Expanded Mesoporous Silicate Films Grown at the AirâWater Interface

Langmuir 2003, 19, 793-800

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Expanded Mesoporous Silicate Films Grown at the Air-Water Interface by Addition of Hydrocarbons J. L. Ruggles,*,† E. P. Gilbert,‡ S. A. Holt,§ P. A. Reynolds,| and J. W. White| Department of Chemistry, University of Queensland, QLD 4072, Australia; ANSTO, Private Mailbag 1, MENAI, NSW 2234, Australia; ISIS, Rutherford Appleton Laboratory, Didcot OX11 0QX, U.K.; and Research School of Chemistry, The Australian National University, Canberra ACT 0200, Australia Received September 20, 2002 We have examined the effect of addition of saturated straight-chain alkanes, Cn (n even and n ) 8-16), and the aromatics toluene and benzene on the formation of mesoporous silicate films, which form at the air-water interface. The silicate films self-assemble by slowly condensing silicate oligomers attaching to templates formed by the alkyltrimethylammonium surfactants CnTAX (n even and n ) 12-18, and X ) Cl- and Br-) in solution. The collection of X-ray and neutron reflectivity profiles from these solutions shows that there is a chain length dependency to the formation of alkane-swollen hexagonal silicate films, which is manifest as a linear increase of the pore sizes of the films with increasing alkane chain length for Cn, n > 8, up to a limit of a hexagonal cell dimension of a100 ) 80.8 Å. The expansion is greatest when using decane as expander for all surfactant chain lengths examined. For C12 chain length surfactants a cubic phase results, C14 and C16 chain lengths form expanded hexagonal films, while C18 chain lengths are relatively unaffected by alkanes. There is also anion dependency, which is revealed in larger cell expansion over a wider range of alkanes for bromide syntheses over the chloride analogue. In contrast to the results obtained with MCM materials, the incorporation of aromatics does not produce swollen films. Examination of the structures in bulk solution using small angle neutron scattering (SANS) shows the formation of rodlike micelles in the bulk before film formation is suppressed by adding alkanes. Fitting SANS data to models showed that, in contrast to the case of the sphere to rod transition normally associated with the formation of hexagonal silicate films at the interface, only expanded spherical micellar structures capable of accommodating several alkane molecules per surfactant molecule exist in solution up to and beyond film formation.

1. Introduction 1

Beck and co-workers showed that the lattice spacing of MCM-41S materials could be expanded by the addition of swelling agents such as 1,3,5-trimethylbenzene. Later, Ulagappan et al.2 used straight chain alkanes as expanders for MCM-41S materials, by adding an equimolar quantity of alkane to the surfactant in the synthesis. Previous work with MCM-41S had shown that incorporation of alkanes3 beyond the length of the longest hydrophobic portion of the surfactant was unsuccessful; hence, experiments were restricted to the addition of alkanes, Cn, with n from 5 to 16. The authors found a linear increase in the pore expansion for alkanes, Cn, with n ) 8 to 16, but below C8 the pore expansion was unpredictable. They proposed a scheme for alkane solubilization based on alkane chains extending the surfactant alkyl chains, which results in a maximum surfactant-to-alkane ratio of 1 to 1. Blin et al.4 also examined the swelling of MCM using decane, and they found that their evidence supported a scheme of alkane as a core within the micelle and arranged among * To whom correspondence should be addressed. E-mail: [email protected]. Fax: (61) 7 3365 4299. † University of Queensland. ‡ ANSTO. § ISIS. | The Australian National University. (1) Beck, J.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C.; Olsen, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 4. (2) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759. (3) Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Rippon, S.; Lubetkin, S. D.; Mulqueen, P. J. Langmuir 1999, 15, 4495. (4) Blin, J. L.; Otjacques, C.; Herrier, G.; Bao-Lin, S. Langmuir 2000, 16, 4229.

the surfactant tails. They varied the molar ratio of surfactant to alkane in the syntheses and found that the maximum expansion of MCM-41S materials occurred at a molar ratio of between 1 and 2 alkane molecules for each surfactant molecule. The authors argued that the decane is solubilized as a core within the micelles but less organized than the scheme proposed by Ulagappan et al. In these experiments the amount of alkane solubilized in the solution containing micelles far exceeds the amount predicted from the small solubility of alkanes in these aqueous systems. According to Agren and co-workers,5 the ability of the silicate film synthesis solution to incorporate organic expanders is enhanced compared to the case of aqueous CTAB systems. The release of substantial amounts of ethanol into the reaction solution, through the hydrolysis of tetraethyl orthosilicate (TEOS), increases the oil solubility in the system 3-fold. Both schemes result in a change of micelle morphology from rods to spheres. There is an increase in the apparent hydrophobic tail length, lc, as the alkane effectively extends the surfactant alkyl chain in the micelle core. As a result, the surfactant packing parameter, pp ) v/lca0o, where v is the alkyl chain volume and a0 the headgroup area, decreases, and the system tends to higher interfacial curvature structures, in this case from rods (1/3 < pp < 1/2) to spheres (pp < 1/3).13 Such behavior is supported by thermodynamic calculations of solubilization of hydrocarbons in aqueous surfactant solutions, which predict a solubilization-induced rod to sphere transition.6-8 Investigators found that the rodlike inorganic-organic ag(5) Agren, P.; Linden, M.; Rosenholm, J. B.; Blanchard, J.; Schuth, F.; Amenitsch, H. Langmuir 2000, 16, 8809. (6) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2834.

10.1021/la0265833 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/03/2003

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gregates in C14TABr/NaSal aqueous systems transformed into spheres at very low concentrations of added decane. There is much in common between the formation of MCM materials in the bulk solution and the formation of silicate films at the air-water interface, including the crystalline structure of the material, the multistage inductive development, and the cooperative self-assembly between silicate oligomers and surfactant assemblies. Hence, we decided to examine the capability of the silicate film synthesis to incorporate organics in direct comparison to the results obtained for MCM materials. We have previously shown that the pore size of the silicate films grown at the air-water interface can be tuned by adjusting the template size via the length of the hydrophobic tail.9 Straight chain alkanes (Cn, n from 8 to 16), and the aromatic species toluene and benzene, were added to silicate film syntheses produced from a homologous series of trimethylammonium surfactants. The resulting films were examined in situ using X-ray and neutron reflectivity measurements. The optimum concentration of alkanes adsorbed, that is, that which achieves the maximum swelling of the hexagonal silicate film for any single alkane chain length/surfactant chain length combination, was also investigated by variation of the amount of decane incorporated into the C16TAX (X ) Br, Cl) silicate film synthesis solutions. The location of the organic species within the micelle, and the morphology of those precursor species formed in bulk solution before growth of the crystalline silicate film at the interface were examined by using isotopic contrast SANS techniques. 2. Experimental Section 2.1. Materials and Conditions. The surfactants dodecyl (C12), tetradecyl (C14), hexadecyl (C16), and octadecyl (C18) trimethylammonium bromide were obtained from Aldrich, and the chloride analogues from Fluka. The surfactants were purified by recrystallization from an acetone/methanol solvent, except C16TACl, which was supplied as a 25 wt % aqueous solution and used as received. Hydrochloric acid, 37% w/w AnalaR grade (BDH), and tetraethyl orthosilicate (TEOS), 99+% (Aldrich), were used as received. The surfactant was added to the Millipore purified water (>18 MΩ) and, where necessary, was stirred and heated to dissolve, after which the solution was allowed to return to room temperature. Alkane was added next, and again the mixture was stirred for several minutes to allow the oil to become incorporated within the micelles before addition of TEOS. Blin et al.10 demonstrated that the maximum pore expansion of MCM materials, when using decane as expander, was obtained with incorporation of oil into micelles before addition of the silicate source. The resultant solution was stirred at room temperature until completely clear (about 3 min) and placed in a Teflon trough on the reflectometer (ca. 10 mL for X-ray work and 25 mL for neutron reflectivity) within a controlled atmosphere at 25 and 30 °C, respectively. The films were grown at the air-solution interface on a meniscus sitting several millimeters above the edge of the Teflon trough, which enabled the specular reflection of radiation from the surface. The relative proportions of components used for those silicate film syntheses closely followed the recipes used for expansion using surfactants of different chain lengths.9 A typical molar composition of a film synthesis solution, used for both reflectivity and SANS experiments, comprised TEOS/surfactant/alkane/HCl/ H2O in the ratio 1:1:1:10:750 for those syntheses using the C16TAX form of the surfactant, except where the amount of added (7) Bayer, O.; Hoffman, H.; Ulbricht, W.; Thurn, H. Adv. Colloid Interface Sci. 1986, 26, 177. (8) Hoffman, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (9) Ruggles, J. L.; Holt, S. A.; Reynolds, P. A.; White, J. W. Langmuir 2000, 16, 4613. (10) Blin, J. L.; Otjacques, C.; Herrier, G.; Bao-Lian, S. Langmuir 2000, 16, 4229.

Ruggles et al. alkane was varied. For surfactants of different chain length the concentration used was adjusted to the same concentration beyond the CMC in solution. 2.2. X-ray and Neutron Reflectivity. The X-ray measurements were made using the Cu rotating anode X-ray reflectometer at the Research School of Chemistry, described11,12 in detail elsewhere. The data were measured, and the final specular reflectivity profiles were produced as described by Brown et al.13 Data were collected over the Qz range 0.03-0.35 Å-1: Qz ) (4π/λ) sin θ, where θ is the incident angle and λ the wavelength of X-rays used. The reflectometer was operated at an angular resolution of ∆Qz/Qz of 1 to 8% (∆Qz is constant). Additionally, neutron reflectivity profiles were collected on the time-of-flight reflectometer SURF at the Rutherford Appleton Laboratory, U.K.14 Data were collected at an incident angle of 1.5° and placed on an absolute scale by calibration against D2O. The instrument was operated at an instrument resolution of approximately 5%, and the data were rebinned at 3%. Each dataset was collected over 20 min as a compromise between the kinetics of film formation and the need for accuracy in the final data.15 The observed high background is due to the contribution of incoherent scattering from the solution, which contains hydrogenous materials. Collection of reflectivity data suitable for modeling during the induction phase of silicate film formation was not possible due to excessive surface activity. This activity is a result of the “Marangoni Effect” of self-adjusting gradients of liquid surface tension brought about by the competition between alkane and the surface-active surfactants.16 The duration of the induction period of film formation (defined as the period from mixing of components to the appearance of Bragg diffraction in the reflectivity profile from a hexagonal phase at the interface) was extended compared to that for synthesis without any added alkane. 2.3. Small Angle Neutron Scattering. SANS profiles were collected for C16TAX (X ) Br and Cl) synthesis solutions without any additives on the LOQ instrument, at the ISIS pulsed neutron source at Rutherford Appleton Laboratory, U.K.17,18 This timeof-flight instrument gives a Q range of scattering vectors from 0.009 to 0.22 Å-1. Samples were mounted in 1 mm thick flat quartz cells, and data were collected for ∼20 min. Aliquots were taken soon after mixing solutions, which indicates the immediate state of the system rather than an evolved structure. Samples were then measured at various times throughout, and beyond that of known film development (see Figure 4). A 100% D2O background was measured in a 1 mm cell and then subtracted from the data. Normalization was by use of a protonated/ deuterated polymer blend standard known as “Bates” polymer. SANS experiments for silicate film syntheses with added alkane were conducted on the small-angle diffractometer SAND, at the Intense Pulsed Neutron Source, Argonne National Laboratory, USA. The Q range covered in the measurements was from 0.006 to 0.6 Å-1. The sample liquids were contained in flat quartz cells with a 2 mm path length. The temperature of the sample was set by a thermostat-controlled circulating water bath, to an accuracy of 0.1 °C. Measured intensities were corrected (11) Brown, A. S.; Holt, S. A.; Thien-Dham; Trau, M.; White, J. W. Langmuir 1994, 10, 6363. (12) Jamie, I.; Dowling, T.; Holt, S. A.; Creagh, D.; Leon, R. J. 3rd Vacuum Society of Australia Congress Proceedings; 1995; p 25. (13) Brown, A. S.; Holt, S. A.; Dam, T.; Trau, M.; White, J. W. Langmuir 1997, 13, 6363. (14) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 2 1997, 93, 3899. (15) Penfold, J.; Lindner, P.; Zemb, T. Neutron X-ray Light Scattering 1991, 223. (16) Ruggles, J. L. A Study of the Formation of Mesoporous Silicate Films at the Air-Water Interface. PhD Thesis, Australian National University, 2001. (17) Heenan, R. K.; King, S. ISIS User Guide; Rutherford Appleton Laboratory: Didcot, 1992. (18) Heenan, R. K.; King, S. M. International Seminar on Structural Investigations at Pulsed Neutron Sources, 1993, Joint Institute for Nuclear Research, Dubna, Russia.

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Table 1. Cell Dimensions of Crystalline Phases Observed with Reflectivity from Silicate Film Syntheses with Added Alkanesa surfactant

no alkane

octane d100/Å

decane d100/Å

dodecane d100/Å

tetradecane d100/Å

hexadecane d100/Å

C12TABr C14TACl C14TABr C16TACl C16TABr C18TACl C18TABr

34.8 38.2 40.3 44.9 46.1 50 50.6

35.9 42.57 45.14 69.8 46.1 53.9 52.36

98, 57.2, 49, 32.6, 28.7 57.2, 54.5 60.7 70 70 53.9 53.93

98, 54, 49, 32.6 54.6 57.2 54.6 68.34 53.9 53.93

52.36 57.2 44.9 60.7 52.36, 41.3 53.93

44.9 62.52 52.36, 41.3 53.93

a

benzene d100/Å

toluene d100/Å

46.2

47.5

Bold preparations were those experiments repeated as SANS experiments conducted on the SAND instrument at IPNS.

for background and empty cell contributions, and again normalization was by use of standard “Bates” polymer samples. The preparations used D2O instead of H2O to maximize the contrast between scattering length densities (SLD) of the micelles, with a SLD of about -0.2 × 10-6 Å2, compared to D2O with a SLD of 6.34 × 10-6 Å2. SANS profiles were collected for a selection of those silicate film syntheses with added alkanes previously observed using reflectivity, and these are marked as the bold values in Table 1. The contrast of neutron scattering length density between the protonated alkyl chain of the surfactant and the deuterated alkane is theoretically sufficient for observation of the alkane configuration within the micelle. Two contrastssdeuterated alkane and deuterated solvent, and protonated alkane and deuterated solventswere examined to increase the information content of data. Some silicate film syntheses had Bragg diffraction associated with a thin coverage of crystalline silicate film on the windows of the SANS cell, but this was not consistently observed and was generally of very low intensity. The silicate film growth environment in SANS experiments is different from that used for reflectivity, as there is only a very small air-water interface (approximately 0.2 cm2) in the neck of the SANS cell, compared to large areas of meniscus in the Teflon troughs used for reflectivity (commonly 100 cm2 and greater). The growth of silicate films has been observed at the liquid-solid interface on substrates such as silicon19 and mica.20

3. Reflectivity Results

Figure 1. Reflectivity profile, collected on SURF, from C12TABr film synthesis with added decane. Peaks are marked with arrows at d spacings of 98, 57.2, 49, 33.2, and 28.7 Å, respectively.

The Bragg peaks from diffraction observed in reflectivity experiments are listed in Table 1. Multiple peaks, apart from the d100 reflection, are only listed for those syntheses where the reflections were not generated from a single hexagonal phase at the interface. The peaks are listed in Table 1 as the dhkl spacing observed, but later some discussion of the swelling of the hexagonal phases throughout the results is in terms of the percentage increase of the a100 cell dimensions of the hexagonal phase. Investigations of the effect of addition of alkanes were restricted to those silicate film syntheses using CnTAX surfactants (n even, n ) 12-18, X ) Br and Cl) known to readily form hexagonal partially crystalline silicate films at the interface under ambient conditions. Thus, C12TACl/ HCl syntheses were not investigated, even though a thick film forms at the air-water interface, because it has been shown to be amorphous.9 The addition of the alkanes caused an extension to the induction period of silicate film formation before the appearance of the Bragg diffraction in all syntheses investigated. 3.1. C12TABr. The C12TABr silicate film syntheses with added alkane were conducted in 3 M KBr D2O solutions. These “salted out” solutions were used because previous reflectivity experiments using C12TABr surfactants had shown that although the development of a crystalline film occurred without salt, this process was very slow, and

increase of solution ionic strength caused rapid film formation with little effect on the film morphology. Once the crystalline film had formed, indicated by the appearance of Bragg diffraction in the reflectivity profiles, pools of pure alkane as well as film material were observed on the surface. The synthesis using added octane produced reflections indexed as the hexagonal phase previously observed in C12TABr syntheses9 swollen by 3%. Both the decane and dodecane, C12TABr, silicate film syntheses produced multiple reflections, indexed as a cubic phase (Figure 1). The reflectivity profile of the C12TABr/decane film synthesis shown in Figure 1 contains five Bragg peaks, corresponding to d spacings of 98, 57.2, 49, 32.6, and 28.7 Å (with the 98 Å peak half the intensity of the 49 Å peak) generated by a cubic form of the silicate film. There may be a preferred orientation of the cube at the interface, giving rise to observation of certain classes of reflections as strong; however, nothing is known of such a textural effect. Similar patterns of peaks have been observed in XRD analysis of materials assigned the two cubic phases, Ia3d and Pm3n, which are commonly associated with the alkyltrimethylammonium halide surfactants used in the silicate film syntheses.21,22 3.2. C14TAX. The cylindrical solution structures necessary for the formation of the hexagonal form of the silicate

(19) Holt, S. A.; Reynolds, P. A.; White, J. W. Phys. Chem. Chem. Phys. 2000, 2, 5667. (20) Aksay, I. A.; Trau, M.; Manne, S.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892.

(21) Kruk, M.; Jaroniec, M.; Ryoo, R.; Joo, S. H. Chem. Mater. 2000, 12, 1414. (22) 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.

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film only exist for the C14TACl system under high ionic strength conditions.9 Hence, the C14TACl alkane addition syntheses were also conducted in “salted out” 2 M NaCl solutions. There was slight swelling of the hexagonal phase with added octane (11.4% increase of cell dimension), whereas with the longer chain length alkanes, C10 to C14, the swelling was significant (ranging from 37% to 49% increase). The C14TACl silicate film synthesis with added decane produced several reflections indexed as contributions from two separate hexagonal phases coexisting at the interface. The two phases are both larger than the hexagonal film formed when no alkane is added. All the C14TAX silicate film syntheses with added alkane resulted in hexagonal silicate films of larger cell dimensions than those formed without added alkanes; see Table 1 for details. The C14TABr syntheses with added alkane had shorter induction periods and larger expanded cell dimensions than the corresponding C14TACl syntheses. The largest expansion of cell dimensions occurred with added decane, and this was 23.5 and 21.9 Å for X ) Br- and Cl-, respectively. 3.3. C16TAX. The maximum swelling of the hexagonal form of the silicate film occurred when using decane as expander for both the C16TABr and the C16TACl syntheses. The expansion using decane was 100% of the possible molecular length of the alkane molecule arranged end to end.23 Further experiments were conducted using decane and C16TAX surfactants to examine the concentration dependency of the process. These involved the addition of between 0.5 and 4 mol of decane per mole of C16TAX surfactant. Throughout the silicate film syntheses there was some inconsistency between repeated identical syntheses due to the low solubility of alkane in the solutions. Evidence of this process came from the observation of pools of alkane on the surface, along with film material, after film growth had occurred. This occurred repeatedly, but not reproducibly, throughout all experiments using alkanes. Related to the appearance of alkane at the interface, there were some indications of multiple phases for the addition of decane and dodecane to the C16TACl syntheses, although the peaks corresponding to the expanded phases were of very low intensity. The addition of octane produced an increase of 28.7 Å over the standard hexagonal cell dimension for C16TACl, significantly longer than the length of two extended octane molecules arranged end-to-end.23 The addition of tetradecane and hexadecane produced no swelling of the standard film, and there was noticeable pooling of alkane at the interface. In contrast to the C16TACl syntheses, those C16TABr syntheses with the addition of octane produced a film with cell dimensions the same as those with alkane-free synthesis. All other alkanes, from decane to hexadecane, produced swollen hexagonal silicate films for C16TABr syntheses. 3.4. C18TAX. There was a slight but consistent swelling (3.8-4.5 Å) of the cell dimensions of the silicate films with the addition of alkanes for C18TAX syntheses. However, in comparison to the results of experiments using the shorter chain length surfactants, this is a small expansion. For those C18TACl silicate film syntheses involving the addition of tetradecane and hexadecane, there were two coexistent phases observed. These phases were indexed as the hexagonal phase, previously observed in reflectivity,9 and the coagel lamellar phase. All C18TABr silicate film syntheses with added alkane produced slightly swollen hexagonal phases, but unlike the case of C18TACl, there were no multiple phases observed. (23) Nyburg, S. C.; Potworowski, J. A. Acta Crystallogr. 1973, B29, 347.

Ruggles et al.

Figure 2. Reflectivity profiles, collected on SURF, for the C18TACl/HCl/TEOS/hexadecane system at 30 °C. Profiles have been offset for clarity: squares, 1st hour; circles, 4th hour; triangles, 5th hour; diamonds, 9th hour after mixing ingredients.

3.5. Aromatics. Experiments were conducted using benzene and toluene added to the silicate film synthesis solution to examine whether they too produced swollen hexagonal phases. The aromatics were added to C16TABr/ HCl silicate film syntheses, after mixing of surfactant solution and before addition of silicate as equimolar quantities of aromatic hydrocarbon and surfactant. Adding toluene to the C16TABr/HCl silicate film synthesis resulted in a slight (1.5 Å) increase in cell dimension but no change in induction period. The addition of benzene resulted in no change in cell dimension or induction period from those of the standard C16TABr/HCl silicate film synthesis. 4. Discussion 4.1. Surfactant Chain Length. The formation of the alkane-swollen form of the hexagonal silicate film only occurs for a narrow range of surfactant chain lengths between C14 and C16. The cell expansion of the C12TABr film, which only occurs for the addition of decane and dodecane, is accompanied by a structural change from the hexagonal to the cubic (see Table 1). Similarly, the longer chain length C18TAX syntheses produced phases little altered from those of syntheses without added alkane, mainly characterized by dissolution of the hexagonal phase for the C18TACl system to the lamellar film. The formation of the crystalline hexagonal form of the silicate film at the interface depends on the formation of silicate-encrusted cylindrical structures, which then pack into the lowest energy conformation of a hexagonal close packed array. The formation of the hexagonal silicate films in aqueous solution under ambient conditions is dependent on surfactant alkyl chain length.9 The addition of alkane disrupts the electrostatic charge balance in the interfacial region, which is most dramatic in systems least favoring the formation of hexagonal phases at room temperature.24 Figure 2 shows the reflectivity profiles for a C18TACl silicate film synthesis with added hexadecane, with the reflections from two different phases coexisting at the interface. Similar multiple hexagonal phases were observed by Linden et al.25 with oil solubilized in hexagonal silicate structures. The two phases grew in unison, (24) Magid, L. J.; Han, Z.; Warr, G. G.; Cassidy, M. A.; Butler, P. D.; Hamilton, W. A. J. Phys. Chem. B 1997, 101, 7919. (25) Linden, M.; Agren, P.; Karlsson, S.; Bussian, P.; Amenitsch, H. Langmuir 2000, 16, 5831.


indicated by a broad fringe appearing just prior to Bragg diffraction peaks. This fringe, from Q ) 0.08 to 0.18 Å-1, was observed over the Q ranges of both hexagonal and lamellar first-order diffraction peaks (d100). However, there is a slight maximum in intensity of the fringe at the Q range of the hexagonal reflection, which was interpreted as an indication that the first phase formed at the interface was the hexagonal. Some of the hexadecane was absorbed by the precursor micelles and resulted in a slightly expanded hexagonal film. As more alkane was incorporated within the micelle, the effect was an increased disturbance of the interfacial region, which resulted in the subsequent formation of a lamellar crystalline phase.26 There are no spherical micellar forms of the C18TAX system in the aqueous phase diagram, which are characteristic of the shorter chain length forms of the surfactant. This phenomenon is due to the increased attraction between the alkyl chains.27 Instead they form lamellar phases of the surfactant as the crystalline form of the surfactant in solution.28 The appearance of the lamellar form of the silicate film indicates that the incorporation of the alkane disturbed the surfactant chemistry, favoring the formation of crystalline surfactant phases at increased threshold temperatures. The transition from rods (hexagonal) to layers (lamellae) is the equivalent of the rod to sphere transition observed by Nagarajan et al.6 for the C16TAX system, which was consistently observed for all shorter chain length surfactants with added alkane. 4.2. Alkane Concentration. A series of syntheses were conducted to examine the concentration dependence of swelling of hexagonal silicate films, using the C16TABr and C16TACl forms of the surfactant, with increasing amounts of added decane. Decane was chosen because it had consistently produced the largest expansion of the hexagonal phase, and it offered a direct comparison with similar experiments using decane addition to the MCM41S synthesis.4 These experiments investigated added decane concentrations, which gave a molar ratio of alkane to surfactant of between 0.5 and 3.5. Blin et al. found that within the range of surfactant to decane molar concentration ratios of 0.5 to 2.5 the cell dimensions of MCM materials increase linearly.4 For surfactant-to-alkane ratios below 0.5 there was no consistent increase of the MCM cell dimensions, and above this range there was broadening and eventual disappearance of the Bragg diffraction from the hexagonal phase. In Figure 3 the increase of silicate film cell d100 spacings is plotted against the molar ratio of surfactant to decane added to the silicate film syntheses. For both chloride and bromide forms of the C16TAX surfactant we observed an overall increase of cell dimension with increasing decane concentration, similar to the results obtained with MCM.10 However, unlike the linear increase in cell dimensions for MCM materials, the silicate film syntheses rapidly achieved a maximum cell expansion. The C16TABr/HCl/decane silicate film syntheses achieved a maximum expansion with a surfactant-to-alkane ratio of 1, and this maximum expansion was maintained throughout the surfactantto-alkane ratio range to 3 mol of alkane per mole of surfactant. In contrast, the C16TACl/HCl/decane syntheses achieved maximum cell expansion at a ratio of around 0.6, and the maximum decreased beyond a ratio of 1.5. At concentrations beyond 3 mol of alkane per mole of (26) Kunieda, H.; Umizu, G.; Aramaki, K. J. Phys. Chem. B 2000, 104, 2005. (27) Israelachvili, J.; Mitchell, D.; Ninham, B. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (28) Kodama, M.; Seki, S. Prog. Colloid Polym. Sci. 1983, 68, 158.

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Figure 3. Cell dimensions of the hexagonal silicate film versus the concentration of added decane to C16TAX syntheses.

surfactant, there was no evidence of a hexagonal silicate phase at the interface. The maximum cell expansion was the same for both chloride and bromide forms of the surfactant, at a cell dimension of a100 ) 80.8 Å. This was 28.9 Å beyond the dimensions of a standard film synthesis, representing an expansion greater than the length of two decane molecules arranged end-to-end in the crystalline state.23 The range of maximum swelling, from a molar equivalent ratio of 1 to 3 alkane molecules per surfactant molecule in solution, represents more than the one to one alignment of alkane and surfactant molecule within the core proposed by Ulagappan et al.2 The data support the alternate model of alkane molecules distributed throughout the micelle, both as a pure alkyl core region and arranged alongside the surfactant chains. Calculations show that the expansion to a maximum cell dimension of 80.8 Å observed with alkane swelling represents an increase in the effective headgroup area of the surfactant from 64 to 180 Å2 in the endcaps of the rodlike micelles.29 In this expanded form the surface charge density of the micelles is reduced, and hence their ability to coordinate sufficient counteranions at the surface for binding with the charged silicate phase is compromised. The increased effective headgroup area is analogous to an increase in the Debye screening length (κ-1) sufficient to disfavor the formation of a rodlike structures, which in turn suppresses the growth of a hexagonal silicate film at the interface.24 4.3. Anion Dependence. Formation of the silicate film differs from the formation of MCM materials in that the nucleating species are a combination of ion trimers (Si+X-Surf+) rather than surfactant-silicate ion pairs. Hence, the acidic silicate film synthesis is highly sensitive to the nature of the mediating anion. We have previously observed this anionic dependency of the silicate film synthesis expressed as larger hexagonal cell dimensions and the longer induction times observed for films using different anions (Br-, Cl-).9 In Table 1 this anion effect is observed in the alkane addition syntheses as a consistent trend toward greater cell expansion for the CnTABr syntheses compared to CnTACl syntheses and as an increased capacity to incorporate alkane (previous section). Similarly, the extension of the induction period for alkane syntheses25 was less for the Br- syntheses than the Clsyntheses. The anion effects are explained in terms of the size of the hydration sphere or structured water surrounding (29) Knock, M. M.; Bain, C. D. Langmuir 2000, 16, 2857.

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Figure 4. SANS profiles collected on the LOQ instrument at ISIS, U.K., for the C16TAX/D2O/TEOS system (X ) Br- and Cl- in parts a and b, respectively). Profiles in parts c and d were collected on the SAND instrument at IPNS Argonne, USA, for the C16TABr/D2O/TEOS/decane system using added d-decane and h-decane, respectively. Data collected before and after film formation (see legend).

the anion in solution, which leads to stronger electrostatic interactions between anion and the positively charged silicate oligomers and micelle surface. The more hydrophobic anion allows closer interaction between surfactant and silicate species, which in turn leads to greater silicate condensation (thicker walls and larger cell dimensions). There were no multiple phases observed for Br- syntheses, whereas there were some examples in all Cl- syntheses examined. These multiple phases (a combination of swollen and unswollen hexagonal phases) are explained in terms of the reduced ability of the system to form the hexagonal phase in the presence of added alkane, due to weaker anion-mediated binding. Figure 4a and b shows the SANS profiles recorded at LOQ, before and after formation of the silicate film, for the system C16TAX/HCl/TEOS/H2O, X) Br- and Cl-, respectively, without added alkane. Figure 4c and d shows the SANS profiles recorded at IPNS, for the alkane syntheses: (c) C16TABr/d-decane; (d) C16TABr/h-decane. We are comparing the bulk species evolution of standard film syntheses (Figure 4a and b) and those with added alkane (Figure 4c and d,). In Figure 4a the profile of the C16TABr system without added alkane has a Q-1 dependence in the low Q region characteristic of long cylindrical structures, which is observed both before and after film

formation. In Figure 4b the profile of the C16TACl system without added alkane is flat in the low Q region, both before and after film formation; these patterns may be successfully modeled to spherical particles. In contrast, Figure 4c and d from the C16TABr system with added deuterated and protonated alkane, respectively, shows a Q-4 dependence, indicating the presence of spherical structures in solution even after film formation. The addition of alkane has suppressed the sphere to rod transition in solution observed in C16TABr syntheses without added alkane. In all profiles shown in Figure 4 there is a noticeable reduction of the intensity of scattering in the low Q region as the film material thickens. This occurs as a result of reduced scattering contrast due to thickening of the siliceous outer shell of the micelle, which is continuing to condense over this period. The SLD of the structure increases, lowering the contrast between the SLD of the solvent (D2O) and the micellar structure. 4.4. Alkane Incorporation. Figure 5 shows the SANS profiles from C16TABr silicate film synthesis solutions with added deuterated alkanes, from C8 to C16. The SANS profiles are displayed as points, and the model fits to the data as continuous lines. The details of the model fits for these and the equivalent protonated syntheses are shown


Langmuir, Vol. 19, No. 3, 2003 799

Table 2. Models of SANS Profiles of C16TABr/HCl/TEOS/Alkane Systems as Length of Alkane Is Increased Recorded Immediately after Mixing Solutions alkane (p-protonated, d-deuterated)

rods or spheres

no alkane d-octane p-octane d-decane p-decane d-dodecane p-dodecane d-tetradecane p-tetradecane d-hexadecane p-hexadecane-aged

rods spheres spheres spheres spheres spheres spheres spheres spheres spheres spheres

radius/Å

contrast: subphase - micelle/ 10-6 Å-2

core radius/Å

contrast: micelle - core/ 10-6 Å-2

20.7(0.6)/275(6) 30.8(1) 30.8(0.7) 30.5(1) 30.5(0.7) 26.7(0.5) 25.0(0.4) 22.3(0.60 22.8(0.6) 24.7(0.50 23.9(0.6)

-6.0 -6.7(0.4) -6.7(0.3) -6.9(0.7) -6.9(0.5) -5.8(0.9) -6.1(0.3) -6.7(0.7) -6.2(0.4) -7.2(0.2) -6.1(0.3)

no 13 13 13 13 13 no no no no no

core 3.6 3.6 4.7 4.7 3.8 core core core core core

Figure 5. SANS profiles and least-squares fits for added protonated alkanes in the C16TABr/D2O/TEOS/d-alkane system with alkanes Cn, n ) 8 to 16, from the bottom profile upward (offset for clarity).

in Table 2. There is a sharp minimum to the fringe observed at approximately Qz ) 0.13 Å-1 in all profiles, which is attributed to scattering contrast arising from a core of pure alkane. This fringe is observed with both deuterated and protonated alkane but is of lower intensity for the protonated alkanes. This fringe is less distinct as the chain length of the alkane increases, and the position of the first minimum moves to higher Q, indicating smaller particles (and the data in Table 2 agree). As the alkane chain length increases, the contrast is reduced and eventually disappears because the longer alkane molecules are distributed more evenly throughout the micelle. In Table 2 the best fits of modeling the data show exclusively spherical structures for those syntheses with added alkane, with radii slightly lower than the cell dimensions of silicate films calculated from diffraction from reflectivity of the same film synthesis. The system was modeled using model independent input of SLD differences of the contrast between micelle palisade region and core, and between subphase and micelle, which were derived from calculated volume fractions and Tanford’s30 formulas. The starting points for the dimensions of cylinder and sphere radii were obtained from the results of reflectivity shown in Table 1, with a subtraction of 5 Å allowing for the silicate walls.31 The internal core radius was fixed at 13 Å, as this was the limit of the experimental resolution, and the radii were allowed to refine for the fit. (30) Tanford, C. J. Phys. Chem. 1972, 76, 3020. (31) Yao, N.; Ku, A. Y.; Nakagawa, N.; Lee, T.; Saville, D. A.; Aksay, I. A. Chem. Mater. 2000, 12, 1536.

Figure 6. SANS profiles for the C12TABr/D2O/TEOS/decane system, collected at IPNS.

The trend observed is for spherical micelles in the bulk solution before silicate film formation, which is in contrast with the case of the same synthesis without alkane. The presence of alkane molecules within the micelle depresses the formation of rodlike micelles in the bulk synthesis solution. The models allow for a separate core, distinct from the slightly hydrogenous core of the surfactant micelle, only up to deuterated dodecane. This disappearance of the separate core is correlated by the observed disappearance of the fringe in the SANS profiles. As the chain length of the alkane added to the synthesis solution increases from C8 to C10, so do the micelle radii. However, for alkanes longer than dodecane, the micelle radius decreases slightly as the length of the alkane chain increases. These results are consistent with cell dimensions obtained from reflectivity of the films formed as the alkane chain length increases, where there was a maximum swelling achieved for syntheses with added decane (see Table 1). The size of spherical structures in solution for the C16TABr system more closely follows the trend observed with reflectivity for the C16TACl system cell dimensions at the interface. This suggests that once the phase at the interface begins to crystallize, there is some further swelling of the cylindrical structures, which is suppressed in the bulk. 4.5. Cubic Phases. The SANS profiles, recorded for the C12TABr/D2O/decane (deuterated and hydrogenous) synthesis solutions soon after mixing, are shown in Figure 6. The hydrogenous sample shows two separate slopes within the intermediate Q region, with a distinct knee in the profile at Q ) 0.11 Å-1. The finite slope extends into the very low Q region, an indication of longer structures.

800


The diffraction observed with reflectivity for this synthesis strongly suggests the formation of a cubic phase, which may be formed by the attachment of short rodlike micelles to spherical micelles. For such a structure it is possible that these SANS profiles are produced by the scattering from a combination of precursor structures in solution, some spherical and some cylindrical. Hence, the unusual profile observed is an average of the scattering from these structures in solution. 4.6. Nonexpansion of Hexagonal Silicate Films Using Aromatics. Syntheses conducted involving the addition of short chain amines and alcohols32 to the MCM41S synthesis resulted in the formation of a lamellar form of MCM. The authors explained that the incorporation of these molecules within the palisade layer of the micelles led to a decrease of curvature of the phase produced. Crystals of the alkyltrimethylammonium bromide surfactants incorporating various aromatic compounds were synthesized and examined by Sawada and co-workers.33 In general they found that the aromatics formed hydrogen bonds with the bromide anion of the surfactant and became sandwiched between alkyl groups of adjacent surfactant molecules immediately below the headgroup region. These results suggest a limited role for such species in expanding silicate films at the air-water interface, as they would be expected to disrupt the inorganic-organic interactions occurring in the interfacial region of the micelles, and not be solubilized within the hydrophobic core of the micelles. The fact that a hexagonal silicate film grew with the standard cell dimensions, and in the same induction period, indicated that the aromatic species were not incorporated within the surfactant micelles. 5. Conclusions We observed a significant swelling of the silicate film cell dimensions with addition of alkanes, up to a maximum cell expansion of 80.8 Å. However, contrary to the results obtained for MCM-41 materials,4 the expansion of the hexagonal form of the silicate films reached a maximum in the increase in cell dimensions as alkane chain length is increased, rather than a continuing linear increase with increased chain length of alkane up to C16. The maximum cell expansion consistently occurred with decane, regardless of the alkyl chain length of the surfactant used in the synthesis. This suggests that the solubility of the alkane in the aqueous mixture also plays a role in the successful swelling of the film at the interface. There is correlation between the SANS evidence that alkane addition suppresses rodlike structure formation in the bulk and the (32) Agren, P.; Linden, M.; Rosenholm, J. B.; Blanchard, J.; Schuth, F.; Amenitsch, H. Langmuir 2000, 16, 8809. (33) Sawada, K.; Kitamura, T.; Ohashi, Y.; Iimura, N.; Hirata, H. Bull. Chem. Soc. Jpn. 1999, 71, 2109.

Ruggles et al.

evidence from reflectivity from films with added alkane that only a narrow range of surfactant chain lengths form swollen hexagonal films. In contrast to alkane-free silicate film syntheses using C18TACl and C12TABr surfactants, the C18TACl syntheses produced the lamellar forms of the surfactant phases with added alkane, and similarly the C12TABr syntheses produced cubic forms of the film, both of which represent a transition induced by alkane addition. There was a counteranion dependence observed when alkanes were added to the silicate film synthesis, which was manifested in slightly larger cell dimensions, shorter induction periods for the bromide syntheses, and an increased tendency for multiple phases to occur in the chloride system. Alkane addition interferes with the electrostatic charge balance in the interfacial region, and the stronger interactions of the bromide system are more able to accommodate such disturbance. The swelling of silicate films increased with alkane concentration, with a maximum in cell expansion occurring for the range of molar ratios of alkane to surfactant between 1 and 3. The evidence supports the model proposed by Blin et al.10 of solubilization within the micelles as a less organized core region, which tends to extend into the palisade region as the alkane chain length increases. The loss of the high Q fringe in the SANS profiles as the chain length of the added alkane increases supports a model of alkanes solubilized as a separate core within the micelle interior, for alkanes of chain length from C8 to C12. However, as the alkane chain length increases, they tend to arrange alongside the surfactant tails, blurring the core. No significant swelling was observed when toluene and benzene were added to silicate film synthesis solutions. There was no change to the cell dimensions or to the induction period of the syntheses. This suggests that no aromatics were incorporated within micelles, as they tend to associate with the polar interfacial region of the micelles and disturb the electrostatic charge balancing which allows formation of cylindrical micellar structures necessary for film formation. Acknowledgment. This work has benefited from the use of the Intense Pulsed Neutron Source at Argonne National Laboratory, which is funded by the U.S. Department of Energy, BES-Materials Science, under Contract W-31-109-ENG-38. Travel grants through the Australian Government ISTAC/ANSTO Access to Major Facilities Program are gratefully acknowledged, as is access to ISIS through an ARC grant. The authors also 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. LA0265833

Expanded Mesoporous Silicate Films Grown at the AirâWater Interface

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