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Polymer-Organosilane Interactions Studied at the Air/Water Interface Mika Linde´n* and Jarl B. Rosenholm Deptartment of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FIN-20500 Turku, Finland Received March 29, 2000. In Final Form: June 7, 2000 Mixed monolayers of octadecyltrimethoxisilane, OTMS, and poly(ethylene oxide), PEO, have been studied at the air/water interface as a function of pH by means of surface pressure and surface potential isotherms. PEO had a strong influence on the isotherms at large monolayer areas, while it was squeezed out of the air/water interface at high surface pressures. The LE-LC transition pressure of unhydrolyzed OTMS was increased by several mN/m in the presence of PEO, while PEO had little influence on the isotherms of hydrolyzed OTMS at small monolayer areas. The results are discussed in terms of attractive van der Waals interactions between methoxy groups of nonhydrolyzed OTMS and PEO.

Introduction The large number of process constraints available on the molecular level with the Langmuir-Blodgett (LB) technique makes it an ideal tool for the investigation of the interactions between insoluble surfactant molecules at the air/water interface.1 Furthermore, several investigators have used this technique to investigate mixed monolayers of different lipids and polymers, both homoand block-co-polymers.2-4 Organized films of lipids with attached polymer backbones has been used in the development of more selective biosensors with the aim to decrease the defects of bilayers deposited directly onto solid surfaces by separating the bilayer from the surface by polymer brushes.5 Recently, the polymerization of different derivatives of alkylsilanes at the air/water interface with various possible condensation levels and different numbers of hydrocarbon chains were reported on.6-14 In its simplest scheme the polymerization reaction is regarded as a two-step reaction: (i) the hydrolysis of the polar silane headgroup and (ii) the subsequent condensation reaction leading to the formation of Si-OSi bonds between adjacent headgroups.15 Poly(ethylene oxide), PEO, has been shown to affect the gelation times of TEOS and have been used to vary the pore size of the final oxide in the sol-gel process.16 Furthermore, it has * To whom correspondence should be addressed. (1) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (2) Aston, M. S.; Herrington, T. M.; Tadros, Th. F. Colloid Polym Sci. 1995, 273, 444. (3) Casas, M.; Baszkin, A. Colloids Surf. 1992, 63, 301. (4) Charron, J.-R.; Tilton R. D. J. Phys. Chem. 1996, 100, 3179. (5) Baekmark, T. R.; Elender, G.; Lasic, D. D.; Sackmann, E. Langmuir 1995, 11, 3975. (6) Ariga, K.; Okahata, Y. J. Am. Chem. Soc. 1989, 111, 5618. (7) Okahata, Y.; Yokobori, M.; Ebara, Y.; Ebato, H.; Ariga, K. Langmuir 1990, 6, 1148. (8) Sjo¨blom, J.; Stakkestad, G.; Ebeltoft, H.; Friberg, S. E.; Claesson, P.; Langmuir 1995, 11, 2652. (9) Barton, S. W.; Goudot, A.; Rondelez, F. Langmuir 1991, 7, 1029. (10) Linde´n, M.; Slotte, J. P.; Rosenholm, J. B. Langmuir 1996, 12, 4449. (11) Vidon, S.; Leblanc, R. M. J. Phys. Chem. 1998, 102, 1279. (12) Kumar, V.; Krishnan, S.; Steiner, C.; Maldarelli, C.; Couzis, A. J. Phys. Chem. 1998, 102, 3152. (13) Britt, D. W.; Hlady, V. J. Phys. Chem. 1999, 103, 2749. (14) Britt, D. W.; Hlady, V. Langmuir 1999, 15, 1770. (15) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (16) Nakanishi, K.; Komura, H.; Takahashi, R.; Soga, N. Bull. Chem. Soc. Jpn. 1994, 67, 1327.

been suggested that PEO will adsorb onto surfaces of colloidal silica, especially at low pH, through hydrogen bonding.17 PEO is a linear polymer that has been extensively used in industrial as well as biotechnological applications.18 Unlike most other polymers PEO is completely soluble in both water and chloroform at room temperature.19 PEO also exhibits intrinsic surface activity and has been shown to form stable monolayers when spread at the air/water interface.20,21 Surface quasi-elastic light-scattering studies of adsorbed and spread films of PEO at the air/water interface has revealed that the structure of both films are identical.22 The equilibrium spreading pressure of PEO with mean molecular weights exceeding 105 has been determined to be about 9.5 mN/m at room temperature and to be insensitive to changes in the mean molecular weight of the polymer, while monolayers of lower Mw PEO will collapse at lower surface pressures.23 However, the monolayer may not be regarded as being purely two-dimensional since molecular models suggest that a close-packed PEO monolayer should occupy an area of 0.16 nm2/polymer segment,21 while areas between 0.09 and 0.11 nm2 have been found in practice by several authors using different experimental techniques.22-24 The aim of this study was to investigate the interactions between OTMS and PEO as a function of pH since the degree of hydrolysis/condensation of OTMS is strongly pH-dependent. Silicate-EO interactions are of interest also in the corresponding 3D systems since block copolymers with EO blocks have been used as structuredirecting agents in the synthesis of mesostructured oxides.25,26 Here, cooperative self-assembly processes lead to the condensation of metal precursors around polymer (17) Rubio, J.; Kichener, J. A. J. Colloid Interface Sci. 1976, 57, 132. (18) Albertson, P. A. Partition of Cell Particles and Macromolecules; John Wiley & Sons: New York, 1986. (19) Bailey, F. E.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976. (20) Glass, J. E. J. Phys. Chem. 1968, 72, 4459. (21) Shuler, R. L.; Zisman, W. A. J. Phys. Chem. 1970, 74, 1523. (22) Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 786. (23) Kuzmenka, D. J.; Granick, S. Macromolecules 1988, 21, 779. (24) Rennie, A. R.; Crawford, R. J.; Lee, E. M.; Thomas, R. K.; Crowley, T. L.; Roberts, S.; Qureshi, M. S.; Richards, R. W. Macromolecules 1989, 22, 3466. (25) Templin, M.; Franck, A.; Chesne, A. D.; Leist, H.; Zhang, Y.; Ulrich, R.; Scha¨dler, V.; Wiesner, U. Science 1997, 278, 1795. (26) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548.

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Figure 1. Compression and surface potential isotherms of OTMS monolayers at pH ) 2 (- - -), pH ) 5.6 (s), and pH ) 13 (- ‚ -), respectively.

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Figure 2. Compression and surface potential isotherms of PEO at pH ) 5.6. Mw ) 35 000.

aggregates. Although inorganic precursors have been used for the preparation of these materials, the synthesis is carried out in the presence of alcohol, which promotes the formation of intermediates containing alkoxy groups.27 Experimental Details Materials. Octadecyltrimethoxysilane (OTMS, 90% purity), n-hexane (99%), and chloroform used as the spreading solvent (1 mg/mL) were supplied by Fluka Chemicals. 29Si NMR was performed on the OTMS sample to ensure that no prehydrolysis/ condensation had occurred before spreading of the monolayer. Poly(ethylene oxide) (PEO) with Mn ≈ 35 000 was supplied by Merck. Dilute solutions of HCl and NaOH were used to adjust the pH. All the chemicals were used as supplied. The distilled water was purified with a Millipore Milli-Q filtering system with two carbon and two ion-exchange stages and finally a filtration through a 0.22-µm Zetapore filter, yielding a water resistivity >18 MΩ cm. The pH of the pure water was 5.7 determined by dissolved CO2. Monolayer Formation. The experimental setup used for measuring the compression and surface potential isotherms has been described previously.28 The compression was started 10 min after spreading, if not otherwise stated, at a constant barrier speed of 0.025 nm2/(molecule‚min).

Results Compression Isotherms. The shape of the surface pressure isotherms of OTMS is critically dependent on pH, as shown in Figure 1. At intermediate pH the compression isotherm shows a distinct transition from the liquid-expanded (LE) to the liquid-condensed (LC) state at a mean molecular area of about 0.44 nm2 and a surface pressure of π ) 9 mN/m. Upon further compression further transition to the condensed state (C) was observed at π ) 19 mN/m. A clear plateau in the compression isotherm was observed at the LC-C transition pressure, which is normally not seen for other film-forming compounds. The mean extrapolated area in the condensed state was about 0.20 nm2 at intermediate pH, corresponding to an arrangement of close-packed hydrocarbon chains. At pH 0.5-3.5 and pH ) 11.5-13.5, however, condensed monolayers with no sign of the LE state was observed. The extrapolated mean molecular area in the condensed state at these pH values increased to about 0.25 nm2. However, the mean molecular area concept does (27) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (28) Linde´n, D. J. M.; Peltonen, J. P. K.; Rosenholm, J. B. Langmuir 1994, 10, 1592.

Figure 3. Compression and surface potential isotherms of mixed monolayers of OTMS/PEO (no/ne ) 0.83) at pH ) 2 (- ‚ -), pH ) 5.6 (s), and pH ) 12.2 (- - -), respectively.

not have any real physical significance for these monolayers due to the aggregation of OTMS into large aggregates and eventually two-dimensional gels, as discussed elsewhere.10 Here, the compression isotherm gives a measure of the compressibility of the aggregates formed rather than that of the compressibility of a monolayer consisting of individual monomers. In the pH intervals 3.5-5 and 10.5-12 the compression isotherms were intermediate between the condensed- and the expanded-type isotherms, possessing some LE-state properties. If PEO was spread at the A/W interface, a continuous increase in the surface pressure was observed, in agreement with what might be expected for high-Mw polymers. The π-A isotherms for PEO are shown in Figure 2. The collapse pressure was about 9.5 mN/m at a surface concentration of about 0.12 nm2/repeat unit of the polymer. These results are in nice agreement with earlier reports in the literature. Changing the pH did not have any effect on the shape of the compression isotherm. Mixed monolayers of OTMS and PEO were spread at the A/W interface at different pHs. The molar ratio, no/ne, of OTMS:PEO repeating units in the mixture was 0.83. The addition of PEO resulted in a clear expansion of the OTMS/PEO monolayer at large areas compared to the isotherms obtained for pure OTMS as shown in Figure 3. However, the pH dependence of the compression isotherms of the mixed monolayers were similar to the general behavior observed for the pure OTMS monolayers. The

Polymer-Organosilane Interactions at A/W Interface

Figure 4. Compression and surface potential isotherms of mixed monolayers of OTMS/PEO at pH ) 5.6: (s) pure OTMS, (- - -) no/ne ) 3.30, (- - - -) no/ne ) 1.65, (‚‚‚) no/ne ) 0.83, (- - -) no/ne ) 0.41, and (s) pure OTMS spread on a subphase containing 1% w/v PEO, respectively.

Figure 5. Compression and surface potential isotherms of mixed monolayers of OTMS/PEO at pH ) 3: (s) pure OTMS, (- - -) no/ne ) 1.65, (- - -) no/ne ) 0.83, (- - -) no/ne ) 0.41, and (- - -) pure OTMS spread on a subphase containing 1% w/v PEO, respectively.

monolayers were very viscous at extreme pH, which lead to errors in the measured compression isotherms at high surface pressures since the monolayer passed over the edges of the trough. This is indicated by dots in Figure 3. Compression isotherms for mixed OTMS/PEO monolayers with different no/ne values at pH ) 5.6 and pH ) 3 are shown in Figures 4 and 5, respectively. The isotherms are plotted versus the mean molecular area occupied by OTMS. The presence of low amounts of PEO did not affect the general phase behavior of OTMS at pH ) 5.6; the LE, LC, and C states were still seen in the compression isotherms. However, the surface pressure in the LE-LC transition region increased with increasing PEO concentration to values exceeding the collapse pressure of the pure PEO monolayer. Identical isotherms were obtained at pH ) 9.3 (results not shown). The increase in the surface pressure above the collapse pressure of PEO in this region might be a nonequilibrium property resulting from a too fast compression of the high molecular weight PEO. The relatively high compression speed used (0.025 nm2/min) was necessary to be able to compare the pH dependence of the hydrolysis/condensation of the mixed OTMS/PEO monolayers with the corresponding pure OTMS monolayers, due to the fast hydrolysis at extreme pH. However,

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only a slight decrease in surface pressure of about 0.8 mN/m was observed within 6 h if a mixed OTMS/PEG monolayer (no/ne ) 0.41) was compressed to 17 mN/m and allowed to undergo surface pressure relaxation at constant area at pH ) 5.6. Furthermore, fluorescence microscopy measurements gave no evidence for any PEO-induced transition of OTMS into the LC state at large nominal areas (results not shown). This indicates a fairly strong interaction between the two components in the monolayer. Moreover, the LC-C transition pressure increased from 19 to about 20.5 mN/m already at low PEO concentrations. Upon further compression the compression isotherms approached asymptotically the isotherms observed for pure OTMS monolayers, suggesting that the PEO was progressively squeezed out of the monolayer with increasing surface pressure. At pH ) 3, a corresponding expansion of the OTMS monolayer occurs in the presence of PEO. A continuous increase in the surface pressure with decreasing area was observed at large nominal mean molecular areas of OTMS. However, at π ) 9.5 mN/m a partial collapse of the mixed monolayer was observed. This pressure corresponds to the collapse pressure of the corresponding pure PEO monolayers. Clearly, at this point the desorption of PEO from the mixed monolayer was initiated. Upon further compression, the condensed state was reached at mean molecular areas close to those observed for the pure OTMS monolayer. Therefore, one may conclude that at large areas, i.e., when the surface pressure was below the collapse pressure of PEO, the isotherm is determined by PEO and at low areas by OTMS. Since PEO is also soluble in water, it may also be added to the subphase instead of being spread at the A/W interface. PEO (1% w/v) was added to the subphase onto which a OTMS monolayer was spread. The measured compression isotherms at pH ) 5.6 and pH ) 3 are also included in Figures 3 and 4, respectively. Since the surface pressure values are measured relative to the surface tension of the subphase, a correction for the difference in surface tension between a pure water subphase and the PEO-containing subphase has to be introduced in order to obtain true surface pressure values. Since the LC-C transition pressure was not sensitive to changes in the PEO concentration of the mixed OTMS/PEO monolayers, this value was taken as the reference value. The measured surface pressure had to be corrected by 9.5 mN/m on subphases containing 1 w/v % PEO, respectively, i.e., exactly the values observed for the close-packed PEO monolayer. This gave clear evidence for the presence of a close-packed adsorbed monolayer of PEO at the air/ water interface on the subphases containing 1% w/v PEO, in agreement with previous studies on adsorbed monolayers of PEO.19,29 At pH ) 5.6 no sign of the LE state was observed in the compression isotherms of the spread OTMS monolayer. The surface pressure did not change upon compression until the nominal mean molecular area of OTMS reached ≈0.4 nm2 at both pHs. Here again, the plateau value of the surface pressure at pH ) 3 coincides with the collapse pressure of PEO at pH ) 3 while the plateau value observed at pH ) 5.6 exceeds the collapse pressure of PEO by several mN/m. Miscibility of OTMS and PEO. To determine the compatibility of PEO and OTMS at the A/W interface, compression isotherms for OTMS/PEO monolayers of different compositions were measured at pH ) 5.6. For an ideally miscible or completely immiscible mixture of (29) Winterhalter, M.; Bu¨rner, H.; Marzinka, S.; Benz, R.; Kasianowicz, Biophys. J. 1995, 69, 1372.

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Figure 6. Deviations from additivity for OTMS/PEO mixed monolayers at the air/water interface at a surface pressure of 4 mN/m.

Figure 7. ∆Vc versus pH for pure OTMS (9), OTMS/PEO (no/ ne ) 0.83) (O), and pure OTMS spread on a subphase containing 1% w/v PEO (4). Regions I-IV are referred to in the Discussion section. See text for details.

components 1 and 2 the following relation should be obeyed,

A12 ) x1A1 + x2A2 where A12 is the observed mean molecular area of the mixed monolayer at a given surface pressure, A1 and A2 are the mean molecular areas observed for the pure components at the same surface pressure, and x1 and x2 are the corresponding molar fractions of component 1 and 2, respectively (x1 + x2 ) 1). The mean molecular areas observed at π ) 4 mN/m and pH ) 5.6 are shown in Figure 6 as a function of the molar fraction of OTMS. The deviation from ideal miscibility expressed in nm2 is used as the abscissa. A linear relationship was observed which suggested that OTMS and PEO where either ideally miscible or totally immiscible at all compositions in the LE state. A similar determination of the miscibility between PEO and OTMS was not carried out at pH ) 3, due to the fast aggregation processes occurring in the monolayer, as discussed previously. Surface Potential Isotherms. The surface potential, ∆V, of the monolayer was measured simultaneously with the surface pressure during compression. The ∆V/A isotherms were reproducible over the entire area range studied. Since a more comprehensive discussion concerning the surface potentials of pure OTMS monolayers is given elsewhere,10 only a brief summary will be given here.

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∆V/A isotherms for OTMS measured at different pHs are included in Figure 1. At pH ) 5.6 a plateau value of about 250 mV was observed in the LE-LC coexistence region. A continuous increase in ∆V occurred when the OTMS monolayer reached the LC state and finally a maximum in ∆V, denoted ∆Vc, of 600 (15 mV was observed in the condensed state. At low and high pH the ∆V/A isotherms resembled the corresponding isotherms of saturated fatty acids giving ∆V values close to 0 mV during most of the G-C coexistence region and then a sharp change in ∆V prior to reaching the C state. The values of ∆Vc as a function of pH are shown in Figure 7. While the compression isotherms at extreme pH were almost identical, large differences in ∆Vc were observed. At low pH ∆Vc was always positive, while strongly negative values were observed at high pH. The reason for this difference is discussed in the next section. The ∆V/A isotherms for PEO spread at the A/W interface showed a continuous increase in ∆V during compression to finally reach a value of about 420 mV at the collapse pressure regardless of the pH. These values are in fair agreement with those reported in a previous study, where a ∆Vc value of 440 mV was given for spread and adsorbed monolayers of PEO.23 The ∆V/A isotherms measured for mixed OTMS/PEO monolayers (no/ne ) 0.83) as a function of pH are included in Figure 3. The ∆V/A isotherms follow nicely the changes in the corresponding compression isotherms. At large areas, where the compression isotherm is governed by PEO, clearly higher ∆V values than what was observed for pure OTMS monolayers were observed. However, upon further compression the ∆V values approached the values observed for pure OTMS monolayers to finally reach exactly the same ∆Vc values. This was true over the entire pH range 0.5-13.5, as shown in Figure 7. In the pH interval 0.5-3 an almost linear decrease in ∆Vc from 200 mV to a local minimum of 70 mV was observed with increasing pH. Between pH ) 3 and pH ) 5.5 a continuous increase in ∆Vc from 70 mV to a plateau value of 600 mV occurred. Above pH ) 10.7 ∆Vc decreased monotonically to finally reach strongly negative values. These results stress the fact that PEO was squeezed out of the monolayer and that PEO had little or no influence on the hydrolysis/condensation behavior of OTMS. ∆V/A isotherms measured for different no/ne ratios at pH ) 5.6 and pH ) 3 are shown in Figures 4 and 5, respectively. At both pHs the ∆V values increased with increasing PEO content of the monolayers. With decreasing monolayer area, however, the ∆V values approached and finally coincided with the values observed for pure OTMS, again supporting the conclusion that PEO was squeezed out of the mixed monolayers at low nominal areas of OTMS. If PEO was added to the subphase at 1% w/v and the ∆V was measured relative to the subphase, a constant difference of about 410 mV, compared to the value observed for OTMS on water throughout the pH range, was found. Here again, the surface potential is measured relative to the surface potential of the subphase and the presence of the adsorbed PEO monolayer should be corrected for. The difference of 410 mV closely resembles the ∆Vc value observed for compressed PEO monolayers and supports the conclusion that PEO has formed a dense monolayer at the A/W interface. It should be noted that the corresponding ∆Vc values given in Figure 7 have been corrected by 410 mV.

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Discussion OTMS might undergo hydrolysis and condensation reactions at the air/water interface. The hydrolysis and subsequent condensation of OTMS may be written

tSi-OCH3 + H2O ) tSi-OH + CH3OH tSi-OH + OH-Sit ) tSi-O-Sit + H2O However, this description is an oversimplification due to several reasons. The rate of hydrolysis is not the same for each hydrolysis step, and condensation between hydrolyzed and unhydrolyzed species may also occur.30,31 Depending on the pH, the fatty silanol may also be ionized, positively at low pH and negatively at high pH.10 The pKa value of Si(OH)4 is 9.815 and C18H37-Si(OH)3 should be an even weaker acid due to the enhancement of the electron density around the polar group.8 The smaller size of the polar headgroup of the hydrolyzed OTMS will affect the shape of the compression isotherm. The suppression of the LE state in the compression isotherms of OTMS at extreme pH may be attributed to this effect. Therefore, it is clear that the OTMS monolayer remained virtually unhydrolyzed during the course of the experiment at intermediate pH, while almost complete hydrolysis had occurred at low and high pH. This interpretation is supported by XPS6 and FT-IR8 measurements carried out on deposited Langmuir-Blodgett films of similar compounds. While the compression isotherms were almost identical at low and high pH, clearly different ∆Vc values were observed in the two pH regimes. Since the value of ∆Vc should be sensitive to any changes in the dipoles at the interface,32 it should be sensitive to both hydrolysis and possible dissociation of the hydrolyzed species. The variation in ∆Vc with pH is clearly consistent with the pH-dependent hydrolysis as suggested by the changes in the compression isotherms. Furthermore, the slight increase in ∆Vc with decreasing pH in the pH region 0.5-3 and the decrease in ∆Vc with increasing pH in the pH region 11-13.5 suggests that the OTMS monolayer was ionized. The question still remains to what extent condensation has occurred in the monolayer. Recent fluorescence microscopy studies suggest that the condensation reaction kinetics is very fast and that condensation occurs almost immediately after hydrolysis.10,24 Due to sterical constraints, a linear oligomer/polymer is formed at both acidic and basic pH.11 On the basis of the above reasoning, we interpret the regions I-IV indicated in Figure 7 as follows. In region I a hydrolyzed/condensed and slightly positively charged monolayer has formed. Region II corresponds to a neutral hydrolyzed/condensed monolayer (pH ) 3). In region III the monolayer remained virtually unhydrolyzed and in region IV the monolayer was hydrolyzed/condensed and slightly negatively charged at high pH. The fact that the presence of PEO did not alter the shape of the ∆Vc/pH curve suggests that PEO did not alter the hydrolysis/condensation kinetics of OTMS on the molecular scale. This is contrary to mixed monolayers of OTMS and methyl stearate, where both the hydrolysis and condensation rate of OTMS were altered compared to those observed for the pure OTMS film.13 However, the fact that the (30) Fontaine, P.; Rondelez, F. Short and Long Chains at Interfaces; Daillant, J., et al., Eds.; Editions Frontieres: Gif-sur-Yvette, France, 1995; Vol. M85, p 207. (31) Sjo¨blom, J.; Helle, M. H.; Friberg, S. E.; Moaddel, T.; Brancewicz, C. Colloids Surf. 1994, 88, 235. (32) Gaines, G. L., Jr. Insoluble Monolayers at the Liquid-Gas Interface; Wiley: New York, 1966.

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hydrolyzed/condensed OTMS monolayer may be compressed to slightly lower nominal mean molecular areas at pH ) 3 in the presence of PEO might suggest that PEO will limit the possibility of interdomain aggregation observed for pure OTMS monolayers,10 by decreasing the surface diffusion of OTMS, thus affecting the aggregation of OTMS on a microscopic scale. It has been shown that PEG has a similar influence on corresponding bulk reactions, where the interparticle aggregation kinetics is slower in the presence of PEG, while the presence of PEG has little or no influence on the kinetics of primary particle formation.33 The strong influence of PEO on the compression isotherm of OTMS at neutral pH is interesting. Normally, mixed monolayers of lipid and polymer will show a plateau at the collapse pressure of the polymer,4,23 as observed in this study at extreme pH. However, the LE-LC and LC-C transitions of unhydrolyzed OTMS were clearly affected by PEO. In a recent study by Charron and Tilton4 on mixed monolayers of DPPC and the block copolymer polystyrenepoly(ethylene oxide) the presence of the polymer was observed to promote the LE-LC transition of DPPC. However, no LC domains were observed at large monolayer areas in our study, even if the surface pressure exceeded the LE-LC transition pressure of OTMS, 9 mN/m. Taylor and co-workers34 has suggested that the LE-LC transition represents the close packing of the methoxy groups of OTMS while further compression will result in an undulation of the OTMS molecules, allowing a closer packing of the monolayer. In a recent paper, it was suggested that the hydrolysis of OTMS is preceded by protonation of the methoxy group on acidic subphases,14 which also could explain the increase in the observed transition pressure for the mixed monolayers at pH ) 5.6. However, identical isotherms were obtained if we carried out the monolayer study at pH ) 9.3, where protonation should not occur, why a protonation reaction cannot explain the features observed in this study. The ideal mixing behavior of OTMS/PEG observed in the LE state at this pH and the observed anomaly in the high surface pressure in the coexistence region suggests that the unhydrolyzed OTMS and PEO formed a homogeneously mixed film at all compositions in the LE state and that the undulation-induced transition required a higher surface pressure in the presence of PEO due to attractive van der Waals interactions between OTMS and PEO. A recent study of the surface pressure-dependent interaction between a bilayer of C18EO5 and poly(oxy ethylene) revealed an attractive polymer surfactant interaction over most of the surface pressure range studied.35 On the basis of the results obtained in the present study and literature data, we therefore suggest that one important driving force for the formation of mesostructured inorganic-co-block polymer composite structures in water-ethanol solutions is the van der Waals interactions between the alkoxy groups of the inorganic precursor and the ethoxide moiety of the polymer. No strong interaction between PEO and hydrolyzed OTMS was observed in the present study since the overall features of the phase behavior were preserved in this case. However, the slightly higher surface pressure in the plateau region of the OTMS/PEG monolayer at pH ) 3 compared to that at pH ) 12 might suggest that some interaction through hydrogen bonding occurs (33) A° gren, P.; Counter, J.; Laggner, P. J. Non-Cryst. Solids 2000, 261, 195. (34) Taylor, D. M.; Gupta, S. K.; Dynarowicz, P. Thin Solid Films 1996, 80, 284. (35) Meier, W.; Ramsden, J. J. J. Phys. Chem. 1996, 100, 1435.

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between PEO and undissociated, hydrolyzed OTMS, as observed for colloidal silica.13 No such hydrogen bonding is expected between PEO and ionized, hydrolyzed OTMS. However, the fast aggregation processes occurring at extreme pH makes a detailed evaluation of this behavior difficult. Summary The influence of PEO on the hydrolysis/condensation of OTMS monolayers at the air/water interface has been examined as a function of PEO content in the pH interval 0.5-13.5. The presence of PEO did not affect the hydrolysis/condensation kinetics of OTMS on the molecular

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scale, but may influence the interdomain aggregation, eventually leading to the formation of a two-dimensional gel. Strong interaction was observed between PEO and unhydrolyzed OTMS, while little or no interaction was observed between PEO and hydrolyzed OTMS. PEO was squeezed out of the mixed OTMS/PEO monolayers at low nominal areas of OTMS at all compositions and pHs. The resemblance between the influence of PEG on the behavior of spread monolayers of an organosilane and the corresponding bulk reactions imply that spread monolayers may be used as a model system for investigating bulk phase interactions. LA000472R