Langmuir 2000, 16, 4613-4619
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Synthesis of Silica Films at the Air/Water Interface: Effect of Template Chain Length and Ionic Strength J. L. Ruggles,* S. A. Holt, P. A. Reynolds, and J. W. White* Research School of Chemistry, The Australian National University, Canberra, Australian Capital Territory 0200, Australia Received November 8, 1999. In Final Form: February 16, 2000 We have grown surfactant-templated silicate films at the air-water interface using n-alkyltrimethylammonium bromide and chloride in an acid synthesis with tetraethyl orthosilicate as the silicate source. The films have been grown with and without added salt (sodium chloride, sodium bromide) and with n-alkyl chain lengths from 12 to 18, the growth process being monitored by X-ray reflectometry. Glassy, hexagonal, and lamellar structures have been produced in ways that are predictable from the pure surfactant-water phase diagrams. The synthesis appears to proceed initially through an induction period characterized by the accumulation of silica-coated spherical micelles near the surface. All syntheses, except those involving C12TACl, show a sudden transformation of the spherical micellar phase to a hexagonal phase. This occurs when the gradually increasing ionic strength and/or changing ethanol concentration is sufficient to change the position of boundaries within the phase diagram. A possible mechanism for this to occur may be to induce a sphere to rod transition in the micellar structure. This transformation, as predicted from the surfactant-water phase diagram, can be induced by addition of salts and is slower for chloride than bromide counteranions. The hexagonal materials change in cell dimension as the chain length is changed in a way consistent with theoretical model predictions. All the materials have sufficiently flexible silica frameworks that phase interconversion is observed both from glassy to hexagonal and from hexagonal to lamellar and vice versa in those surfactant systems where multiple phases are found to exist.
Introduction 1
Since the discovery of MCM materials in 1992, it has been understood that surfactant molecular assemblies can function as templates for the growth of siliceous mesoporous materials. Aksay et al.2 demonstrated the synthesis of thin mesoporous films at the air/water interface, from the cooperative self-assembly of a surfactant template (cetyltrimethylammonium bromide, C16TABr) and a slowly condensing silica source (tetraethyl orthosilicate, TEOS). This synthesis is conducted with the surfactant concentration in excess of the critical micelle concentration (cmc). The bulk solution contains micelles in equilibrium with monomers, and there is a surface excess of surfactant molecules with polar headgroups in the solution and the tails pointing upward out of the solution.3 This supramolecular assembly forms the initial template structure for film growth. The synthesis of MCM-41, -48, and -50,4 mesoporous materials of hexagonal, cubic, and lamellar structures, occurs in bulk solution at elevated temperature and at far lower surfactant concentrations, whereas this film synthesis is conducted at low pH, high surfactant concentrations. and at room temperature. In situ neutron and X-ray reflectometry experiments on the growing film from a C16TABr preparation have been performed. At the start of the reaction they show only a monolayer of surfactant at the air-water interface, and an underlying concentration gradient of micelles ca. 70-150 Å deep.5 Later, structure slowly develops, cor* Correspondingauthor.E-mail:
[email protected];jww@rsc. anu.edu.au. Telephone: Int +61 2 62493575. Fax: Int +61 2 62490750. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Aksay, I. A.; Trau, M.; Mann, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (3) Penfold, J.; Thomas, R. K., J. Phys Conden. Matter 1990, 2, 1369. (4) Edler, K. J.; Reynolds, P. A.; White, J. W.; Cookson, D. J. Chem. Soc., Faraday Trans. 1997, 93, 199.
responding to increased layering at the surface in which silica-rich and surfactant-rich layers alternate. Finally a highly oriented partly crystalline structure with a regular repeat distance of approximately 45 Å and three orders of pseudo-Bragg diffraction6 appears very rapidly after this induction period.5 Brown et al.7 proposed that initially the surfactant, at a concentration in excess of the cmc, forms a structure at the surface with the surfactant hydrocarbon tail 11(1) Å thick, with a 12(2) Å aqueous sublayer of counterions and headgroups. The model proposed an ordering of either surfactant bilayers or micelles beneath the surface, initially heterogeneous surface coverage followed by complete coverage reaching progressively deeper into solution. This ordering of surfactant micelles is accompanied by expulsion of water to be replaced by condensing silicate species which surround the external part of the micelles. After an induction period, rapid growth of the film down into the subphase occurred to give a crystalline structure. Grazing incidence X-ray diffraction experiments support the model of a final partly crystalline two-dimensional hexagonal p6m phase with rodlike channels oriented parallel to the surface.8,9 The films can also be removed from the growth interface after several days and placed onto various substrates for further analyses. Synchrotron X-ray experiments on films placed on mica and silicon substrates, and dried samples placed into capillaries, have also revealed in some syntheses a transformation from an unusual initial Pm3n cubic to a p6m hexagonal phase10 as the film dries. (5) Brown, A. S.; Holt, S. A.; Reynolds P. A.; Penfold, J.; White, J. W. Langmuir 1998, 14, 5532. (6) Hyde, S. Curr. Opin. Solid State Mater. Sci. 1996, 653-662. (7) Brown, A. S.; Holt, S. A.; Dam, T.; Trau, M.; White, J. W. Langmuir 1997, 13, 6363. (8) Ruggles, J. L.; Holt, S. A.; Reynolds, P. A.; White, J. W. Personal communication, 1999. (9) Holt, S. A.; Foran, G. J.; White, J. W. Langmuir 1999, 15, 2540.
10.1021/la9914559 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/13/2000
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Table 1. Summary of Synthesis Conditions surfactant
T/°C
cmc/mM
C12TACl C12TABr C14TACl C14TABr C16TACl C16TABr C18TACl C18TABr C18TABr
25 25 25 25 25 25 25 and 40 25 25 and 40
20 16 4.5 2.1 1.3 0.92 0.3 0.3
surfactant concn/mM
surf:Si
surf:H2O × 10-3
302 304 143 147 68 73 19.7 2.5 0.69
0.999 0.997 0.91 0.999 0.979 1.04 0.749 1.08 0.925
6.52 6.76 2.85 2.85 1.28 1.3 0.36 1.45 0.4
Krafft temp/°C 2a 0a 10b 8b 28.517 15.520 3320 3320
a Rubingh and Holland, Eds. Cationic Surfactants; Surfactant Science Series; Marcel Dekker: New York, 1990; p 15. b Warnheim et al. J. Colloid Interface Sci. 1988, 125, 2.
The generality of the mechanism for film formation described above is tested in this paper by variation of both surfactant molecule and the ionic strength. Whereas in previous syntheses we have used C16TABr/C16TACl (Cl ) chloride) as the surfactant templates, here we attempt to grow similar films at the interface from surfactants of varying alkyl chain length, CnTABr from n ) 12 to18. The growth of films has been studied at various ionic strengths by use of X-ray reflectometry. The tendency to form lyotropic phases of these surfactants in water is well understood,11-13 and the earlier reflectivity results show that these phase transitions are largely maintained, although occurring at far lower threshold concentrations and in a more complex system (see Figure 2). The effect is a shift to the left of the boundaries in the usual phase diagram of the system. A key focus of the present work is to elucidate the nature of the induction process. Although it was acknowledged that some partially organized layering of surfactant molecules intercalated by silicate-rich layers existed at the surface, the process of rapid crystallization was not understood. It was proposed that the induction period involved either bilayers or cylindrical micelles with their long axes parallel to the interface beneath the surface coverage, and crystallization involved removal and replacement of subphase in this region by silicated surfactant species. However, at such low pH conditions (pH ) 0.25), where TEOS hydrolysis is almost immediate on contact with water, micelles are known to rapidly acquire a sheath of partially condensed silica, and further condensation of this silica is slow.14 There are four ethanol molecules produced during hydrolysis for each TEOS molecule. The alcohol molecule is known to arrange itself partially buried within the micelles, with the hydroxyl group between the surfactant headgroup, causing an increase in the effective headgroup area, ao.14 It is proposed that the induction period may represent the gradual decrease of alcohol molecules in the headgroup region of the micelles as some alcohol-catalyzed condensation of silicate oligomers occurs.15 We now propose there is an increase in silicated micelles density and concomitant expulsion of water in the near surface region, accompanied by a sphere to rod micellar transition of all micelles in the solution. Finally there is rapid growth of p6m phase, triggered by the transition and caused by increasing density of silicate species and/or change in ethanol concentration. We will also demonstrate that the silicate framework is sufficiently flexible that this transition can occur by cycling between a p6m to lamellar phase, which occurs with the C18 homologue. Experimental Section Surfactants dodecyl (C12), myristyl (C14), cetyl (C16), and octadecyl (C18) trimethylammonium bromide were obtained from
Aldrich, chloride compounds were obtained from Fluka, and and all were recrystallized from an acetone/methanol solvent for purification. Purification was indicated by a sharp Krafft transition temperature. Hydrochloric acid, 37% (w/w) AnalaR grade (BDH), and tetraethyl orthosilicate (TEOS) 99+% (Aldrich) were used. The syntheses, unless specified otherwise,16 were prepared by dissolving surfactant in Millipore water, acidifying with hydrochloric acid, and finally adding tetraethyl orthosilicate. The resultant solution was stirred at controlled speed for 3 min and then placed into a trough for measurement. Syntheses were conducted at 25 °C; however for C18TAX (X ) Br, Cl), syntheses were also conducted at the Krafft temperatures of 37 and 40 °C. At 25 °C these surfactants exhibit lamellar gel phases. The surfactant/water and surfactant/silica ratios of our previous syntheses10 using C16TAX were approximately maintained for this experiment (Table 1). In the case of C12TABr and C14TABr, higher concentrations of surfactant were necessary to maintain a level of surfactant well above the cmc since the critical micelle concentration increases with reduction in alkyl chain length. Synthesis of the films was conducted in specially prepared Teflon troughs attached to a hollow brass base through which temperature-controlled water flowed. They were hermetically sealed with a Perspex lid fitted with Mylar windows to allow for transmission of X-rays. Measurements of specular reflection were conducted using the Research School of Chemistry X-ray reflectometer.6 Reflectivity probes the changes in refractive index of the system perpendicular to the surface by measuring specular reflection of X-rays as a function of the angle of reflection. For the elastic scattering of X-rays, the scattering vector is |QZ| ) (4π/λ) sin θ, where θ is the angle of incidence and λ is the wavelength. The reflectometer has an angular resolution ∆QZ/ QZ of variable resolution of 1% at the highest Q values to 8% near the critical angle for the external reflection from the water surface, when taken over the full Q range from 0.01 to 0.48 Å-1. This is further reduced to 1 to 4% over the reduced Q range used in these experiments, i.e., 0.03-0.35 Å-1. There was still some loss of solution to the atmosphere inside the lid due to the saturation of the atmosphere within the lid, and hence small surface height changes occur during data collection. After film growth begins, this loss of height is greatly reduced, as the partially grown film tends to reduce evaporation from the surface. The syntheses conducted at higher temperature suffered more from rapid evaporation, even using sealed troughs. The height loss caused a shift to lower Q values over the collection times for the reflectometer and is the major source of variation in d spacing measurements in this experiment, causing uncertainties of approximately 2%. (10) Ruggles, J. L.; Holt, S. A.; Reynolds, P. A.; Brown, A. S.; Creagh, D. C.; White, J. W. Phys. Chem. Chem. Phys. 1999, 1, 323. (11) Miyagishi, S.; Asakawa, T.; Kurimoto, H. Bull. Chem. Soc. Jpn. 1995, 68, 135. (12) Kodama, M.; Seki., S. J. Colloid Interface Sci. 1987, 117, 485. (13) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216. (14) Di Renzo, F.; Testa, F.; Chen, J. D.; Cambon, H.; Galarneau, A.; Plee, D.; Fajula, F. Micro. Meso. Mater. 1999, 28, 437. (15) Assink, R. A.; Kay, B. D. Colloids Surf., A 1993, 74, 1-5. (16) Note: Some experiments involved either heating of solutions to dissolve reactants or slow stirring for a specified period of time. These descriptions are within the text of the results and discussion where relevant.
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Langmuir, Vol. 16, No. 10, 2000 4615 Table 2. List of Hexagonal d spacings and Induction Times for the Syntheses Conducted
Figure 1. C12TACl synthesis: (a) reflectivity 2.5 h after mixing; (b) 5 h and fringe has developed; (c) after 42 h growth, same fringe; (d) C12TACl synthesis with 6 mL of 5 M NaCl added to solution after 24 h on a reflectometer.
Figure 2. Schematic phase diagram for the system C16TAB/ water. Note the cmc2 where sphere to rod transition occurs in micellar solution and that at room temperature exists as crystals in water. Arrow shows the position of this synthesis in a binary diagram.18 For some films we performed X-ray diffraction in reflection geometry using a Siemens Cu-tube source diffractometer in θ/2θ configuration, and the X-ray diffraction pattern was recorded from θ ) 2° to 10°. This was to search for higher orders of the film structure perpendicular to the surface and also for any powder scattering from misorientated crystallites.
Results C12TABr/Cl Syntheses. The C12TACl synthesis produced a well-formed film almost a micrometer thick (which could be removed physically) after similar growth times to that of the standard preparation. After a similar induction period to that observed in standard C16TABr/Cl syntheses, reflectivity showed no Bragg peaks, only a welldeveloped fringe centered at approximately 0.15 Å-1 which increased in intensity over time. Figure 1 shows a series of reflectivity profiles from a film grown from C12TACl synthesis over a 30 h period. After approximately 24 h, 6 mL of 5 M NaCl solution was added to the reduced bulk of solution (some solution lost to atmosphere through evaporation). This only produced a fringe of higher intensity. This film growth without rapid crystallization
surfactant
T/°C
d-spacing
C12TACl C12TABr C14TACl C14TABr C16TACl C16TABr C18TACl C18TACl C18TABr C18TABr
25 25 25 25 25 25 25 40 25 40
34.8 38.2 40.3 44.9 44.9 46.2 40.8 50 41 50.6
Bragg peak predicteda 34.7 36 39.8 41 44.8 46
induction time/min >1500 >1500 360 400 180 0
50 51.3
0 150
contrasts with C16TABr/Cl, where Bragg peaks grow after induction period and reflectivity generally decreases with time. The film was removed and a portion placed flat onto a silicon (111) slice and examined by X-ray diffraction in reflection geometry. The scattering pattern showed a broad peak, characteristic of glassy amorphous structures, centered at a d spacing of about 37 Å. The C12TABr synthesis without added salt produced the same reflectivity as C12TACl, no Bragg peaks in the reflectivity profile but an optically obvious film at the surface. Syntheses conducted in 3 M NaBr solution produced Bragg peaks in the first profile recorded, at d spacings of 34.7 and 17.4 Å. Ozeki et al.33 predicts a sphere to rod transition of C12TABr in 3 M solutions. It is believed that the immediate appearance of Bragg peaks indicates the elimination of the induction period. C14TABr/Cl Syntheses. The syntheses using C14TABr produced Bragg peaks at 40.6 and 20.3 Å, which appeared after an induction period of approximately 330 min in the Teflon trough at 25 °C (Table 2). The reflectivity profile for the C14TACl synthesis showed a fringe, as with C12TACl, with a broad maximum in the profile corresponding to a d spacing of approximately 40 Å, but no appearance of any Bragg reflections after 24 h. After the C14TACl synthesis had been on the reflectometer for 24 h without peak development, 5 mL of solution was removed and replaced with 5 mL of 5 M NaCl solution. This is equivalent to adjusting the NaCl concentration of the solution to about 1.7 M NaCl, given that total volume of the trough is approximately 15 mL. The very next reflectivity profile showed Bragg diffraction peaks of d spacing of 38.2 Å but no associated second-order peak, even after several hours. Reflectivity profiles were then collected for syntheses with C14TAX at salt concentrations known to induce the sphere to rod micellar transitions in binary aqueous solutions. The C14TACl synthesis involved the use of 3 M NaCl solution, and Bragg diffraction peaks were observed upon the first reflectivity profile recorded, at maximum 20 min after mixing, at 38.2 and 19.1 Å. The C14TABr synthesis used 0.2 M NaBr solution, and again Bragg diffraction peaks were observed in the first profile at 40.6 Å. Finally a further C14TABr synthesis was conducted in a 0.2 M NaBr solution without any silica source (TEOS) added, to check that the reflectivity profile showed no evidence of the characteristic Bragg diffraction unless there was a silica source to decorate and stabilize the surfactant phase. There was no observed significant scattering in the profile, and subsequent modeling revealed the most sensible model consisted of a 12(1) Å layer of surfactant tails at the surface and a 11(1) Å layer of counterions and headgroups immediately below. Modeling also revealed an associated 180 Å region below the surface excess, slightly less dense than the subphase, indicating some accumulation of micelles, in agreement with previous
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studies of water/surfactant systems with concentrations in excess of the cmc.17 C16TABr/Cl Syntheses. Under standard conditions (25 °C, concentration far in excess of the cmc and a surfactant/ silica source ratio of approximately 1) a visible film grew, and Bragg diffraction peaks appeared after approximately 180 min for C16TABr and 400 min for C16TACl in “saltfree” mixtures. The Bragg peaks were at 46/44.9 Å and 23/22.8 Å, respectively, which were assigned as the (1,0) and (2,0) reflections from a hexagonal array of rods lying parallel to the interface. Reflectivity profiles were taken of several C16TAX syntheses to ascertain whether an induced sphere to rod micellar transition would eliminate the “induction” period before Bragg reflections appear. Extensive studies have been conducted on the phase behavior of C16TAX18-21 surfactants in binary water/surfactant systems, and further syntheses were conducted in sufficiently concentrated salt solutions known to induce the transformation from sphere to rod. A synthesis involving C16TABr in a 0.5 M NaBr solution showed a peak in the first reflectivity profile recorded. A synthesis of C16TABr was also conducted in which the silica source was combined with half the water used in synthesis and stirred gently at room temperature for 30 min before mixing with other ingredients. This was done to check if the same effect of immediate Bragg diffraction from the film could be obtained by a silica source which was already partially condensed, i.e., consisting of a mixture of Q1, Q2, Q3, and Q4 species. There was no sign of Bragg diffraction for over 100 min, which indicates that condensation of the silicate does not appear to significantly alter the phase relationships to the same degree as altering the ionic strength of the solution. C18TABr/Cl Syntheses. The C18TABr syntheses were conducted at two temperatures (25 and 40 °C) because of the existence of lamellar gel phases at 25 °C12 rather than the isotropic micellar phase of lighter homologues.21,22 The isotropic micellar solution, which is the precursor solution for film growth in all other CnTAX syntheses, only exists above 40 °C for C18TABr22 in the binary water/surfactant system and 28 °C for C18TACl23 synthesis. There are two effects of the raised temperature which impinge on the results of the experiment. These are (a) raising the temperature reduces the time taken for the growth of a film to occur and for the appearance of the Bragg diffraction peaks and (b) the loss of solution during time taken for data collection causes film height changes; hence inaccuracies in determinations of Q occur. Synthesis at 40 °C using C18TABr produced peaks at 50.6 and 25.5 Å in the first reflectivity profile recorded. Further C18TABr syntheses were conducted at room temperature, and peaks were recorded at 40.8 and 20.6 Å corresponding to the known coagel phase. The temperature of the Teflon trough was raised to 40 °C, and after equilibration Bragg diffraction peaks were then observed at the larger d spacing of 50.5 Å. Figure 3 shows reflectivity profiles of a synthesis where the temperature was cycled between 25 and 40 °C. Initially, with the trough (17) Simister, E. A.; Lee, E. M.; Thomas, R. K. J. Phys. Chem. 1992, 96, 1373. (18) Imae, T.; Kamiya, R.; Ikeda, S. J. Colloid Interface Sci. 1985, 108, 215. (19) Lu, J. R.; Li, Z. X.; Smallwood, J.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1995, 99, 8233. (20) Fontell, K.; Khan, B.; Lindstrom, B.; Maciejewska, D.; PuangNgern, S. Colloid Polym. Sci. 1991, 269, 727. (21) Mancini, G,; Schiavo, C,; Cerichelli, G. Langmuir 1996, 12, 3567. (22) Kodama, M.; Tsujii, K.; Seki, S. J. Phys. Chem. 1990, 94, 815. (23) Kodama, M.; Seki, S. Prog. Colloid Polym. Sci. 1983, 68, 158.
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at 40 °C, we observed a small peak developing at a Q range of the diffraction from the 50.5 Å peaks. The temperature of the trough was then lowered to 25 °C, and a peak developed immediately at 40.8 Å. The temperature was next raised above the 40 °C transition temperature from gel phases to isotropic micellar solution, and the next reflectivity profile contains peaks of reduced intensity at two d spacings, 40.8 and 50.5 Å, representing the lamellar and hexagonal phases, respectively. Further profiles at this temperature showed only the Bragg diffraction from the 50.5 Å hexagonal phase. The temperature of the trough was then again lowered to 25 °C, and the reflectivity profile showed a peak at 40.8 Å after approximately 6 h. Subsequent X-ray diffraction (XRD) analysis of the film grown at 25 °C, in both transmission and reflection geometry, confirmed a 41 Å repeat lamellar phase. A control study was made of the reflectivity from C18TABr in acidified aqueous solution without addition of silica source (TEOS), to investigate the role of the silica. A Bragg peak appears immediately at room temperature at 43.6 Å, and after the temperature is raised to 40 °C, the peaks disappear leaving a reflectivity profile characteristic of a surface excess of surfactant. When the temperature is again lowered to room temperature, there is no evidence of any peaks for several hours, after which the 43.6 Å peak reappears. Syntheses were conducted using C18TACl as surfactant where the temperature was cycled between 25 and 40 °C. With the trough at 25 °C, the reflectivity profile contained two peaks at 40.8 and 20.6 Å. The temperature was raised to 40 °C, and a peak appeared at 50 Å in the first reflectivity profile collected. Discussion Film Structure. For the hexagonal phase identified by three orders of Bragg diffraction, there is a linear increase in cell size for the series CnTAX, n ) 14-18. The increase is about 5 Å for each additional pair of CH2 groups added to the alkyl chain (Figure 4). This is in agreement with the predictions of micelle size using Tanford’s theoretical models,24 which are included in Table 2. There is an increase in length of the chain of 1.27 Å per CH2 group. Importantly in a micelle, there are two chains interdigitated at the center, therefore actually four CH2 groups contribute, these are added and 1.27 × 4 ) 5.08 Å is the overall increase in micelle size. There are noticeable counterion-dependent differences in hexagonal cell sizes for syntheses of silicate films (Table 2) which follow a linear pattern. The films grown using the bromide form of the surfactant are consistently larger in cell size. For C14TAX syntheses this difference is 2 Å, C16TAX 1 Å, and C18TAX 0.5 Å. The bromide ion is less hydrated than the chloride and may produce a better ion pair with the trimethylammonium. The tendency of a counterion to bind closely to micelles follows the tendency for these ions to be hydrated by water molecules, i.e., their shape and charge. The counterions used in these syntheses are Cl- < Br-, in the order of Hofmeister’s25 lyotropic series, reflecting the ion charge and hydrated radius. Magid et al.26 conducted calorimetric studies of the binding of various anions to the C16TA+ micelles and concluded that the distance of closest approach to the charged micellar surface determines the (24) Tanford, C. J. Phys. Chem. 1972, 76, 3020. (25) Swanson-Vethamuthu, M.; Almgren, M.; Karlsson, G.; Bahadur, P. Langmuir 1996, 12, 2173. (26) Larson, W. L.; Magid, L. J. J. Am. Chem. Soc. 1974, 96, 5774.
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Figure 3. C18TACl synthesis: (a) initially T at 25 °C and lamellar phase peak at d ) 41 Å appears immediately; (b) as T is increased both phases briefly coexist, peaks at 50 Å and 41 Å (hexagonal and lamellar phases); (c) once T equilibrates at 40 °C only peak at 50 Å (hexagonal); (d) T is lowered again to 25 °C, and after 8-10 h peak reappears at 41 Å (lamellar).
Figure 4. Hexagonal cell d spacings observed, showing linear increase in cell size with added alkyl chain. Open circles are chloride homologues; filled squares are bromides.
effectiveness of binding of counterions. The bromide ion is known to be less hydrated in acidic aqueous solution and hence is able to approach the charged micellar surface more easily.
In the cases where Bragg scattering failed to develop, such as with C12TACl, while a visible film grew, the X-ray diffraction indicates a glassy structure. Further XRD8 shows that the alternative of a highly crystalline sample inappropriately oriented to show any Bragg peaks is not truesthe XRD shows a ring characteristic of glasses. The observed maximum in diffraction at ca. 37 Å is consistent with a glass of touching micelles. Film Growth. Film growth was observed in all syntheses, although the higher temperature synthesis of C18TAX had produced a very thin coverage after approximately 24 h which showed Bragg peaks. This contrasts with the very thick and optically reflective films produced from the C12TACl syntheses which showed no Bragg diffraction. The peak width in all hexagonal patterns is about constant with a full width at half-maximum (fwhm) of 0.01 Å-1 of the (1,0) peak at 0.137 Å-1. There were counterion-dependent differences in induction times, these are displayed in Table 2. Those syntheses using the surfactant with chloride ion counterion had consistently longer induction times. The C12TACl produced no peaks after 72 h growth, although C12TABr transformed after addition of salt. C14TACl required addition of salt after 24 h with no peak growth, whereas C14TABr grew peaks in 360 min without requiring any supplementary salt. C16TACl took 400 min as against 180 min for C16TABr without added salt, and finally with C18TAX grown
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at elevated temperatures, peaks appeared before data collection began less than 30 min after mixing. Understanding of these results requires an understanding of the sphere to rod transition for these micellar mixtures. The tendency for amphiphilic molecules, such as the cationic surfactants used in this synthesis, to form stable lyotropic mesophases in dilute aqueous systems has been quantified by the surfactant packing parameter,27 g, which accounts for the competing hydrophilic and hydrophobic forces
g ) v/aolc
(1)
where v is the volume occupied by the hydrophobic tail, lc is the effective length of the surfactant often taken as 80-90% of the fully extended hydrocarbon chain, and ao is the effective area of the charged hydrophilic headgroup.27,28 For ionic surfactants ao varies with solution ionic strength, temperature, and level of counterion binding.28-30 Values of g of 0.33, 0.5, and 1 favor the formation of spherical, rodlike, and lamellar morphologies forming in dilute solutions. Our surfactants CnTAX used at concentrations ∼10-100 times above the cmc, as we have, are known to form phases with spherical micelles when mixed only with water.31,32 However altering the ionic strength of the solution is known to induce a sphere to rod transition for CnTABr at n g 12, and at n g 14 for CnTAClforthealkyltrimethylammoniumhalidesurfactants,33-35 and at successively lower ionic strengths as the chain length increases. TEOS, the silicate source used in this synthesis, is hydrophobic, so initially hydrolysis is restricted to the TEOS droplet/water interface. However after 3 min of controlled stirring the solution clears indicating the establishment of a microemulsion. This occurs due to the lowering of interfacial tension by the ethanol produced from the rapid hydrolysis of TEOS. At very low pH and high water/Si ratio, the hydrolysis of TEOS is almost immediate and condensation is extremely slow. Di Renzo et al.14 propose the idea that rapid hydrolysis produces a coating of monomeric silicate species on the micellar surface. These silica coatings are largely uncondensed, containing silanol groups which makes the micelle more soluble, and the micelle acts as a reservoir of TEOS for further film growth, after hydrolysis, to silicic monomers. In our quiescent aqueous acidic synthesis the oligomeric silicate species is believed to acquire a positive charge due to one of the silicon atoms becoming six-coordinated.36 In the induction period, before transformation to rod morphology and crystallization of the hexagonal phase occurs, these oligomers gradually replace water molecules surrounding the counterions and headgroups in the region of the growing film. For C12TACl this is all that occurs, spherical micelles accumulate at the growing surface and are coated with silicate from TEOS either within the micelle or free in solution. This results in a phase which, (27) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (28) Hyde, S. T. Langmuir 1997, 13, 842. (29) Blackmore, E. S.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1115. (30) Linden, M.; Schacht, S.; Schuth, F.; Steel, A.; Unger, K. K. J. Porous Mater. 1998, 5, 177. (31) Malliaris, A,; Lang, J,; Zana, R. J. Colloid Interface Sci. 1986, 110, 237. (32) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 1898. (33) Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1982, 87, 424. (34) Ikeda, S.; Ozeki, S.; Tsunoda, M. J. Colloid Interface Sci. 1980, 73, 27. (35) Ozeki, S.; Ikeda, S. Bull. Chem. Soc. Jpn. 1981, 54, 552. (36) Iler, R. K. “The Chemistry of Silica; Wiley: New York, 1979.
while optically observable as similar to the hexagonal phase films, shows no characteristic diffraction. However, the replacement of water for silicate (or addition of salt) may induce a micellar sphere to rod transition in the higher homologues, since the ionic strength of the mixture is steadily increasing. Another explanation of the transition from sphere to rod morphology is that as the silicate species surrounding the micelles gradually condense, steric restrictions on the headgroup area develop which in turn restrict the effective headgroup area, ao. There are two possible explanations: (i) local concentration fluctuations drive the system into a completely different region of the phase diagram, or (ii) a combination of the effects of small local concentration fluctuations and a sphere to rod transition caused by altered packing forces. The first scenario leaves the phase diagram unchanged, while the second involves a shift of the boundaries of the phase diagram to the left (see Figure 2). The successful growth of a hexagonal film in the C12TABr synthesis and inability to do so with C12TACl demonstrate the role of the counterion. A 20 times stronger ionic strength solution is necessary for the transition to occur in binary water/surfactant systems of chloride compared to bromide counterion for C14TAX syntheses, and similarly in these syntheses there may be a tendency for the necessity for a greater density of polymeric protonated silica oligomers to replace water molecules in the regions between surfactant arrays. These strong correlations of induction times, added salt effects, counterions, and final film nature with the sphere to rod transition are good evidence for the proposed mechanism. Film flexibility. For the lamellar to p6m transition to occur must require substantial flexing and reorganization of condensed silicate. Our earlier work on the Pm3n to p6m transition10 showed that the silica framework is sufficiently flexible that this transition can occur, though with some degradation in crystal quality. We are fortunate that in our C18TAX temperature cycling experiments we can also demonstrate such large silicate framework flexibility. However these films are in early stages of growth, and silicate is believed to contain many uncondensed sites occupied by hydroxyls. Films appear to continue to grow thicker for up to a week, depending on conditions, and over this period much of the silicate is expected to have condensed, thereby reducing this flexibility. On heating a C18TABr synthesis from 25 to 40 °C, we observe a transition from a lamellar to the hexagonal phase, confirmed by recent synchrotron XRD7 experiments. These syntheses were conducted at a surfactant concentration of approximately 1 wt % of solution, while the transitions from gel phases to micellar solutions in water/surfactant binary mixtures, at 40 and 28 °C for C18TABr and C18TACl, respectively, occur at or above 17% weight of surfactant. However the highly protonated environment of a pH 0.2 solution, and the presence of the partially polymerized, protonated silica oligomers and the increased local micellar concentration at the air-water interface, has altered the phase boundaries locally. The same synthesis mixture can be cycled 40 f 25 f 40 f 25 °C, with the phase transitions hexagonal f lamellar f hexagonal f lamellar. This shows that the silica framework is indeed very flexible. The first reflectivity profile after heating from 25 to 40 °C was unusual in that two separate phases were identifiable in the same reflectivity profile, and these diffraction signals were both produced from structures at the air/water interface (see Figure 3b). Reflectivity measures the variation in refrac-
Silica Film Synthesis
tive index of the very topmost region of the solution, and at q ) 0.155 Å-1 Als-Nielsen et al.37 report that the penetration depth can be up to 106 Å, although absorption may reduce this by several orders of magnitude. We propose that as the temperature is raised, the two phases briefly coexist as the temperature of the solution equilibrates, and the silica framework reorganizes in this region of condensed phase adjacent to the surface. Concluding Remarks Raimondi and Seddon,38 in a recent review article on mesoporous material syntheses, concluded that these materials are the result of a pre-existing liquid crystal phase which is subsequently coated with slowly condensing silica. They also point out that this type of synthesis more closely resembles natural growth as it occurs in the confined membrane bound areas within cells, similar to the microenvironment of the micelle-rich region near the surface template in these syntheses. Our experiments support this view that the conditions necessary for the formation of a particular surfactant liquid crystal phase are little modified in these acidic quiescent preparations from that in pure surfactant-water mixtures and that the silica coating does not strongly influence these conditions. However we are able to grow films with characteristic liquid crystal phases forming at greatly reduced bulk concentration. This appears to be due to an accumulation of micelles at the surface, increasing the (37) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (38) Raimondi, M. E.; Seddon, J. M. Liq. Cryst. 1999, 26, 305.
Langmuir, Vol. 16, No. 10, 2000 4619
local concentrations. We can produce glassy, hexagonal, and lamellar structures in ways predictable from pure surfactant-water phase diagrams. This may involve manipulation of the micellar sphere to rod transition, by change in surfactant chain length and by a change in ionic strength of the synthesis mixture, or temperature cycling. The syntheses of silicate films all appear to proceed initially by an accumulation of silica-coated spherical micelles at the surface region. The C12TACl synthesis showed no further change, but C12TABr and all the longer chain materials show a sudden transformation to a hexagonal phase. The mechanism whereby this occurs is not yet fully understood, but there are indications that the shift in the phase diagram to region of stable rodlike micelles may be involved. This, as predicted, can be accelerated by addition of salts and is slower for chloride than bromide counteranions. The hexagonal materials change in cell dimension as the chain length is changed in a way consistent with previous micellar models. All the materials have sufficiently flexible silica frameworks that phase interconversion is observed both from glassy to hexagonal and from hexagonal to lamellar and vice versa in those surfactant systems where multiple phases exist. Acknowledgment. The authors wish to gratefully acknowledge the financial contribution from the Australian Research Council toward the construction and operation of the Research School of Chemistry X-ray reflectometer.StephenHoltacknowledgestheAustralianSynchrotron Research Program for the provision of a fellowship. LA9914559
