Grazing Incidence Synchrotron X-ray Diffraction of Polymerizing

Nov 7, 2003 - Octadecyltrimethoxysilane (OTMS) was spread on the water surface of a ... with the ordering found in bulk OTMS by powder X-ray diffracti...
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Grazing Incidence Synchrotron X-ray Diffraction of Polymerizing Langmuir Monolayers Stephen R. Carino,†,§ Royale S. Underhill,†,| Holger S. Tostmann,† Andrew M. Skolnik,† Jennifer L. Logan,† Mark R. Davidson,‡ Jeffrey T. Culp,† and Randolph S. Duran*,† Butler Polymer Laboratory, Department of Chemistry, and Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 Received March 14, 2003. In Final Form: August 25, 2003 Octadecyltrimethoxysilane (OTMS) was spread on the water surface of a Langmuir trough. Utilizing the high-intensity synchrotron X-ray radiation at the Advanced Photon Source at Argonne National Laboratory, the kinetics of film growth of OTMS as a function of the subphase pH was studied. In situ grazing incidence X-ray diffraction experiments at the air/water interface revealed that OTMS self-assembles into small domains and that the number of these ordered domains increases as OTMS undergoes hydrolysis/ condensation reactions. The hexagonal packing of alkyl chains found in the OTMS monolayer is consistent with the ordering found in bulk OTMS by powder X-ray diffraction.

Introduction Self-assembled monolayers (SAMs) have received wide acceptance as a means of modifying the surface properties of many materials. The ease of preparation of SAMs and the ability to easily introduce and control specific molecular interactions have led to their application in surface patterning,1,2 molecular lubrication,3 and corrosion prevention.4 Commonly used SAM systems are derived from amphiphilic compounds of alkanethiols, fatty acids, and organosilanes such as alkylchlorosilanes and alkylalkoxysilanes.5 Unlike SAMs of alkanethiols and fatty acids in which the ultimate film structure is determined by substrate-molecule and van der Waals chain-chain interactions, the reactive headgroups in organosilanes introduce potential lateral film growth mechanisms at substrates. While it is well-known that SAM films resulting from alkylsilanes are semicrystalline and the alkyl chains orient normal to the surface with little or no tilt, the mechanism of lateral growth of these SAMs at solids has been debated over the years. On SiO2, alkylsilanes have been suggested to undergo island growth as manifested by the invariant film thickness over the duration of deposition by Dutta et al.6 and others.7-9 Studies by Wasserman et al.10 and * To whom correspondence should be addressed. E-mail: [email protected]. † Butler Polymer Laboratory, Department of Chemistry. ‡ Department of Materials Science and Engineering. § Current address: Pfizer Corp., 7000 Portage Road, Kalamazoo, MI 49001. | Current address: DRDC Atlantic, Emerging Materials Section, P.O. Box 99000 Station Forces Halifax, Halifax, Nova Scotia, B3K 5X5, Canada. (1) Finnie, K. R.; Haasch, R.; Nuzzo, R. G. Langmuir 2000, 16, 6968. (2) Zheng, J. W.; Zhu, Z. H.; Chen, H. F.; Liu, Z. F. Langmuir 2000, 16, 4409. (3) Tsukruk, W. Adv. Mater. 2001, 13, 95. (4) Jennings, G. K.; Munro, J. C.; Yong, T. H.; Laibinis, P. E. Langmuir 1998, 14, 6130. (5) Ulman, A. Chem. Rev. 1996, 96, 1533. (6) Richter, A. G.; Durbin, M. K.; Yu, C. J.; Dutta, L. Langmuir 1998, 14, 5980. (7) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (8) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354.

Vallant et al.11 indicate homogeneous growth modes, where the alkyl chains are initially randomly oriented and gradually align with increasing deposition time. Overall results seem to favor island growth, the mechanism involving oligomeric alkylsilanes that physisorb at the hydration layer of the oxide surface. Eventually, these species chemically anchor to the substrate. This was originally proposed by Sagiv12 and corroborated by other groups.13,14 The lateral film growth of alkylsilanes can be effectively isolated from the influence of a solid substrate in a Langmuir trough. Under these conditions, the chemistry intrinsic to the alkylsilane molecules may be investigated, as the water surface is fluid and will not pin reactive species to a specific site. The positional ordering in a floating Langmuir monolayer more closely reflects the van der Waals interchain interaction and the intrinsic lateral chemical bonding of the silane headgroups than such a film self-adsorbed to a solid. Vertical film growth modes that yield a multilayered structure are impossible. Furthermore, the surface pressure imposed by displacing the barrier in a Langmuir trough can be conveniently controlled to vary the surface density and energy involved in the chemisorption of the silane molecules to the water substrate.15 Since the first reported grazing incidence X-ray diffraction (GIXD) studies at the air-water interface by Dutta et al.16 and Kjaer et al.,17 the focus of most synchrotron monolayer studies has been in situ investi(9) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143. (10) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (11) Vallant, T.; Kattner, J.; Brunner, H.; Mayer, U.; Hoffmann, H. Langmuir 1999, 15, 5339. (12) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (13) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (14) Resch, R.; Grasserbauer, M.; Friedbacher, G.; Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H. Appl. Surf. Sci. 1999, 140, 168. (15) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (16) Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulos, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58, 2228. (17) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Laxhuber, L. A.; Mohwald, H. Phys. Rev. Lett. 1987, 58, 2224.

10.1021/la034444v CCC: $25.00 © 2003 American Chemical Society Published on Web 11/07/2003

GIXD of Polymerizing Langmuir Monolayers

gations of compression and temperature effects on the structure. Very little has been reported regarding X-ray studies of time-dependent effects in floating monolayers. The main problem is the weakness of the scattered signal, which imposes the need for high-intensity X-ray sources so that the data acquisition can be done in the required time and at an adequate resolution. Dynamic properties of Langmuir-Blodgett multilayer films transferred to solids, on the other hand, have been more routinely investigated since they have significantly larger scattering cross sections. For example, Rapp investigated the multilayer spacing and total film thickness of a 28-monolayer-thick film of magnesium stearate during heating with a time resolution of 10 s.18 Using a multiple imaging-plate detector system and focusing monochromator, Foran and co-workers were able to achieve a signal-to-noise ratio far higher than that of conventional scanning methods.19,20 Their instrumental design allowed them to do time-resolved grazing incidence diffraction studies of fatty acid metal salts.21 In the case of Langmuir films, the scattering cross section from the monolayer-thick film is reduced drastically such that a typical scan usually lasts anywhere from 20 min to 1 h, depending on the desired resolution. This significantly limits the type of time-resolved studies to dynamic systems that take hours to complete or reach equilibrium. Fortunately, certain reactions in monolayers have such characteristic kinetics. For instance, the photoisomerization in certain azobenzene surfactants can take up to 1000 s (at Π ) 14 mN m-1) to reach completion.22 Similarly, the UV-induced condensation reactions in dilinoleoylphosphatidylethanolamine monolayers occur equally slowly.23 These systems are good candidates where real-time GIXD can be used to investigate the structural transformations occurring during the reaction. Dynamics in monolayers can also involve other processes. Previously, Lin et al. investigated the timedependent effects that are observed as amphiphiles are compressed. They have identified for the first time that a microscopic relaxation mechanism occurs as the monolayer reorganizes from a pseudohexagonal structure, formed upon compression, to an undistorted hexagonal structure.24 The series of experiments that will be discussed hereafter was designed to exploit and further improve the GIXD technique as a method of investigating the structural transformation in real time. The alkoxysilane system provides an excellent platform in which these goals can be achieved since its chemistry is easily controlled by subphase pH and surface concentration. Many groups, including ourselves, have investigated the reactions of alkylsilanes in Langmuir monolayers.25-29 (18) Rapp, G.; Koch, M. H. J.; Hohne, U.; Lvov, Y.; Mohwald, H. Langmuir 1995, 11, 2348. (19) Foran, G. J.; Peng, J. B.; Steitz, R.; Barnes, G. T.; Gentle, I. R. Langmuir 1996, 12, 774. (20) Foran, G. J.; Garrett, R. F.; Gentle, I. R.; Creagh, D. C.; Peng, J. B.; Barnes, G. T. J. Synchrotron Radiat. 1998, 5, 500. (21) Foran, G. J.; Gentle, I. R.; Garrett, R. F.; Creagh, D. C.; Peng, J. B.; Barnes, G. T. J. Synchrotron Radiat. 1998, 5, 107. (22) Kim, I.; Rabolt, J. F.; Stroeve, P. Colloids Surf., A 2000, 171, 167. (23) Viitala, T.; Peltonen, J. Biophys. J. 1999, 76, 2803. (24) Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P.; Thomas, B. N.; Buontempo, J.; Rice, S. A. J. Chem. Phys. 1989, 90, 2393. (25) Vidon, S.; Leblanc, R. M. J. Phys. Chem. B 1998, 102, 1279. (26) Britt, D. W.; Hlady, V. J. Phys. Chem. B 1999, 103, 2749. (27) Carino, S. R.; Duran, R. S.; Baney, R. H.; Gower, L. A.; He, L.; Sheth, P. K. J. Am. Chem. Soc. 2001, 123, 2103. (28) Sjo¨blom, J.; Stakkestad, G.; Ebeltoft, H.; Friberg, S. E.; Claesson, P. Langmuir 1995, 11, 2652. (29) Brosseau, J.-L.; Vidon, S.; Leblanc, R. M. J. Chem. Phys. 1998, 108, 7391.

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In particular, 2D viscometry has shown that a cross-linked network results from the reaction of n-octadecyltrimethoxysilane [OTMS, CH3(CH2)17Si(OCH3)3] monolayers under acidic conditions. The kinetics of this polymerization process and the evolution of the resulting mechanical properties with the extent of reaction are reproducible. Previously, we have shown that standard Langmuir trough monolayer techniques coupled with synchrotron GIXD have sufficient time-resolution to follow structural changes induced by a polymerization reaction.30 We have also reported that a nucleation-type growth precedes the 2D gelation of the material. In this paper, synchrotron GIXD is utilized to further investigate the structural transformations occurring in this reacting monolayer system. Experimental Methods General. Chloroform solutions of OTMS (95%, Gelest) were spread at air/water interfaces to accomplish the monolayer polymerization under isobaric conditions as described previously.27 Reacted silane compounds used for powder X-ray diffraction were prepared from compounds obtained from Gelest and used without further purification. About 1 mL of an alkoxysilane was added dropwise to a 0.1 M HCl solution. The resulting whitish suspension was reacted for 12 h, filtered, washed with ethanol and copious amounts of deionized water, and vacuum-dried. Wideangle X-ray diffraction was obtained from a Siemens Platform diffractometer with a Hi-Star GADDS (General Area Diffraction System) using a Cu KR source (λ ) 1.542 Å) and operating at 50 kV and 40 mA. The detector was positioned 60 cm from the sample, and data were collected for at least 1 h. To prepare samples for X-ray diffraction, a small amount of the dry powder was placed between two pieces of adhesive tape and mounted in transmission geometry in front of the X-ray source. After data acquisition, the image was unwarped and plots of 2θ versus intensity were obtained by integration along the chi (χ) direction. Subsequent background subtraction using scattering data obtained at the same time duration as the sample removes the broad scattering caused by the adhesive tape. Grazing incidence X-ray diffraction experiments were performed at the Materials Research Collaborative Access Team (MRCAT) of the Advance Photon Source (APS) on the Sector 10-ID Beamline.30 GIXD Instrumentation. The X-ray beam from the undulator was monochromatized by a double silicon crystal monochromator tuned to provide X-ray energy of 10 or 11 keV (λ ) 1.13 or 1.24 Å, respectively). To achieve surface sensitivity,31 the angle of incidence was kept at approximately 85% of the critical angle for total external reflection at the air-water interface, which is 2.0 or 2.2 mrad at 11 or 10 keV, respectively. A schematic of the experimental setup has been previously published by Carino et al. as supplemental information.30 All synchrotron GIXD data are shown with pure Lorentzian fits and the background already subtracted.

Results and Discussion Grazing Incidence Diffraction of a Polymerized OTMS Monolayer. Figure 1 shows the GIXD data obtained from a floating Langmuir film of reacted OTMS. The data were obtained from a monolayer spread on a pH 1.5 subphase and reacted under isobaric conditions at 8 mN m-1 for 2 h. At this surface pressure, the monolayer has an initial average molecular area of 43.5 ( 0.7 Å2 corresponding to a liquid expanded phase. It reacts to occupy (see Figure 11) 21 ( 2 Å2, a value similar to that (30) Carino, S. R.; Tostmann, H.; Underhill, R. S.; Logan, J.; Weerasekera, G.; Culp, J.; Davidson, M.; Duran, R. S. J. Am. Chem. Soc. 2001, 123, 767. (31) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 252.

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Carino et al. Table 1. Data from the Diffraction Patterns in Figure 2 2θ (λ ) 1.542 Å)

k, nm-1

d-spacing, Å

3.28

2.33

26.97

5.11 21.28

3.63 15.05

17.30 4.17

2.45

1.74

36.11

4.97 21.38

3.53 15.12

17.80 4.16

compound C, n-octadecylmethyldiethoxysilane

21.44

15.16

4.14

compound D, n-octadecyldimethylmethoxysilane

18.16

12.86

4.89

21.68 23.20

15.33 16.39

4.10 3.83

silane compound A, n-octadecyltrimethoxysilane

compound B, octadecyl(3-trimethoxysilylpropyl)ammonium chloride

Figure 1. First-order peak detected from an OTMS Langmuir monolayer. The inset shows the approximate hexagonal lattice.

of Fontaine et al.32 At pH 1.5, OTMS rapidly undergoes hydrolysis and condensation, such that the reactions are complete in 1.5 h.32 The diffraction peak in Figure 1 has a full width at half-maximum (fwhm) of 0.51 ( 0.12 nm-1 centered at kxy ) 15.48 ( 0.04 nm-1 and can be indexed to a hexagonal lattice. If this peak is assigned to reflection {10}, the second-order diffraction peak corresponding to {11h } should appear at x3kxy or 27 nm-1. Further scans toward higher kxy did not detect any other peaks; therefore no higher reflection orders could be assigned. A quick scan along the kz direction did not reveal any other peak aside from the primary reflection referred to earlier. If the observed diffraction peak is assigned as the triply degenerate {10} + {01} + {11h } peak for hexagonal packing, the Bragg d-spacing can be calculated using d ) 2π/kxy and the lattice spacing from a ) (2/x3)d, yielding d ) 4.06 ( 0.02 and a ) 4.69 ( 0.03 Å. These values are characteristic for close-packing of alkyl chains,33 though deviating slightly from the reported value of 4.76 ( 0.02.32 Assuming exponential decay of positional correlation with increasing separation, that is, e-r/ξ, the positional correlation length ξ can be estimated from

ξ)

2 fwhm(kxy)

(1)

which yields 4 nm for the uncorrected fwhm. In comparison, small amphiphiles such as fatty acids are known to have correlation lengths ranging from 10 nm33-35 to 500 nm, in the case of phospholipids.36 While the monolayer reaction conditions differ somewhat from similar data reported in the literature, the ordering can be interpreted in a manner consistent with previous discussions, as illustrated in Figure 1. Comparison with Bulk Silanes. Wide-angle X-ray powder diffraction data of reacted and dried silanes are shown in Figure 2. Table 1 summarizes the data obtained via Lorentzian fits to these peaks. These silanes all have the same number of methylene groups in the alkyl chain with the exception of compound B which contains a (32) Fontaine, P.; Goldmann, M.; Rondelez, F. Langmuir 1999, 15, 1348. (33) Tippmann-Krayer, P.; Kenn, R. M.; Mohwald, H. Thin Solid Films 1992, 210, 577. (34) Lin, B.; Shih, M. C.; Bohanon, T. M.; Ice, G. E.; Dutta, P. Phys. Rev. Lett. 1990, 65, 191. (35) Kenn, R. M.; Bohm, C.; Bibo, A. M.; Peterson, I. R.; Mohwald, H.; Als-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991, 95, 2092. (36) Seul, M.; Eisenberger, P.; McConnell, H. M. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 5795.

quaternary amine and an extra propyl moiety in the alkyl chain. Compounds A, C, and D differ in the number of methyl substitution in the silane headgroups which have three, two, and one methyl groups, respectively. While compound C is diethoxy substituted, this difference should not influence the ultimate packing since the ethoxy group is eliminated during hydrolysis. Bulk OTMS shows a strong peak at 15.05 nm-1 and minor peaks at 3.63 and 2.33 nm-1. These kxy values correspond to d spacings of 4.17, 17.30, and 26.97 Å, respectively. The first agrees with chain-chain packing measured from GIXD of the OTMS monolayer in this study and that reported by Tippmann-Krayer.33 The other peaks close to the small-angle region indicate some tendency of this compound to self-assemble, forming lamellar structures in the bulk. The d ) 26.97 Å peak matches the expected length of 26-27 Å obtained from molecular models of a fully extended OTMS molecule. The d-spacing of 17.30 Å is difficult to assign in the absence of other supporting data. Parikh,37 however, reported that a headto-head bilayer structure of OTMS yielded a primary reflection with d ) 52.4 Å; the d-spacing corresponding to the third-order or (003) peak should appear at approximately the same location of the second peak in Figure 2A. The only other silane that exhibited lamellar structure is compound B. A strong peak was observed at 1.74 nm-1, and a minor peak at 3.53 nm-1. The resulting d-spacing suggests that the second peak is a second-order reflection of the peak at 1.74 nm-1. This strong manifestation of a lamellar structure is most likely due to the strong hydrophilic character of the quaternary amine. Compound C showed no evidence of lamellar structure. In general, all silanes investigated showed wide-angle diffraction consistent with typical alkane chain closepacking, with d-spacings around 4.14-4.17 Å. Even compound C, which has a methyl substitution at the headgroup, exhibited a diffraction peak consistent with chain-chain packing close to that of OTMS. Apparently, the presence of the methyl group at the silane headgroup does not significantly distort the crystal lattice. The presence of two methyl groups in compound D, however, disrupts the hexagonal lattice observed in the other silanes, breaking the degeneracy of the first-order peak and yielding the three distinct peaks in the region around 15 nm-1. Compression-Induced Ordering of the Monomer Monolayer. In situ diffraction studies were performed (37) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135.

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Figure 2. X-ray diffractograms of selected alkoxysilanes. The chemical structure of each is also depicted and noted in the text as compounds A, B, C, and D.

on a monolayer as its surface pressure was progressively increased. By investigating OTMS on a subphase of unbuffered water (pH ∼ 5.5) at 25 °C, we hoped to be able to probe the ordering in an unreacted monomer monolayer. Figure 3 depicts the monomer isotherm (Figure 3a) and a series of diffraction scans (Figure 3b) obtained as the monolayer was compressed in a stepwise manner. Mean molecular areas (MMAs) ranging from 80 to 40 Å2, a region corresponding to an isotropic liquid phase, gave rise to diffraction scans with no detectable structure. The first evidence of an emerging structure was detected at Π ) 16 mN m-1 or a MMA of about 30 Å2. Upon further compression, the peak grows steadily (see Figure 4). The increase in scattering intensity with rising surface pressure is usually attributed to an increased population of alkyl chains in the trans conformation. The population of more ordered chains can then presumably pack within ordered regions. Conformational transformations of this type have been suggested in previous GIXD studies38 and confirmed by reflection-absorption Fourier transform infrared spectroscopy experiments with pentadecanoic acid.39 In the latter study, analysis of C-H stretching vibrations demonstrated a disordered arrangement within the monolayer, where the liquid expanded (LE) region contained a large population of gauche conforma(38) Barton, S. W.; Thomas, B. N.; Flom, E. B.; Rice, S. A.; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Chem. Phys. 1988, 89, 2257. (39) Sinnamon, B. F.; Dluhy, R. A.; Barnes, G. T. Colloids Surf., A 1999, 146, 49.

Figure 3. (a) Π-A isotherm of OTMS monomer on a pH 5.5 subphase at 25 °C. (b) Diffraction spectra obtained at various pressures along the isotherm in panel a. Featureless diffraction curves for Π < 10 mN m-1 are not shown.

tions, while the condensed phases (C and S) consisted of mostly trans chains. A similar experiment using polarization modulation external infrared reflection-absorption spectroscopy40 also showed a gauche-trans transformation as the monolayer underwent a liquid-solid phase transformation when cooled.

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Figure 4. Intensities of the diffraction peak at about kxy ≈ 15 nm-1 vs surface pressure. Figure 6. Grazing incidence diffraction spectra of OTMS at Π ) 21 mN m-1. The inset is an expanded view of the 23-29 nm-1 region.

Figure 7. Space-filling molecular model of an OTMS molecule showing the relative cross sections of the tail and headgroup.

Figure 5. Peak center of the first-order peak (top) and the corresponding peak widths (bottom) as determined from the Lorentzian fit. The right axes of the two graphs are the calculated Bragg d-spacing (top) and the positional correlation length (bottom).

Figure 5 depicts the relation between surface pressure and fwhm and peak location, three parameters which are relevant to molecular organization and packing. At the lowest pressure, where the first diffraction peak was observed, the molecules have a larger d-spacing and lower positional correlation. As the pressure is increased, the d-spacing is reduced and the positional correlation increases. This implies that the molecules initially form loose, organized domains that grow or coalesce as the pressure is increased. Once the condensed phase has formed, the size of the organized domains approaches that of the instrumental resolution and further compression does not alter the peak width. As seen in Figure 6, at Π ) 21 mN m-1, a scan to higher kxy revealed a broad peak at 26 nm-1. This contrasts the polymerized OTMS on a pH 1.5 subphase where no peak was observed in this region. The second peak is close to the expected {11} peak for a structure with a first-order {10} peak at kxy ) 15.24 nm-1. At this pH value, the

monolayer likely consists of a small amount of partially hydrolyzed (silanol-containing) material and predominantly consists of alkyltrialkoxysilanes (see Figure 7). We expect that the alkyl tailgroups must reorient and possibly tilt the lattice to accommodate the larger cross-sectional area occupied by the headgroup. Experimental evidence to prove this point is not available, but similar behavior has been observed in fatty acids containing chain branching at the 2-position.41 Overall, the data show that a monolayer of unreacted OTMS monomer can be organized into sufficiently ordered regions to give rise to X-ray diffraction. The onset of such ordering appears at a pressure of 16 mN m-1, corresponding to a MMA of approximately 30 Å2. An additional wideangle diffraction peak, indicating packing order, appears at higher applied surface pressures. In addition, we observed a mixture of partially hydrolyzed molecules, and the bulky methoxy groups of unhydrolyzed material may limit the crystalline domain size, but they do not impede crystallization of the alkyl chains. Interestingly, no evidence of tilted phases, commonly observed in many other low molar mass amphiphiles with large headgroups, has yet been observed. Insights on the Kinetics of Self-Organization and Film Formation Real-Time Grazing Incidence X-ray Diffraction. Initial experiments were done at a constant pressure of Π ) 8 mN m-1, T ) 27 °C, and subphase pH ) 4.0, where the reaction is sufficiently slow to be followed by conventional diffraction scans at the MRCAT undulator beamline. Unlike the GIXD experiments described earlier, data collection commenced immediately after the monolayer was spread. The first 100 min is characterized by (40) Alonso, C.; Blaudez, D.; Desbat, B.; Artzner, F.; Berge, B.; Renault, A. Chem. Phys. Lett. 1998, 284, 446. (41) Brezesinski, G.; Dietrich, A.; Struth, B.; Bohm, C.; Bouwman, W. G.; Kjaer, K.; Mohwald, H. Chem. Phys. Lipids 1995, 79, 145.

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Figure 8. Real-time X-ray diffraction peaks of the reacting OTMS system (from data published elsewhere (ref 30)). The inset cartoon depicts placement of the three reactive groups looking down the alkyl chain of an OTMS molecule and possible bonds formed from 2D hexagonal packing of several molecules.

the absence of X-ray scattering peaks and no molecular ordering. Afterward, a symmetrical peak at kxy ) 15.24 nm-1 indicates the onset of ordering, becoming resolvable at an average macroscopic molecular area of 32.6 ( 2.3 Å2. The peak intensity increases as the reaction proceeds. The chemically interesting feature of the data is that the position of the peak does not change substantially with time, indicating that the spacing between the alkyl chains in the organized regions is constant. It is reasonable to assume that in the beginning of the reaction, small domains of ordered alkyl chains form, giving rise to a weak scattering peak. Also, the onset of a detectable diffraction peak occurs at surface areas significantly larger than the MMA attributed to the crystalline alkyl chain as reported by Fontaine et al.32 As these areas are large and correspond to only about 50% conversion, the alkyl chains might be expected to be unordered and liquidlike. Initial measurements published elsewhere30 have illustrated that diffraction occurs prior to the gel point of the reacting system. The appearance of diffraction so early in the reaction, corresponding to such a large overall MMA, supports the notion of the formation of organized domains consisting of partially condensed silane molecules dispersed in primarily disordered and partially reacted OTMS. It is useful to consider the chemical consequences of the local positional order, which is schematically shown in the inset to Figure 8. First, it is likely that the diffraction signal arises mainly from packing of the octadecyl alkyl chains, and from these data the exact contribution from any positional order of the silicon atoms is unclear. Eventually, anomalous diffraction studies at the Si absorption edge might address this question. Second, when one looks down the axis of the alkyl chains, as depicted in Figure 8, one conclusion becomes clear. Each alkyl chain has six nearest neighbors and is covalently attached to one silicon atom. Furthermore, each silicon atom has sp3 geometry and three reactive methoxy groups. Therefore,

if a 2D monolayer structure is retained, a maximum of 50% of the reactive groups can possibly condense with each other to form Si-O-Si bonds and the remaining 50% or more of these reactive groups may remain unreacted. Of course, these remaining reactive groups are available for reaction with surfaces and/or molecular substrates in self-assembly or templating reactions. The unreacted bonds will surely act to frustrate the long-range positional order in the floating monolayer however. Such results clearly demonstrate the value of obtaining direct structural data by GIXD to elucidate the mechanism of film formation on a molecular level. Clearly, the appearance of a diffraction peak indicates that OTMS undergoes a phase transition as the methoxy groups are hydrolyzed and the headgroups become smaller. At a MMA of 32.6 Å2, the monolayer exists in a state analogous to the condensed phase in unreacted OTMS, labeled C in Figure 3b. The initial appearance of order occurred at a similar MMA as when the unreacted monolayer was being compressed. The diffraction peak close to the onset of the condensed phase is at a position indicative of hexatic order.42 Unlike truly crystalline structures, positional correlation in a hexatic structure exists over a few lattice units43 but is then lost at larger distances (although long-range orientational order normal to the interface remains throughout the entire sample). This difference in molecular packing is depicted in Figure 9.44 For the hexatic order to arise, the molecular area must be larger than that of close-packed alkyl chains to allow for the larger degrees of freedom inherent in these structures. An indication of this possibility is shown in (42) Kaganer, V. M.; Mohwald, H.; Dutta, P. Rev. Mod. Phys. 1999, 71, 779. (43) Kaganer, V. M.; Brezesinski, G.; Mohwald, H.; Howes, P. B.; Kjaer, K. Phys. Rev. E 1999, 59, 2141. (44) Kaganer, V. M.; Loginov, E. B. Phys. Rev. E 1995, 51, 2237.

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Figure 9. Molecular packing in (a) a crystalline lattice and (b) a hexatic lattice as seen normal to the interface. The shaded ovals and circles indicate the average molecular orientation in the domain. Adapted from Kaganer and Loginov (ref 44). Figure 11. Intensity of the peak around kxy ) 15 nm-1 plotted against reaction time at subphase pH ) 1.5. The superimposed right axis corresponds to the area relaxation curve of OTMS spread on the same subphase.

Figure 10. Variation in the peak center (left axis) and molecular area calculated from the lattice parameter a (right axis) with reaction time.

Figure 10, which illustrates that the molecular area is larger at the onset of diffraction and shrinks by about 2% after 300 min. This is comparable to the decrease in molecular area observed by Kaganer in octadecanol monolayer as it undergoes LS f S transition.43 Although there have been doubts regarding the assignment of certain phases as possessing hexatic order,45 some indirect evidence has been reported in the literature. Rivie`re46 has shown that this phase is optically isotropic and possesses one first-order diffraction peak in the powder pattern. The structure could thus be a 2D liquid system analogous to a smectic liquid crystal, a hexagonal crystal, or one that is hexatic. The fact that the usual crystal peak width would be narrower than what was observed renders hexatic order to be the most plausible structure in this phase.42 At this point, results have indicated that both unreacted OTMS and its product exhibit ordering at the air-water interface. While the unreacted OTMS shows evidence of a distorted hexagonal crystal lattice, its reaction products exhibit packing characteristic of hexatically packed alkyl chains. In the following section, it will be shown that the molecular packing at the onset of the diffraction is less dense than that of the final structure, further supporting the notion of hexatic order in at least the early part of film formation. Rapid Grazing Incidence X-ray Diffraction. The experiments described in the previous section were performed on a slowly reacting system. As the pH is lowered, the rate of hydrolysis and condensation reactions (45) Sirota, E. B. Langmuir 1997, 13, 3849. (46) Rivie`re, S.; Henon, S.; Meunier, J.; Schwartz, D. K.; Tsao, M. W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 10045.

can change drastically such that at pH 1.5 the reaction is complete after 1 h.32 To be able to monitor the transformation in these systems, a modification of the scanning technique was made. In the absence of a wide-area detector such as a CCD or an image plate, these experiments were limited to the use of a scintillation counter point detector. The conventional method of scanning with a point detector at a predetermined angular step is painfully time-consuming. In addition to the time required to move the detector arm and acquire the data, extra time is needed to allow the water surface to settle down after the detector arm has finished moving to the desired angle. This additional wait period can account for 5-10 s per step and slow the complete scan by as much as 5 min. The alternative approach is to sweep the detector arm continuously (“quickscan”) at a predetermined scan rate. The implementation of the quickscan scheme involves an alteration in the input/output controller (IOC) routines during data acquisition. In the conventional step-scan technique, the control software sets up the scaler (Joerger) hardware so that the IOC stops and resets the data acquisition after a certain number of counts are detected. In the quickscan scheme, the 10 MHz internal clock of the scaler is used as the time base that defines an acquisition gate. The IOC then retrieves the data for the duration of the preset acquisition time. This process decreases the detected intensity by a factor of 10-100. However, this is inconsequential since the incident beam has sufficient intensity required for the experiment. Using quickscan routines, the reaction of OTMS monolayers at pH 1.5, 2.0, 3.0, and 3.5 was investigated. The fastest scan rate that was attained using this new scanning scheme was approximately 56 s, a considerable improvement compared to the best scan rate of 17 min for the conventional step scanning. Figure 11 depicts a typical plot of peak intensity versus time acquired during approximately 1 h from a reacting OTMS system at pH 1.5. Similar to the observation concerning OTMS reacted at pH 4, the peak intensity at pH 1.5 increases as the reaction time progresses and reaches an asymptotic value. In this case, however, the ordering occurs very rapidly as indicated by the early onset of diffraction at 100 s. Also shown in Figure 11 is the result of an independent area relaxation experiment. Since the reaction is very rapid at pH 1.5, the MMA decreases before the desired surface pressure is attained. In this case, the initial value that

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Figure 12. Intensity of the peak around kxy ) 15 nm-1 plotted against reaction time at subphase pH ) 3.5. The superimposed right axis corresponds to the area relaxation curve of OTMS spread on the same subphase.

was measured is already around 25 Å2. The remarkable observation apparent here is that there seems to be a complete correspondence between the two curves, even though they characterize different length scales. While the intensity data represent a microscopic description of the system, the mean molecular area curve is a representation of a gross macroscopic trend. In contrast to the low-pH data, at higher pH the two datasets do not coincide. As shown in Figure 12, the diffraction peak does not appear until after 30 min into the reaction. At this point, the monolayer has a MMA of ∼30 Å2, a value very close to the onset of diffraction observed at pH 4.0.30 At t ) 120 min, the diffraction peak intensity started to plateau, while the MMA curve has not yet reached its asymptote of 21 Å2. This observation indicates that the hydrolysis occurs in random sites and that a significant amount of hydrolysis needs to occur before the molecules can self-assemble into ordered domains. The growth of the peak and its fwhm with respect to time can be seen in Figure 13. These data also mirror the results obtained from slower kinetics at pH 4.0. The only difference is that the initial peak center was detected at kxy ) 14.6 nm-1 which is equivalent to a molecular area that is larger than previously measured. The decrease in the correlation length has been suggested by Fontaine to be due to the disruption of the lattice brought about by the polymerization reaction. For any set of scattering atoms, the intensity of diffraction is proportional to the square of the structure factor A(K) which can be expressed as

A(K) )

∑j

fj(K) exp(iK‚rj)

(2)

where K is the scattering vector equal to the difference of the incident wave vector kin and the scattered wave vector kout, while fj(K) represents the scattering factor of the jth atom located at rj.43 Depending on the crystal system, the structure factor can be a simple sum of fj or zero, the latter occurring in certain combinations of hkl indices where the selection rule does not apply.47 When the structure factor is nonzero, the increase in the diffraction intensity is proportional to the density of scattering elements. For this reacting monolayer system, the increasing intensity would therefore result from either (47) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley: Reading, MA, 1978.

Figure 13. Variation in the peak centers (top) and calculated fwhm (bottom) for the reacting OTMS on a pH 1.5 subphase. The superimposed right axis corresponds to the molecular area of the ordered phase and the correlation length, respectively, calculated from the data.

an increase in the number of ordered domain nuclei or the continuous growth of existing ones. In the former case, the correlation length of the domains would remain the same. In the latter case, the correlation length would eventually increase. Based on the data presented in Figure 13, our study suggests the former to be true. Although an increase in the correlation length was observed, the change is very insignificant as to consider domain growth to have a strong contribution in the increase of peak intensity. It is therefore implied that the increase in peak intensity by the reacting OTMS must be due to the growing number of ordered domains. The ultimate size of these domains is most likely limited by topological limitations since not all of the reactive silane groups can form Si-O-Si bonds. Conclusion The studies discussed here have shown that highbrilliance synchrotron radiation can be used to acquire sufficiently fast in-plane diffraction scans to follow a surface reaction in situ. The alkyl chains in OTMS spread on a clean water subphase self-assemble into small domains demonstrating a characteristic in-plane structure. Such a mechanism appears possible at fluid interfaces and thus in the absence of any anchoring to a solid substrate. The short-range positional order between the alkyl chains does not significantly change during the course of the reaction. The increase in intensity of the diffraction peaks with time can be most likely interpreted as an increase in the number of such ordered domains as the polymerization progresses. Overall, these kinetic experiments have illustrated the potential of fast GIXD scans to study the surface dynamics of reactive amphiphile systems. The experimental setup described for these initial measurements is not yet

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optimized for highest brilliance and overall experimental speed. Further optical and detection enhancements such as the use of an area detector being currently planned should easily allow the scan time required for similar quality diffraction data to be reduced significantly. Acknowledgment. Financial support was provided by DOE-BES (Grant Number DE-FG02-96ER45589). Additional funding was provided by the Engineering Research Center (ERC) for Particle Science and Technol-

Carino et al.

ogy at the University of Florida and the National Science Foundation (NSF) (Grant Numbers EEC-94-02989 and NSF-CPE 80005851). The NSF Graduate Fellowship Program is acknowledged for J. Logan. The UF Gibson Dissertation Fellowship is also acknowledged for S. R. Carino. The authors thank Dr. Jeremy Kropf of ANL for technical assistance and discussions useful to generating the data for this article. LA034444V