Formation of a Freely Suspended Membrane via a Combination of

Sep 20, 2005 - AC2, The University of Ulm, Polymer Physics, BASF-Aktiengesellschaft, Ludwigshafen, and. Physical Chemistry, Chemnitz University of ...
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Langmuir 2005, 21, 10475-10480

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Formation of a Freely Suspended Membrane via a Combination of Interfacial Reaction and Wetting Cathy E. McNamee,† Manfred Jaumann,† Martin Mo¨ller,†,‡ Ailin Ding,| Steffen Hemeltjen,| Susanne Ebert,| Wolfgang Baumann,| and Werner A. Goedel*,†,§,|,⊥ Organic and Macromolecular Chemistry, OC3, The University of Ulm, German Wool Research Institute at the University of Technology Aachen, Inorganic Chemistry - Materials & Catalysis, AC2, The University of Ulm, Polymer Physics, BASF-Aktiengesellschaft, Ludwigshafen, and Physical Chemistry, Chemnitz University of Technology Received April 6, 2005. In Final Form: July 1, 2005 Applying poly(ethoxysiloxane) (a liquid non-water-soluble polymer that can be hydrolyzed and crosslinked by diluted acids) to an air/pH 1 water interface gave rise to thin homogeneous solid layers. These layers were strong enough to be transferable to electron microscopy grids with holes of dimensions up to 150 µm and covered the holes as freely suspended membranes. No homogeneous layers were formed at an air/pH 5 water interface. Brewster angle microscopy images show that the poly(ethoxysiloxane) is not spontaneously forming a wetting layer on water. It initially forms lenses, which slowly spread out within several hours. We conclude that the spreading occurs simultaneously with the hydrolysis and cross-linking of the poly(ethoxysiloxane) and that the reaction products finally assist the complete wetting of the water surface.

Introduction The ability of a liquid to form wetting layers at an interface is important both technologically and in biological systems, for example, to prevent fogging1 and to safeguard against the deposition of dirt,2 or guarantee the integrity of protective or decorative coatings. Wetting is influenced by short-range forces, such as hydrogen bonding and donor/acceptor interactions, and by long-range dispersion forces. Depending on the relative strengths of these forces, one can observe dewetting, complete wetting, or partial wetting. In the latter case, the competition between favorable short-range forces and unfavorable long-range forces gives rise to the formation of a wetting layer of a limited thickness that coexists with lenses, formed by the excess of the liquid.3 A similar situation can arise if a liquid is applied to a layered system, e.g., a substrate composed of a thin solid layer with favorable dispersion forces on top of a bulk medium with unfavorable dispersion forces.4-6 Dispersion forces can be estimated from the Lifschitz theory;7,8 as a rule of thumb, * Corresponding author. Physical Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany. Tel: +49(0)371 531-1477. Fax: +49(0)371 531-1371. http://www.tu-chemnitz.de/chemie/physchem/werner.goedel@ chemie.tu-chemnitz.de. † Organic and Macromolecular Chemistry. ‡ German Wool Research Institute at the University of Technology Aachen. § Inorganic Chemistry - Materials & Catalysis, AC2. | Polymer Physics, BASF-Aktiengesellschaft. ⊥ Chemnitz University of Technology. (1) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature (London) 1997, 388, 431. (2) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (3) Berg, J. C. Wettability; Marcel Dekker: New York, 1993; p 94. (4) Chaudhury, M. K.; Whitesides, G. M. Langmuir 1991, 7, 1013. (5) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Pariku, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (6) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (7) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1994; p 179.

a liquid with a high refractive index is unlikely to wet a medium with a low refractive index. Given the relatively low refractive index of water, there are only a few liquids that form wetting layers on a water surface. Low molecular weight materials, such as alkanes, have been shown to undergo a wetting transition when the physical parameters of the liquid subphase were altered.9-11 For example, the temperature dependence of the refractive index of pentane differs from that of water. Thus, pentane does not wet a water surface at room temperature but wets at elevated temperatures.12,13 Octane can be made to wet water by adding sucrose to the subphase, which increases the refractive index of the aqueous subphase.11 There are reports indicating that poly(dimethylsiloxane),14,15 poly(phenylmethylsiloxane),16 and methyl/phenyl-substituted poly(cyclosiloxanes)17 form wetting layers or at least multilayers on a water surface. However, as in the case of low molecular weight substances, complete wetting of a water surface by a polymer is the exception rather than the rule. The ability to form a polymer wetting layer at an air-liquid interface, (8) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: New York, 1989; pp 153156. (9) Pfohl, T.; Mo¨hwald, H.; Riegler, H. Langmuir 1998, 14, 5285. (10) Bertrand, E.; Dobbs, H.; Broseta, D.; Indekeu, J.; Bonn, D.; Meunier, J. Phys. Rev. Lett. 2000, 85 (6), 1282. (11) Pfohl, T.; Riegler, H. Phys. Rev. Lett. 1999, 82 (4), 783. (12) Bertrand, E.; Dobbs, H.; Broseta, D.; Indekeu, J.; Bonn, D.; Meunier, J. Phys. Rev. Lett. 2000, 85 (6), 1282. (13) Ragil, K.; Meunier, J.; Broseta, D.; Indekeu, J. O.; Bonn, D. Phys. Rev. Lett. 1996, 77, 1532. (14) Arslanov, V. Russ. Chem. Rev. 1994, 63, 1. The statement of complete wetting is based on thermodynamic analysis of the equilibrium spreading pressure. (15) Oberservations by Brewster angle microscopy shown by Mann, E. K.; Henon S.; Langevin D.; Meunier, J. J. Phys. (Paris) 1992, 2, 1683 and our own observations with Brewster angle microscopy contradict the conclusions of ref 14. (16) Granick, S.; Kuzmenka, D. J.; Clarson, S. J.; Semlyen, J. A. Langmuir 1989, 5, 144; Macromolecules 1989, 22, 1878. (17) Buzin, A. I.; Godovsky, Yu. K.; Makarova, N. N.; Fang, J.; Wang, X.; Knobler, C. M. J. Phys. Chem. B 1999, 103, 11372-11381.

10.1021/la0509089 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/20/2005

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however, can be important technologically, for example, for the production of freely suspended membranes. In previous investigations, we prepared freely suspended membranes from monolayers of liquid polymers with ionic headgroups, initially stabilized by short-range interactions between the ionic headgroups and the water phase, and subsequently solidified and transferred to a porous support.18-20 However, their thicknesses needed to be tuned by synthesizing polymers of different chain lengths. For each chain length, only a limited range of layer thicknesses could be realized.21 In search of a liquid that completely wets a water surface and that can be solidified to form thin homogeneous freely suspended membranes, we applied poly(ethoxysiloxane), a liquid polymer that can be converted into solid amorphous silica upon hydrolysis in acidic pH, to a water surface. At low enough pH, this approach indeed yielded thin solid layers floating on the water surface. These layers were stable enough to be transferred to porous supports as freely suspended membranes.

Figure 1. Molecular structure of poly(ethoxysiloxane) (PAOS) (-[SiO(OCH2CH3)2]-).

5.2

5.3

Experimental Section

1.405 1.1831

1.410 1.1473

2.1. Materials and Instruments. Hyperbranched poly(ethoxysiloxane) of the approximate sum formula of (CH3CH2O)2SiO was synthesized by the polycondensation reaction of acetoxy triethoxysiloxane, as described elsewhere.22 Two samples of poly(ethoxysiloxane) with different molar masses were used: poly(ethoxysiloxane)4.2 (Mw 4.2 kg mol-1) and poly(ethoxysiloxane)7.7 (Mw 7.7 kg mol-1). The molar masses were determined via size exclusion chromatography, as described in ref 22. The densities were determined by weighing a known volume of the liquid. Pentane (GR for analysis grade, Merck, Germany) and hydrocholoric acid (GR for analysis grade, Merck, Germany) were used as received. The water was purified with a Millipore-Q Plus 185 ultrapure water system, giving water with a conductance of 18.2 MΩ cm and a total organic carbon content of 5 ppm. Transmission electron microscopy (TEM) grids (Au) (Plano, Germany) with holes of sizes 45 or 150 µm were used as received. An optical microscope (Axioplan 2 imaging, Zeiss, Germany) with a camera attached was used to take the optical microscope images. A Brewster angle microscope (Minibam, Nanofilm Surface Analysis, Germany) was used to image the structures formed by the poly(ethoxysiloxane) film at the air-water interface.23,24 Cross-sections of the freely suspended membranes (coated with 3 nm layer of Pt) were imaged using a high-resolution scanning electron microscope (ultrahigh-resolution scanning microscope S-5200, Hitachi, Japan). The dielectric constant of poly(ethoxysiloxane) was measured with the equipment described in ref 25. A refractometer (Abbe´ refractometer Carl Zeiss Jena, Germany) was used to measure the refractive indices at 20 °C. Infrared transmission spectra were recorded in transmission using a Fourier transform spectrometer (IFS48, Bruker, Germany). Electron probe microanalysis was conducted with the analyzer SX100 of CAMECA. Membranes were transferred to metal grids, and electrical contact to the brass sample holder was established using silver ink. Two specimens obtained from independent hydrolysis experiments were each analyzed at ten different spots using electron energies of 25 keV and 5 keV and a beam diameter of 20 µm, and the values were averaged. (18) Goedel, W. A.; Peyratout, L.; Ouali, V.; Scha¨dler, V. Adv. Mater. 1999, 11, 213. (19) Mallwitz, F.; Grassmu¨ller, M.; Ismeier, J. R.; Eckelt, R.; Nuyken, O.; Goedel, W. A. Macromol. Chem. Phys. 1999, 200, 5, 1014. (20) Goedel, W. A.; Heger, R. Langmuir 1998, 14, 3470. (21) Baltes, H.; Schwendler, M.; Helm, C. A.; Heger, R.; Goedel, W. A. Macromolecules 1997, 30, 6633. (22) Jaumann, M.; Rebrov, A.; Kazakova, V. V.; Muzafarov, A. M.; Mo¨ller, M.; Goedel, W. A. Macromol. Chem. Phys. 2003, 204, 7, 1014. (23) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (24) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (25) Heinrich, W.; Stoll, B. Colloid Polym. Sci. 1985, 263, 873.

Table 1. Physical Properties of Poly(ethoxysiloxane) physical properties molar mass (kg mol-1) average number of repeat units per molecule relative dielectric constant at 1 MHz refractive index density (g cm-3)

poly(ethoxysiloxane)4.2 poly(ethoxysiloxane)7.7 4.2 31

7.7 57

Lateral pressure to area isotherms of poly(ethoxysiloxane) monolayers were recorded using a 927 cm2 rectangular Teflon Langmuir trough (FW2 Lauda, Germany) equipped with a floating barrier to detect the lateral pressure. Monolayers were prepared by spreading either a solution of poly(ethoxysiloxane)4.2 or poly(ethoxysiloxane)7.7 in pentane (with a 4.18 × 10-2 g/L and 7.67 × 10-2 g/L concentration, respectively) onto the water surface. The subphase was maintained at 20 °C by circulating water of a regulated temperature through the base of the trough. A waiting time of 15 min was given for the pentane to evaporate, before the monolayer was compressed. 2.2. Method of Film Preparation. Monomolecular-thick layers of poly(ethoxysiloxane) were prepared by spreading solutions in pentane onto a water surface. We spread the amount needed (surface coverages of 0.130 and 0.126 mg m-2 for poly(ethoxysiloxane)4.2 and poly(ethoxysiloxane)7.7, respectively) to reach a surface concentration corresponding to the onset of a nonzero lateral pressure. For thicker layers, liquid poly(ethoxysiloxane) was applied onto the water surface without the aid of an organic solvent. The hypothetical layer thickness of the non-hydrolyzed poly(ethoxysiloxane) was estimated from the volume spread divided by the area of the water surface. Layers were prepared on water surfaces using as the container for the water the Langmuir trough, 7 cm diameter Petri dishes, and 25 cm × 35 cm rectangular plastic containers that were loosely covered by lids to protect against dust particles. The films were transferred to TEM grids by horizontal transfer.26

Results and Discussion In the first series of experiments, we applied various amounts of poly(ethoxysiloxane) (see Figure 1 for the structure and Table 1 for the characterization data) to unbuffered water (pH 5 to 6) and to diluted hydrochloric acid (pH 1). In regular intervals, the surface was disturbed by a spatula; the layer was considered “solid” if small dust specks in centimeter distance from the spatula followed the movement of the spatula. At pH values of 5 to 6, no “solid” layer formed, even after waiting for a period of up to 7 days. At pH 1, solid layers formed on the surface within a few hours at a surface concentration of 0.130 10-3 gm-2 and within up to 3 days for surface concentrations exceeding 1 gm-2 of polyalkoxysiloxane. These solid layers could be transferred to electron microscopy grids, covering the openings as freely suspended membranes. (26) Araki, T.; Oinuma, S.; Iriyama, K. Langmuir 1991, 7, 738.

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Figure 3. Two alternative mechanisms for the formation of silica membranes from poly(ethoxysiloxane). Path A-B-D shows the formation of a wetting layer of the poly(ethoxysiloxane) at the water surface, followed by the hydrolysis reaction. In path A-C-D, the conditions initially are unfavorable for the formation of a layer. The subsequently formed reaction products gradually assist the formation of a layer. Figure 2. Optical microscopy pictures of membranes derived from hydrolyzed poly(ethoxysiloxane) on electron microscopy grids. Figure 2A-C shows membranes derived from surface coverages of 1.147 gm-2 poly(ethoxysiloxane)7.7, 0.115 gm-2 poly(ethoxysiloxane)7.7, and a poly(ethoxysiloxane)7.7 monolayer on 45 µm holes. Figure 2D,E shows membranes derived from surface coverages of 1.147 gm-2 and 0.115 gm-2 poly(ethoxysiloxane)7.7 on 150 µm holes. Figure 2F shows membranes derived from a surface coverage of 1.183 gm-2 poly(ethoxysiloxane)4.2 on 45 µm holes.

The transferred membranes could be shattered into pieces with irregular sharp borderssconfirming our interpretation that the layers on the water surface were solid. In the case of the higher molar mass poly(ethoxysiloxane), i.e., poly(ethoxysiloxane)7.7, solid layers derived from 1.147 gm-2 (hypothetical layer thickness of poly(ethoxysiloxane) 1000 nm) and 0.115 gm-2 (hypothetical layer thickness of poly(ethoxysiloxane) 100 nm) were able to cover 45 µm holes; see Figure 2A and B, respectively. But layers derived from an initial surface concentration corresponding to a monolayer (0.130 10-3 gm-2) could only cover a few windows; see Figure 2C. Holes of 150 µm magnitude were covered only by the thickest layers (1.147 gm-2); see Figure 2D. A thinner layer (0.115 gm-2) only covered the corners of the holes; see Figure 2E. In the case of the lower molecular weight poly(ethoxysiloxane), i.e., poly(ethoxysiloxane)4.2, complete coverage of the holes was only possible when smaller holes (45-µm-sized holes), and the highest surface concentration (1.183 gm-2) was used (figure not shown). A lower surface coverage (118 10-3 gm-2) caused the holes to be either not covered by the film at all or only partially covered; see the arrows in Figure 2F. A possible mechanism of the formation of a solid layer on acidic water might be the creation of a complete poly(ethoxysiloxane) wetting layer on top of the water surface followed by hydrolysis and cross-linking to a solid product, as indicated by the path A f B f D f E shown in Figure

Figure 4. Lateral pressure vs area per molecule (bottom axis) and surface coverage (top axis) isotherms for poly(ethoxysiloxane) spread at an air-water interface. s and s, poly(ethoxysiloxane)4.2 on a pH 5 subphase; ‚‚‚ and ‚‚‚, poly(ethoxysiloxane)7.7onapH5;- - -and- - -,poly(ethoxysiloxane)4.2 on a pH 1 subphase; and -‚- and -‚-, poly(ethoxysiloxane)7.7 on a pH 1 subphase. The blue and red lines correspond to the area per molecule and surface coverage data, respectively. The solid arrow indicates the conditions used during the monolayer transfers.

3. Such a wetting layer requires favorable short- and longrange forces.27 In a Langmuir film balance experiment, we can detect a significant lateral pressure at monolayer coverage; see Figure 4. At higher surface concentrations, we can measure a positive equilibrium spreading pressure (26.9 and 26.8 mN m-1 for poly(ethoxysiloxane)4.2 and poly(ethoxysiloxane)7.7, respectively, at pH 5). We therefore conclude that at least a monolayer is easily formed and thus that there are favorable short-range interactions. We can obtain information about the influence of the longrange forces by calculating the Hamaker constant of this (27) Nightingale, M. P.; Indekeu, J. O. Phys. Rev. B 1985, 32, 3364.

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Table 2. Hamaker Constants of Various Systems at T ) 298 K Calculated Using the Lifshitz Theory and Our Experimentally Determined E and n Values system

A123 (total) (J)

air-poly(ethoxysiloxane)4.2 - pH 1 subphase air-poly(ethoxysiloxane)7.7 - pH 1 subphase air-poly(dimethylsiloxane) - pH 1 subphase air-poly(ethoxysiloxane)4.2 - silica air-poly(ethoxysiloxane)7.7 - silica

5.07 × 10-21 5.71 × 10-21 5.16 × 10-21 -1.58 × 10-20 -1.54 × 10-20

system from the refractive indices and dielectric constants of the materials involved (see Appendix 1 for further details on the calculation and Table 1 for our measured refractive indices and dielectric constants of the liquid poly(ethoxysiloxane)). One obtains positive Hamaker constants (see Table 2); these values are comparable to the Hamaker constant we calculate for poly(dimethylsiloxane) (silicone oil) at an air-water interface. This positive Hamaker constant indicates unfavorable (attractive) long-range forces between the two interfaces of poly(ethoxysiloxane) on water. Thus, in accordance with similar experiments conducted with poly(dimethylsiloxane),15 the formation of a thick homogeneous wetting layer of poly(ethoxysiloxane) should not occur, and thus, the path A f B f D f E in Figure 3 is unlikely. To obtain further insight into the spreading of poly(ethoxysiloxane), we observed the formation of the solid layer on a pH 1 water subphase via Brewster angle microscopy. Directly after applying the liquid poly(ethoxysiloxane)7.7, we indeed do not see a homogeneous layer, but observe the formation of lenses (Figure 5A). Judging from the interference patterns in Figure 5A, these lenses have a thickness of at least several times the wavelength of visible light. Only with time do these lenses disappear (Figure 5B-E), until finally, the stage of a homogeneous layer is reached (Figure 5F). This process also occurs if we use poly(ethoxysiloxane) of a lower molecular weight (i.e., poly(ethoxysiloxane)4.2, Figure 5H). However, it does not occur if we apply poly(ethoxysiloxane) to the surface of pure water with pH 5-6 (Figure 5G). The time requirement for poly(ethoxysiloxane) to form a homogeneous layer on pH 1 water increased with the surface concentration of PAOS (larger expected film thicknesses). This suggests that the wetting mechanism is coupled to a comparatively slow process. On the basis of the above observations, we propose the following mechanism for the membrane formation at the air/pH 1 water interface: In accordance with the observed equilibrium spreading pressure, the lenses of liquid poly(ethoxysiloxane) initially coexist with a thin layer of poly(ethoxysiloxane) of equilibrium thickness. This thin layer is completely or at least partially hydrolyzed into amorphous silica, due to the acid-catalyzed reaction

[SiO(OCH2CH3)2]x + xH2O f xSiO2 + 2xHOCH2CH3 silica poly(ethoxysiloxane) (1) The thin layers of hydrolyzed, cross-linked PAOS thus created are subsequently soaked with or covered by additional material that spreads out from the droplet. Simultaneously, this material is hydrolyzed. This procedure continues until all the liquid poly(ethoxysiloxane) is consumed and converted into a cross-linked solid. This proposed mechanism requires the wetting of the silica (or silica/poly(alkoxysiloxane) hybrid) by the poly(ethoxysiloxane). Indeed, we calculate a negative Hamaker constant for the air-poly(ethoxysiloxane)-silica system (see Table 2). We expect that the poly(ethoxysiloxane) forms a wetting

Figure 5. Brewster angle microscope pictures of poly(ethoxysiloxane) spread at a water surface. Figure 6A-F shows poly(ethoxysiloxane)7.7 initially, and 1, 32, 125, 1200, and 4320 min after spreading 1.147 gm-2 poly(ethoxysiloxane)7.7 at an air/pH 1 water interface. Figure 6G shows the nonspreading of 1.147 gm-2 poly(ethoxysiloxane)7.7 at an air/pH 5 water interface. Figure 6H shows a layer of poly(ethoxysiloxane)4.2 film at an air/pH 1 water interface after 4320 min (1.183 gm-2).

Figure 6. High-resolution scanning electron microscope picture of a cross-section of a silica membrane derived from a surface coating of 1.147 gm-2 poly(ethoxysiloxane)7.7.

layer of limited thickness on top of the cross-linked layer. This thickness increases with the steadily growing silica layer, until all the lenses are consumed. Alternatively, it is as well possible that the poly(alkoxysiloxane) and/or water swell the partially cross-linked layer and that further hydrolysis occurs within the layer rather than at one of its surfaces. To further understand their inner structure, the membranes were cut perpendicularly to their surface and the cross-section investigated by the high-resolution scanning

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Table 3. Thicknesses of Layers of Hydrolyzed Poly(ethoxysiloxane) hydrolyzed poly(ethoxysiloxane) poly(ethoxysiloxane)4.2 poly(ethoxysiloxane)7.7

thickness of poly(ethoxysiloxane) derived film (nm) hypothetical thickness of a wetting layer of poly(ethoxysiloxane) on water, calculated by dividing the volume spread by the covered area of the water surface (estimated thickness) actual thickness of the final transferred membrane, as measured by high-resolution scanning electron microscopy (measured thickness) calculated from eq A2.3 assuming no porosity and a literature value for the density calculated from eq A2.3 assuming a water content equal to SiO2‚4H2O

electron microscope. Figure 6 shows as an example a membrane derived from a surface coverage of 1.147 gm-2 of poly(ethoxysiloxane)7.7. As can be expected, if hydrolysis occurs and thus the ethoxy side groups are lost, the membrane thickness (approximately 700 nm) is smaller than the hypothetical thickness of a wetting layer of nonhydrolyzed poly(ethoxysiloxane) (1000 nm); see Table 3.28 If one assumes that the poly(alkoxysiloxane) is completely hydrolyzed and converted into silica, the expected membrane thickness can be estimated from the applied amount, the changes in the molar mass upon hydrolysis and the density of silica (see Appendix 2). From eq A2.3 and the literature value of the density of silica of 2.0 g mL-1,29 we estimate the thickness of silica layers resulting from a surface coverage of 1.147 gm-2 of poly(ethoxysiloxane) to be approximately 260 nm (see Table 3). The measured values are significantly larger than this value. It is known that silica derived from the hydrolysis of alkoxysiloxanes may contain pores30 or be strongly hydrated,31 causing its density to be less than that of fused quartz. Thus, it is reasonable to assume that our membranes also contain pores. No clear indication of pores can be seen by high-resolution SEM. Therefore, either most of the assumed pores must be smaller than the resolution of the electron microscopy (18 nm) or the pores were smeared out by the cutting process, or the material may be strongly hydrated. If one assumes that the membrane is composed of a water-containing silica xerogel of composition (SiO2)‚4H2O and density 1.65 g cm-3 like the one described in ref 32, one obtains quite a good agreement between the experimental thickness and the thickness calculated from eq A2.2 (see Table 3). On the other hand, the larger than expected thickness might be due to the fact that the poly(ethoxysiloxane) is only partially hydrolyzed and still contains a significant fraction of ethoxy groups. To clarify that question, a freely suspended membrane (transferred to a metal grid) was investigated by transmission infrared spectroscopy and electron probe microanalysis. The infrared spectrum of the membrane (See Figure 7) indeed reveals peaks in the spectral range of 3000 cm-1 to 2850 cm-1 corresponding to methyl and methylene groups. Furthermore, the analysis of the elemental composition by electron probe micro-

Figure 7. Transmission infrared spectrum of a freely suspended membrane of poly(alkoxysiloxane) hydrolyzed on an aqueous subphase of pH 1 and subsequently transferred to a metal grid. Peaks in the spectral range of 3000 cm-1 to 2850 cm-1 indicate the presence of alkyl groups.

1000

1000

708

716

265 685

257 664

analysis reveals a significant content of carbon (the elemental composition corresponds to a sum formula of Si1+0.1O2+0.2C0.8+0.1Hx33). One can thus conclude that the solid membranes obtained in this report are composed of partially hydrolyzed, cross-linked poly(ethoxysiloxane). Since we did not know the density of the partially hydrolyzed PAOS, we were unable to calculated a theoretical thickness and add it to Table 3. But we are convinced that the observed partial hydrolysis is indeed contributing to the higher than expected layer thickness. Conclusions We have shown that the liquid polymer poly(ethoxysiloxane) is a suitable liquid precursor for the preparation of solid membranes on a water surface. Although it does initially not wet the water surface, applying poly(ethoxysiloxane) to the surface of acidic water finally yields continuous solid membranes. This presumably occurs via a simultaneous surface reaction combined with the slow spreading of nonreacted material, aided by the presence of the already formed thin layer of cross-linked material. Acknowledgment. The authors would like to kindly thank Aziz Muzafarov for stimulating discussions concerning the synthesis of the polymers, Paul Walther for help in the high-resolution electron microscopy, Marlies Fritz for the TGA measurements, and Bernhard Stoll for the dielectric measurements. Cathy McNamee thanks the Alexander von Humboldt Foundation for a fellowship. This work was supported by the Deutsche Forschungsgemeinschaft through SSP “Wetting & Structure Formation at Interfaces”. Werner Goedel thanks B. Rieger, K. Landfester, R. Iden, and H. Auweter for support. Appendix 1. Calculation of the Hamaker Constant of a Three-Phase System The Lifshitz theory of the van der Waals forces treats the materials of a three-phase system as continuous media. This allows the Hamaker constant to be calculated in terms of such bulk properties of the materials as their refractive index and dielectric constant. The expression of a Hamaker constant, A123, for a three-phase system is given by37 (28) For details on the calculation of the thickness, see Appendix 2. (29) Ketelson, H. A.; Pelton, R.; Brook, M. A. Langmuir 1996, 12, 1134. (30) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Tokyo, 1990; p 366. (31) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Tokyo, 1990; p 244. (32) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, 1990; p 538. (33) Electron probe microanalysis is insensitive to hydrogen; thus, no number could be assigned to the hydrogene content. During electron probe microanalysis, the sample is subjected to ultrahigh vacuum; thus, it is possible that part of the incorporated water is evaporated before measurement. Since the element of interest was carbon, the specimen was not coated by a carbon conduction layer. Thus, charging effects may cause an additional systematic error to the elemental composition. (34) Rochow, E. G. Silicon and Silicones; Springer-Verlag: Berlin, 1987; p 106.

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A123(total) ≈ 3hνe

(

)(

)

A - L S - L 3 + kT 4 A + L S + L (n2A - n2L)(n2S - n2L)

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formula and density of silica. The thickness of a film, hi, is determined from the surface coverage (mass/area), Γi, at the air-water interface and the substance density, Fi, as expressed in eq A2.1

2 2 1/2 2 2 1/2 2 2 1/2 2 2 1/2 8x2 (nA + nL) (nS + nL) [(nA + nL) + (nS + nL) ]

where A, L, and S are permittivity values of air (layer 1), layer 2, and the substrate (layer 3), respectively. nA, nL, and nS are the refractive indices of air, layer 2, and the substrate. k, T, h, and νe are Boltzmann constant, absolute temperature, Planck constant, and main electronic absorption frequency in UV (∼3 × 1015 s-1). The dielectric constants and refractive indices of water, silica, and poly(dimethylsiloxane) were taken from refs 34-37. The corresponding data for the poly(ethoxysiloxane) and diluted hydrochloric acid were experimentally determined using the equipment described in ref 25. It should be noted, however, that this method does not take into account the distance dependence due to retardation, thus especially in the case of layers of several hundred micrometer thickness, the exact values might be smaller than the values given here. Appendix 2. Calculation of Thickness of Poly(ethoxysiloxane) Membrane after Hydrolysis The complete loss of the ethyl groups from the poly(ethoxysiloxane) should result in a material with a sum (35) Iler, R. K. The Chemistry of Silica Solubility, Polymerisation, Colloid and Surface Properties, and Biochemistry; John Wiley & Sons: Brisbane, 1979; p 19. (36) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1994; p 41. (37) Israelachvili, J. Intermolecular & Surface Forces; Academic Press: London, 1994; p 184.

hi )

Γi niFwi m ) ) Fi AiFi AiFi

(A2.1)

where Fwi, ni, m, and Ai are the formula weight of the compound (or the repeat unit in the case of a polymer), the number of moles applied, the mass applied, and the surface area of the film of substance i. If there is no loss of silicon atoms, then the mass of silica can be given by

mpoly(ethoxysilane) mSiO2 ) FwSiO2 Fwpoly(ethoxysilane)

(A2.2)

where the subscripts SiO2 and poly(ethoxysiloxane) refer to silica and poly(ethoxysiloxane), respectively. Substituting eq A2.2 into eq A2.1 gives the thickness of the silica membrane (hSiO2) that is expected for complete conversion of poly(ethoxysiloxane) into silica; see eq A2.3.

hSiO2 )

LA0509089

FwSiO2

mpoly(ethoxysilane)

Fwpoly(ethoxysilane)

FSiO2A

(A2.3)