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Binary Monolayer Mixtures: Modification of Nanopores in Silicon-Supported Tris(trimethylsiloxy)silyl Monolayers Alexander Y. Fadeev1 and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 Received April 2, 1999. In Final Form: June 22, 1999 Chemically grafted monolayers were prepared by reaction of tris(trimethylsiloxy)chlorosilane (trisTMSCl) with silicon wafers both in the vapor phase and in toluene solution. Denser (more closely packed) monolayers, as assessed by carbon content (determined using X-ray photoelectron spectroscopy) and contact angle analysis, were obtained using the vapor phase reaction. Contact angles (θA/θR) for water, methylene iodide, and hexadecane on the vapor phase modified surfaces (96°/87°; 66°/56°; 33°/31°) indicate the hydrophobic and oleophobic nature of this surface. Dynamic contact angles of 30 different probe fluids were measured on the tris-TMS surfaces. A plot of contact angle hysteresis (the difference between advancing and receding contact angles) versus molar volume of the probe fluid shows a sharp decrease in hysteresis in the region of 180-190 cm3/mol for tris-TMS monolayers prepared in vapor phase. Probe fluids with lower molecular volume exhibit hysteresis of 8-12 degrees, while liquids of larger molecular volume show hysteresis of 2-3°. This size-exclusion contact angle hysteresis behavior argues for the presence of holes (nanopores) with a cross section of ∼0.5 nm2 that are accessible to the probes of smaller dimension. No size-exclusion effect was found for the tris-TMS monolayer (of lower degree of surface coverage) prepared by liquid-phase silanization. Contact angle hysteresis on this surface (8-13°) decreases gradually with increasing molecular volume of the probe fluid. The results suggest that penetration of molecules of fluid into the monolayers is responsible for contact angle hysteresis. The holes in the tris-TMS monolayers expose silanols that can be reacted with smaller silanizing reagents to yield uniformly mixed (one molecule of silane per hole) binary monolayers. Several binary monolayers were prepared by subsequent modification of tris-TMS surfaces with alkyl-, bromoalkyl-, fluoroalkyl-, and aminoalkyl-functionalized silanes. Functional silane/ tris-TMS binary monolayers are formed with a ∼1:10 molar ratio from the vapor phase synthesized trisTMS surface. Mixed monolayers with ∼1:4 molar ratio of silane/tris-TMS were obtained for binary monolayers prepared from the liquid phase synthesized tris-TMS surface. The extent of incorporation of the subsequently reacted silane is controlled primarily by the density of the tris-TMS monolayer and is almost independent of the chemical nature of the reagent used subsequently.
Introduction Chemical modification of metal oxide2-8 and polymer9,10 surfaces by covalent attachment of organosilanes is a versatile technique that can control adhesion, wettability, adsorption, catalytic, and other surface properties of solids. Formation of ordered monolayers with predictable and well-defined structures (self-assembly) is of interest for a variety of applications.11 Methods for the covalent attachment of a wide variety of chemical compounds (totaling several thousand) to surfaces include representatives from almost every class of stable compound. The vast majority of synthetic methods described in the literature, however, produce either mixtures of the desired surface group and other surface functionalities of unknown composition or * To whom correspondence may be addressed. E-mail:
[email protected]. (1) On leave from Chemistry Department, M. V. Lomonosov Moscow State University, 119899 Moscow, Vorob. Gory, Russia. (2) Leyden, D. E., Ed. Silanes, Surfaces, and Interfaces; Gordon and Breach: New York, 1986. (3) Lisichkin, G. V., Ed. Chemically Modified Silicas in Adsorption, Chromatography and Catalysis; Khimiya: Moscow, 1986. (4) Plueddemann, E. P. Silane Coupling Agents, 2nd ed.; Plenum: New York, 1991. (5) Mittal, K. L., Ed. Silane and Other Coupling Agents; VSP: Zeist, 1992. (6) Pesek, J. J., Leigh, I. E., Eds. Chemically Modified Surfaces; Royal Society of Chemistry: Cambridge, 1994. (7) Murray, R. Electroanal. Chem. 1984, 13, 191. (8) Amati, D.; Kovats, E., sz. Langmuir 1988, 4, 329. (9) Chaudhury, M. K.; Whitesides, G. M. Langmuir 1991, 7, 1013. (10) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1998, 14, 5586. (11) Ulman, A. Chem. Rev. 1996, 96, 1533.
surfaces composed of a single component. For numerous applications, controlled dilution of surface functionality with an inert component or mixing of functionalities in definite proportions is desired to adjust the surface properties precisely. To meet this goal, several approaches have been described in the literature. The most widely used approach involves the treatment of a surface with a mixture of reagents (competitive chemisorption). By use of mixtures of organosilanes, binary monolayers of aryl (phenyl,12,13 naphthyl,14 pyrenyl15)/alkyl, alkyl/alkyl,13,16-20 fluoroalkyl/alkyl,21-23 ester/alkyl,24 haloalkyl/alkyl,12,24-26 (12) Crowther, J. B.; Hartwick, R. A. Chromatographia 1982, 16, 349. (13) Erard, J. F.; Nagy, L.; Kovatz, E., sz. Colloids Surf. 1984, 9, 109. (14) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3446. (15) Lochmuller, C. H.; Hunnicutt, M. L. J. Phys. Chem. 1986, 90, 4318. (16) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015. (17) Fatunmbi, H. O.; Brunch, M. D.; Wirth, M. J. Anal. Chem. 1993, 65, 2048. (18) Wirth, M. J.; Fatunmbi, H. O. Anal. Chem. 1992, 64, 2783. (19) Wirth, M. J.; Fairbank, R. W. P. J. Liq. Chromatogr., Relat. Technol. 1996, 19, 2799. (20) Erard, J. F.; Kovatz, E., sz. Anal. Chem. 1982, 54, 193. (21) Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 1341. (22) Lagutchev, A. S.; Song, K. J.; Huang, J. Y.; Yang, P. K.; Chuang, T. J. Chem. Phys. 1998, 226, 337. (23) Huang, J. Y.; Song, K. J.; Lagoutchev, A.; Yang, P. K.; Chuang, T. J. Langmuir 1997, 13, 58. (24) Fryxell, G. E.; Rieke, P. C.; Wood, L. L.; Engelhard, M. H.; Williford, R. E.; Graff, G. L.; Campbell, A. A., Wiacek, R. J.; Lee, L.; Halverson, A. Langmuir 1996, 12, 5064. (25) Heise, A.; Stamm, M.; Rauscher, M.; Duschner, H.; Menzel, H. Thin Solid Films 1998, 199, 327.
10.1021/la9903806 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/10/1999
Binary Monolayer Mixtures
phthalimidoalkyl/alkyl,27 diphenylphosphino/alkyl,13 cyanoalkyl/alkyl,28,29 and amino acid (valine)/alkyl30 groups were prepared. Preparation of mixed monolayers, in which one of the components is physically adsorbed, was described for alkyltrichlorosilanes with fatty acids21,31 and alkyltrichlorosilanes with dyes.32 Mixed organosilane monolayers can be prepared at the air-water interface and transferred to solid substrates using LangmuirBlodgett techniques.21,31 Questions of how uniformly the functionality is mixed in monolayers prepared by competitive adsorption and how solution composition correlates with surface composition are of importance. It cannot generally be predicted that the mixing will be uniform. Few studies regarding these issues have been carried out, and the results obtained are controversial, which illustrates the complexity of the problem and the importance of experimental parameters on the results of mixing. A comprehensive study of mixed grafted monolayers prepared by competitive chemisorption of R1R2R3-N,N-dimethylaminosilanes has been reported.13 The authors conclude that if the molar surface area of individual substituents is not too different, the total surface concentration reflects an ideal mixture. Enrichment of binary monolayers with one component, with respect to the solution composition, has been reported in some cases,22,23,25,26,28 while uniform mixing has been reported in other systems.12,14,18,19,29,30 For example, uniform mixing, with respect to solution composition, was found for binary monolayers of 17-bromoheptadecyltrichlorosilane and hexadecyltrichlorosilane on Si wafers,24 while surface enrichment of the bromoalkylsilane was observed for binary monolayers of 11-bromoundecyltrichlorosilane and undecyltrichlorosilane on the same substrate.25 Considerable deviation from the solution composition was reported for binary monolayers prepared from 1-bromomethyldimethylchlorosilane and trimethylchlorosilane as well as 3-bromopropyltrichlorosilane and octyltrichlorosilane on porous silica.26 Distributions of alkyl/fluoroalkyltrichlorosilanes in monolayers prepared by Langmuir-Blodgett methods (the transfer of the monolayer from water onto a solid substrate) were found to be patchy in structure, while uniform mixing was reported for monolayers prepared by competitive chemisorbtion of the same silanes on silica.21,31 The dependence of surface composition on the nature of the solvent used for the coadsorption has been reported.13,24 The surface composition of binary self-assembled monolayers formed from alkyltrichlorosilanes of different chain length and branching structure was proposed to be under kinetic control.16 The more rapidly reacting n-alkyltrichlorosilane (regardless the chain length) enriched the surface with respect to the slower reacting tert-butyltrichlorosilane, while the mixing was uniform if silanes of the same reactivity (both with linear chains) were used.16 Considering that surface silanization (even for a single reagent) is extremely sensitive to small variations in reaction conditions (temperature, solvent, water content (26) Fadeev, A. Y.; Lisichkin, G. V. In Adsorption on New and Modified Inorganic Sorbents; Dabrowsky, A., Tyertykh, V. A., Eds.; (Ser.: Studies in surface science and catalysis, vol. 99), Elsevier: Amsterdam, 1995; p 191. (27) Heid, S.; Effenberger, F.; Bierbaum, K.; Grunze, M. Langmuir 1996, 12, 2118. (28) Colmsjo, A.; Ericsson, M. W. Chromatographia 1987, 24, 683. (29) Chartier, A.; Gonnet, C.; Morel, D.; Rocca, J. L.; Serpinet, J. J. Chromatogr. 1988, 438, 263. (30) Feibush, B.; Cohen, M. J.; Karger, B. L. J. Chromatogr. 1983, 282, 3. (31) Takahara, A.; Koijo, K.; Ge, S.; Kajiyama, T. J. Vac. Sci. Technol., A 1996, 14, 1747. (32) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92.
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of the system and nature of substrate),33-39 predictability and reproducibility of the composition and properties of mixed monolayers prepared by competitive chemisorption cannot be generally expected. An alternative approach for the preparation of “mixed surfaces” involves subsequent reaction of partially modified surfaces (submonolayers) with another silane.14,21,26 Submonolayers can be prepared by kinetic control of the surface modification reaction. Compared to competitive chemisorption, this technique is more time-consuming and potentially more complex as additional steps (isolation, washing, drying, two reactions instead of one) are required. This approach, however, is more general as a greater number of functional group mixtures can be prepared. For example, binary monolayer mixtures of functional groups that would react with one another in solution can be prepared.31 Another potential advantage of the twostep technique is that individual reactions using single components can be more controllable and reproducible yielding more ordered and better packed monolayers than modification with mixtures.27 The distribution of components in sequentially prepared mixed monolayers is determined by the distribution of the component reacted first. If the first component forms islands, the resulting binary monolayer will be islandlike; if the first component reacts randomly with the surface, the binary mixture will be random. The structure and the spatial organization of submonolayers have been studied intensively only for octadecyltrichlorosilane (OTS) on planar substrates. Most authors accept that OTS films form by islandlike growth and incomplete monolayers have patchy structures.23,40-45 The mechanism of monolayer growth depends on temperature, with island growth taking place at low temperatures (40 °C).45 Island growth for several other organosilanes at submonolayer coverages has been reported.15,26,46 Homogeneous organosilane submonolayers (random filling of the surface) have also been observed.14,47 In general, however, the distribution of an organosilane in a submonolayer on a given substrate cannot be predicted, thus there are currently no well-prescribed routes to binary monolayers with defined composition and organization. We have investigated a route to overcome the uncertainty of the distribution in submonolayers using the technique we report here. As described in Figure 1, it involves preparing complete monolayers of bulky orga(33) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367. (34) Davidovits, J. V.; Pho, V.; Silberzan, P.; Goldmann, M. Surf. Sci. 1996, 352-354, 369. (35) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102, 4441. (36) McGorven, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (37) Hair, M. L.; Tripp, C. P. Colloids Surf., A 1995, 105, 95. (38) LeGrange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (39) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (40) Flinn, D. H.; Guzonas, D. A.; Yoon, R.-H. Colloids Surf. 1994, 87, 163. (41) Banga, R.; Yarwood, J.; Morgan, A.; Evans, B.; Kells, J. Langmuir 1995, 11, 4393. (42) Britt, D. W.; Hlady, V. J. Colloid Interface Sci. 1996, 178, 775. (43) Davidovits, J. V.; Pho, V.; Silberzan, P.; Goldmann, M. Surf. Sci. 1996, 352-354, 369. (44) Richter, A. G.; Durbin, M. K.; Yu, C.-J.; Dutta, P. Langmuir 1998, 14, 5980. (45) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102(23), 4441. (46) Chuvilin, A. L.; Moroz, B. L.; Zaikovskii, V. I., Likholobov, V. A.; Yermakov, Yu. I. J. Chem. Soc., Chem. Commun. 1985, 733. (47) Hall, L. D.; Waterton, J. C. J. Am. Chem. Soc. 1979, 101, 3697.
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Figure 1. Pictorial representation of the two-step silanization procedure.
nosilanes that are well packed but contain “holes” in the array of grafted groups that can be “filled” by reaction with oranosilanes that are smaller than the cross-sectional area of these holes. We describe the preparation and properties of monolayers of tris(trimethylsiloxy)silane and their use as patterns for the synthesis of binary monolayers of organosilanes on oxidized silicon wafers. Experimental Section General Information. Ethanol, 2-propanol, hexane (all HPLC grade), sulfuric acid, hydrogen peroxide, sodium dichromate (all from Fisher), ethyldiisopropylamine, and anhydrous toluene (both from Aldrich) were used as received. Organosilanes 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, and other liquids (obtained from Aldrich and used as received, unless stated otherwise): acetone, acetonitrile, dimethyl sulfoxide, formamide, N,N-dimethylformamide, ethylene glycol, tetrahydrofuran, 1,4-dioxane, 1,2-dichloroethane, chlorobenzene, benzene, toluene, cyclohexane (Aldrich, 99.8%), methylene bromide, methylene iodide, nitromethane, nitrobenzene, 1,2,4trichlorobenzene, p-xylene, n-octane, n-nonane, n-decane, ndodecane, n-tetradecane, n-hexadecane, trans-decahydronaphthalene, 1,3-dimethyladamantane, bicyclohexyl (Aldrich, anhydrous, 99+%), mesitylene (J. T. Baker, 99+%). Dynamic advancing (θA) and receding (θR) angles were recorded while the probe fluid was added to and withdrawn from the drop, respectively. The values reported for contact angle hysteresis were obtained on two independently prepared samples of each surface and were averages of four to eight measurements made on different areas of each sample. 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, and the thickness of the native silicon oxide was estimated from ellipsometry to be 2.0-2.5 nm. 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 five to seven 50 mL aliquots of water, and placed in a clean oven at 120 °C for 2 h. Silanization reactions were carried out immediately after treating the plates in this fashion. Reaction of Silicon Wafers with Tris(trimethylsiloxy)chlorosilane (tris-TMSCl) in Solution. Dried plates were covered with anhydrous toluene (5-10 mL) containing ethyl-
diisopropylamine (0.17 mL; 10-3 mol). tris-TMSCl (0.35 mL; 10-3 mol) was added by syringe. Reactions were carried out at 60-70 °C for 3 days. The plates were isolated, were rinsed (in this order) with 1 × 10 mL of toluene, 2 × 10 mL of 2-propanol, 2 × 10 mL of ethanol, 1 × 10 mL of ethanol-water (1:1), 1 × 10 mL of water, and 1 × 10 mL ethanol, and were dried in an oven at 120 °C for 10 min. Reaction of Silicon Wafers with tris-TMSCl in the Vapor Phase. Silicon wafers were suspended using a sample holder in a flask containing 0.5 mL of tris-TMSCl (the wafers did not make contact with the liquid). Reactions were carried out at 60-70 °C for 3 days. The modified wafers were rinsed and handled as described for the liquid-phase synthesis. Subsequent Modification of tris-TMS Monolayers with Organosilanes. The wafers treated with tris-TMSCl as described in the above two paragraphs were covered with anhydrous toluene (5-10 mL) containing ethyldiisopropylamine (0.17 mL; 10-3 mol). Chloroorganosilane reagent (10-3 mol) was added by syringe. Reactions were run at room temperature for 3 days. The plates were rinsed and handled as described above. In the case of 4-aminobutyldimethylmethoxysilane no ethyldiisopropylamine was added.
Results and Discussions Monolayers of tris-TMS on Silicon Wafers. Monolayers of tris-TMS were prepared by the reaction of trisTMSCl with Si wafers. Immobilization is assumed to occur exclusively through reaction between the chlorosilyl group and surface silanol groups (eq 1). Two experimental
procedures were compared: reaction with the silane solution in toluene containing ethyldiisopropylamine at 60-70 °C for 3 days and reaction of the neat chlorosilane in the vapor phase at the same temperature for the same duration. A higher yield of surface modification, indicated by a higher carbon content (XPS analysis) as well as the higher contact angles, was found for the vapor phase reaction (Table 1). This is consistent with reported results48,49 comparing solution and vapor reactions of other monofunctional organosilanes. To show that siloxane bonds in tris-TMSCl do not react with surface silanols under the mild conditions used, Si wafers were treated with hexamethyldisiloxane under the same conditions; no significant hydrophobization of the wafers was observed. (48) Duchet, J.; Chabert, B.; Chapel, J. P.; Jerard, J. F.; Chovelon, J. M.; Jaffrezic-Renault, N. Langmuir 1997, 13, 2271. (49) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759.
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Langmuir, Vol. 15, No. 21, 1999 7241 Table 1. XPS and Contact Angle Data for Tris-TMS Monolayers
XPS atomic concentration (%)a C Si O
reaction conditions vapor phase toluene solution a
31.82 9.82 26.15 9.10
27.52 48.31 28.30 48.80
40.56 41.91 47.20 42.17
contact angles θA/θR (deg) CH2I2 C16H34
compositionb
H2O
C1Si0.86O1.27
96/87
66/56
33/31
C1S1.08O1.80
76/65
65/53
24/16
Upper and lower numbers are 15° and 75° takeoff angle, data, respectively. b Based on 15° takeoff angle data.
Table 2. Contact Angle Data for Various Fluids on Tris-TMS Monolayers (θA/θR, in Degrees) Prepared by Solution Phase and Vapor Phase Reactions
fluid
molecular volume (mL/mol)
water formamide nitromethane acetonitrile ethylene glycol methylene bromide dimethyl sulfoxide acetone dimethylformamide 1,2-dichloroethane methylene iodide tetrahydrofuran 1,4-dioxane benzene chlorobenzene nitrobenzene toluene cyclohexane p-xylylene 1,2,4-trichlorobenzene mesitylene trans-decalin octane nonane 1,3-dimethyladamantane bicyclohexyl decane dodecane tetradecane hexadecane
18.0 39.7 48.0 52.2 55.8 70.2 70.9 73.1 77.7 78.8 80.5 81.1 85.2 89.3 101.7 102.9 106.5 108.0 122.6 124.8 139.0 158.9 162.5 178.6 185.4 190.4 194.9 227.1 260.0 292.9
Contact angles for water, methylene iodide, and hexadecane on tris-TMS surfaces (Table 1) are relatively high, indicating the formation of hydrophobic and oleophobic layers on the surfaces of the Si wafers. The values for the samples prepared in the vapor phase, however, are ∼510° lower than those reported for densely packed layers of trimethylsiloxy (TMS) groups on Si wafers.49 Solution phase prepared samples exhibit even lower values. The lower contact angles of tris-TMS surfaces indicate lower degrees of surface coverage and more accessible (to the probe fluids) silanols than the TMS surface. The Israelashvili equation (eq 2),50 which is suggested to be more accurate than the Cassie equation for heterogeneous surfaces
(1 + cos θ)2 ) f1(1 + cos θ1)2 + f2(1 + cos θ2)2 (2) f1 + f2 ) 1 with molecular scale separation, was used to analyze these data. Treating the tris-TMS surfaces as mixtures of TMS (θ1 ) 108°) and silanol (θ2 ) 0°) indicates that 91% and 72% of the tris-TMS surfaces are covered with TMS groups for vapor phase and solution phase prepared samples, respectively. Although these tris-TMS monolayers show residual silanols, as revealed by water and other probe fluid contact
tris-TMS (vapor)
tris-TMS (solution)
96/87 82/72 51/43 38/30 64/53 48/40 57/45 19/10
76/65 79/67 45/36 33/23 57/44 42/32 53/41 12/0
40/31 66/56
38/28 65/52
40/31 24/15 21/12 54/46 22/14 15/5 20/12 45/37 22/14 20/11 10/0 12/5 24/21 21/17 16/13 19/17 26/23 33/31
36/23 16/4 18/6 50/41 12/2 12/0 11/0 36/27 21/11 19/10 spreads 12/2 20/11 16/8 16/7 19/10 21/13 24/16
tris-TMS (vapor) reacted with TMSCl 98/95 85/78 52/45 39/36 74/72 50/42 60/50 22/14 46/38 42/33 69/64 20/12 42/36 27/20 25/22 56/51 24/19 17/15 25/22 49/46 28/25 23/20 10/8 14/11 18/15 25/22 22/20 25/22 29/26 34/31
angles, we stress that these layers should be considered complete monolayers with respect to tris-TMS moieties and the conditions of preparation. These silanols are not reactive to tris-TMSCl under the conditions of reaction, which have been optimized (in terms of the most dense monolayers) for reaction of Si wafers with monochlorosilanes.49 The contact angle data indicate, however, that these layers contain defects that are penetrated by molecules of smaller size. Figure 1 describes this graphically; the size of the defects is obviously proportional to the cross-sectional area of the grafted molecules. To estimate the cross-sectional area of these “holes”, we measured the contact angle hysteresis on tris-TMS surfaces for 30 different probe fluids of different molecular volumes (Table 2). This approach was used successfully by Zisman for estimating the pore size in a condensed monolayer of 17-(perfluoroheptyl)heptadecanoic acid.51 Figure 2 shows a plot of contact angle hysteresis vs molar volume of the probe fluid for the tris-TMS monolayer prepared in the vapor phase. There is a sharp drop in hysteresis at a molar volume of ∼170 cm3/mol. Smaller probe fluids exhibit hysteresis of ∼8-12°, revealing a heterogeneous surface, while larger probe fluids show (50) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (51) Timmons, C. O.; Zisman, W. A. J. Colloid Interface Sci. 1966, 22, 165.
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Figure 2. Contact angle hysteresis vs molar volume of the probe fluid for the tris-TMS monolayer prepared in the vapor phase.
almost no hysteresis (2-3°), which indicates a homogeneous surface. The smaller molecules penetrate the pores in the monolayer and sense silanol functionality while the larger molecules are excluded. The size of these pores lies between 0.49 and 0.54 nm2, which are the crosssectional areas of trans-decaline and 1,3-dimethyladamantane, calculated, respectively, from their molar volumes assuming spherical shapes of the molecules. Treatment of the vapor phase prepared tris-TMS monolayer with trimethylchlorosilane fills these reactive hydrophilic holes with TMS groups (that have crosssectional areas52,53 of ∼0.35-0.4 nm2) rendering a homogeneous lyophobic surface that exhibits low (2-3°) hysteresis for most fluids studied (Figure 3). The hysteresis remains high (5-12°) for fluids of very small (less than ∼100 cm3/mol) molar volume. This reveals the presence of holes with cross sectional areas of ∼0.38 nm2 (cross section of cyclohexane) and smaller, which are too small to be penetrated by trimethylchlorosilane. Liquids which associate (water, formamide, ethylene glycol, nitromethane, and acetonitrile) deserve special comment, as they show anomalously low hysteresis on this surface (see Table 2, Figure 3) with respect to their molar volumes. This is not the case if degrees of association of 6 for water and of 2 for ethylene glycol, formamide, nitromethane, and acetonitrile are assumed; the anomalous points fall in line with the others. Zisman proposed that water interacts with hydrophobic surfaces as a hexamer and that the other associating liquids interact as dimers.51 The data in Figure 3 offer a strong argument for Zisman’s proposal. Tris-TMS monolayers prepared by the solution phase reaction show relatively high contact angle hysteresis (813°) and behave like hetrogeneous surfaces to all probe fluids studied. Only a slight and gradual decrease in hysteresis with increasing molar volume is observed. These data not plotted but are presented in Table 2. Treatment of this tris-TMS monolayer with trimethylchlorosilane also produces a homogeneous lyophobic surface that exhibits hysteresis of ∼2-3° for fluids of molar volume greater than ∼100 cm3/mol. (52) Szabo, K.; Ha, N. L.; Schneider, P.; Zeltner, P.; Kova´ts, E., sz. Helv. Chim. Acta 1984, 67, 2128. (53) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86, 5208.
Fadeev and McCarthy
Figure 3. Contact angle hysteresis vs molar volume of the probe fluid for a surface prepared by reacting a vapor phase prepared tris-TMS monolayer with TMSCl. The triangles are data for probe fluids that associate (water, formamide, ethylene glycol, nitromethane, and acetonitrile); the squares indicated molecular volumes of associated complexes.
Preparation of Binary Monolayer Mixtures by Subsequent Reaction of tris-TMS Monolayers with Functionalized Organosilanes. Binary monolayer mixtures were prepared by subsequent silanization of trisTMS monolayers with several organofunctional silanes. The silanes were chosen to fit the size of pores in the tris-TMS monolayers (∼0.49-0.54 nm2, see above). The cross-sectional areas53 of dimethylalkylsilanes are generally 0.4-0.45 nm2 (we used 1-bromomethyldimethylsilyl, 3,3,3-trifluoropropyldimethylsilyl, and 4-aminobutyldimethylsilyl; a cross-sectional area of 0.51 nm2 for 2-(perfluorooctyl)ethyldimethylsilyl has been determined.54 Each of these silanes contains a heteroatom (F, Br, or N) which is not present in the tris-TMS monolayers, so the formation of binary monolayers can easily be assessed by XPS and these data can also be used to estimate the chemical composition of binary monolayers. XPS and contact angle data for monolayer mixtures prepared from the vapor phase synthesized tris-TMS surface and the solution phase prepared tris-TMS monolayers are summarized in Tables 3 and 4, respectively. Estimates of mixture compositions were made by “mixing” the chemical compositions (15° takeoff angle data) of homogeneous trisTMS monolayers and homogeneous monolayers of the functionalized silanes (adjusting X and Y in eq 3) to obtain
CaSibOcZd ) X(CkSilOm) tris-TMS mixed monolayer
+
Y(CqSirOsZt) monolayer of Zfunctionalyzed silane (3)
best fits to the composition data of the binary mixtures. This approach assumes that single layers as well as the binary layers are the same thickness and that the mean free path values for photoelectrons are the same in single and binary monolayers. The 15° data was used, instead of the 75° data, because a higher percentage of the data is from the monolayer (as opposed to the substrate) at this takeoff angle. All of these monolayers are less than 10 Å thick. (54) Fadeev A. Y.; Yeroshenko V. A. Colloid. J. 1996, 58, 654.
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Table 3. XPS Atomic Composition and Contact Angle Data for Binary Monolayers Obtained by Subsequent Silanization of the Tris-TMS Monolayer Prepared in the Vapor Phase silane reacted with tris-TMS surface
C
ClSi(CH3)3 ClSi(CH3)2(CH2)2CF3 ClSi(CH3)2(CH2)2C6F13 ClSi(CH3)2CH2Br CH3OSi(CH3)2(CH2)4NH2 a
40.49 19.70 32.78 9.48 34.95 14.69 45.49 19.70 46.69 18.57
XPS atomic concentration (%)a Si O 24.39 46.18 26.26 47.40 24.41 44.51 20.39 44.18 22.04 44.95
39.96 33.82 38.67 41.62 36.70 39.52 32.96 36.82 30.09 39.67
contact angle (θA/θR, deg) H2O C16H34
F/Br/N
2.29 1.30 3.81 1.91 1.26 0.50 1.25 0.80
98/95
34/31
97/94
36/29
108/100
48/44
89/79
34/18
97/55
26/9
Upper and lower numbers are 15° and 75° takeoff angle data, respectively.
Table 4. XPS Atomic Composition and Contact Angle Data for Binary Monolayers Obtained by Subsequent Silanization of the Tris-TMS Monolayer Prepared in the Liquid Phase silane reacted with tris-TMS surface
C
ClSi(CH3)2(CH2)2CF3 ClSi(CH3)2(CH2)2C6F13 ClSi(CH3)2CH2Br CH3OSi(CH3)2(CH2)4NH2 a
32.62 11.01 31.92 11.69 42.49 15.70 45.52 17.28
XPS atomic concentration (%)a Si O 25.26 45.94 16.90 41.51 23.39 44.18 20.06 40.65
36.24 40.22 24.90 37.52 38.56 40.82 32.10 40.65
contact angle (θA/θR, deg) H2O C16H34
F/Br/N 5.89 2.83 25.94 9.12 1.51 0.85 2.31 1.81
96/92
37/20
112/103
48/44
86/82
20/15
83/52
14/8
Upper and lower numbers are 15° and 75° takeoff angle data, respectively. Table 5. Chemical Composition of Binary Monolayer Mixtures silane reacted with tris-TMS surface
observed XPS atomic composition
calculated best fit (eq 3) composition
tris-TMS:functional silane ratio
ClSi(CH3)2(CH2)2CF3 ClSi(CH3)2(CH2)2C6F13 ClSi(CH3)2CH2Br CH3OSi(CH3)2(CH2)4NH2
Vapor Phase Prepared tris-TMS Monolayers C14Si11O17F C13Si11.5O17.5F C20Si14O21F2.2 C17Si14O21F2.3 C36Si22O36Br C34Si23O37Br C31Si17O28N C30Si17O28N
10:1 13:1 9:1 12:1
ClSi(CH3)2(CH2)2CF3 ClSi(CH3)2(CH2)2C6F13 ClSi(CH3)2CH2Br CH3OSi(CH3)2(CH2)4NH2
Solution Phase Prepared tris-TMS Monolayers C5.5Si4.3O6F C5.3Si4.5O7F C28Si15O22F23 C31Si15O21.5F23 C30Si17O28Br C27Si17O28Br C20Si9O14N C21Si9O14N
3:1 5:1 4:1 4:1
The ratios of tris-TMS to functional silane (X:Y), determined using eq 3, as well as the experimental and calculated (based on X:Y) compositions are shown in Table 5. Note that binary monolayer mixtures prepared from the vapor phase tris-TMS pattern are mixed in ∼1:10 ratios (functional silane/tris-TMS) and that those prepared from the solution phase tris-TMS pattern are mixed in ∼1:4 ratios. It is evident that the surface concentration of functional silane is controlled primarily by the tris-TMS pattern and not the chemical nature of the functional silane. These ratios are consistent with the Israelashvili analysis of contact angles described above (eq 2), which showed that the vapor phase and solution phase prepared surfaces contain 9% and 28% accessible silanols, respectively. Summary The structure of chemically grafted monolayers prepared by reaction of tris-TMSCl with silicon wafers depends on the reaction conditions. Denser (more closely packed) monolayers are formed with vapor phase reactions than with solution phase reactions. Vapor phase prepared tris-TMS monolayers on Si wafers contain nanopores that are too small for tris-TMSCl to penetrate. Size exclusion
contact angle analysis using 30 different probe fluids of varying molecular volume indicates that these nanopores have a cross-sectional area of ∼0.5 nm2. A plot of contact angle hysteresis versus molar volume of the probe fluid shows a sharp decrease in hysteresis in the region of 180190 cm3/mol for this surface. tris-TMS monolayers prepared in solution phase do not exhibit this size exclusion contact angle behavior, and all fluids show relatively high hysteresis. The penetration of molecules of fluid into the nanopores is responsible for contact angle hysteresis. The nanopores contain accessible silanols that can be reacted with smaller silanizing reagents to yield uniformly mixed binary monolayer mixtures. Binary monolayers are formed with a ∼1:10 molar ratio from the vapor phase synthesized tris-TMS surface and a ∼1:4 molar ratio from the liquid phase synthesized tris-TMS surface. Acknowledgment. We thank Presstek Corporation, the Office of Naval Research, and the NSF-sponsored Materials Research Science and Engineering Center for financial support. LA9903806
