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Two-Dimensional Polymerization in Langmuir Films: A PM-IRRAS Study of Octadecyltrimethoxysilane Monolayers D. Blaudez,*,† M. Bonnier,§ B. Desbat,‡ and F. Rondelez§ #Centre
de Physique Mole´ culaire Optique et Hertzienne, UMR 5798 du CNRS, Universite´ Bordeaux 1, 33405 Talence, France, Laboratoire de Physico-Chimie Mole´ culaire, UMR 5803 du CNRS, Universite´ Bordeaux 1, 33405 Talence, France, and Laboratoire de Physico Chimie Curie, UMR 168 du CNRS, Institut Curie, Section de Recherche, 26 rue d’Ulm, 75248 Paris Cedex 05, France Received March 21, 2002. In Final Form: October 1, 2002 We provide a detailed in situ investigation of the two-dimensional polymerization of n-octadecyltrimethoxysilane Langmuir monolayers by PM-IRRAS (polarization-modulated infrared reflection absorption spectroscopy). The formation of the siloxane bonds at the headgroup level is unequivocally assessed by the appearance of a characteristic Si-O-Si band in the 1000-1150 cm-1 region. The aliphatic chains are shown to tilt toward the normal to the interface during polymerization. The process is sterically controlled and can be totally suppressed by replacing the hydrocarbon chain by its fluorocarbon homologous. The chain cross section is then too large to allow the reticulation of the silanol groups between neighboring molecules.
I. Introduction Langmuir films spread at the air-water interface are ideally suited to study the polymerization processes of organic molecules in low dimensionalities. When adsorbed on surfaces or interfaces, the molecules are naturally aligned due to their amphiphilic character, with their reactive sites in register. Consequently the polymerization mechanism is faster than in solutions or in bulk materials. In addition the molecular density can be varied over a wide range by a simple mechanical compression of the monolayer. Two popular Langmuir films are the polydiacetylenes and the n-alkyltrimethoxysilanes that polymerize under ultraviolet irradiation1 and acidic or basic conditions,2-4 respectively. A similar process is also encountered in selfassembled monolayers of n-alkyltrichlorosilanes on oxidized silicon wafers. Albeit these substrates are solid, the process is controlled by the physisorbed liquid water layer present on this hydrophilic surface.5,6 Experimentally the polymerization is detected through an irreversible change in one of the monolayer macroscopic properties, e.g., its compressibility,7,8 viscoelasticity,9,10 * To whom correspondence should be addressed. Telephone 335 56 84 89 97. Fax 33- 5 56 84 69 70. E-mail: blaudez@ cpmoh.u-bordeaux.fr. † Centre de Physique Mole ´ culaire Optique et Hertzienne. ‡ Laboratoire de Physico-Chimie Mole ´ culaire. § Laboratoire de Physico Chimie Curie. (1) Go¨bel, H. D.; Gaub, H. E.; Mo¨hwald, H. Chem. Phys. Lett. 1987, 138, 441-446. (2) Linden, M.; Slotte, J. P.; Rosenholm, J. B. Langmuir 1996, 12, 4449-4454. (3) 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, 767-768. (4) Fontaine, P.; Goldmann, M.; Rondelez, F. Langmuir 1999, 15, 1348-1352. (5) Parikh, A. N.; Allara, D. L.; Ben Azouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577-7590. (6) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367-4373. (7) Peltonen, J. P. K.; He, P.; Linden, M.; Rosenholm, J. B. J. Phys. Chem. 1994, 98, 12403-12409. (8) Huilin, Z.; Weixing, L.; Shufang, Y.; Pingsheng, H. Langmuir 2000, 61, 2797-2801.
or phase diagram.7,11 The onset of a network structure also modifies the average intermolecular spacing and the degree of positional order of the amphiphilic molecules.3-6 However none of the many different techniques used so far can probe the nature of the chemical bonds and a fortiori the chemical changes induced in the film by the polymerization. In this paper we investigate the two-dimensional reticulation of n-octadecyltrimethoxysilane Langmuir monolayers by in situ polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The whole mid-infrared spectral range is monitored and direct molecular information is obtained. The disappearance of the alkoxy groups and their replacement by hydroxyl groups is clearly detected, together with the subsequent formation of polysiloxane bonds. The influence of the chain cross section relative to the headgroup size is demonstrated by comparing the results between hydrocarbon and fluorocarbon chains. The main difference between PM-IRRAS and infrared reflection absorption spectroscopy (IRRAS) developed by Dluhy and co-workers12 is the periodic modulation of the polarization of the incident electromagnetic field. It allows for the compensation of the absorption of the infrared beam in the 1200-1800 cm-1 range by the water vapor above the monolayer. In addition it allows for the accurate probing of the orientation of the transition dipole moments in favorable cases.13,14 This is due to a specific surface selection rule according to which in-plane (out of plane) transition moments M B result in positive (negative) absorption bands and of the largest possible intensity. For arbitrary orientations of M B , the positive and negative (9) Barton, S. W.; Goudot, A.; Rondelez, F. Langmuir 1991, 1, 10291030. (10) Miyano, K.; Veyssie´, M. Phys. Rev. Lett. 1984, 52, 1318-1320. (11) Vidon, S.; Leblanc, R. M. J. Phys. Chem. B 1998, 102, 12791286. (12) Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. Soc. 1988, 110, 712718 and references therein. (13) Blaudez, D.; Buffeteau, T.; Cornut, J.-C.; Desbat, B.; Escafre, N.; Pe´zolet, M.; Turlet, J.-M. Appl. Spectrosc. 1993, 47, 869-874. (14) Blaudez, D.; Turlet, J.-M.; Dufourcq, J.; Bard, D.; Buffeteau, T.; Desbat, B. J. Chem. Soc., Faraday Trans. 1996, 92, 525-530.
10.1021/la0257642 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2002
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Figure 1. Pressure-area isotherm of OTMS recorded on a water sub-phase at pH ) 5.5 (curve A) and pH ) 2 (curve B).
contributions add up algebraically. There is almost exact cancellation for a 39° tilt relative to the surface normal, and the sensitivity to the orientation is then maximal. II. Materials and Methods II.1. Langmuir Monolayers. Octadecyltrimethoxysilane (OTMS) and 1,1,2,2-tetrahydro perfluorodecyltriethoxysilane (PDTS) were obtained commercially from Aldrich. These compounds are sensitive to hydrolysis and were kept away from water traces. The purity was shown by 1H NMR to be higher than 98% after distillation under reduced pressure (3 10-2 Torr). The samples were separated in small aliquots and kept in sealed ampules under vacuum until just before use. Small droplets of dilute (10-4 M) chloroform solutions were spread with an Hamilton syringe at the free surface of a trough filled with ultrapure water (ELGA Prima-Maxima or Millipore, resistivity > 18 MΩ‚ cm) at pH ) 5.5. The Langmuir monolayers formed spontaneously following the evaporation of the solvent. The troughs used for the surface pressure isotherms and the PM-IRRAS experiments were made of Teflon and had similar dimensions of 30 × 15 × 1 cm. The monolayer surface density was adjusted by sweeping a computer-controlled impermeable Teflon barrier. The maximum barrier speed was 2 Å2/molecule.min. The surface pressures were measured by the Wilhelmy hanging plate method, using a filter paper plate attached to a sensitive force transducer. The accuracy was 1 mN/m. To initiate the polymerization process, the pH of the subphase was lowered from 5.5 to 2 by addition of hydrochloric acid (Prolabo, Normapur quality). Care was taken to keep constant the surface pressure: a slight compression of the monolayer was generally required, which indicates spontaneous compaction of the monolayer during polymerization. II.2. Infrared Spectroscopy. The PM-IRRAS spectra were recorded on a Nicolet 740 FT-IR spectrometer with a spectral resolution of 8 cm-1. The details of the optical setup, the experimental procedure and the two-channel processing of the detected intensity have already been described.13 To ensure good signal-to-noise ratio, 200 interferograms were coadded. The spectra are then normalized to the spectrum of the aqueous subphase and multiplied by the total area occupied by the monolayer. As a result, the band intensities do not depend on the number of molecules probed by the infrared beam.
III. Results and Discussion III.1. Surface Pressure Isotherm of n-Octadecyltrimethoxysilane (OTMS). The surface pressure isotherm for OTMS monolayers at pH ) 5.5 and T ) 20 °C is shown in Figure 1, curve A. Three distinct regions are observable as the monolayer is compressed and the mean molecular areas decrease from 80 to 20 Å2. Above 50 Å2, the monolayer is in the liquid-expanded (LE) phase and the surface pressure increases continuously as the monolayer is compressed. Between 50 and 26 Å2, the monolayer is in a two-phase regime (liquid-condensed/liquid-ex-
Figure 2. Experimental PM-IRRAS spectrum of an OTMS monolayer spread on a pure water sub-phase at pH ) 5.5 and compressed at four different surface pressures (5, 11, 14, and 20 mN/m), and calculated PM-IRRAS spectrum of an OTMS monolayer spread at the water surface (dotted line).
panded) and the surface pressure levels off to a plateau value of 11 mN/m. Below 26 Å2, the monolayer enters a new, liquid-condensed, phase which is much less compressible than the liquid-expanded phase: this explains the sharp rise in surface pressure. The noticeable kink at 23 Å2 and 18 mN/m indicates a transition to a second solidlike phase. The monolayer is mechanically stable until a pressure of 28 mN/m. As long as this pressure is not exceeded, the isotherm is fully reversible. The isotherm of OTMS at pH 2 is shown in Figure 1, curve B. It appears very different from the isotherm at pH 5.5 since (1) the surface pressure remains nonmeasurable down to a mean molecular area of 25 Å2, (2) the surface pressure at maximum compression is larger, of the order of 50 mN/m at 22 Å2, and (3) the isotherm is not reversible if the monolayer is maintained at full compression during 30 min. This is an indication that polymerization has occurred during the time course of the measurement. For this reason, the reported isotherm should not be taken as very accurate since it is certainly affected by the polymerization occurring during compression. III.2. Infrared Spectra of OTMS Monolayers at Different Surface Pressures. III.2.1. 3000-2800 cm-1 Spectral Region. The PM-IRRAS spectrum of an OTMS monolayer at pH ) 5.5 and four different surface pressures (5, 11, 14, and 20 mN/m) is presented in Figure 2. This 3000-2800 cm-1 spectral region covers the symmetric (νsCH2) and antisymmetric (νaCH2) stretching bands of the methylene groups. One observes an abrupt increase of the band intensities when the surface pressure is 14 mN/m or higher. Since the corresponding transition moments are perpendicular to the alkyl chain, this intensity increase indicates a tilt of the aliphatic chains toward the normal to the air-water interface as the surface pressure exceeds 11 mN/m. This is just above the pressure for the liquid-condensed and the liquid-expanded coexistence region: the reorientation of the long chain axis occurs during the mechanical compression in the homogeneous liquid-condensed region. The increase in intensity of the CH2 bands between 11 and 14 mN/m is accompanied by a shift of the maximum peak position from 2922.5 to 2919 cm-1 and from 2853 to 2851 cm-1 for the νaCH2 and νsCH2 bands, respectively.
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There is also a concomitant decrease of the full width at half-maximum (fwhm) from 22 to 18.5 cm-1 and from 19 to 12.5 cm-1 for νaCH2 and νsCH2, respectively. Such a behavior is well-known in the ordering process of alkyl chains and indicates that the reorientation of OTMS chains is accompanied by a decrease of the number of gauche conformational defects.15,16 At surface pressures higher than 14 mN/m and up to the maximum measured pressure of 20 mN/m, there is no further evolution in the infrared peak intensities. The only detectable changes in the spectra are in the fwhm. One observes a strong narrowing of the νaCH2 peak from 18.5 to 14.5 cm-1, whereas the fwhm of the νsCH2 peak is reduced from 12.5 to 10 cm-1. Such a decrease can be due to two different factors, either a decrease in the static broadening due to the local variations in the molecular packing around the oscillating dipoles or an increase in the number of molecules in the all-trans conformation. In our case, since the peak position stays unchanged, we believe that the decrease in the static broadening is the major contributing effect. The monolayer thus tends toward a two-dimensional crystalline phase at the highest surface pressures. The peak positions are similar to those obtained in a polyethylene crystal17,18 and the fwhm are on the upper side of those measured for organic crystals at room temperature. The dashed curve in Figure 2 displays the calculated PM-IRRAS spectrum for an OTMS monolayer of thickness 26 Å and with alkyl chains perpendicular to the surface. An homemade software was used19 and the optical constants were taken from ref 20 for poly-n-octadecylsiloxane crystallites formed by precipitation from a solution. The calculated spectrum is almost identical to the experimental one at π ) 20 mN/m. This confirms that the OTMS monolayer at this pressure is crystalline and that the alkyl chains are perpendicular to the interface. Interestingly, a simulation using alkyl chains tilted at 11° from the surface normal leads to peak intensities 30% lower than the ones experimentally observed. III.2.2. 1500-900 cm-1 Spectral Region. The data shown in Figure 3 have been measured at four different surface pressures (6, 7.2, 11, and 20 mN/m). Three distinct peaks at 1075, 1192, and 1468 cm-1 are observed in all cases. They have markedly different intensities and are assigned to the stretching mode (νSi-(O-CH3)3) of the trimethoxysilane headgroup, the symmetric bending mode (δsCH3) of the 4 terminal methyl, and the scissoring mode (δ CH2) of the methylene, respectively. Each one of these peaks provides important information on the conformation of the amphiphilic molecules in the various states of the monolayer. The strong increase in the 1468 cm-1 peak intensity between 7.2 and 11 mN/m is consistent with a tilt of the chains toward the normal in the liquid-condensed phase. In addition, the narrowness of the band confirms our previous conclusion that the monolayer gets crystalline at high surface pressures. Since the peak has a single component, an orthorhombic packing can be dismissed17,18 (15) Buontempo, J. T.; Rice, R. A. J. Chem. Phys. 1993, 99, 70307037. (16) Flach, C. R.; Brauner, J. W.; Mendelsohn, R. Biophys. J. 1993, 65, 1994-2001. (17) Tasumi, M.; Shimanouchi, T. J. Chem. Phys. 1965, 43, 12451258. (18) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A, 395-406. (19) Buffeteau, T.; Blaudez, D.; Pe´re´, E.; Desbat, B. J. Phys. Chem. B 1999, 103, 5020-5027. (20) 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 3. PM-IRRAS spectrum of an OTMS monolayer spread on a pure water sub-phase at pH ) 5.5 and compressed at different surface pressures (6, 7.2, 11, and 20 mN/m).
and the symmetry is most probably hexagonal. This finding also agrees with an earlier conclusion drawn from X-ray diffraction data.4 The broad band between 1000 and 1150 cm-1 yields the molecular state of the polar trimethoxysilane Si-(OCH3)3) headgroups that control the in-plane polymerization of the monolayer at low acidic pH, as will be shown later. This intense band can be decomposed in two components centered at 1089 and 1065 cm-1, respectively. Their relative contribution is a strong function of the monolayer surface pressure. The ratio between these two components changes from 0.8 at 7.2 mN/m to 2 at 11 mN/ m, leading to a switch in the shape of the band. The integrated peak intensity is also affected by the changes in surface pressure: it drops markedly between 6 and 7.2 mN/m, but increases again slightly at 11 and 20 mN/m. This indicates that reorientation of the polar headgroups occurs during the mechanical compression of the monolayer. However, the detailed analysis of this vibration band is quite complex because it involves several atomic groups coupled together. In this respect, the δsCH3 band at 1192 cm-1 is simpler to analyze because the three methyl groups contributing to this band are independent from each other.21 Its peak intensity decreases between 6 and 7.2 mN/m and then remains approximately constant at all higher surface pressures. On one hand, this indicates that the methoxy groups tilt toward the normal to the interface upon compression. On the other hand, since the peak never gets negative the tilt angle of the transition moment has to remain larger than 39° according to the PM-IRRAS selection rule. It is also interesting to note that the pressure at which this reorientation is observed by PM-IRRAS corresponds precisely to the location of the liquid-expanded to liquid-condensed transition region (see Figure 1). In this coexistence region the mean molecular area change from 50 to 30 Å2 as the monolayer is compressed. To accommodate for this decrease the bulky Si-(O-CH3)3 headgroups have to close down on themselves in an “umbrella mode”. The change from a spread-out to a folded structure is schematized in Figure 4. (21) We neglect here the contribution of the extra methyl group present at the alkyl chain end to the 1192 cm-1 peak. For vertical chains its transition moment is at 35° from the surface normal, which is close to the angle of 39° at which the PM-IRRAS signal vanishes. Another argument for neglecting this contribution is given by the absence of signal when the trimethoxysilane headgroups are converted into trihydroxysilane headgroups by hydrolysis (see curves b and c in Figure 5 of the next paragraph).
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Figure 4. Schematic representation of the spread (I) and folded (II) configuration of the trimethoxysilane polar headgroup of OTMS. Figure 6. PM-IRRAS spectrum of an OTMS monolayer spread on a water sub-phase at (a) pH ) 5.5, (b) pH ) 2 (30 min after addition of HCl), and (c) pH ) 2 (90 min after addition of HCl). The surface pressure is kept to 25 mN/m. The spectral simulation is displayed as a dotted line.
Figure 5. Experimental PM-IRRAS spectrum of an OTMS monolayer spread on a water subphase at (a) pH ) 5.5, (b) pH ) 2, and 30 min after addition of HCl and (c) pH ) 2, and 90 min after addition of HCl, and calculated PM-IRRAS spectrum of a polymerized OTMS monolayer spread at the water surface (dotted line).
III.3. Hydrolysis and Polymerization of OTMS Monolayers. The PM-IRRAS spectra in the 1500-850 cm-1 spectral domain are strongly dependent on the pH of the subphase. Figure 5a-c corresponds to spectra taken before, and 30 and 90 min after, injection of dilute HCl to lower the pH of the subphase from 5. 5 to 2.0. In all experiments, the surface pressure has been kept constant at 25 mN/m, by barrier control. When comparing curve b to curve a, one observes (1) the disappearance of the peak at 1192 cm-1, (2) the replacement of the two peaks at 1089 and 1192 cm-1 by a broad multicomponent band extending from 950 to 1150 cm-1, and (3) the formation of a broad band centered at 905 cm-1. These spectral changes can be interpreted as resulting from the hydrolysis of the trimethoxysilane Si(O-CH3)3) headgroups and their conversion in trihydroxysilanes Si-(OH)3), followed by the formation of intramolecular siloxane bonds and polymerization of the monolayer. The 905 cm-1 band corresponds to the Si-O stretching mode of Si-O-H groups (νSi-O-H)22 whereas the band extending from 950 to 1150 cm-1 corresponds to the Si-O-Si stretching mode (νSi-O-Si) of siloxane bonds.22 These processes are slow, which explains the differences between curve c, taken 90 min after injection, and curve b. The intensity of the (νSi-O-H) band has decreased slightly, whereas the one of the (νSi-O-Si) band has increased, and its shape has changed, with the growth of a peak at 1035 cm-1. These two variations are obviously coupled since the hydroxyl groups are consumed by the condensation reaction. Spectra recorded at still longer times show no further evolution of the monolayer after 90 min. From the intensity ratio of the νSi-O-H (22) Socrates, G., Eds. Infrared Characteristic Group Frequencies; John Wiley and Sons: New York, 1980; p 128.
and bands of spectrum c, one can estimate that 30% of the OH groups are still present. As suggested by Ulman,23 all the three hydroxyls are not available for condensation because of steric hindrances that limits the extent of intermolecular cross-linking. On average, each silane molecule is linked to two neighbors and forms a part of a linear polymeric chain. The presence of two separate components at 1095 and 1035 cm-1 in the νSi-O-Si band in curve c supports this interpretation. Such a band splitting is typical of polymer chains and corresponds to in-phase and out-of-phase coupled vibrations of the chain monomers.24,25 It has also been observed by Allara and co-workers20 on dry powders of polymerized n-octadecylsiloxane. Using their determination of the optical constants and assuming an isotropic orientation of the transition moments, we have simulated the PM-IRRAS spectrum of a polymerized OTMS monolayer at the water surface. The result is shown as the dotted line in Figure 5 and appears to be in good agreement with the experimental spectrum. The only difference is the much stronger intensity of the νSi-O-H in the experimental spectrum. This suggests that polymerization is not as complete when molecules are floating on water and are constrained to polymerize in a 2D configuration. The polymerization affects the intensity and width of the νsCH2 band at 2850 cm-1. Analysis of the spectrum measured after 90 min (not shown) reveals that the integrated intensity has decreased from 11 to 9.6 and that the fwhm has been reduced from 11 to 10 cm-1. Albeit small, these changes indicate a tilt in the orientation of the alkyl chains and a decrease in the local fluctuations in molecular packing. Infrared spectroscopy therefore shows that the polymerization increases the degree of order within the monolayer. It should be remarked at this point that IR vibrations are sensitive to the local, and not to the long-range, order. The correlation length for crystalline order in such Langmuir film has been measured by X-ray diffraction and found to be of the order of 10 molecular bonds.4 This is already a very long distance for the infrared technique. The PM-IRRAS spectra in the 1800-1500 cm-1 spectral domain for the three different characteristic molecular states of the monolayer are presented in Figure 6. The main feature is the peak at 1468 cm-1 that corresponds to the scissoring mode (δ CH2) of the methylene groups (23) Ulman, A. Adv. Mater. 1990, 2, 573. (24) Zbinden, R. Infrared Spectroscopy of High Polymers; Academic Press: New York, 1964; Chapter 3. (25) Desbat, B.; Huong, P. V. J. Chem. Phys. 1983, 78, 6377-6384.
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Figure 7. PM-IRRAS spectrum of a PDTS monolayer spread on a water/HCl sub-phase (pH ) 2) (a) 20 min after deposition (π ) 5.5 mN/m), (b) 45 min after deposition (π ) 10 mN/m), and (c) 75 min after deposition (π ) 30 mN/m).
in the aliphatic chains. No change can be detected between an OTMS monolayer at pH 5.5 (curve a) the same monolayer fully hydrolyzed at pH 2 after 30 min (curve b or the partially polymerized monolayer after 210 min (curve c). This was to be expected since the alkyl chains are not affected by the chemical changes. All three curves also show a broad dip centered at 1650 cm-1. This feature is an optical effect peculiar to PMIRRAS, and that originates from the difference in the optical response for covered and uncovered water surfaces.14 The effect is particularly pronounced in spectral regions where the aqueous subphase presents an adsorption band, and therefore a large dispersion in its refractive index. The scissoring mode (δ OH2) of liquid water is the cause for the dip at 1650 cm-1. Similarly, we have detected a dip at 3400 cm-1 (not shown here) due to the (νOH) stretching mode. We have performed a spectral simulation, taking the OTMS values of 26 Å and 1.5 for the monolayer thickness and refractive index, respectively. This is shown as a dotted line in Figure 6. Since the parameters used to calculate the spectrum does not vary strongly with the details of the headgroup chemical composition or the monolayer structure,26 the same simulated curve has been compared to the three experimental curves. The agreement is excellent with curves a or b but not with curve c. This difference is most probably due to the reorientation of the underlying water molecules in the case of the polymerized monolayer. The oxygen atoms of the polysiloxane network creates a hydrophilic surface onto which the water molecules adsorb with a preferred orientation. A similar ordering over several molecular thickness has been observed for water molecules physisorbed on glass slides or surrounding tubular micelles.27 Our results are consistent, according to the PM-IRRAS selection rule, with a rotation of the water dipole toward the normal to the interface. III.4. Influence of the Steric Hindrance of Alkyl Chains on Polymerization (PDTS). The PM-IRRAS spectrum of a 1H, 1H, 2H, 2H-perfluorodecyl triethoxysilane (ODTS for short) monolayer in the 1450-850 cm-1 spectral region is shown in Figure 7, as a function of the surface pressure. Contrary to the case of the OTMS monolayer all spectra have been run on an acidic water (26) Handbook of Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, 1999. (27) Desbat, B.; Oda, R. Langmuir, in press.
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subphase (pH ) 2) exclusively. The headgroups of the molecules are thus trihydroxylsilane rather than trimethoxysilane. It is actually difficult to study the monolayer in its non-hydrolyzed form, even at pH ) 5.5, because the headgroups are extremely reactive to water and prone to hydrolysis. This is the consequence of the strong electronegativity of the fluorine atoms. The antisymmetric (νaCF2) and symmetric (νsCF2) stretching modes of the fluoromethylene groups are very prominent in the 1100-1250 cm-1 spectral region. They are located at 1205 and 1150 cm-1, respectively. On one hand, one observes a significant increase in their peak intensities as the surface pressure increases from 5.5 to 10 and 30 mN/m. This indicates a tilt of the chains toward the normal to the interface as the surface density increases. On the other hand, the fwhm of the CF2 bands changes only slightly with compression (from 31.8 to 27 cm-1 and from 21 to 17.3 cm-1 for νaCF2 and νsCF2, respectively). This shows that the chain reorientation is not accompanied by a large increase of the molecular order, contrary to the case of the hydrogenated chains. It is also to note that the fwhm of these bands are close to the ones measured (∆ν ) 28 cm-1 for νaCF2 and ∆ν ) 15.7 cm-1 for νsCF2) in well-organized Teflon films.28 These two results can be explained by the natural rigidity of the fluorinated chains due to their helical structure.29 The molecular vibrations of the trihydroxysilane polar headgroups are located in the 1200-850 cm-1 spectral range. The single broad band observed at 920 cm-1 can be assigned to the νSi-O-H, just as in the previous case of OTMS. Its position is independent of surface pressure and does not change with time. On the other hand there is no evidence for extra peaks in the 1120-960 cm-1. In particular there is no evidence for an νSi-(O-CH3)3 twocomponent band at 1075 cm-1, or for a νSi-O-Si multicomponent band at 1095-1035 cm-1. This negative result allows to draw the important conclusion that the monolayer is fully hydrolyzed but that it has not led to in-plane polymerization. Contrary to what was happening with the hydrocarbon chains, condensation between neighboring fluorinated molecules has not taken place although hydrolysis is complete. Obviously the bulky fluorinated chains create a steric hindrance which prevent the polar heads to come close enough for the polymerization to take place. According to the surface pressure isotherms, the mean molecular area is 30 Å2 for the fluorocarbon chains at maximum surface density. This corresponds to an intermolecular distance of 5.8 Å,30 to be compared with 4.8 Å4 for hydrocarbon chains. An increase in the minimum approach distance of 1 Å thus appears sufficient to suppress the possibility of condensation between the neighboring hydroxyl groups. IV. Conclusion Our experiments show that the infrared vibration bands in the reacted and unreacted states during polymerization of n-octadecyltrimethoxysilane are clearly distinguishable from each other and are also reasonably intense so that monolayer sensitivity is achievable. At acidic pH, our observations in the 1200-1800 cm-1 frequency range give evidence for the total conversion of the trimethoxysilane into trihydroxylsilane headgroup, followed by the forma(28) Desbat, B.; Nguyen, H. T.; Brunet, M. Soft Material, The Journal 2002. In press. (29) Sperati, C. A.; Starkweather, H. W. Adv. Polym. Sci. 1961, 2, 465. (30) Acero, A. A.; Li, M.; Rice, S. A.; Goldmann, M.; Ben Azouz, I.; Goudot, A.; Rondelez, F. J. Chem. Phys. 1993, 99, 7214-7220.
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tion of siloxane bonds between neighboring molecules if the monolayer is close-packed. A detailed analysis of the band structure also reveals a tilt of the molecules toward the normal to the interface during polymerization. Crosslinking is extensive but not total: on average 70% of hydroxyls are involved in a siloxane bond. This is due to steric hindrance in the plane of the interface. We have shown that the polymerization can be totally suppressed in the case of fluorinated chains that have much larger molecular cross section than hydrocarbon chains.
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At neutral pH the monolayer neither hydrolyzes nor polymerizes. An interesting effect has nevertheless been detected by PM-IRRAS infrared spectroscopy. As the monolayer is compressed, the three methoxy of the bulky headgroup fold in an umbrella mode to minimize the steric constraints. As a result, the mean area per molecule in the most compact state before monolayer collapse can get as low as 22 Å2. LA0257642
