Langmuir 2004, 20, 1311-1316
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Isostructural Self-Assembled Monolayers. 2. Methyl 1-(3-Mercaptopropyl)-oligo(ethylene oxide)s David J. Vanderah,* Thomas Parr, Vitalii Silin, Curtis W. Meuse, Richard S. Gates,† and Hongly La Biotechnology Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8313
Gintaras Valincius Institute of Biochemistry, Mokslininku 12, Vilnius 2600, Lithuania Received September 30, 2003. In Final Form: November 26, 2003 The structural order and ordering conditions of the self-assembled monolayers (SAMs) of HSCH2CH2CH2O(EO)xCH3, where EO ) CH2CH2O and x ) 3-9, on polycrystalline gold (Au) were determined by reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and electrochemical impedance spectroscopy. For x ) 5-7, RAIRS and SE data show that the oligo(ethylene oxide) [OEO] segments adopt the near single phase, 7/2 helical conformation of the folded-chain crystal polymorph of crystalline poly(ethylene oxide), oriented normal to the substrate. These SAMs exhibit OEO segment structure and orientation identical to that found in a previous isostructural series [HS(CH2CH2O)6-8C18H37 SAMs. Vanderah, D. J., et al. Langmuir 2003, 19, 3752] and are anisotropic films for surface science metrology where structure is constant and thickness increases in 0.30 nm increments. In addition, this is the first example of OEO SAMs to attain this highly ordered, helical conformation where the (EO)x segment is separated from the Au-sulfur headgroup by a polymethylene chain. For x ) 4, 8, and 9, the SAMs are largely helical but show evidence of nonhelical conformations and establish the upper and lower limits of the isostructural set. For x ) 3, the SAMs are largely disordered containing some all-trans conformation. SAM order as a function of immersion time from 100% water and 95% ethanol indicates that the HSCH2CH2CH2O(EO)5-7CH3 SAMs order faster and under a wider range of conditions than ω-alkyl 1-thiaolio(ethylene oxide) [HS(EO)xCH3] SAMs, reported earlier (Vanderah, D. J., et al. Langmuir 2002, 18, 4674 and Vanderah, D. J., et al. Langmuir 2003, 19, 2612).
Introduction The conformational order in oligo(ethylene oxide) [OEO] segments plays a significant role in determining the properties and potential uses of surfaces modified by this structural motif.1-3 Surfaces covered with poly(ethylene oxide)s [PEO]4-7 or ω-(CH3 or H) [OEO] self-assembled monolayers (SAMs)1,3,8-15 exhibit significant resistance * To whom correspondence should be addressed. Email: david.
[email protected]. † Ceramics Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD. (1) Vanderah, D. J.; Valincius, G.; Meuse, C. W. Langmuir 2002, 18, 4674. (2) Gong, X.; Dai, L.; Griesser, H. J.; Mau, A. W. H. J. Polym. Sci., Polym. Phys. 2000, 38, 2323. (3) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426. (4) Kenausis, G. L.; Vo¨ro¨s, J.; Elbert, D. L.; Huang, N.; Hofer, R.; Ruiz-Taylor, L.; Textor, M.; Hubbell, J. A.: Spenser, N. A. J. Phys. Chem. B 2000, 104, 3298. (5) Kingshott, P.; Griesser, H. J. Curr. Opin. Solid State Mater. Sci. 1999, 4, 403. (6) Malmsten, M.; Emoto, K.; Van Alstine, J. M. J. Colloid Interface Sci. 1998, 202, 507. (7) Harris, J. M. Poly(ethylene glycol) Chemistry. Biotechnical and Biomedical Applications; Plenum Press: New York, 1992. (8) Herrwerth, S.; Eck, W.; Reinhardt, S.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 9359. (9) Benesch, J.; Svedham, S.; Svensson, S. C. T.; Valiokas, R.; Liedberg, B.; Tengvall, P. J. Biomater. Sci., Polym. Ed. 2001, 12, 581. (10) Papra, A.; Gadegaard, N.; Larsen, N. B. Langmuir 2001, 17, 1457. (11) Harder, P.; Grunze, M.; Waite, J. H. J. Adhes. 2000, 73, 161. (12) Ostuni, E.; Chapman, R. G.; Holmlin, E.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605.
to protein adsorption and desirable biocompatibility properties, when the OEO segments adopt more than one conformation and remain conformationally mobile. Alternatively, surfaces with ordered OEO segments are potentially useful in thin film metrology.16 OEO SAMs with near single phase, helical (EO)x segments, oriented normal to the substrate, have been prepared from compounds of the general formula HS(EO)xR, where R ) CnH2n+1.1,17,18 Film thickness is independent of structure for SAMs where (1) R is held constant, (2) x varies over a range of numbers (e.g., 3 f 4 f 5 f etc.), and (3) the (EO)x segments are in the 7/2 helical conformation, oriented normal to the substrate. This has recently been reported, where R ) C18H37 and x ) 6-8,16 and is, to the best of our knowledge, the first example of anisotropic, isostructural films where film thickness correlates with crystallographic dimensions.19 In this paper, we report the structures of a new series of OEO SAMs, HSCH2CH2CH2O(EO)xCH3 SAMs (hereafter designated C3EOx20), where x ) 3-9. Of interest is the effect the short polymethylene (C3) chain would have (13) Silin, V.; Weetall, H.; Vanderah, D. J. J. Colloid Interface Sci. 1997, 185, 94. (14) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (15) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12. (16) Vanderah, D. J.; Gates, R. S.; Silin, V.; Zeiger, D. N.; Meuse, C. W.; Valincius, G.; Nickel, B. Langmuir 2003, 19, 2612. (17) Vanderah, D. J.; Pham, C. P.; Springer, S. K.; Silin, V.; Meuse, C. W. Langmuir 2000, 16, 6527. (18) Vanderah, D. J.; Meuse, C. W.; Silin, V.; Plant, A. L. Langmuir 1998, 14, 6916. (19) Takahashi, Y.; Tadokoro, H. Macromolecules 1973, 23, 672.
10.1021/la035829g CCC: $27.50 © 2004 American Chemical Society Published on Web 01/16/2004
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on the SAM order as compared to HS(EO)xCH3 SAMs (designated EOx, where x ) 3-6) where it was found that (1) methyl-terminated OEO SAMs adopt the highly ordered, helical conformation for x g 5;21 (2) the OEO segments in the helical conformation exhibited reduced protein resistance,1 providing validation of an earlier hypothesis;22 (3) small deviations from the highly ordered, helical conformation yielded highly protein resistant surfaces; and (4) structural change is not sequential over the range of x from 3 to 6. On one hand, the C3 chain might inhibit the formation of the helical conformation resulting in a series with sequential structural changes as a function of x. This possibility is supported by the fact that OEO SAMs on Au with longer polymethylene (undecanyl) chains between the sulfur headgroup and the OEO segment are disordered, with the OEO segments adopting both helical and nonhelical conformations.3,9,14,23,24 If this is found to be the case, such a series might (a) better define the structure-function relationship between order and protein resistance than the HS(EO)6CH3 SAMs1 and (b) better afford optimization of parameters for biotechnology strategies, ranging from protein immobilization to smallmolecule molecular-recognition sensing strategies that employ (EO)x segments as linkers or matrixes. On the other hand, the C3 chain might facilitate the formation of the helical conformation resulting in a series where structural order is independent of x. This possibility is supported by the fact that more ordered SAMs on Au have been obtained when short alkyl segments are present between the sulfur headgroup and certain endgroups.25 If this is found to be the case, an isostructural series is possible. Such a series would provide prospective metrology reference materials/standards in a different thickness range from those mentioned above. Materials and Methods26 Synthesis. Tri-, tetra-, penta-, and hexaethylene glycol mono methyl ether and methoxy(poly(ethylene glycol)), av MW ) 350, were purchased from TCI America (Portland, OR) and ICN Biomedicals, Inc. (Aurora, OH), respectively. THF (Mallinckrodt AR) was purchased from North Strong Scientific (Phillipsburg, NJ). All other chemicals were purchased from Aldrich Chemical Co. (Milwaukee, WI). The THF was distilled from calcium hydride under N2 immediately before use. The compounds HSCH2CH2CH2O(CH2CH2O)3-9CH3 were synthesized along established schemes described previously.1,15,21 For the x ) 3-6 compounds, the commercially available tri-, tetra-, penta-, and hexa(ethylene oxide) monomethyl ethers were first alkylated (NaH-allyl bromide/THF). Conversion of the allyl groups to the final 1-(mercaptopropyl) groups via the thiol acetates was carried out using established procedures.15 For the x ) 7-9 compounds, a mixture of oligo(ethylene oxide) monom(20) In these compounds, the length of the OEO segment, x, is determined from the following definitions. (1) An EO unit is CH2CH2O. (2) The preceding atom, Y (YCH2CH2O), must be an atom other than carbon (e.g., S, N, or O, etc.). (3) An (EO)x segment begins at Y. Using definitions 1, 2, and 3, x, in these series, is consistent with previous literature (see refs 3, 8, 11, 15, 21, 23, and 24). (21) Vanderah, D. J.; Arsenault, J.; La, H.; Gates, R. S.; Silin, V.; Meuse, C. W.; Valincius, G. Langmuir 2003, 3752. (22) Jeon, S. I.; Andrade, J. D. J. Colloid Interface Sci. 1991, 142, 159. (23) Tokumitsu, S.; Liebich, A.; Herrwerth, S.; Eck, W.; Himmelhaus, M.; Grunze, M. Langmuir 2002, 89, 8862. (24) Valiokas, R.; Svedham, S.; Svensson, S. C. T.; Liedberg, B. Langmuir 1999, 15, 3390. (25) Tao, Y. T.; Wu, C. C.; Eu, J. Y.; Lin, W. L. Langmuir 1997, 13, 4018. (26) Certain commercial equipment, instruments, or materials are identified in this paper to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment are necessarily the best available for that purpose.
Vanderah et al. ethyl ethers, av MW ) 350, was alkylated in an analogous fashion to the x ) 3-6 compounds. The x ) 7, 8, and 9 allyl monomethyl ethers were obtained pure by repeated flash chromatography (silica gel) and then converted to the final methyl 1-(mercaptopropyl) hepta-, octa-, and nona(ethylene oxide) as just described. Structural assignments were made from proton (1H) nuclear magnetic resonance (NMR) and high-resolution mass spectrometry data.27 Sample purity (>98%) was determined from thinlayer chromatography (TLC) analysis (one spot by TLC) and 1H NMR data. Sample Preparation. All reflection-absorption infrared spectroscopy (RAIRS) and spectroscopic ellipsometry (SE) experiments used substrates prepared on silicon (1 0 0) wafers (Silicon Inc., Boise, ID) coated initially with chromium (∼2 nm) and then with gold (∼200 nm) by magnetron sputtering (Edwards Auto 306, U.K.) at a base pressure of ∼1.3 × 10-6 mbar as described previously.18 The monolayers were prepared by immersing the gold substrates in ∼0.5 × 10-3 mol/L solutions for times varying from 1 h to 3 days. Reflection-Absorption Infrared Spectroscopy. The RAIRS data were obtained using a Nicolet Magna-IR model 750 Series II spectrometer (Thermo Nicolet, Madison, WI) with a model FT-85 (85° grazing angle) Spectra-Tech external reflection accessory (Thermo Spectra-Tech, Shelton CT) as described previously.16 Visible Spectroscopic Ellipsometry. Multiple wavelength ellipsometric measurements were performed on a J. A. Woollam Co., Inc. (Lincoln, NE) M-44 spectroscopic ellipsometer aligned at a nominal incidence angle of ∼70° from the surface normal, as described previously.21 Electrochemical Impedance Spectroscopy (EIS) Measurements. The EIS measurements were obtained using a Solartron electrochemical impedance system (model 1286 potentiostat, model 1250 frequency response analyzer, computer, and software) (Farnborough, U.K.), as described previously.16
Results and Discussion C3EO3 to C3EO9 were assembled from 100% water and 95% ethanol at room temperature (22 ( 2 °C). For each compound, SAM formation and evolution to the final, thermodynamic, highest-order state, hereafter referred to simply as the final state, was monitored by RAIRS and SE data as a function of immersion time. The final state was deduced when consecutive, essentially identical RAIRS spectra were obtained exhibiting the highest population of the (EO)x segments in either the helical or the all-trans conformation, as discussed previously.1,3,16-18,21 In general, those SAMs that attained the near single phase, all-helical conformation did so from both solvents with minor variance in time. As a result, all the spectra for the series from both solvents are not presented in the following figures. Figures 1 and 2 show RAIRS data for C3EO3 to C3EO9, in their final state, from 1400 to 750 cm-1 and from 3050 to 2700 cm-1, respectively. By simple inspection, the data in these figures indicate that SAM structural order varies, as has been observed for other OEO-SAM series.1,9,16-18,21 For convenience, we begin by analyzing the spectra of C3EO6 (spectra D, Figures 1 and 2), which exhibit the characteristics of highly ordered, near single phase, helical (27) MS: C3EO3: HR FAB [M + H]+ calcd for C10H23O4S, 239.1317; found, 239.1327. C3EO4: HR FAB [M + H]+ calcd for C12H27O5S, 283.1579; found, 283.1578. C3EO5: LR FAB found [M + H]+ 327.41, as was [M + Na]+ 349.40. HR FAB [M + Cs]+ calcd for C14H30O6CsS, 459.0817; found, 459.0811. C3EO6: HR FAB [M + H]+ calcd for C16H35O7S, 371.2104; found, 371.2115. C3EO7: FAB [M + H]+ calcd for C18H39O8S, 415.2366; found, 415.2373. C3EO8: FAB [M + H]+ calcd for C20H43O9S, 459.2628; found, 459.2641. C3EO9: FAB [M + H]+ calcd for C3EO9 C22H47O10S, 503.2890; found, 503.2875. 270 MHz 1H NMR for all compounds: δ (relative to tetramethylsilane) 1.38, 1H, t, J ≈ 6.2 Hz, HSCH2CH2CH2O(EO)xCH3; 1.88, 2 H, pentet, HSCH2CH2CH2O(EO)xCH3; 2.62, 2H, dt, J ≈ 6.5 and 6.2 Hz, HSCH2CH2CH2O(EO)xCH3; 3.38, 3H, s, HSCH2CH2CH2O(EO)xCH3; 3.5-3.7, m with broad major peak at 3.65, (4x + 2)H, HSCH2CH2CH2O(CH2CH2O)xCH3.
Isostructural Self-Assembled Monolayers
Figure 1. RAIRS spectra of C3EO3 to C3EO9 from 1400 to 750 cm-1 in their final state. A ) C3EO3; B ) C3EO4; C ) C3EO5; D ) C3EO6; E ) C3EO7; F ) C3EO8; G ) C3EO9. Spectra are offset for clarity.
Figure 2. RAIRS spectra of C3EO3 to C3EO9 from 3050 to 2700 cm-1 in their final state. A ) C3EO3; B ) C3EO4; C ) C3EO5; D ) C3EO6; E ) C3EO7; F ) C3EO8; G ) C3EO9. Spectra are offset for clarity.
OEO monolayers. First, the absorption bands at 1347, 1244, 1118, and 964 cm-1 (Figure 1), which correspond to those vibrations with transition moments parallel to the chain axis [A2 (4) to (7)28], and the absence of bands at 1360, 1280, 1234, 1149, 1116, 1061, 947, and 843 cm-1, respectively, which correspond to those vibrations with transition moments perpendicular to the chain axis [E1 (8) to (15)28], indicate that the (EO)x segments are in a 7/2 helical conformation, oriented normal to the substrate.18 Second, the absence of any higher wavenumber shoulders on the A2 bands, particularly the 1118 cm-1 band, indicates the absence of nonhelical OEO conformations.17 Third, highly ordered, helical OEO SAMs exhibit prominent absorption at ∼2893 cm-1 (parallel (EO)x methylene symmmetric stretch) and a weak band at ∼2740 cm-1 (parallel (EO)x combination vibration).23 Both of these bands are clearly present in Figure 2. Fourth, earlier work (28) Kobayashi, M.; Sakashita, M. J. Chem. Phys. 1992, 96, 748760.
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has shown that the methyl rocking (∼1200 cm-1, Figure 1) and stretching bands (∼2980 and ∼2815 cm-1, Figure 2) absorb at lower frequencies (2-5 wavenumbers) in ordered, helical SAMs than those in disordered SAMs.21 The methyl rocking band at 1201 cm-1 (Figure 1) and the methyl stretching bands at 2980 and 2815 cm-1 (Figure 2) are consistent with a well-ordered ω-CH3-OEO SAM. In contrast, the spectra for C3EO3 (spectra A, Figures 1 and 2) show strong absorption between 1150 and 1140 cm-1 (C-O stretching bands) [Figure 1] along with the attenuation and/or absence of the bands at 1347, 1244, 1118, and 964 cm-1 (Figure 1) and the bands at 2892 and 2741 cm-1 (Figure 2). These spectral features are characteristics of a disordered SAM with increasing components of trans, trans, trans3 or amorphous conformations in the OEO segment.1,17,18,21 In addition, the methyl rocking band at 1203 cm-1 (Figure 1) and the methyl stretching bands at 2982 and 2818 cm-1 (Figure 2) are 2-5 wavenumbers higher than those in C3EO6, indicative of a lessordered ω-CH3-OEO SAM. The RAIRS data of C3EO4, C3EO5, and C3EO7-9 (spectra B, C, and E-G, respectively, Figures 1 and 2) indicate that these SAMs more closely resemble the ordered C3EO6 (spectra D) than the disordered C3EO3 (spectra A). Close inspection reveals that, except for expected increases in intensities for the 2892, 1347, 1244, 1118, and 964 cm-1 bands, the spectra for C3EO5, C3EO7, and C3EO8 (spectra C, E, and F, respectively) appear to be nearly identical to that of C3EO6 suggesting that the SAMs, over the range of x ) 5-8, might be isostructural. [Further analysis of this possibility is given later (vide infra).] In contrast, the spectra for C3EO4 and C3EO9 (spectra B and G, respectively, Figures 1 and 2) exhibit small, higher wavenumber shoulders on the prominent 1118 cm-1 band (Figure 1) indicating that these SAMs contain small amounts of nonhelical conformations, as discussed above and previously.1,17,18,21 The fact that the spectra of C3EO4 and C3EO9 are identical to those of C3EO5-8 in all respects except for the differences around the 1118 cm-1 band illustrates the significance of the spectral features from 1150 to 1120 cm-1 in the assessment of helical OEO SAM order and the presence of nonhelical conformations.29 However, it is important to note that (EO)x conformations cannot be assigned solely on absorbance bands in this narrow 30 wavenumber region. Helical bands depend strongly on the orientation of the (EO)x segment with respect to the substrate30 with an absorbance band at ∼1149 cm-1 [E1 (11)] expected for orientations other than normal to the substrate. In these SAMs, this possibility is eliminated because of the absence of other prominent E1 bands in the 1400-750 cm-1 region, namely, the medium E1 (9) and the strong E1 (15) bands at 1280 and 843 cm-1, respectively, as discussed above and shown in Figure 1. SE data for C3EO3 to C3EO9 (Table 1) support the conclusions derived from the RAIRS data. As expected, SAM thickness increases as a function of time (films progressing from lower coverage, disordered states to higher coverage, ordered final states) and increasing x. For C3EO5 to C3EO8, final-state film thickness values are in excellent agreement with the calculated values for the SAMs using an all-7/2 helical structural model for both the EOx and the C3 (CH2CH2CH2) segments [see subse(29) Higher wavenumber shoulders are observed on the 1347, 1244, and 964 cm-1 bands when the OEO SAM is more disordered than C3EO4 and C3EO9 (see the 1347 cm-1 band, 1 h, Figure 3). These bands are not as discernible as those on the 1118 cm-1 band because of their relative intensities. (30) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37, 2764.
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Table 1. Calculated and Experimental Spectroscopic Ellipsometric Thickness, d, for the C3(EO)3 to C3(EO)9 SAMs d (experimental) at various times (nm ( 0.10 nm) compound
d, calcda (nm)
C3(EO)3
1.46
C3(EO)4
1.76
C3(EO)5
2.06
C3(EO)6
2.36
C3(EO)7
2.66
C3(EO)8
2.96
C3(EO)9
3.26
solvent
1h
2h
3h
EtOH H2O EtOH H2O EtOH H2O EtOH H2O EtOH H2O EtOH H2O EtOH H2O
1.25 1.52 1.46 1.27 1.37 1.69 1.78 2.28 1.67 2.20 2.17
1.35 1.53 1.57 1.32 1.56 1.97 2.21 2.35 1.85 2.36 2.73
1.34 1.54 1.51 1.53 1.78 1.99 2.40 2.35 2.07
2.38
2.42
4h
2.46 2.71
6h
2.67 2.68
1 day 1.49 1.56 1.65 1.68 1.96 2.10 2.38 2.37 2.68 2.68 2.92 2.98
a Film thickness calculations assume a 7/2 helical conformation for both the EO and the C segments (see the text). d [C (EO) ] ) x 3 3 6 d(C-C-C-O) + calcd HS(EO)6CH3. d(C-C-C-O) ) 0.36 nm [4(0.09 nm/atom; 0.28 nm (ethylene oxide unit in 7/2 helix) (ref 19) ÷ 3)]. Calcd HS(EO)6CH3 ) 1.99 nm (ref 1). d (other SAMs) ) d [C3EO6] + (0.3 nm)(x - 6).
quent discussion]. In addition, for these SAMs, relative film thickness increase/EO unit is 0.30 nm ( 0.04 nm, in excellent agreement with that predicted on the basis of the 7/2 helical structural model28 and the unit cell dimensions of crystalline PEO19 of 0.28 nm/EO unit. The data in Figures 1 and 2 and Table 1 indicate that the C3EO5-8 SAMs are the first examples of highly ordered, helical OEO SAMs where the (EO)x segment is separated from the Au-S headgroup by a polymethylene chain. Table 1 shows that for all SAMs, the final-state thicknesses are equal to or less than the calculated thicknesses except C3EO3. This is consistent with the RAIRS data, which indicates that C3EO3 has a significant fraction of the EO segments in the trans, trans, trans conformation and is, therefore, not adequately described by the 7/2 helical or slightly disordered 7/2 helical structural model. EO units in a trans, trans, trans conformation have a larger thickness contribution (0.36 nm/EO)3 than EO units in the helical conformation (0.28 nm/EO)19 and thus may give rise to SAMs thicker than the all-helical model. The structural changes as a function of x, in this series, differ in two respects from those found for the EOx series. First, although both series have disordered SAMs for small values of x, the change from disorder to order (x ) 5) is more gradual in the C3EOx series. C3EO4 is almost ordered, whereas EO4 is largely disordered. Second, highly ordered, helical SAMs are obtained faster (e1 day) and over a wider range of assembly conditions (100% water and 95% ethanol)31 in the C3(EO)x series, whereas ordered EOx (x ) 5 or 6) samples were obtained only from 95% ethanol after several days. The more gradual changes from order to disorder for x ) 5 f 4 f 3, reduced time to the ordered final state, and increased range of assembly conditions to the ordered final state for the C3EOx films indicate that the C3 spacer facilitates the formation of an (EO)x helical structure. Although structural changes from x ) 3 f 5 may be more gradual in the C3EOx series, the changes are not incremental. From x ) 4 f 5, the change is small, whereas the change is much larger from x ) 3 f 4. Thus, a series of SAMs with small changes that might more precisely define the structure-function relationship between order and protein resistance appears to be limited to x ) 4 and 5. (31) C3EO6 SAM assembly from 50% ethanol/water yielded results similar to those obtained from 100% water and 95% ethanol.
Figure 3. RAIRS spectra of C3EO6, assembled from 100% water, from 1400 to 900 cm-1 as a function of time.
Figure 4. RAIRS spectra of C3EO6, assembled from 100% water, from 3050 to 2700 cm-1 as a function of time.
Figures 3 and 4 show the RAIRS spectra for C3EO6 from 1400 to 900 cm-1 and from 3050 to 2700 cm-1, respectively, as a function of immersion time and are similar to those observed for EO5 and EO6, assembled from 95% ethanol.21 After 1 h, multiple C-O stretching bands are found from 1190 to 1100 cm-1 with maxima above 1120 cm-1 and the bands characteristic of the (EO)x segment in the helical conformation (Figure 3) are attenuated or absent. In
Isostructural Self-Assembled Monolayers
Figure 5. RAIRS spectra of C3EO6, assembled from 100% water, from 1200 to 1050 cm-1 after 3 h and 1 day.
Figure 6. RAIRS spectra of C3EO5 to C3EO8 from 1400 to 900 cm-1.
addition, the methyl rocking band appears at 1203 cm-1. The C-H stretching region (Figure 4) exhibits a broad range of methylene stretching bands from 2960 to 2930 cm-1, methyl stretching bands at 2983 and 2820 cm-1, and no absorption at 2740 cm-1. After 2 h, the bands at 1347, 1343, 1118, and 964 cm-1 are present and the methyl rocking band has shifted 2 wavenumbers to 1201 cm-1. However, the discernible higher wavenumber shoulder on the 1118 cm-1 band indicates a small amount of nonhelical conformations is still present. After 3 h, C3EO6 shows increased absorption band intensities in both Figures 3 and 4 and no shoulder on the 1118 cm-1 band. Comparison of the spectra of C3EO6 SAMs after 3 h and 1 day (Figure 5) shows that the 1118 cm-1 band does not change in intensity but exhibits a slight reduction in the full width at half-maximum. These data reveal the progression from a lower coverage, disordered ω-methyl OEO SAM (1 h) to the final state, a highly ordered, helical ω-methyl OEO SAM (1 day), and, to a first approximation, determine the kinetics of C3EO6 formation. Similar data were obtained for C3EO5 to C3EO7 (data not shown), with minor variations with regard to solvent and time. Further assessment of C3EO5 to C3EO8 as an isostructural series was carried out similarly to the previous isostructural series [(EO)6-8C18].16 Figures 6 and 7 show the RAIRS data and peak areas (∆’s) of the 1347, 1244, 1201, 1118, and 964 cm-1 bands, respectively, for C3EO5 to C3EO8. By inspection of Figure 6, one sees that the 1118 cm-1 peak height increase is smaller and the peak
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Figure 7. C3EO5 to C3EO8 peak areas for the bands in the 1400-900 cm-1 region.
Figure 8. RAIRS spectra of C3EO6, (EO)6C18, and (EO)7C18 from 1400 to 900 cm-1.
width broader than those for C3EO5 to C3EO7. Figure 7 supports the data presented in Figure 6. The peak area (∆) increase of the 1347, 1244, 1118, and 964 cm-1 bands from C3EO7 to C3EO8 is less than those found for C3EO5 to C3EO7. On the basis of these data, C3EO8 is slightly different from and, therefore, no longer considered isostructural with C3EO5 to C3EO7. Noteworthy in these figures is that the methyl rocking bands (1201 cm-1) are essentially constant for all the SAMs. While the position of this band is dependent on SAM order, as mentioned above, the intensity of this band is dependent only on the packing density. Thus, although C3EO8 differs from C3EO5-7, the packing density is essentially equal. Because the methyl rocking band intensity is independent of SAM order, correlation of the peak area from ∼1210 to ∼1195 cm-1 to other peak areas in the 1400-900 cm-1 region may be a useful way of further quantifying SAM order. EIS and RAIRS data indicate structural order in the C3 segment. The complex capacitance EIS spectrum of C3EO5 is a single nearly perfect semicircle with the real component of the admittance (Y)/frequency (ω) f 0 at low frequencies (data not shown, see Figure 1c-e in ref 16). EIS spectra with these features indicate a highly uniform film (dielectric layer) that behaves as a nearly ideal capacitor. The specific capacitance for C3EO5 was found to be 3.09 ( 0.11 µF/cm2, with the constant phase element (CPE) exponent of 0.990 ( 0.001, which yields a dielectric constant for the monolayer of 7.19 ( 0.26, identical to
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obtained for the previous isostructural series [(EO)6-8C18].16 As a result, the calculated film thicknesses in Table 1 are derived using an all-helical model for both the EO and the C3 segments and previous thickness calculations.1,34 The data in Figure 9, coupled with the RAIRS and SE data discussed earlier, are further evidence that C3EO5-7 are isostructural. In addition, the correlation of the peak areas of C3EOx with EOx+1C18 for x ) 5-7 (e4%, Table 2) indicates that the two series are isostructural with each other.
Figure 9. C3EO5 to C3EO7 peak areas for the 1347, 1244, 1118, and 964 cm-1 bands normalized per EOx+1 (see the text). The error bar on the C3EO6 bands is equal to one standard deviation (3% for the 1118 cm-1 band). Table 2. Peak Area (∆) of the 1118 cm-1 Band of the C3EO5-7 and (EO)6-8C18 SAMs compound ∆ (arbitrary units)a compound ∆ (arbitrary units)a C3EO5 C3EO6 C3EO7 a
0.7115 0.8193 0.9182
(EO)6C18 (EO)7C18 (EO)8C18
0.7395 0.8145 0.9609
∆’s determined from 1188 to 1057 cm-1.
that found for (EO)6.1 This indicates that the two adjacent methylenes of the C3 segment and the first oxygen [HSCH2CH2CH2O(CH2CH2O)5CH3] are in the same helical conformation as the EO units in HS(CH2CH2O)6CH3. If this is the case, the peak areas of the 1347, 1244, 1118, and 964 cm-1 bands, due solely to the helical conformation, for a given x should be comparable to x + 1 in compounds of the general formula A(EO)x+1B, where A and B * EO. Figure 8 compares the RAIRS spectrum of C3EO6 to that of (EO)6C18 and (EO)7C18 from 1400 to 900 cm-1, and Table 2 gives the 1118 cm-1 band ∆’s for C3EO5-7 and (EO)6-8C18.32 Both the figure and the table show that x, in the C3(EO)x series, corresponds to x + 1 in the (EO)xC18 series. For example (from Table 2), ∆ for C3EO6 (0.8193) ≈ ∆ for (EO)7C18 (0.8145) [the ∆ difference (δ∆) is 0.6% and within the one standard deviation (3%) of the ∆Av for (EO)6-8C18 of 0.1199 (normalized /EO unit)].33 The ∆’s for C3EOx (x ) 5-7) are normalized to x + 1 (Figure 9) and are constant for the above four bands, similar to that (32) Peak areas (∆’s) for the other A2 bands in the 1400-900 cm-1 region are expected to exhibit constant absorbance increases/EO unit similar to the 1118 cm-1 band. The 1118 cm-1 band was analyzed because of its relative intensity and because of the effects nonhelical conformations exhibit from 1150 to 1120 cm-1. (33) The average 1118 cm-1 band peak area difference (δ∆) is 11.2% [C3(EO)x to (EO)xC18] versus 2.9% [C3(EO)x to (EO)x+1C18].
Conclusions RAIRS and SE data indicate that C3EOx, where x ) 3-9, vary in structure and order. Highly ordered SAMs are obtained for x ) 5-7. These SAMs appear to be isostructural with the OEO segments, oriented normal to the substrate, in the 7/2 helical conformation of the foldedchain crystal polymorph of PEO and show film thickness increases/EO unit similar to that found in previous isostructural SAMs [(EO)6-8C18].16 The x ) 5-7 SAMs order as a function of immersion time faster and over a wider range of assembly conditions, including assembly from 100% water, compared to the helical (EO)x SAMs reported earlier.1 For x ) 4, 8, and 9, order decreases but the (EO)x segments are still predominantly helical. In contrast, for x ) 3 the SAM is largely disordered with the (EO)x segment containing appreciable amounts of all-trans conformations. Our data suggest that the C3 segment facilitates (EO)x segments to adopt the helical conformation. Comparison of the integrated band peak areas of the isostructural SAMs in this series with the corresponding band peak areas of the previous isostructural SAMs indicates that the oxygen and the two sequential methylene groups in the propylene oxide segment adopt the same helical conformation as the OEO segment. The identification of isostructural films in this series (thickness range, 2.1-2.7 nm), that are also isostructural with earlier isostructural films (thickness range, 4.0-4.6 nm), provides additional metrology materials/standards for measurements and instrument calibration in ∼0.3 nm increments. Acknowledgment. This work was funded in part by the Environmental Management Science Program (EMSP) of the U.S. Department of Energy under Contract DE-A107-97ER62518 and the NIST intramural Advanced Technology Program. The North Atlantic Treaty Organization (NATO) Science Program and the NSF-NIST summer research program (2001 and 2002) supported Gintaras Valincius and Thomas Parr, respectively. LA035829G (34) It is unlikely that the order in the C3 segment of C3EO5-7 exists to the same extent, or at all, in the lesser-ordered SAMs. Thus the calculated thicknesses (Table 1) for these SAMs are likely to have larger errors.
