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Wettabilities of Organosiloxane Thin Films Derived from SiCl3-Terminated Carbosilane Dendrons on Mica Chi Ming Yam, Aurelie Mayeux, Alexandra Milenkovic, and Chengzhi Cai* Department of Chemistry, University of Houston, Houston, Texas 77204-5003 Received August 15, 2002. In Final Form: October 21, 2002 We prepared submonolayers, monolayers, and multilayers of organosiloxanes derived from the second to fourth generation (G2-G4) SiCl3-terminated carbosilane dendrons on mica by spin-casting. The surface properties of these films, such as wettabilities, surface tensions, works of adhesion with various liquids, pore size, and surface coverage, were investigated by means of contact-angle measurements. All G2-G4 films exhibit high hysteresis of ∼20°, probably due to molecular scale chemical heterogeneity and surface roughness. The surface tensions of the G2-G4 monolayers are similar: γS (total) ) ∼29 mJ/m2, γSd (dispersive) ) ∼24 mJ/m2, and γSp (polar) ) ∼5 mJ/m2. However, only γSd (∼21 mJ/m2) contributes to the surface tensions of the multilayers, suggesting that the underlying polar mica substrate is significantly screened by the dendron multilayers. Works of adhesion (WSL) for water, glycerol, dimethylformamide, methylene iodide, squalane, and hexadecane on the films were measured. The multilayers exhibit a lower WSLd (dispersive) than the monolayers (difference of ∼3 mJ/m2), consistent with a decrease of γSd from monolayers to multilayers. Furthermore, both the average pore size and the surface coverage of the G2-G4 monolayers were estimated to be 0.31-1.1 nm2 and ∼77%, respectively.
Introduction Dendrimers are monodisperse molecules with a regular three-dimensional structure and a high degree of branching.1-9 The structures and properties and the dendrimers can be tailored by modification of the core and the periphery of the dendrimers. In recent years, they have received extensive attention because of their potential applications in the fields of chemical and biological sensors, microelectronic and biomimetic systems, coatings and adhesives, and nanolithography.3,10 Basically, dendrimer thin films can be prepared at the air-water interface by Langmuir-Blodgett (LB) techniques and at the air-solid interface by self-assembly and spin-casting techniques.2,3,10 For example, dendrimer films of poly(benzyl ether), poly(amido amine) (PAMAM), poly(propyleneimine), carbosilane, and hyperbranched polystyrene have been reported so far.2,3,10 The dendrimers bind onto a variety of surfaces by either physisorption or chemisorption via the terminal groups at the periphery of the dendrimers, such as alkyl, amino, hydroxyl, carboxylic acid, thiol, and chlorosilane groups. The physical and chemical properties of dendrimers in solution phase have been intensively studied.1-3 However, * To whom correspondence should be addressed. Tel: 713-7432710, Fax: 713-743-2709, E-mail:
[email protected]. (1) Grayson, S. K.; Frechet, J. M. J. Chem. Rev. 2001, 101, 3819. (2) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (3) Tsukruk, V. V. Adv. Mater. 1998, 10, 253. (4) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638. (5) Hawker, C. J.; Frechet, J. M. J. J. Chem. Soc., Chem. Commun. 1990, 1010. (6) Tomalia, D. A.; Naylor, A. M.; Goddard III, W. A. G. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (7) Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466. (8) Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (9) Newkome, G. R.; Yao, Z.-Q.; Baker, G. R.; Gupta, K. J. Org. Chem. 1985, 50, 2003. (10) Tully, D. C.; Frechet, J. M. J. J. Chem. Soc., Chem. Commun. 2001, 1229.
the study of the surface properties of dendrimers, such as wettability which is very sensitive to the molecular structure at interfaces, has just been started.1-3,11-20 For example, Frechet and co-workers11-14 have reported monolayers of poly(benzyl ether) dendrimers with carboxylic acids at either the focal point or the periphery on aminated silicon, whereas Crooks and co-workers15-20 have reported monolayers of thiol-terminated dendrimers on gold. We have recently reported21,22 a new type of siloxane film derived from second to fourth generation (G2-G4) carbosilane dendrons23-27 with periphery consisting of multiple SiCl3 groups. Figure 1 displays the structural formula of the G2-G4 dendrons containing 9, 27, and 81 SiCl3 terminal groups, respectively. Previous studies have shown that dendrimers tend to flatten and spread out (11) Saville, P. M.; White, J. W.; Hawker, C. J.; Wooley, K. L.; Frechet, J. M. J. J. Phys. Chem. 1993, 97, 293. (12) Saville, P. M.; Reynolds, P. A.; White, J. W.; Hawker, C. J.; Frechet, J. M. J.; Wooley, K. L.; Penfold, J.; Webster, J. R. P. J. Phys. Chem. 1995, 99, 8283. (13) Tully, D. C.; Wilder, K.; Frechet, J. M. J.; Trimble, A. R.; Quate, C. F. Adv. Mater. 1999, 11, 314. (14) Tully, D. C.; Trimble, A. R.; Frechet, J. M. J.; Wilder, K.; Quate, C. F. Chem. Mater. 1999, 11, 2892. (15) Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 3988. (16) Tokuhisa, H.; Crooks, R. M. Langmuir 1997, 13, 5608. (17) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120, 4492. (18) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323. (19) Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364. (20) Lackowski, W. M.; Campbell, J. K.; Edwards, G.; Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 7632. (21) Xiao, Z.; Cai, C.; Deng, X. J. Chem. Soc., Chem. Commun. 2002, 1442. (22) Xiao, Z.; Cai, C.; Mayeux, A.; Milenkovic, A. Langmuir 2002, 18, 7728. (23) Frey, H.; Schlenk, C. Top. Curr. Chem. 2000, 210, 69. (24) Terunuma, D.; Kato, T.; Nishio, R.; Matsuoka, K.; Kuzuhara, H.; Aoki, Y.; Nohira, H. Chem. Lett. 1998, 59. (25) Kim, C.; Park, E.; Kang, E. Bull. Korean Chem. Soc. 1996, 17, 419. (26) Zhou, L. L.; Roovers, J. Macromolecules 1993, 26, 963. (27) Vandermade, A. W.; Vanleeuwen, P. J. Chem. Soc., Chem. Commun. 1992, 1400.
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Figure 1. Structural formula of the second generation dendron (G2) and the idealized molecular formula of the dendron G2, G3, and G4.
upon adsorption on a surface, especially when the interaction with the surface is strong.2,3,10,18,28-30 Accordingly, upon hydrolysis of the terminal SiCl3 to Si(OH)3 by the surface-bound water on a polar surface such as mica under ambient conditions,31 the G2-G4 dendrons are likely to flatten and spread out on the substrate surfaces to maximize the interaction between the periphery silanol groups of the dendrons and the polar mica surface. We have characterized the surface morphology of the dendron films using atomic force microscopy (AFM).22 The AFM images revealed that the surface morphology of the films was highly dependent upon the thickness and the generation of the dendrons, and molecularly flat monolayers could be obtained under suitable conditions. In this work, we investigated the wetting properties relating to the effect of the surface morphology and/or chemical structure of the new type of dendron films. Contact angle measurements have been extensively used to determine the wetting properties of solid surfaces and also of dendrimer films.32,33 Herein, we prepared the G2G4 carbosilane films from low coverage to multilayer on mica surface by spin-casting and employed contact angle measurements, using water (H2O), glycerol (GL), N,Ndimethylformamide (DMF), methylene iodide (MI), squalane (SQ), and hexadecane (HD) as probe liquids, to monitor the wetting behavior, surface tension, dispersive and polar works of adhesion with various liquids, surface coverage, and pore size of the dendron film surface as a function of film thickness. (28) Li, J.; Piehler, L. T.; Qin, D.; Baker, J. R., Jr.; Tomalia, D. A.; Meier, D. L. Langmuir 2000, 16, 5613. (29) Li, J.; Swanson, D. R.; Qin, D.; Brothers, H. M.; Piehler, L. T.; Tomalia, D.; Meier, D. J. Langmuir 1999, 15, 7347. (30) Mansfield, M. L. Polymer 1996, 37, 3835. (31) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149. (32) Ulman, A. In An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (33) Ulman, A. In Characterization of Organic Thin Films; Butterworth-Heinemann: Boston, 1995.
Experimental Section Materials. All probe liquids, GL, DMF, MI, SQ, and HD were purchased from commercial suppliers of highest purity and were used without further purification. The SiCl3-terminated carbosilane dendrons G2-G4 were synthesized according to the reported procedures.22,24 The freshly prepared dendrons were diluted with anhydrous THF to a series of concentrations (10-7-10-3 M). These diluting solutions were used immediately for preparation of dendron films on mica. Preparation of Dendron Films on Mica. Dendron films were prepared by spin-casting under ambient conditions with relative humidity of 40% using a model WS-400A-6NPP spin coater (Laurell Tech). The spincasting procedures involved (i) adding a drop of the dendron solution on a freshly cleaved muscovite mica (Structure Probe), (ii) spinning the substrate immediately with a spin rate reaching 2000 rpm within 5 s and continuing at 2000 rpm for 115 s, and (iii) curing the film in an oven at 115 °C overnight and slowly cooling to room temperature. Contact Angle Measurement. The probe liquids, H2O, GL, DMF, MI, SQ, and HD, were dispersed onto the dendron film surfaces using a Matrix Technologies microElectrapette 25. Advancing and receding contact angles were measured with a goniometer (Rame-Hart model 100). During the measurements, the pipet tip should be kept in contact with the drop. Both edges of three drops of the contacting liquids were measured across the surface for each sample, and the average values were reproducible within (1°. Results and Discussion Contact Angle Measurement. We examined advancing and receding contact angles of a variety of probe liquids, including H2O and GL (dipolar protic); DMF (dipolar aprotic); and MI, SQ, and HD (nonpolar aprotic), on the surfaces of G2-G4 dendron films deposited with solutions
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Yam et al. Table 1. Advancing Contact Angle Data (θa) for Various Probe Liquids on Bare Mica and G2, G3, and G4 Monolayers and Multilayers on Mica at the Indicated Concentration dendron concn (M)
advancing contact angle θa ((1°) H2O GL MI DMF SQ HD
G2 G3 G4
8 × 10-6 2 × 10-6 4 × 10-7
80 82 81
G2 G3 G4 mica
10-4 10-4 10-4
multilayer 105 100 105 100 105 100 5 15
dendron film
Figure 2. Advancing contact angle θa of various probe liquids on G2 dendron on mica over a series of dendron concentrations.
of a series of concentrations ranging from 10-7 to 10-3 M. These probe liquids were chosen because of their different polarity or molecular size. Figure 2 displays advancing contact angle (θa) of these probe liquids on the G2 film. θa of H2O increases from 30 to 105° as the concentration of G2 increases from 10-7 to 10-4 M and remains constant over a concentration range of 10-4-10-3 M. We note that this pattern is consistent with those observed for GL, DMF, MI, and SQ. However, the pattern is different from that observed for HD, which exhibits an average θa ) 20° over a concentration range of 10-7-10-5 M, followed by decreasing to 12° at 10-3 M. The measured wettability data for all probe liquids on G2 film over the concentration range are consistent with the formation of submonolayers at low concentration (10-5 M), as shown by the AFM study.22 At submonolayer concentration, the enhanced wettability of the film (low θa) arises from the exposure of polar mica surface uncovered by the dendrons, which raises the surface free energy. At monolayer concentration, the wettability is likely caused by the exposure of a minor proportion of uncovered mica surface. Note that the monolayer in the present system is referred to a layer of dendron molecules that is laterally interconnected to form a two-dimensional siloxane network. However, this network does not fully cover the mica surface due to presence of gaps and holes in the network as shown by the AFM22 and the contact angle studies presented here. Due to the flexible structure and depending on the density of the molecules, the dendron molecules in the monolayers can flatten to a various degree to maximize the interaction of the polar periphery groups with the mica surface and to maintain a two-dimensional network. The surface of the flattened dendron monolayer should mainly consist of lying CH2CH2CH2-Si chains (similar to methylene groups) and a small portion of phenyl groups. Therefore, all of the three components (CH2CH2CH2-Si chains, phenyl groups, mica) contribute to the contact angle of the monolayer. It was reported that monolayers of C6H5SH on gold and C6H5(CH2)3OH on Si/SiO2 gave similar H2O (85°) and HD (15°) contact angles.34,35 At multilayer concentration, the wettability arises mainly from the exposure of the CH2CH2CH2-Si chains at the interface. The wettability data for H2O, DMF, MI, and (34) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (35) Yam, C. M.; Tong, S. S. Y.; Kakkar, A. K. Langmuir 1998, 14, 6941.
monolayer 78 71 78 69 79 68 80 80 80 30
60 58 60
48 44 48
20 20 20
74 72 74 0
52 49 53 25
12 12 12 20
Table 2. Contact Angle Hysteresis of G2-G4 Monolayers and Surface Tension of Various Probe Liquids of Increasing Molecular Size (Cross-Sectional Area) probe molecular molecular hysteresisa liquid vol (mL/mol) size (nm2) ∆θ (( 2°) H2O GL DMF MI HD SQ
18.0 73.1 77.7 80.5 292.9 528.5
0.12 0.30 0.31 0.32 0.75 1.1
>60 >60 22 22 18
surface tension (mJ/m2)39,55-57 γL γLd γLp 72.8 64.0 36.8 50.8 27.5 30.7
21.8 34.0 28.7 50.8 27.5 30.7
51.0 30.0 8.1 0 0 0
a
∆θ of H2O, GL, and HD could not be measured accurately due to the low θr (60°) even though its molecular size (0.30 nm2) is larger than H2O. This suggests that the pore size is still large enough for GL molecules, as both hydrogen bond donors and acceptors, to reach and form a hydrogen bonding network connecting the underlying mica surface. DMF and MI have a size similar to GL, and hence, they should be able to reach the mica surface through the pores. However, they exhibit a lower ∆θ (∼22°) than GL. This is probably due to the fact that DMF and MI cannot form a hydrogen bond network connecting the underlying mica surface, and hence these probe liquids are easier to recede than H2O and GL. Even though HD is nonpolar and its molecular size (0.75 nm2) is larger than DMF and MI, it was difficult to determine ∆θ of HD because while its θa was about 20°, its θr was below 10° which could not be measured accurately. ∆θ continuously decreases to ∼18° as the molecular size of the probe liquid (SQ) further increases to 1.1 nm2. However, SQ also exhibits a similar ∆θ of ∼18° on all of the G2-G4 multilayers. Assuming that the mica surface was completely covered by the multilayer film, one of the possible reasons for the same hysteresis of SQ on both the monolayer and multilayer is that the pore size of the monolayer may have reached the upper limit (1.1 nm2). If this is true, the average size of these pores/gaps will fall approximately in the range of 0.31-1.1 nm2. Surprisingly, there is no significant difference of pore size from G2 and G3 to G4 monolayers, prepared from 8 × 10-6, 2 × 10-6, and 4 × 10-7 M concentration, respectively. This implies that the higher generation dendron molecules (G3 and G4) are still flexible enough to intercalate with adjacent molecules in the monolayer. It is conceivable that the ability of the probe liquid to penetrate the monolayer and to establish contact with the mica surface is, besides the pore size, also influenced by other structural and chemical nature of the absorbed dendron molecules and the probe molecules. Thus, the pore size value is only a rough estimate. Overall, the contact angle hysteresis data indicate that both the small molecular size of the probe liquid and the presence of a hydrogen bonding network between the probe liquid and the underlying mica surface result in high ∆θ for H2O and GL. All G2-G4 monolayers exhibit a relatively high ∆θ, which probably arises from molecular scale chemical heterogeneity and surface roughness such as pores/gaps due to formation of a cross-linked network.32 Surface Tensions and Works of Adhesion. To probe the physical origins of the wettabilities of the G2-G4 monolayers and multilayers, we calculated their respective surface tensions and works of adhesion between the probe liquids and the film surfaces. Using the Young-Dupre equation46,47
WSL ) γL(1 + cos θ)
(1)
the work of adhesion (WSL) between a solid surface (S)
and a liquid (L) can be calculated, where γL is the surface tension of the liquid and θ is the advancing contact angle of the liquid on the solid surface. Furthermore, Using the Good-Girifalco-Fowkes relation48-51
WSL ) WSLd + WSLp
(2)
the total work of adhesion (WSL) can be separated into dispersive (WSLd) and polar (WSLp) components. According to Fowkes,52 Owens and Wendt,53 and Kaelble54
WSLd ) 2(γSdγLd)1/2
(3)
WSLp ) 2(γSpγLp)1/2
(4)
γS ) γSd + γSp
(5)
where γS (γL) is the solid (liquid) surface tension as a summation of the respective dispersive (γSd, γLd) and polar (γSp, γLp) components. Hence, when nonpolar aprotic probe liquids are used (MI, SQ, and HD), i.e., γLp ) 0, and WSL ) WSLd, eqs 1 and 3 become equivalent, so that
γSd ) γL2(1 + cos θ)2/4γLd
(6)
When the given values39,55-57 of γL, γLd, and γLp for H2O, GL, DMF, MI, SQ, and HD (Table 2), and the above equations are employed, average values of γS, γSd, and γSp and WSL, WSLd, and WSLp of various probe liquids on the G2-G4 films can be estimated. We employed MI, SQ, and HD to estimate γSd and H2O, GL, and DMF to estimate γSp and γS. All G2-G4 monolayers exhibit similar surface tensions: γS ) ∼29, γSd ) ∼24, and γSp ) ∼5 mJ/m2, consistent with similar wettabilities as previously discussed. The measured γS values are similar to those reported for polyethylene films (∼30 mJ/m2),39,56 in accord with a methylene-rich surface as expected for films derived from such carbosilane dendrons.22 We attribute the polar component γSp mainly to the uncovered mica surface that is accessible to the probe liquid through the pores/gaps. Surface tensions (γS) of the G2-G4 films decrease from ∼29 for monolayers to ∼21 mJ/m2 for multilayers. The surface tensions of the multilayers mainly arise from the dispersive component (γSd), i.e., γSp ) 0, due to the screening of the underlying mica surface by the dendron multilayers. A decrease of γSd from ∼24 (monolayers) to ∼21 mJ/m-2 (multilayers) probably relates to an increasing surface area of the nonpolar dendron wedge. We further measured the works of adhesion (WSL, WSLd, and WSLp) for various probe liquids on the G2-G4 films. All G2-G4 monolayers exhibit similar average WSL, WSLd, and WSLp values for the following probe liquids: H2O (84.2, 45.5, 38.7 mJ/m2), GL (76.9, 56.9, 20.0 mJ/m2), DMF (55.6, (46) Young, T. Proc. R. Soc. London 1804, December. (47) Dupre, A. In Theorie Mecanique de la Chaleur; GauthierVellars: Paris, 1869. (48) Good, R. J.; Girifalco, L. A. J. Phys. Chem. 1960, 64, 561. (49) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538. (50) Dann, J. R. J. Colloid Interface Sci. 1970, 32, 302. (51) Dann, J. R. J. Colloid Interface Sci. 1970, 32, 321. (52) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40. (53) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (54) Baier, R. E. In Adhesion in Biological Systems; Manly, R. S., Ed.; Academic Press: New York, 1970. (55) Fletcher, P. D. I.; Nicholls, R. J. Phys. Chem. Chem. Phys. 2000, 2, 361. (56) Lee, L.-H. Langmuir 1996, 12, 1681. (57) Fowkes, F. M.; Riddle, F. L., Jr.; Pastore, W. E.; Weber, A. A. Colloids Surf. 1990, 43, 367.
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Figure 3. Dispersive works of adhesion WSLd for various probe liquids on G2, G3, and G4 monolayers and multilayers on mica.
52.1, 3.5 mJ/m2), MI (68.7 mJ/m2), HD (53.3 mJ/m2), and SQ (51.7 mJ/m2). There is no significant difference in the corresponding values from G2 to G4, consistent with similar surface tensions of these films. Due to negligible contribution from WSLp, only WSLd for the G2-G4 multilayers were determined. As shown in Figure 3, a lower WSLd for multilayers (a difference of ∼3 mJ/m2) was observed, consistent with a decrease of γSd from monolayers to multilayers. Monolayer Surface Coverage. We employed the Israelashvili equation45,58
Figure 4. Surface coverage of G2 dendron on mica over a series of dendron concentration employing various probe liquids for measurement.
(1 + cos θ)2 ) f1(1 + cos θ1)2 + f2(1 + cos θ2)2 (7) f1 + f2 ) 1
(8)
which was suggested to be more accurate than the Cassie equation59 for those heterogeneous surfaces such as our systems, to estimate monolayer surface coverage, where θ is the advancing contact angle of a liquid on the heterogeneous surface composed of a fraction of one type of molecule (f1) and a fraction of a second type (f2), and θ1 (θ2) is the advancing contact angle (θa) of this liquid on the respective pure homogeneous surface. We treat the surface of the dendron films on mica as a mixture of carbosilane and mica, employing θa of multilayer film as θ1 and θa of freshly cleaved mica as θ2 (Table 1). As shown in Figure 4, the surface coverage of G2 film increases with the dendron concentration until reaching 100% in the concentration range of 10-4-10-3 M in which multilayers were formed (see our previous AFM studies22). G3 and G4 films exhibit a similar pattern over the concentration range. We employed various probe liquids to measure the surface coverage of the G2-G4 monolayers (Figure 5). HD was not chosen because the difference between θ1 (20°) and θ2 (12°) was small and would introduce a relatively large error in the calculation. All G2-G4 monolayers exhibit similar surface coverage: 78.0 ( 4.4% for G2, 76.0 ( 3.6% for G3, and 76.5 ( 2.9% for G4, suggesting a similar surface morphology, probably due to the flexible structure of the carbosilane dendrons allowing intercalation of the adjacent molecules during thin film formation. Conclusion Well-defined submonolayers, monolayers, and multilayers of carbosilanes derived from G2-G4 SiCl3-termi(58) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (59) Cassie, A. B. D. Discuss. Faraday Soc. 1952, 75, 5041.
Figure 5. Surface coverage of G2, G3, and G4 monolayers prepared with concentration of 8 × 10-6, 2 × 10-6, and 4 × 10-7 M, respectively, on mica, employing various probe liquids for measurement.
nated dendrons were constructed on mica by spin-casting. Various probe liquids were employed to estimate the wettabilities and the surface tensions of these films. High hysteresis of ∼20° was observed for all films, probably due to molecular scale chemical heterogeneity and surface roughness. All G2-G4 monolayers exhibit similar γS (∼29 mJ/m2), which is contributed from γSd of ∼24 mJ/m2 and γSp of ∼5 mJ/m2. Surface tensions of the multilayers are mainly contributed from dispersive component (γS ≈ γSd ) ∼21 mJ/m2), due to the screening of the underlying mica surface by the dendron multilayers. Dispersive works of adhesion (WSLd) of various liquids on the monolayers were measured to be ∼3 mJ/m2 higher than those on the multilayers, consistent with larger γSd on the monolayers. Finally, the G2-G4 monolayers exhibit similar average pore size of 0.31-1.1 nm2 and similar surface coverage of ∼77%. Acknowledgment. This work was supported by the Texas Advanced Research Program under Project Number 003652-0365-1999, Robert A. Welch Foundation, and Petroleum Research Fund administrated by the American Chemical Society. LA026415A
