and hexadecane, purified by vacuum distillation. Dynamic advancing (θA) and receding angles (θR) were recorded while the probe fluid was added to and withdrawn from the drop, respectively. The values reported are averages of 5-10 measurements made on different areas of the sample surface. The modified surfaces reported here exhibited very homogeneous surfaces as evidenced by contact angle, and all measurements for all surfaces were within (2° of the averages. Ellipsometric measurements were made with a Rudolph research model auto SL-II automatic ellipsometer. The light source was a He-Ne laser with λ ) 632.8 nm. The angle of incidence (from the normal to the plane) was 70°. Measurements were performed for three to five different spots on each sample. The thicknesses of the layers were calculated from the ellipsometric parameters (∆ and Ψ) using DafIBM software. For silicon-supported monolayers, calculations were performed for the transparent double-layer model (silicon substrate/silicon oxide/alkylsilane layer/air) with the following parameters: air, n0 ) 1; alkylsilane layer, n1 ) 1.45; silicon oxide layer, n2 ) 1.462; the thickness of this oxide layer was fixed at 2.1 nm (see the next section); silicon substrate, nS ) 3.858, kS ) 0.018 (imaginary part of the refractive index).25 Pretreatment of Silicon Substrates. Silicon wafers were obtained from International Wafer Service (100 orientation, P/B doped, resistivity from 20 to 40 Ω cm). They varied in thickness from 450 to 575 µm. Disks (100 mm) were cut into 1.5 × 1.5 cm pieces. The samples were held in a custom designed (slotted hollow glass cylinder) holder and were rinsed with water and put in a freshly prepared mixture of 7 parts of concentrated sulfuric acid containing dissolved sodium dichromate (∼3-5 wt %) and 3 parts of 30% hydrogen peroxide. Upon preparation, the solution turns from red-brown to green, warms to 80-90 °C, and foams extensively due to the formation of oxygen and ozone. Plates were submerged in the solution overnight, rinsed with 5-7 50 mL aliquots of water, and placed in a clean oven at 120 °C for 1-2 h. Silanization reactions were carried out immediately after treating the plates in this fashion. The thickness of the silicon oxide overlayer on the silicon substrates was calculated using ellipsometric parameters (∆ and Ψ) and Daf IBM software with the following settings (single layer model): air, n0 ) 1; silicon oxide layer, n2 ) 1.462; silicon substrate, nS ) 3.858, kS ) 0.018 (imaginary part of the refractive index). The thickness of silicon oxide for bare samples was 1.9 ( 0.05 nm (average of three samples), and it increased slightly (2.1 ( 0.05 nm) after the oxidative cleaning treatment. No detectable change in silicon oxide layer thickness was found after silicon wafers were held in closed vials at 70 °C for 3 days, which modeled the conditions of the reactions with alkylchlorosilanes. Reaction of Silicon Wafers with Alkylchlorosilanes (Diand Trichloro-) in Solution in the Presence of Amine. Dried plates (hot) were covered with anhydrous toluene (5-10 mL) containing ethyldiisopropylamine (2 mmol). The chlorosilane

Langmuir, Vol. 16, No. 18, 2000 7271 (0.15-0.5 mL, depending on the formula weight of chlorosilane (1 mmol)) was added by syringe. Reactions were run at 70 °C for 3 days. The plates were isolated, were rinsed (in this order) with 2 × 10 mL of toluene (hexane was substituted for long-chain silanes and octadecyl samples were extracted with hexane in a Soxhlet apparatus for 2 h), 3 × 10 mL of ethanol, 2 × 10 mL of ethanol-water (1:1), 2 × 10 mL of water, 2 × 10 mL of ethanol, and 2 × 10 mL of water, and were dried in a clean oven at 120 °C for 10 min. The plates were then placed in covered flasks (one plate per flask) prior to contact angle, ellipsometry and XPS analyses. Reaction of Silicon Wafers with Alkyltrichlorosilanes in Solution (Self-Assembly). Dried plates were allowed to cool to room temperature and were transferred to a flask containing carbon tetrachloride (5 mL). The trichlorosilane (0.1 mL) was added by syringe. Reaction mixtures were held at room temperature for 1 h. The plates were isolated, rinsed, and handled as described above for the solution-phase synthesis. Reaction of Silicon Wafers with Alkylchlorosilanes in the Vapor Phase. Dried plates were allowed to cool to room temperature and were transferred to a flask containing 0.5 mL of chlorosilane. There was no direct contact between liquid and the wafers during reaction. Reactions were carried out at 70 °C for 3 days. The modified wafers were rinsed and handled as described above for the solution-phase synthesis.

Results and Discussion Reaction Conditions. We have used two types of reactions to prepare covalently attached layers of di- and trichlorosilanes on silicon wafers: (1) in toluene at 70 °C for 3 days and (2) in the vapor phase at 70 °C for 3 days. We also prepared several samples of self-assembled monolayers from trichlorosilanes for comparison. The choices of the conditions and the silicon pretreatment deserve some comment. Toluene was chosen as solvent because we used it to prepare monolayers from monochlorosilanes26 and want to compare the data. We note that high binding densities can be consistently obtained using toluene. Ethyldiisopropylamine (EDIPA) was chosen as a base to prevent the silylation of the amine, which can take place with less sterically congested amines. It is well established7,8,48 that the reaction between chlorosilanes and surface silanols (covalent attachment) is facilitated by the presence of amines. Amine-catalyzed reactions generally give higher surface coverage and require lower temperatures and duration than noncatalyzed ones. The role of the base is more complex than merely neutralizing HCl; its structure, pKb, and the ratio of base to silane also affect the reaction yield (bonding density). Most previous studies of the vapor-phase reactions of di- and trichloroalkylsilanes were carried out with evacuated silica; such a pretreatment gives a surface of silanol groups and no adsorbed water.60 We chose conditions that provide the presence of an equilibrium adsorbed water layer on the surface of the silicon wafers in order to promote polycondensation of the chlorosilanes. Layer Thickness. A series of 10 n-alkylmethyldichlorosilanes and 9 n-alkyltrichlorosilanes with the alkyl chain ranging in length from methyl to octadecyl were used to prepare layers on silicon wafers under both of the conditions described above. The thickness of the layers was determined by ellipsometry and is presented in Table 1. There are obvious differences in the thickness behavior as a function of alkyl chain length for samples prepared in solution and in the vapor phase. The thickness of the monolayers prepared in solution from the alkylmethyldichlorosilanes is on the order of the single molecule size; thus the layers are monomolecular, and the thickness (60) Hair, M. L. In Silanes, Surfaces and Interfaces; Leyden, Ed.; Gordon and Breach: New York, 1985; p 25.

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Table 1. Layer Thicknesses (from Ellipsometry) in Angstroms R

RMeSiCl2 solutiona

RMeSiCl2 vaporb

RSiCl3 solutiona

CH3 C2H5 C3H7 C4H9 C6H13 C7H15 C8H17 C10H21 C12H25 C18H37

3.5 5.2 6.0 6.2 9.5 9.3 12.0 12.5 14.8 18.2

25.5 17.2 15.8 15.0 13.2 10.1 10.5 15.2 16.1 18.0

8.0 9.3 13.2 10.5 11.8 13.2 13.5 15.8 20.1

RSiCl3 vaporb

RSiCl3 S-Ac

114 88 71 65 38.5 22.5 20.0 15.8 18.2

9.5, 9.8d 12.3d 13.6d 16.5, 14.8d 17.4d 19.9d 28.1, 27.5d

Toluene, EDIPA, 70 °C, 3 days. b Vapor phase, 70 °C, 3 days. c Self-assembly, CCl , 25 °C, 1 h. d Data from: Wasserman, S. R.; 4 Tao, Yu-Tai; Whitesides, G. M. Langmuir 1989, 5, 1074. a

increases gradually with alkyl chain length. The thickness of these monolayers is consistently less than the length of the fully stretched alkylsilane, which argues for a disordered monolayer structure. The alkylsilyl layers prepared in the vapor phase from alkylmethyldichlorosilanes show a complex thickness relationship with alkyl chain length. Layers prepared from short and medium chain length alkylsilanes are much thicker than monomeric and must be oligomeric. The degree of oligomerization (observed thickness/monomolecular thickness) decreases from ∼7 for dimethyldichlorosilane to ∼1.4 for hexylmethyldichlorosilane. We propose that oligomerization of dichlorosilanes takes place at the silicon dioxide surface in the presence of adsorbed water, yielding surfacegrafted alkylsiloxanes (presumably both linear and cyclic). The thickness of the layer of heptylmethylsilane (and silanes with longer chains) prepared in the vapor phase coincides with the thickness of the monolayers prepared in toluene (within the accuracy of the technique). The decrease in the degree of polymerization observed with increasing length of the alkyl chain is likely due to the lower vapor pressure of the high molecular weight compounds. The lower concentration of dichlorosilane will disfavor oligomerization. Similar trends are observed for the alkyltrichlorosilanederived monolayers. The thickness of monolayers prepared from these reagents should be thicker than those prepared from the corresponding alkylmethyldichlorosilanes because of higher bonding density. The layers prepared in solution increase in thickness with increasing alkyl chain length. The methyl, ethyl, and butyl surfaces are thicker than monomeric and are approximately dimeric. The hexyl and longer chain silanes are monomeric and slightly thicker than those prepared from alkylmethyldichlorosilanes, but thinner than the fully stretched alkyl chains. Data for self-assembled monolayers, which consist of closely packed, fully stretched alkyl chains, are included in Table 1 for comparison. The layers prepared in the vapor phase decrease in thickness with alkyl chain length and are oligomeric, except for the dodecyl- and octadecylsurfaces that are monomolecular. The degree of oligomerization (observed thickness/monomolecular thickness) decreases from ∼30 for methyltrichlorosilane to ∼1.5 for decyltrichlorosilane. This behavior is similar to that seen for alkylmethyldichlorosilanes and we propose the same explanation for this behavior: polycondensation with adsorbed water in competition with reaction with surface silanols. The decrease in thickness with increasing alkyl chain length is explained by a decreasing vapor pressure of the higher boiling silanes. The layers produced from trichlorosilanes are significantly thicker than those made from dichlorosilanes, and this is logical because of the

Table 2. Water Contact Angle Data (θA/θR) in Degrees (deg) for Surfaces Prepared by Reaction in Solutiona R

RMe2SiClb

RMeSiCl2

RSiCl3

CH3 C2H5 C3H7 C4H9 C6H13 C7H15 C8H17 C10H21 C12H25 C18H37

105/91 104/91 103/91 104/92 103/93 103/91 103/91 104/92 104/92 103/91

92/85 94/75 94/78 99/91 103/90 106/93 103/90 104/92 104/92 104/89

80/66 90/75

RSiCl3 (S-A)c

93/61

109/92

108/95 103/89 106/92 106/92 103/85

110/95 110/94 110/94 110/98

a Toluene, EDIPA, 70 °C, 3 days. b Data from ref 26. c Selfassembly, CCl4, 25 °C, 1 h.

functionality difference. The dichlorsilane polymerization should self-terminate before the trichlorosilane polymerization. We note that these calculations of film thickness from ellipsometry data are based on the assumption that all of the films have the same refractive index (1.45). This is the number accepted by many groups for self-assembled monolayers of octadecyltrichlorosilane. We point out that there is some uncertainty here, because the refractive index obviously depends on the alkylsilane (e.g., 1.40 for (CH3)2SiCl2 and 1.46 for C18H37SiMeCl2) and on the structure of the layer as well. The exact value of the refractive index cannot be derived with an acceptable degree of accuracy from ∆/Ψ data for such thin layers. We note, however, that the replacement of the refractive index we used (1.45) with, say, 1.40 gives only small changes (∼10% increase) to the values reported in Table 1 and does not affect the conclusions made above. Wettability. Water contact angle data for the covalently attached layers is reported in Tables 2 (solution reactions) and 3 (vapor phase reactions). Included in these tables for comparison are data for alkyldimethylchlorosilanes30 and self-assembled monolayers. Several points warrant comment. Contact angles for alkyldimethylchlorosilanederived monolayers prepared by solution reaction (Table 2) are independent (θA/θR ) 103-105°/91-92°) of chain length, and this indicates that the probe fluid does not interact with residual silanols on the surface. This is not the case for the alkylmethyldichlorosilane- and alkyltrichlorosilane-derived monolayers. The contact angles increase with chain length. Water penetrates the layer and interacts with silanols on the surfaces of the monolayers with shorter (methyl-butyl) alkyl groups but does not penetrate the alkyl groups of the longer chains (hexyl and higher). Very similar surfaces (as assessed by water contact angle) are formed, independent of whether the silane is mono-, di-, or trifunctional, for octyl and longer alkyl chains. The alkyl group completely controls the wettability and water interacts with a mixture of methyl and methylene groups. The self-assembled monolayers have higher contact angles because water interacts with only methyl groups. This difference supports the ellipsometry data that suggests that the covalently attached monolayers are disordered. Equation 1, the IsraelachviliGee equation,61 can be used to assess the different group

(1 + cos θ)2 ) f1(1 + cos θ1)2 + f2(1 + cos θ2)2 (1) f1 + f2 ) 1 contributions to wettability. We treat the surfaces prepared from long-chain silanes as a mixture of methyl and (61) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288.

Surface Modification

Langmuir, Vol. 16, No. 18, 2000 7273

Table 3. Water Contact Angle Data (θA/θR) in Degrees (deg) for Surfaces Prepared by Reaction in the Vapor Phasea R

RMe2SiClb

RMeSiCl2

RSiCl3

CH3 C2H5 C3H7 C4H9 C6H13 C7H15 C8H17 C10H21 C12H25 C18H37

108/96 110/98 104/93 105/93

104/103 103/95 106/101 106/104 104/101 106/101 107/101 107/100 106/100 104/92

104/73 102/75

106/99 106/95 106/99

110/93 112/100 110/100 111/97 110/100 112/98 104/80

Table 4. Hexadecane Contact Angle Data (θA/θR) in Degrees (deg) for Surfaces Prepared by Reaction in Solutiona

RSiCl3 (S-A)c

109/92 108/95 110/95 110/94 110/94 110/98

a Vapor phase, 70 °C, 3 days. b Data from ref 26. c Self-assembly, CCl4, 25 °C, 1 h.

methylene surfaces (contact angles for pure surfaces of methyl and methylene are θ1 ) 110° and θ2 ) 94°, respectively). Equation 1 gives, for the advancing contact angle 103° (the lowest value), a mixture of 65% methyl and 35% methylene groups, and for the advancing angle 106° (the highest value), a mixture of 75% methyl and 25% methylene groups. We note that the hysteresis is consistently higher for the surfaces prepared from alkyltrichlorosilanes than the surfaces prepared from alkyldimethylchlorosilanes and alkylmethyldichlorosilanes. We attribute this to molecular scale roughness and rigidity that is a result of the cross-linked structure that the trifunctional reagent forms. We have discussed this issue in a previous publication.26 The surfaces prepared in the vapor phase show very interesting wettability data (Table 3). The layers formed from alkylmethyldichlorosilanes exhibit higher contact angles than the surfaces prepared in solution (Table 2) and very little contact angle hysteresis. The short (methyl-hexyl) chain alkylmethyldichlorosilanes form oligomeric layers, and the silicones effectively shield surface silanols. The surface prepared from dimethyldichlorosilane shows negligible hysteresis and water (and hexadecane and methylene iodide) drops slide very easily on this surface. We have described this surface as “ultralyophobic” in another publication,62 and this property is due to the molecular level smoothness and flexibility of this surface. The longer chain (heptyl-dodecyl) dimethyldichlorosilane-derived monolayers also show low hysteresisslower than self-assembled monolayers and alkyldimethylchlorosilane-derived monolayers. The reason for this is not apparent, and we offer no explanation. The octadecyl surface exhibits higher hysteresis; this may be due to an incomplete reaction (because of the low vapor pressure of octadecylmethyldichlorosilane). Surfaces prepared from the short alkyl chain alkyltrichlorosilanes are oligomeric, and residual silanols are apparent by the low receding contact angles. The surfaces prepared from the longer chain alkyltrichlorosilanes (butyl-dodecyl) have as high as or higher contact angles and lower hysteresis than the self-assembled monolayers. Most of these surfaces are oligomeric (butyl-decyl), but the dodecyl surface is monomeric. The octadecyl surface has anomolously low contact angles, likely due to an incomplete reaction. Contact angles were also measured using hexadecane and methylene iodide as probe fluids, and the data are reported in Tables 4-7. The differences between alkylmethyldichlorosilane-derived surfaces prepared under different conditions are noticeable for short alkyl chains (methyl-butyl); compare Tables 4 and 5 and Tables 6 and (62) Chen, W.; Fadeev, A. Y.; Hsieh, M.; Oner, D.; Youngblood, J.; McCarthy, T. J. Langmuir 1999, 15, 3395.

R

RMe2SiClb

RMeSiCl2

RSiCl3

CH3 C2H5 C3H7 C4H9 C6H13 C7H15 C8H17 C10H21 C12H25 C18H37

28/12 22/10 20/10 20/12

28/18 22/10 18/6 18/7 16/8 16/9 17/10 12/7 13/7 15/8

24/8 20/5

a

15/10 9/