Infrared Reflectance−Absorbance Spectroscopy of Thin Films Formed

Oct 1, 2008 - Permanent address: Department of Chemistry, Pacific University, Forest ... of Chemistry, University of Washington, Seattle, WA 98195-170...
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Anal. Chem. 2008, 80, 8012–8019

Infrared Reflectance-Absorbance Spectroscopy of Thin Films Formed by Forced Dewetting of Solid-Fluid Interfaces Shinobu Tsuruta Heier,† Kevin E. Johnson,‡ Anoma Mudalige, Domenic J. Tiani,§ Vanessa R. Reid,| and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721 An infrared reflectance-absorbance spectroscopy method for characterizing the ultrathin fluid film retained on a surface upon forced dewetting from a fluid has been developed for investigation of interfacial molecular structure at reflective substrates. This report details the optical considerations and constraints necessary to acquire IR spectral data from nanometer-thick films retained upon forced dewetting of a solid substrate from a fluid into a vapor-saturated environment. The feasibility of this method is demonstrated through successful spectral acquisition from Ag surfaces modified with 11-mercaptoundecanol forcibly dewet from water. The IR spectral results clearly illustrate that information is acquired only from the interfacial region with no contribution from the bulk liquid. Residual layer thicknesses calculated from IR absorbance values are substantiated by ellipsometry. The spectra make clear that the molecular structure of the residual layer is distinctly different from that of the bulk liquid, confirming that this method is viable for interfacial structure elucidation of thin fluid films at a variety of solid substrates. Infrared reflectance-absorbance spectroscopy (IRRAS) is a valuable tool for elucidation of the nature, structure, and orientation of molecules at surfaces and within interfaces.1,2 During the past three decades, numerous in situ studies have been carried out utilizing IRRAS for study of thin organic film structure and for investigation of the electrode-electrolyte interface. Although the application of IRRAS to adsorbate-covered surfaces in vacuum has become routine, its use in fluid or even concentrated vapor environments is considerably more challenging. Mitigation of these challenges has been achieved through the development of specific modifications to the original IRRAS approach in which * To whom correspondence should be addressed. Phone: 520-621-8245. E-mail: [email protected]. † Current address: Thermo Fisher Scientific, 1400 Northpoint Parkway, Suite 10, West Palm Beach, FL 33407. ‡ Permanent address: Department of Chemistry, Pacific University, Forest Grove, OR 97116. § Current address: Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599. | Current address: Department of Chemistry, University of Washington, Seattle, WA 98195-1700. (1) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211–357. (2) Gillie, K. J.; Hochlowski, J.; Arbuckle-Keil, G. A. Anal. Chem. 2000, 72, 71R–79R.

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another variable is modulated to discriminate interfacial from bulk phase species. As one example, in electrochemical environments, this discrimination has been achieved by electrochemically modulated infrared spectroscopy (EMIRS),3 relying on the potential dependence of adsorption or the presence of the interfacial species of interest. Many of these studies employed a thin-layer cell geometry in which the electrode surface was pressed against a flat,4 trapezoidal, or hemispherical prism/window3,5-10 to create a thin solution layer on the order of one to several micrometers in thickness to minimize interferences and intensity loss caused by bulk solution absorption. Although this technique has been effective for the acquisition of interfacial IRRAS spectra from many systems, it provides only a difference spectrum between two electrode potentials, and it is difficult to relate this spectrum to the “true” or “absolute” IR spectrum at a given potential.11,12 A second approach to surmount the difficulties of normal IRRAS is polarization modulation (PM-IRRAS). This approach is based on differences in electric field amplitudes at a surface upon reflection of p- and s-polarized IR radiation. At the surface, the electric field amplitude of a p-polarized beam is enhanced upon reflection whereas that of s-polarized radiation is nulled.13,14 The absence of electric field amplitude for s-polarized radiation at the surfaces allows discrimination of the spectral response for surface species from that of bulk solution species. However, since electric field amplitude for s-polarized radiation becomes significant at distances from the surface beyond about a quarter of the wavelength of the radiation being used,15 this approach requires (3) Bewick, A.; Kunimatsu, K.; Pons, B. S. Electrochim. Acta 1980, 25, 465– 468. (4) Weaver, J. D; Roth, M. J. J. Electroanal. Chem. 1991, 307, 119–137. (5) Faguy, P. W.; Marinkovic, N. S. Anal. Chem. 1995, 67, 2791–2799. (6) Richmond, W. N.; Faguy, P. W.; Jackson, R. S.; Weibel, S. C. Anal. Chem. 1996, 68, 621–628. (7) Pettinger, B.; Lipkowski, J.; Hoon-Khosla, M. J. Electroanal. Chem. 2001, 500, 471–478. (8) Li, N.; Zamlynny, V.; Lipkowski, J.; Henglein, F.; Pettingger; B, J. Electroanal. Chem. 2002, 524-525, 43–53. (9) Nichols, R. J.; Burgess, I.; Young, K. L.; Zamlynny, V.; Lipkowski, J. J. Electroanal. Chem. 2004, 563, 33–39. (10) Doneux, T.; Buess-Herman, C.; Lipkowski, J. J. Electroanal. Chem. 2004, 564, 65–75. (11) Roe, D. K.; Sass, J. K.; Bethune, D. S.; Luntz, A. C. J. Electroanal. Chem. 1987, 216, 293–301. (12) Kunimatsu, K. J. Electroanal. Chem. 1982, 140, 205–210. (13) Greenler, R. G. J. Chem. Phys. 1966, 44, 310–315. (14) Golden, W. G.; Dunn, D. S.; Overend, J. J. Phys. Chem. 1978, 82, 843–844. (15) Seki, H.; Kunimatsu, K.; Golden, W. G. Appl. Spectrosc. 1985, 39, 437– 443. 10.1021/ac801019r CCC: $40.75  2008 American Chemical Society Published on Web 10/01/2008

spectral correction for bulk contributions in most real situations in solution. Nonetheless, PM-IRRAS has been successfully used to investigate solid-liquid interfaces in thin-layer cells with solution layers on the order of hundreds of nanometers to micrometers in thickness.16,17 An alternative to these two approaches that serves as the basis for the work reported here is IRRAS on residual fluid films formed by forced dewetting. This approach is described herein by the simple designation “emersion”18,19 and relies on the retention of an ultrathin fluid film (typically θc ) 43°.

While the same function describes reflectivity at both the internal and external interfaces of the prism, Snell’s law dictates a different dependence on angle of incidence. Figure 2c illustrates the reflectance at the prism-gas interface, RP, plotted versus internal incidence angle, θBaF2 (solid line), and external incidence angle, θgas (long dashed line). At internal incident angles greater than the critical angle, θBaF2 > θc, total internal reflection occurs. The relevant calculations for this prism design require consideration of the transmitted intensity (i.e., Tp ) [1 - RP]) through the four prism-gas interfaces that the IR beam encounters: the gas-prism interface at the entrance to the prism (interface 1), at which external reflection at a 0° angle of incidence occurs, the prism-gas interface (interface 2), at which internal reflection occurs as the IR beam exits the prism just prior to incidence on the sample surface, the gas-prism interface (interface 3), at which external reflection occurs as the beam re-enters the prism, and the prism-gas interface (interface 4) at which internal reflection at a 0° angle of incidence occurs as the beam leaves the cell for the detector. Assuming that the sample surface is a perfect reflector (i.e., reflectivity 100%), the intensity emanating from the prism on its way to the detector relative to the intensity incident on interface 1 is simply the product of the transmission at each of the four prism-gas interfaces:

I ) I0

4

∏

4

(1 - Rn) )

1

∏T

(2)

n

1

where I0 is the IR beam intensity as it first enters the prism, I is the intensity as it exits the prism on the way to the detector, Rn is the reflectivity of the nth interface, and Tn is the transmittance of the nth interface. For the conditions described above for the BaF2 prism used here, the short dashed line in Figure 2c shows this normalized intensity behavior as a function of the angle of incidence of the IR beam at the sample surface-gas interfaces (interfaces 2 and 3.) The analysis of McIntyre30 and Hayden28 is used to calculate the magnitude of the absorption in the IRRAS experiment for p-polarized radiation for a two-phase system. Similar calculations were utilized by Pettinger.7 The total absorption intensity28 in the IRRAS experiment, ∆RRRAS, is calculated by

∆RIRRAS )

(EP⊥⁄E Pi)2 cos θIRRAS

(3)

where is the EP⊥ is the electric field component of p-polarized radiation calculated from the complex index of refraction of the surface and incident angle θIRRAS, and EPi is the incident electric field. The inverse cosine factor accounts for the increase in the area of the surface sampled as the incident angle is increased. The dashed line in Figure 3 shows this ∆RIRRAS function for a Ag surface at 2000 cm-1 as a function of angle of incidence, θIRRAS; this function exhibits a maximum near θIRRAS ) 87°. As noted, in the emersion geometry, ∆RIRRAS is modified by losses due to reflection at the four prism-gas interfaces. Thus, for a two-phase system in emersion, the total absorption intensity (defined as [∆RIRRAS]EM) becomes

[∆RIRRAS]EM )

(EP⊥⁄E P0)2 · cos θIRRAS

4

∏T

n

(4)

1

where E0p is the electric field amplitude of the IR radiation incident on the first face of the prism. This product is plotted as the solid line in Figure 3.

Figure 3. Calculated total absorbance for p-polarized light: dependence of standard IRRAS absorbance (- - -) and emersion IRRAS absorbance (s s) on incident angle, θIRRAS, for Ag at 2000 cm-1. Inset shows emersion IRRAS absorbance as a function of θBaF2. Arrows indicate the design angle used for the prism in this work.

It should be noted that, in the presence of an absorbing adsorbate layer, a full three-phase treatment requires proper inclusion of the adsorbate layer optical constants. In this case, Greenler13 has defined an absorption function A ) ∆R/R0 ) (R0 - R)/R0, where R0 is the reflectivity of the interface in the absence of the adsorbate layer and R is the reflectivity of the interface in the presence of the adsorbate layer. Since the functional form of the A - θ behavior for a three-phase systems is the same as that for the two-phase system, only the two-phase treatment is shown here. This [∆RIRRAS]EM function is shown as an inset to Figure 3 and exhibits a maximum at θIRRAS ≈ 80°. In order to achieve this incident angle in the emersion configuration, an incident angle at the prism-gas interface of θBaF2 ) 42.7° is required. This value is within 1° of the critical angle for total internal reflection at the prism-gas interface (θc ) 43.6°) at 2000 cm-1. Not only would this alignment be challenging from an engineering perspective, but the real situation is further complicated by virtue of the fact that the IR beam entering the sample compartment has a divergence from center of ∼4° in this FT-IR spectrometer. To avoid any potential loss from slight misalignment of the beam and to adequately compensate for beam divergence, the final design incorporates an internal angle at the prism-gas interface of θBaF2 ) 36.4° (∼7° less than the critical angle), which results in an incident angle at the sample surface of θIRRAS ) 60°. For this geometry, [∆RIRRAS]EM calculated at the center of the beam is ∼40% of that at the optimum angle of incidence (see arrow in Figure 3). The prism design illustrated in Figure 2a has surfaces for the entering and exiting IR beam cut at 36.4° with respect to the prism base such that these beams will enter and exit the prism with minimal reflectivity and no change in angle due to refraction. A thin mask between the prism and the sample surface was used to define the beam size and location on the surface. This mask restricts the sampling region to include only the emersed film, thus avoiding any edge effects. To optimally utilize the emersion cell design in commercial FT-IR instrumentation, the sampling beam must be focused on the sample surface and directed into the prism at the appropriate angle; the beam reflected from the sample through the prism must then be redirected into the detector. In the Nicolet Magna 550 FT-IR instrument used here, the beam is focused to a spot of ∼5mm diameter at the center of the sampling chamber using a source aperture setting of 32. Figure S1 (Supporting Information, SI) illustrates the optical design implemented for this instrument. The source beam is directed to the sample by two mirrors. The beam is first defocused by a convex, 49-mm radius of curvature, 12.5-mm-diameter mirror (mirror 1 in Figure S1, SI) and refocused onto the sample by a concave, 230-mm-radius of curvature, 25.4mm-diameter mirror (mirror 2 in Figure S1, SI). The two mirrors that redirect the reflected beam onto the detector (mirrors 3 and 4 in Figure S1, SI) are symmetric with the two that direct the source beam to the surface. The positions of mirrors 1 and 4 are determined only by the instrument beam position and, for that reason, were mounted together in a fixed relative position. Mirrors 2 and 3 and the sample cell were mounted separately. The four (30) McIntyre, J. D. E. In Advances in Electrochemistry and Electrochemical Engineering; Muller, R. H., Delahay, P., Tobias, C. W., Eds.; John Wiley & Sons: New York, 1973; Vol. 9,pp 61-166.

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independent mounts were each attached to x-y-z translation stages. All mirrors have fixed angles designed to match the prism geometry. Position and focus of the incident and reflected IR beam can be accomplished with only x-y--z adjustment of the mirrors and sample cell. Careful alignment of the optical bench to maximize beam throughput is critical. When compared to a typical IRRAS geometry (at 80° incidence angle) in which a well-focused beam is incident on the sample surface in the middle of the sample compartment, the intensity throughput for emersion IRRAS is smaller. Not only is some intensity lost due to internal reflection at the prism-vapor interface, low throughput is also due to loss of intensity upon entry of the beam into the prism. Moreover, the sampling area is smaller than the total beam area that passes through the prism due to the need for a mask between the prism and the sample to define the sampling region and to avoid sampling the substrate edge. For the geometry outlined in Figures 2 and S1 (SI) with a polarizer in the beam path, an intensity throughput of only ∼10% of that experienced in simple transmission is achieved. When corrected for surface sampling area, this value compares favorably to the intensity throughput of 80% achieved in a conventional IRRAS experiment (with polarizer) using an IRRAS sampling attachment with a fixed incident angle of 80°. Although the absolute magnitude of the absorbance is unaffected by throughput intensity, the ultimate sensitivity of the measurement depends on total intensity throughput as it affects observed S/N. The overall sensitivity of emersion IRRAS is smaller than that for IRRAS at an 80° incident angle because of loss of amplification of the electric field at the surface by lowering the incident angle to avoid internal reflection. The extent of this decrease is considerable and is given by the difference in ∆RIRRAS at 80° as given by the solid line for standard IRRAS and that for a 60° incident angle given by the dashed line in Figure 3. The loss predicted by these plots is ∼75%. Comparison of the loss experienced experimentally with emersion IRRAS with that expected was assessed by acquiring spectra of the ν(C-H) region for a self-assembled monolayer of octadecanethiol on Ag (Figure S2, SI). The spectrum acquired using the emersion IRRAS configuration is only ∼30% as intense as that acquired using standard IRRAS conditions. The agreement of the magnitude of decreased intensity confirms that the optical aspects of the emersion IRRAS experiment are well-defined with the existing cell configuration and well understood. Emersion IRRAS on Self-Assembled Monolayers (SAMs). Ag surfaces modified with SAMs of 11-MUD emersed from water were investigated using this new cell design. This system was chosen for the initial test of emersion IRRAS, since it was characterized previously in this laboratory using both emersion ellipsometry and emersion surface Raman spectroscopy.31 Moreover, water is a strong IR absorber, in both the liquid and the vapor states, and provides a reasonable test of this new approach to emersion IRRAS. Emersion requires the introduction of a renewable solvent drop to the bottom of the substrate surface with visual confirmation of substrate position and drop formation. The drop is produced using (31) Tiani, D. J.; Yoo, H.; Mudalige, A.; Pemberton, J. E. Langmuir, submitted.

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Figure 4. IRRAS of 11-MUD SAM on Ag. (a) SAM in N2 environment (black), (b) SAM in water-saturated N2 environment (red), and (c) SAM emersed from water (blue).

an ultralow flow peristaltic pump that introduces fluid through a glass capillary of ∼0.4-mm i.d. to a substrate rotating at 0.024 cm/s. The spectrum of an 11-MUD SAM in an inert N2 environment (Figure 4a) shows peaks at 2916, 2848, and 3400 cm-1 corresponding to the νasym(CH2), νsym(CH2), and ν(OH) modes, respectively, of 11-MUD in good agreement with previous literature,32 and with spectra obtained using standard IRRAS in our laboratory (not shown). Thiophenol-modified Ag is used for the background spectrum. Thiophenol signatures are well-studied33,34 and fall in quiet regions of the 11-MUD spectrum and, thus, can be accurately subtracted from sample spectra. An increase in the ν(OH) band is observed when 11-MUDmodified Ag is exposed to H2O-saturated N2 (Figure 4b). This observation of a condensed H2O layer on 11-MUD-modified Ag is consistent with studies of static condensation undertaken in this laboratory using ellipsometry.31 It should be noted, however, that the surface hydroxyl group reorientation upon interaction with surface H2O or H2O vapor may contribute to the intensity changes in this band. When the 11-MUD-modified Ag surface is emersed from H2O (Figure 4c), a further increase of the ν(OH) band absorbance is observed from retention of a residual H2O film on the 11-MUD-modified surface. The thickness of the emersed water layer can be assessed from the Beer-Lambert law by relating emersed layer thickness (d) to absorption path length (b) by

∆RIRRAS )

(EP⊥⁄E Pi)2 cos θIRRAS

(5)

with θIRRAS as defined above. In order to evaluate the thickness of the films, the molar absorptivity of the interfacial water film must be obtained; this process is complicated by the presence of oriented electric fields in IRRAS. From the bulk optical constants, n and k, a simulated IRRAS spectrum of a perfectly isotropic water film can be calculated using the approach by Hansen35 and others.36,37 This approach accounts for the presence of oriented (32) Pan, S.; Belu, A. M.; Ratner, B. D. Mater. Sci. Eng. 1999, C7, 51–58. (33) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979–9984. (34) Wan, L.; Terashima, M.; Noda, H.; Osawa, M. J. Phys. Chem. B 2000, 104, 3563–3569. (35) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380–391. (36) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62–67.

Figure 5. (a) Transmission spectrum of water in the ν(O-H) region, (b) simulated IRRAS spectrum for a 1-nm-thick, isotropic layer of water on Ag, and (c) IRRAS spectrum of 1.9-nm-thick emersed water layer on 11-MUD/Ag with spectral contributions from the SAM film in dry N2 subtracted. Curve fits using Gaussian line profiles shown beneath each ν(O-H) envelope as thin solid lines. Sum of fit peaks shown as thick dashed lines.

electric fields on the predicted spectrum. An IRRAS spectrum for a 1-nm-thick isotropic H2O film on Ag substrates modified with a thin organic film was simulated using the matrix method for a multilayer isotropic system described by Hansen35 and Allara.37 A four-layer model was used in which air, the H2O film, the SAM, and the Ag substrate are the first to fourth layers, respectively. An incident angle of 60° was used for this calculation. Optical constants n and k for H2O were obtained from the transmission spectrum (Figure 5a). Values of the real part of the refractive index, n, were obtained from the bulk IR spectrum of water utilizing a numerical solution of the Kramers-Kronig transformation.38 In this calculation, the SAM film was assumed to be 2 nm thick.39 The refractive indices of the SAM was assumed to be constant at 1.45 (value for neat thiols) with no imaginary component after the manner of Engquist,40 neglecting possible absorption by the film for simplicity. Optical constants for Ag in the IR region were obtained from Palik.41 The simulated spectrum is shown in Figure 5b and exhibits an integrated absorbance for the ν(OH) band that is of the same order of magnitude as that (37) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 11, 1215–1220. (38) Ohta, K.; Ishida, H. Appl. Spectrosc. 1988, 42, 952–957. (39) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D. Langmuir 1992, 8, 2707–2714. (40) Engquist, I.; Lundstrom, I.; Liedberg, B.; Parikh, A.; Allara, D. L. J. Chem. Phys. 1997, 106, 3038–3048. (41) Palik, E. D. Handbook of Optical Constants of Solids; Academic: London, 1985.

reported by Engquist40 for the D2O/SAM-Au system with slight differences due to differences in the angle of incidence and the optical constants of the substrate and the liquid. From this spectrum, the integrated molar absorptivity of the ν(OH) envelope in the simulated spectrum is 3.25 × 104 M-1 cm-2, comparable to the value of 3.40 × 104 M-1 cm-2 determined from the transmission spectrum of bulk H2O and similar to that reported in the literature.42,43 The thicknesses of condensed and emersed H2O layers can be estimated using the integrated molar absorptivity obtained from the simulated IRRAS spectrum after subtraction of spectral contributions from the dry SAM film with the assumption that the absorptivity for the SAM is constant irrespective of environment. The thicknesses obtained from this analysis are only approximate because of the additional implicit assumptions of bulk H2O density and bulk H2O molar absorptivity. Neither of these assumptions may be strictly valid based on previous studies of water (H2O or D2O) dosed onto SAMs in UHV that showed a nonlinear increase in ν(OH) (or ν(OD)) absorbance with water thickness for the first few monolayers at methyl- and hydroxylterminated surfaces.40,44 As shown in Figure 4b, condensation occurs on 11-MUD due to favorable interactions with water vapor. The average condensed layer thickness is 0.5 ± 0.1 nm on the 11-MUD surface, corresponding to ∼1 monolayer of condensed water and similar to the ellipsometric measure of condensed layer thickness of 0.26 ± 0.06 nm.31 Similarly, the emersed H2O layer thickness from the spectrum in Figure 4c is 1.9 ± 0.2 nm on 11-MUD-modified Ag, similar to the value of 1.6 ± 1.1 published previously for this system.31 Although the exact thicknesses obtained by emersion IRRAS and ellipsometry differ somewhat, the values are of a similar magnitude and comparable, especially considering the standard deviations in ellipsometry measurements. Figure 5a shows the ν(OH) envelope centered at 3400 cm-1 for a transmission IR spectrum of bulk H2O. The ν(OH) envelope has traditionally been decomposed into three45 or four bands.46 Two major bands are located at ∼3200 and 3400 cm-1. The band at 3200 cm-1 is generally attributed to the νs(OH) mode of tetrahedrally coordinated, symmetrically hydrogen-bonded water similar to the structure found in ice46-48 and is often referred to as the “icelike” mode. The band at ∼3400 cm-1 is ascribed to water that is in an incomplete tetrahedral coordination (or slightly disordered hydrogen-bonded structure)47,48 and is referred to as the “liquidlike” mode, since it is the prominent contributor to this envelope for liquid water. The non-hydrogen-bonded ν(OH) of water (free OH) and very weakly hydrogen-bonded water are thought to contribute to the high-frequency shoulder of the broad ν(OH) envelope.49 Water (42) Bertie, J. E.; Lan, Z. Appl. Spectrosc. 1996, 50, 1047–1057. (43) Venyaminov, S. Y.; Prendergast, F. G. Anal. Biochem. 1997, 248, 234– 245. (44) Engquist, I.; Liedberg, B. J. Phys. Chem. 1996, 100, 20089–20096. (45) Boissiere, C.; Brubach, J. B.; Mermet, A.; Marzi, G. D.; Bourgaux, C.; Prouzet, E.; Roy, P. J. Phys. Chem. B 2002, 106, 1032–1035. (46) Gonzalez-Blanco, C.; Rodriguez, L. J.; Velazquez, M. M. Langmuir 1997, 13, 1938–1945. (47) Raymond, E. A.; Tarbuck, T.; Richmond, G. L. J. Phys. Chem. B 2002, 106, 2817–2820. (48) Scatena, L. F.; Richmond, G. L. Science 2001, 292, 908–912. (49) Zhelyaskov, V.; Georgiev, G.; Nickolov, Z.; Miteva, N. J. Raman Spectrosc. 1990, 21, 203–205.

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Table 1. Peak Frequencies, Integrated Absorbance Values, and Assignments for Bands from Spectral Decomposition of ν(OH) Region νs(OH)a (icelike)

νa(OH) + νs(OH) (liquidlike)

νs(OH) (weakly H-bonded)

νs (OH) (monomer)

frequency frequency frequency frequency Int Abs, au Int Abs, au Int Abs, au Int Abs, au (fwhm)b,b cm-1 cm-1 (fraction) (fwhm), cm-1 cm-1 (fraction) (fwhm), cm-1 cm-1 (fraction) (fwhm), cm-1 cm-1 (fraction)

system transmission H2O simulated H2O H2O/11-MUD

3218 ± 5 (258 ± 13) 3273 ± 7 (259 ± 1) 3287 ± 5 (260 ± 1)

293 ± 18 (0.32 ± 0.02) 0.23 ± 0.01 (0.32 ± 0.01) 0.19 ± 0.01 (0.47 ± 0.02)

3411 ± 4 (261 ± 9) 3451 ± 3 (228 ± 7) 3466 ± 2 (211 ± 6)

517 ± 23 (0.56 ± 0.25) 0.41 ± 0.02 (0.57 ± 0.03) 0.18 ± 0.07 (0.44 ± 0.02)

3529 ± 7 (141 ± 11) 3531 ± 3 (127 ± 7) 3537 ± 5 (113 ± 16)

81 ± 8 (0.09 ± 0.01) 0.06 ± 0.018 (0.08 ± 0.01) 0.02 ± 0.01 (0.05 ± 0.02)

3617 ± 5 (87 ± 13) 3615 ± 2 (85 ± 1) 3618 ± 5 (91 ± 13)

27 ± 8 (0.03 ± 0.01) 0.02 ± 0.00 (0.03 ± 0.01) 0.01 ± 0.00 (0.03 ± 0.01)

a ν ) stretch, s ) symmetric, and a ) asymmetric; assignments taken from references.46-50 b Bands fit with 100% Gaussian profile; standard deviations for bulk and simulated spectra determined from three independent fits of single spectrum; standard deviations for spectra of residual water on 11-MUD determined from fits of spectra from three independent samples.

in this environment is often described as monomeric, because it is loosely hydrogen-bonded through a few hydrogen-bonding sites, mostly through one or both of its lone pairs. Here, the shoulder of the ν(OH) envelope is fit with two weak bands; the one at lower frequency is attributed to weakly hydrogen-bonded water, and the other represents non-hydrogen-bonded water or monomeric water. The transmission spectrum shown in Figure 5a is decomposed into four 100% Gaussian bands51 centered at 3218, 3411, 3529, and 3617 cm-1. Table 1 summarizes the peak frequency, full width at half-maximum (fwhm), integrated absorbance, fractional integrated absorbance, and assignment for each fit band. The bands at 3218 and 3411 cm-1 are assigned to the icelike and liquidlike modes, respectively. Two weaker bands at 3529 and 3617 cm-1 are assigned to weakly hydrogen-bonded water and free, or monomeric water, respectively. Peak frequency and fwhm values reported here are comparable to those reported in the literature for the water ν(OH) envelope.46 The shape of the ν(OH) envelope in the simulated spectrum shown in Figure 5b is clearly different from that of the transmission spectrum. Based on the previous work of Greenler52 and Allara,37 this mismatch is due to large values of the extinction coefficient, k, with a concomitant small value of the index of refraction, n, value for the ν(OH) envelope. Greenler52 has previously evaluated simulated spectra compared to transmission spectra and found that for strongly absorbing bands (k > 1) of thin films (d , λ), peaks in the simulated spectrum are shifted from those of the k maximum in the transmission spectrum. A similar observation was made by Allara,37 who further noted that the shifts are always to higher frequency, and for sharp bands (fwhm ∼5 cm-1) with small k values (k < 0.1), the shifts are finite but several times smaller than those for broad bands. For broad bands, especially those with moderately large k values (k > 0.1), significant distortion is expected with frequency shifts of up to tens of wavenumbers due to the effect of the small refractive index. The decomposition shown in Figure 5b was performed using the bulk fit parameters from the bulk water spectrum as a starting point for the fits. The fractional integrated areas of each mode were kept similar to those from the bulk spectrum, although in reality, such a thin layer of water might be expected to have a composition different from the bulk because of a different surface(50) Whalley, E.; Klug, D. D. J. Chem. Phys. 1986, 84, 78–80. (51) Bertie, J. E.; Lan, Z. J. Mol. Struct. 1997, 413-414, 333–363. (52) Greenler, R. G.; Rahn, R. R.; Schwartz, J. P. J. Catal. 1971, 23, 42–48.

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to-volume ratio and because the density53 and orientation54,55 of water vary at water-vapor and water-solid interfaces. Four submodes are observed at 3273, 3451, 3531, and 3615 cm-1 and assigned to the icelike, liquidlike, weakly bonded, and monomer modes, respectively. The peak frequencies of weakly hydrogenbonded and monomer modes are comparable to those of bulk H2O. In contrast, peak frequencies for the icelike and liquidlike modes are shifted 55 and 40 cm-1, respectively, to higher frequencies relative to those for bulk H2O. These differences clearly demonstrate distortion in the ν(OH) band shape due to the effects noted. Figure 5c shows the results from Gaussian decomposition of the ν(OH) region for emersed H2O on 11-MUD-modified Ag with spectral contributions from the dry SAM film subtracted. Changes due to reorientation of surface hydroxyl groups were assumed to be minimal in performing. The icelike, liquidlike, weakly hydrogenbonded monomer and free monomer modes are observed at 3287, 3466, 3537, and 3618 cm-1, respectively, for the H2O layer on 11MUD-modified Ag. These peaks are higher in frequency than those of the corresponding simulated spectrum by 14, 15, 6, and 3 cm-1, respectively, suggesting weaker hydrogen-bonding in the emersed layer than in an isotropic, bulklike film. These observed peak shifts are similar to those observed by Tiani31 for emersed D2O films on 11-MUD using Raman spectroscopy, although he observed slightly larger increases in frequency for all four modes (29, 16, 13, and 39 cm-1) compared to bulk D2O. Although the overall trends in peak frequencies agree, the peak intensity ratios determined here differ somewhat from those observed with Raman spectroscopy.31 Specifically, a less intense liquidlike mode and a more intense monomer mode were observed in the surface Raman spectroscopy than observed here. These differences may be the result of the distance dependence of the surface enhancement compared to IRRAS. Surface enhancement decays very rapidly with distance away from the surface in Raman spectroscopy,56-58 whereas enhancement is relatively constant with distance for thin films in IRRAS. The emersed H2O films are ∼2 nm thick; therefore, the outer edge of the emersed (53) Schwendel, D.; Hayashi, T.; Dahint, R.; Pertsin, A.; Grunze, M.; Steitz, R.; Schreiber, F. Langmuir 2003, 19, 2283–2293. (54) Eisenthal, K. R. Chem. Rev. 1996, 96, 1343–1360. (55) Richmond, G. L. Chem. Rev. 2002, 102, 2693–2724. (56) Ye, Q.; Fang, J.; Sun, L. J. Phys. Chem. B 1997, 101, 8221–8224. (57) Compagnini, G.; Galati, C.; Pignataro, S. Phys. Chem. Chem. Phys. 1999, 1, 2351–2353. (58) Murray, C. A.; Allara, D. L. J. Chem. Phys. 1982, 76, 1290–1303.

film is ∼4 nm from the Ag substrate accounting for the thickness of the SAM. Decay of the surface enhancement between 2 and 3 nm from surface has been reported to be somewhere between 3057 and 65%56 for rough Ag substrates. Assuming that the spectral differences observed here are due to differences in surface enhancement distance dependence in the Raman spectra, the results imply that water molecules immediately adjacent to the 11-MUD surface are in a more ordered and monomeric environment than those at distances greater than 1 nm from the surface. In addition, the larger frequency shifts observed in the surface Raman spectrum may be due to enhanced contributions from water molecules closer to surface hydroxyl groups. It is proposed here that water molecules adjacent to surface hydroxyl groups (or directly hydrogen-bonded to surface hydroxyl groups) are more weakly hydrogen-bonded and contain more monomeric hydroxyl groups. This proposed picture contrasts with a previous conclusion of strong hydrogen-bonding at hydrophilic oxide surfaces by SFG.59 This difference is good evidence for a considerably greater degree of hydrophobicity of the 11-MUD surface, despite its hydrophilic terminal group, that may cause the less extensive and weaker hydrogen-bonding observed here. Such hydrophobic character of the 11-MUD surface may be the result of exposure of the underlying methylene groups upon reorganization of the terminal hydroxyl groups to facilitate their lateral hydrogen-bonding as has been observed in recent structure simulations.60 The hydrophobicity of 11-MUD is demonstrated by a significant (∼30°) increase in contact angle upon exposure of a freshly prepared 11-MUD film to water.61 Finally, given the

different surface selection rules for surface Raman and IRRAS, it is also possible that the Raman spectrum contains more contributions from terminal hydroxyl groups that reorient upon exposure to water. The integrated absorbance ratio of the icelike to liquidlike modes is calculated to be 1.06 ± 0.01 for the residual water layer on the 11-MUD film, significantly greater than the value of 0.56 ± 0.02 observed in the simulated spectrum of the isotropic water film. This result indicates that the residual water layer on 11-MUD after forced dewetting prefers more icelike hydrogen-bonding and is more structured than isotropic water. In summary, we have shown here the first successful demonstration of IRRAS for investigating interfacial fluid structure within residual thin films formed by forced dewetting at the solid-fluid interface. Since only a few techniques are available to study the molecular structural characteristics of fluid-solid interfaces under ambient conditions of temperature and pressure, emersion IRRAS is a powerful new addition to this arsenal.

(59) Shen, Y. R.; Ostroverkhov, V. Chem. Rev. 2006, 106, 1140–1154. (60) Tasic, U.; Day, B. S.; Yan, T.; Morris, J. R.; Hase, W. L. J. Phys. Chem. C 2008, 112, 476–490. (61) Evans, H. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156–161.

Received for review May 19, 2008. Accepted August 26, 2008.

ACKNOWLEDGMENT The authors acknowledge support for this research by the National Science Foundation through a grant to J.E.P. (CHE0317114) that included a supplement for K.E.J. to spend a sabbatical year at the University of Arizona. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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