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Langmuir 2008, 24, 13890-13896
Self-Assembly and Magnetic Alignment in Cetyltrimethylammonium Bromide/Sodium Perfluorooctanoate Surfactant Mixtures Investigated Using 2H Nuclear Magnetic Resonance Spectroscopy Todd M. Alam* and Sarah K. McIntyre Department of Electronic and Nanostructured Materials, Sandia National Laboratories, Albuquerque, New Mexico 87185-0886 ReceiVed May 30, 2008. ReVised Manuscript ReceiVed October 15, 2008 Deuterium nuclear magnetic resonance (2H NMR) spectroscopy has been used to investigate the phase behavior for mixtures of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the anionic surfactant sodium perfluorooctanoate (FC7) for total surfactant concentrations ranging from 1 to 25 wt %. The deuterated methyl of the quaternary methyl-ammonia group in CTAB-γ-d3 gives rise to a superposition of spectral components in the 2H NMR spectra allowing for the identification and quantification of the different phases present. The CTAB/FC7 mixture exhibits a coexisting two-phase region composed of an isotropic (Iso) micelle/vesicle phase along with a lamellar (LR) phase for all of the composition ranges investigated. The variation of the phase composition as a function of sample temperature, total wt % surfactant, and surfactant molar ratio are presented. In addition, the 2H NMR reveals that the LR phase spontaneously aligns in the magnetic field, with the extent and distribution of magnetic alignment being determined. The 2H NMR results are discussed in light of previously reported surfactant-templated material synthesis involving the CTAB/FC7 mixture.
Introduction Surfactant templates have found widespread application in the synthesis of nanostructured materials.1-4 For example, materials based on inorganic-organic composites where the selfassembled organic surfactant provides a template for the growth of the inorganic framework continue to receive extensive efforts.5-10 Numerous morphologies and network types have been observed and are often under direct control of the organic/ inorganic precursor ratios as well as a variety of other synthetic parameters.11,12 Mixed or multisurfactant systems have been used to template a variety of different materials and allow additional control over the templating phase and ultimately the microstructures of the final material. Using mixtures with oppositely charged surfactant head groups or surfactant mixtures with hydrocarbon and fluorocarbon chains can lead to stable selfassembled phases not observed in either of the pure surfactants (1) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (2) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138–1143. (3) Barton, T. J.; Bull, L. M.; Klemperer, W. G.; Loy, D. A.; McEnaney, B.; Misono, M.; Monson, P. A.; Pez, G.; Scherer, G. W.; Vartuli, J. C.; Yaghi, O. M. Chem. Mater. 1999, 11, 2633–2656. (4) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56–77. (5) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548–552. (6) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. Nature 1992, 359, 710–712. (8) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366–368. (9) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152–155. (10) Akdogan, Y.; Uzum, C.; Dag, O.; Coombs, N. J. Mater. Chem. 2006, 16, 2048–2055. (11) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843. (12) Sakamoto, Y.; Kaneda, M.; Tersaki, O.; Zhao, D. Y.; Kim, J. M.; Stucky, G.; Shin, H. J.; Ryoo, R. Nature 2000, 408, 449–453.
solutions, plus in many cases may extend the concentration range for which a particular self-assembled phase exist. Examples of mixed surfactant systems used for material templating include mixtures of cetyltrimethylammonium tosylate (CTAT) and sodiumbenzenesulfonate (SDBS) or cetyltrimethylammonium bromide (CTAB) and sodium octylsulfate (SOS) to produce polymeric spheres of styrene and divinylbenzene13 and the use of CTAT or dodecyltrimethylammonium bromide (DTAB) with SDBS to polymerize shells of polystyrene.14 The production of hollow silica nanospheres using the catanionic mixture of CTAT and SDBS or mixtures of CTAB and sodium perfluorooctanoate (FC7) have also been reported.15 More recently 1:1 CTAB/FC7 mixtures have been used as synthetic templates to produce platinum nanowheels.16 These templating studies strongly suggest that the self-assembled phase either controls or impacts the final material morphology. The phase behavior of the mixed CTAB/ FC7 surfactant system is the primary focus of this paper. The phase diagram of the CTAB/FC7 mixture has been reported15,17 and is dominated by a two-phase region composed of vesicles (V) in equilibrium with a lamellar (LR) phase. For high FC7 mole fractions, δ(FC7), a unilamellar lobe is observed in the phase diagram and has been the focal point for the majority of previous studies. Cryo transmission electron microscopy studies of CTAB/FC7 mixtures between 1 and 4 total wt % surfactant have shown a mixture of spherical unilamellar vesicles, flat disks, and bilayer cylinders.18-21 For FC7-rich mixtures, with δ(FC7) between 0.75 and 0.85, the fraction of cylindrical bilayer cylinders (13) McKelvey, C. A.; Kaler, E. W.; Zasadzinski, J. A.; Coldren, B.; Jung, H. T. Langmuir 2000, 16, 8285–8290. (14) Morgan, J. D.; Johnson, C. A.; Kaler, E. W. Langmuir 1997, 13, 6447– 6451. (15) Hentze, H.-P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069–1074. (16) Song, Y.; Dorin, R. M.; Garcia, R. M.; Jiang, Y.-B.; Wang, H.; Li, P.; Qiu, Y.; van Swol, F.; Miller, J. E.; Shelnutt, J. A. J. Am. Chem. Soc. 2008, 130, 12602–12603. (17) Kaler, E. W.; Herrington, K. L.; Iampietro, D. J.; Coldren, B. A.; Jung, H.-T.; Zasadzinski, J. A. Surf. Sci. Ser. 2005, 124, 289–337. (18) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. Proc. Nat. Acad. Sci. 2001, 98(4), 1353–1357.
10.1021/la801681n CCC: $40.75 2008 American Chemical Society Published on Web 11/20/2008
CTAB/FC7 Surfactant Mixtures
increases while the fraction of flat disks decreased at higher FC7 concentrations. The mean size distribution of these spherical vesicles, flat disks, and cylindrical vesicles remained unchanged over the limited composition range investigated. Near equal molar concentrations a mixed V + LR phase is argued but has not been fully explored. Also of interest is the use of magnetic fields to align the liquid crystalline phase of the templating surfactants in order to induce long-range order within the final nanostructured material. Magnetic alignment of bulk silicate-surfactant composites has been described,22,23 and more recently magnetically aligned thin films produced using the evaporation induced self-assembly (EISA) method have been reported.24 Characterizing the phases produced, the extent and direction of magnetic alignment, along with the kinetics of alignment are all important in controlling the final properties of the templated material. Understanding and manipulating the magnetic alignment kinetics becomes increasingly crucial when the time scale of material formation (condensation, evaporation, nucleation, etc.) is similar to the time scale of the alignment process. Deuterium (2H) NMR provides an excellent tool to probe phase structure and magnetic alignment kinetics and has been used extensively to study liquid crystalline systems including CTAB containing mixtures25-29 and CTAB-templated composites.2,22,23,30 The deuterium electrical field gradient is averaged by internal and molecular motions making the 2H NMR line shape very sensitive to the dynamics and structure of both local and mesoscopic environments. In this paper 2H NMR is used to characterize the CTAB/FC7 mixed surfactant system (primarily near the 1:1 molar ratio) by monitoring the phase and magnetic alignment behavior of CTAB in CTAB/FC7 mixtures as a function of total surfactant weight, relative CTAB/FC7 molar ratio and temperature.
Experimental Section Sample Preparation. Monomethyl deuterated (99% 2H enrichment) CTAB was purchased from CDN Isotopes (Pointe-Claire, Quebec); sodium perfluorooctanoate (97%), FC7, was obtained from Alfa Aesar (Ward Hill, MA). Both materials were used as received. The surfactant mixtures were prepared by mixing an appropriate amount of CTAB-γ-d3 in 18 MΩ deionized water. After 10 min of sonication in a water-bath sonicator, FC7 was added to the CTABγ-d3/water mixture, and the sample was sonicated for an additional 10 min. Samples were epoxy sealed in 5-mm tubes to prevent evaporation, were allowed to equilibrate for 24 h in a 338 K water bath, and were followed by 2H NMR analysis. Additional experiments with equilibration up to 72 h in the water bath did not show any additional spectral changes over the 24 h equilibrated sample. Within (19) Zasadzinski, J. A.; Jung, H. T.; Coldren, B.; Kaler, E. W. Spontaneous Curvature Effects on Spontaneous Vesicles. In Self-Assembly; Robison, B. H., Ed.; IOS Press: 2003; pp 432-442. (20) Kang, S.-Y.; Seong, B.-S.; Han, Y. S.; Jung, H.-T. Biomacromolecules 2003, 4, 360–365. (21) Jung, H.-T.; Lee, S. Y.; Kaler, E. W.; Coldren, B.; Zasadzinski, J. A. PNAS 2002, 99(24), 15318–15322. (22) Firouzi, A.; Schaefer, D. J.; Tolbert, S. H.; Stucky, G. D.; Chmelka, B. D. J. Am. Chem. Soc. 1997, 119, 9466–9477. (23) Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264–268. (24) Yamauchi, Y.; Sawada, M.; Sugiyama, A.; Osaka, T.; Sakka, Y.; Kuroda, K. J. Mater. Chem. 2006, 16, 3693–3700. (25) Rapp, A.; Ermolaev, K.; Fung, B. M. J. Phys. Chem. B 1999, 103, 1705– 1711. (26) Cheng, C.-Y.; Hwang, L.-P. J. Chin. Chem. Soc. 2001, 48, 953–962. (27) Krishnaswamy, R.; Ghosh, S. K.; Lakshmanan, S.; Raghunathan, V. A.; Sood, A. K. Langmuir 2005, 21, 10439–10443. (28) Macdonald, P. M.; Strashko, V. Langmuir 1998, 14, 4758–4764. (29) Clawson, J. S.; Holland, G. P.; Alam, T. M. Phys. Chem. Chem. Phys. 2006, 8, 2635–2641. (30) Firouzi, A.; Atef, F.; Oertli, A. G.; Stucky, G. D.; Chmelka, B. D. J. Am. Chem. Soc. 1997, 119, 3596–3610.
Langmuir, Vol. 24, No. 24, 2008 13891 the time frame of the NMR experiments the samples are considered to be in a steady state condition and did not change. Since aging effects and long-term equilibration times are known to impact the phase behavior of CTAB systems, this preparation protocol was kept constant for all samples studied unless otherwise noted. This surfactant system was investigated at 1, 2.5, 5, 10, 16, 20, and 25 wt % in a 1:1 CTAB-γ-d3/FC7 ratio and also at 16 wt % for 2:1, 3:1, and 4:1 CTAB-γ-d3/FC7 ratios. The results throughout the remainder of the paper explicitly utilized CTAB-γ-d3 such that all sample mixtures will simply use the CTAB/FC7 designation. NMR Spectroscopy. All 2H NMR spectra were collected on a Bruker Avance 400 spectrometer (9.6 T) operating at 61.43 MHz. Spectra were obtained on a static wide-line probe (NMR Resonance, Germany) equipped with a 5-mm coil, using a quadrupolar echo pulse sequence, a 4-µs π/2 pulse, a 20-µs echo delay, and a 3-s recycle delay with 1024 or 2048 scan averaging. Samples were heated in 5 K increments from 298 to 358 K with spectra being obtained after 5 min of equilibration at each temperature. Simulation of the 2H NMR spectra, including line broadening, relative concentrations, and alignment distributions were obtained using MATLAB and an in-house written simulation program. Theoretical Background. The 2H NMR spectra in surfactant mixtures is dominated by the quadrupolar interaction of the electrical field gradient (EFG) tensor. The quadrupolar splitting between singularities for an axially symmetric 2H EFG tensor is described by
∆νQ ) 3/2CQP2(cos Θ)
(1)
where the quadrupolar coupling constant CQ (e2qQ/h) is ∼170 kHz for methyl deuterons and P2 is the second Legendre polynomial with Θ being the angle between the largest principal component of the 2 H EFG tensor (approximately along the C-2H bond vector) and the static magnetic field. Internal dynamics and different molecular motions will produce an averaging of the observed quadrupolar interaction. In the case of CTAB it will be assumed the dynamics can be expressed in terms of individual order parameters (Si) for each of the motional frames and is given by
∆νQ )
3CQ S S S S P (cos θ) 2 C3V CN mol wobb 2
(2)
These internal frames include the rapid C3V motion about the methyl group symmetry axis (β ) 70.5°), averaging due to motion of the entire trimethylammonia group around the CN bond (γ ) 70.5°), rapid rotational diffusion around the long axis of the CTAB molecule (mol, φ ) 35.25°), fluctuations or wobble (wobb, R) of this molecular axis about an average director 〈nˆ〉 (wobble with a cone angle R), while θ describes the orientation of the average director with respect to the static magnetic field.28 These different averaging motions are shown schematically in Figure 1S of Supporting Information. These separate order parameters can also be combined into an overall motional order parameter SCD to give
∆νQ )
3CQ S P (cos θ) 2 CD 2
(3)
For methyl C3V rotation (SC3V ) 1/3), CN rotation (SCN ) 1/3), fluctuations about the molecular axis (Smol ) 1/2) and no additional averaging due to wobble (Swobb ∼ 1), a SCD ) 0.056 is predicted for the self-assembled LR phase.28 Fluctuations of the director or additional averaging motions would further reduce SCD. In the case of the hexagonal (H) phase, rapid diffusive motion of the local molecular director around the surface of the cylindrical aggregate produces a new director axis that is parallel to the hexagonal cylindrical axis (ψ ) 90°), resulting in an additional factor of 2 averaging in the observed quadrupolar frequency with a predicted SCD ≈ 0.028.
Results 2
The H NMR spectra for the different CTAB/FC7 mixtures were obtained as a function of temperature, total surfactant concentration, and relative mole fraction of the two surfactants.
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Figure 1. The 2H NMR spectra and simulations (dashed line) for the 16 wt % total surfactant 1:1 CTAB-γ-d3/FC7 molar ratio mixture as a function of temperature. These NMR spectra are a superposition of an isotropic (Iso) resonance and a signal from both an unaligned random lamellar (LRrandom) and magnetically aligned (θ ) 0°) lamellar (LRaligned) phase.
Figure 1 shows the representative spectra for a 1:1 CTAB/FC7 mixture with a 16 wt % total surfactant concentration as a function of temperature. At low temperatures the NMR spectra are a superposition of a central isotropic (Iso) resonance and a Pake powder pattern arising from CTAB species in the unaligned or ). With randomly oriented self-assembled lamellar phase (Lrandom R increasing temperatures an additional doublet is observed due ). to CTAB within a magnetically aligned lamellar phase (Laligned R Simulations of these 2H NMR spectra allowed the relative amounts of the different phases and associated SCD to be measured. An exponential line broadening was introduced in the spectral simulation to reflect differences in T2 relaxation which is a function of both sample concentration and temperature. The observed line broadening varied between 100 and 400 Hz for the Iso spectra, and 750-900 Hz resonance, 500-600 Hz for the Laligned R for the LRrandom spectral component. In addition, the angle of magnetic alignment (θ) and corresponding Gaussian distribution width (σ) were determined. Figure 2 shows an example of the 2H NMR spectral simulation, and the different spectral components that contribute to the overall experimental line shape. The magnetically aligned and unaligned LR phase gives rise to distinct spectral features. Through a series of simulations it was determined that relative concentration of the Laligned phase must be >10% to be distinguished from the R phase. In addition, simulations show that σ must be 20 h (at 338 K). The orientation of the aligned phase has an average director (bilayer normal) parallel (θ ) 0°) to the magnetic field. The final orientational distributions after 14 h of thermal cycling within the magnetic field were σ ) (7° (298 K) and σ ) (4° (338 K).
Formation of Two-Phase Region. The 2H NMR spectra shown in Figures 1S and 3S of Supporting Information demonstrate that the 1:1 CTAB/FC7 mixture between 1 and 25 wt % total surfactant and over a wide temperature range forms a binary phase mixture composed of Iso phase in equilibrium with the LR phase. The isotropic spectral component (singlet) arises from micelles or vesicles that are rapidly tumbling in solution along with fast lateral diffusion over the curved micelle/vesicle surface that effectively averages the quadrupolar interaction of the 2H nuclei, while the Pake line shape arise from CTAB species that experience anisotropic motions within the LR phase. An overview of the different internal and molecular motions that can lead to the averaging of the 2H quadrupolar interaction was detailed in the experimental section. For the CTAB/FC7 mixtures studied here, the magnitude of the order parameters (SCD) observed range from 0.05 to 0.03 (Figure 3), which are larger than the ideal hexagonal (H) and nematic (N) limit of ∼0.028, thereby confirming the identification as a LR phase. The H and N phases observed in pure CTAB mixtures have SCD e 0.028 as expected.29 The ordered LR phase predicts a SCD ≈ 0.056, such that the experimentally observed SCD values (Figure 3) for the selfassembled CTAB/FC7 LR phase reveals the presence of additional small internal or molecular motions (Swobb < 1).28 The twocomponent Iso + LR phase observed in the 2H NMR spectra is consistent with the V + LR two-phase region designated in the CTAB/FC7 phase diagram.15,17 Our NMR results allow the extension of the biphasic Iso +LR region to higher wt % along the equimolar CTAB/FC7 axis up to 11.4 wt % CTAB and 13.6 wt % FC7. The consistency of the SCD temperature variations (Figure 3) with surfactant concentration also shows that the dynamics of CTAB within the LR phase are independent of the surfactant concentration and that the LR phase does not undergo a gradual variation from the ordered bilayer structure to the highly curved micelle/vesicle present in the Iso phase with changing surfactant concentration. The observation of a LR phase in the 2H NMR spectra for total surfactant concentrations as low as 1.0 wt % (Figure 3S of Supporting Information) in the 1:1 CTAB/FC7 mixtures (0.46 wt% CTAB and 0.54 wt% FC7) results from interactions between the oppositely charged surfactant head groups leading to aggregation. The critical micelle concentration (CMC) of CTAB is very small (0.9 mM or 0.03 wt%) in comparison to the CMC of CF7 (36.7 mM or 1.6 wt%)17 or the analogous CF5 (127 mM or 5.7 wt%).31 The role of headgroup interactions can be seen in CTAB/FC5 mixtures that show a critical aggregation concentration (CAC) near 90 µM, which is almost 10 times smaller than the CMC of CTAB and over a 103 times smaller than FC5.31 The low CAC values support the argument that the Iso phase observed in the 2H NMR spectra arises from a rapidly tumbling micelle/vesicle and not simply the CTAB monomer dissolved in solution. The observation of a LR phase containing CTAB at very low surfactant concentrations in the CTAB/FC7 mixtures was also distinctly different than the phase behavior of pure CTAB solutions. For CTAB solutions between 9-11 wt % a spherical micelle to rodlike micelle transition has been argued32,33 but remains a singlet in the 2H NMR spectra. Only above 20 wt % CTAB (at room temperature) is either a binary Iso + N, a Iso + H, or a pure N phase observed, while above 30 wt % a single component H phase is observed.29 (31) Iampietro, D. J.; Kaler, E. W. Langmuir 1999, 15, 8590–8601. (32) Lindblom, G.; Lindman, B.; Mandell, L. J. Colloid Interface Sci. 1973, 42(2), 400–409. (33) Texter, J.; Horch, R. F.; Qutubbin, S.; Dayalan, E. J. Colloid Interface Sci. 1990, 135(1), 263–271.
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Figure 5. The 2H NMR and simulations (dashed lines) for the 16 wt % 1:1 CTAB-γ-d3/FC7 surfactant mixture as a function of thermally cycling within the magnetic field. The time designation is the total amount of time the sample has been equilibrated at 338 K. The sample temperatures were stepped between 298 and 338 K with between 1 and 2 h for each temperature step.
Effect of Temperature. Several 2H NMR spectral changes are observed in the CTAB/FC7 mixture with temperature. First, for the surfactant concentrations investigated (1 to 25 wt %) higher temperatures increases the Iso phase concentration while decreasing the LR concentration. For total surfactant concentrations 360 K) was not reached due to experimental difficulties maintaining the sample seal. The LR f Iso phase transition temperatures in the CTAB/
FC7 mixtures are much higher than the temperature for either the N f Iso or the H f Iso transitions observed for pure CTAB, which range between 295 and 324 K for 20-30 wt % CTAB.29 The increased thermal stability of the LR structures in the CTAB/ FC7 mixtures results from the highly nonideal mixing behavior of these surfactants and is a combination of the free energy gained from isolation of the hydrophobic tails from water within the LR structure and the free-energy minimization due to electrostatic static paring of oppositely charge head groups in nearly equimolar amounts. At higher temperatures this stabilization is overcome
CTAB/FC7 Surfactant Mixtures
leading to high curvature micelle/vesicle structures. The high isotropic transition temperature for the CTAB/FC7 mixtures suggests that the LR phase is thermodynamically favorable and that the presence of the Iso phase at lower temperatures reflect samples that have not reached full equilibrium. Previous investigations of the CTAB/FC531 and CTAB/FC717 phase diagrams show that the sample equilibration at RT time varied from less than a day for the micelle samples to months for the vesicle and lamellar samples. After these extended time periods the sample behavior was stable for months.19 For these reasons, all of the samples in Figures 1-4 were thermally treated under identical conditions and times, were in a steady state on the time scales of the reported NMR experiments, but do not represent a fully equilibrated phase condition. The focus of the present paper was the phase behavior of CTAB/FC7 mixtures under conditions reported for templating of Pt nanostructures16 or under conditions expected for material templating using magnetic alignment.22-24,30 Studies involving equilibration for extended time periods would be required to address this issue and were not pursued here. The second change observed in the 2H NMR spectra with increasing temperature was a linear decrease in the SCD (see Figure 3) with an average slope of -2.7 e-4 K-1. This decrease in SCD reflects the increased averaging due to molecular fluctuations or director wobble with higher thermal energies. Within experimental error the SCD temperature variations are very similar for all concentrations, even though higher surfactant concentrations tend to have a slightly smaller temperature variation. The SCD thermal variations in the CTAB/FC7 mixtures are an order of magnitude larger than those observed for selfassembled phases of CTAB. For example, for pure CTAB in the H phase the SCD temperature variation were -5.3 e-5 K-1 (28.4 wt %) and -3.9 e-5 K-1 (32.9 wt %).29 This difference result from the higher surfactant concentrations in the pure CTAB solution but may also reflect differences in the molecular packing of the LR phase of the CTAB/FC7 mixture vs the H phase of pure CTAB. Appearance of Magnetically Aligned Lamellar Phase. One of the questions we wanted to address was at what temperature the different CTAB/FC7 mixtures begin to magnetically align as directed toward future production of magnetically oriented selfassembled materials. A representation of the different 2H NMR spectral components experimentally observed as a function of temperature and time within the magnetic field is presented in Figure 6. As the sample temperature is ramped up magnetic alignment of the LR phase is observed at temperatures above 2 + Laligned H NMR ∼325 K leading to 3-component Iso + Lrandom R R aligned phase is oriented spectra. For the CTAB/FC7 mixtures the LR with the bilayer normal parallel to the magnetic field (θ ) 0°) due to a positive magnetic susceptibility anisotropy ∆χ. The onset temperature of magnetic alignment varies with total surfactant concentration with the lowest onset temperature (∼325 K) being observed for surfactant concentrations near 15 wt %. Recall the motional order parameters SCD are very similar for the entire surfactant concentration range (Figure 3), arguing against any major structural changes (chain packing, restricted molecular rotations, etc) that could lead to these differences in the onset temperature of magnetic alignment. These differences in onset temperatures may reflect differences in solution viscosities or domain size within the mixtures. Impact of the CTAB/FC7 Molar Ratio. The impact of the CTAB/FC7 molar ratio on the observed 2H NMR spectra (Figure 4) is more complex. For pure CTAB at 16 wt % only the Iso phase is observed, as previously reported.29 That would suggest
Langmuir, Vol. 24, No. 24, 2008 13895
Figure 6. Representation of the different 2H NMR spectral components observed for the CTAB/FC7 1:1 molar ratio mixture (equilibrated for 24 h at 338 K outside the magnetic field) as a function of temperature and time within the magnetic field. The observed components include an isotropic (Iso) micelle/vesicle phase, a randomly oriented lamellar (LRrandom) phase and the magnetically oriented (θ ) 0°) lamellar (LRaligned) phase.
that for CTAB/FC7 mixtures with increasing CTAB concentrations the Iso phase should become dominant. This trend was indeed observed for the 4:1 molar ratio mixture (δ(FC7) ) 0.2, 16 wt % total surfactant, 12.3 wt % CTAB, 3.7 wt % FC7) where an ∼10% increase in the Iso phase was observed in comparison to the 1:1 mixture (δ(FC7) ) 0.5, 16 wt % total, 7.3 wt % CTAB, 8.6 wt % FC7). By use of a simplistic picture of a 1:1 interaction between the CTAB and FC7 head groups in the LR phase, it could be argued that increasing the CTAB concentration would result in excess CTAB being incorporated into the micellar/vesicle phase. Experimentally the observed 10% increase in the Iso phase concentration for the 4:1 mixture does not correlate with the 3-fold excess of CTAB. The dominance of the Iso in the 4:1 mixture may also reflect being near the boundary between the mixed Iso + LR phase and a rodlike micelle (R) phase within the phase diagram.15,17 The dramatic decrease (∼50% to 60%) in the Iso phase concentration for the 2:1 and 3:1 CTAB/FC7 mixtures compared to the 1:1 mixture also argues against a simple picture of the excess CTAB being incorporated solely into the micelle/vesicle Iso phase. The observed 30% reduction in SCD for the 2:1 and 3:1 CTAB/FC7 mixtures compared to the 1:1 mixture suggest that there are local changes in the bilayer composition that lead to the increased internal motional averaging or larger director fluctuations with changing surfactant ratios. The observed SCD remains between 0.25 and 0.275 for the 2:1, 3:1, and 4:1 CTAB/ FC7 mixtures showing that for these compositions the LR structures are very similar. The decrease in the Iso phase concentration for the 2:1 (δ(FC7) ) 0.33) and 3:1 (δ(FC7) ) 0.25) in comparison to the 4:1 (δ(FC7) ) 0.20) mixture is also consistent with moving away from the rodlike micelle boundary within the phase diagram. Also of interest is the observation that the 1:1 CTAB/FC7 mixtures were the only samples that underwent magnetic alignment for temperatures 90%) alignment. It appears for the current CTAB/FC7 mixture thermal cycling within the magnetic field increases the relative concentration of the LR phase compared to thermal treatment outside the magnetic field by allowing the smaller isotropic micelles to become associated within the extended and thermodynamically preferred LR phase. In oriented silicate-surfactant mixtures a transient hexagonal-lamellar transition has been observed and was attributed to the unique epitaxial relationship between these different domains.22 Such a phase interconversion is not expected for the CTAB/FC7 mixture since the hexagonal phase was not observed. This cooperative alignment behavior of the CTAB/ FC7 mixture within the magnetic field is also distinctly different than previous studies of magnetically aligned CTAB (at 28 wt %) where disruption of the self-assembly process is observed when the sample is thermally cycled through the isotropic phase transition followed by magnetic alignment.29 In the pure CTAB samples thermal cycling tends to produce a mixed nematic/ hexagonal phase where the CTAB vesicles have difficulty growing to or evolving to the longer length and ordering present in the pure hexagonal phase. The distribution of alignment for these phases, σ ) 7° (298 K) or σ ) 4° (338 K), CTAB/FC7 Laligned R are similar to those observed for CTAB mixtures in magnetically aligned H and LR phases. For example, σ ≈ 7.5° has been reported for the aligned H phase in CTAB-silicate composites,23 σ ≈ 6.3° for the oriented LR CTAB-silicate composite with hexanol, and σ ≈ 7.1° for the oriented LR CTAB-silicate composite with benzene.22 Again these results demonstrate that the CTAB/FC7 mixture produces magnetically aligned LR phases with comparable distributions to other surfactant mixtures. (34) Sanders, C. R.; Landis, G. C. Biochemistry 1995, 34, 4030.
Alam and McIntyre
Conclusions This paper presents a 2H NMR investigation into the phase and dynamical behavior of CTAB in the mixed CTAB/FC7 surfactant system. For the concentration and temperature ranges investigated, a two-phase region was observed and was composed of an isotropic micelle/vesicle phase and a self-assembled lamellar phase. The relative amounts of these two components were a function of total surfactant concentration, the molar ratio of CTAB and FC7 surfactants, and temperature. The internal dynamics for CTAB in the LR phase is independent of the surfactant concentration (between 1 and 25 wt %), with the motional order parameter decreasing linearly at higher temperatures. This paper also demonstrated that the mixed 1:1 CTAB/FC7 system undergoes magnetic alignment with the bilayer normal parallel (θ ) 0°) to the static magnetic field, with thermal cycling within the magnetic field producing an almost completely magnetically aligned phase. The use of the CTAB/FC7 surfactant mixture for materials templating have predominantly concentrated on the unilamellar vesicle phase in the FC7-rich region of the phase diagram to produce spherical nanostructures. Silicate materials templated in the biphasic V + LR phase region resulted in layered structures, consistent with the lamellar structure observed in the 2H NMR spectra.15 The Pt nanowheels templated in 1:1 CTAB/FC7 mixtures16 are highly suggestive of a discoidal nematic bicellelike structure, which is also consistent with the unique magnetic alignment behavior observed here for the 1:1 CTAB/FC7 mixtures. The observation of magnetic alignment with the CTAB/FC7 mixtures may provide a way of increased long-range order control in templating materials, but the relatively high temperatures and significant times required for magnetic alignment will need to be considered with respect to the conditions for material production. Acknowledgment. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear SecurityAdministrationunderContractNo.DEAC04-94AL85000. This work is supported entirely through the Basic Energy Sciences (BES) from the Department of Energy. Supporting Information Available: Supporting Information includes a description of the internal molecular motions, simulations of aligned 2H NMR spectra as a function of alignment distributions, and the 2H NMR spectra for different surfactant concentrations. This information is available free of charge via the Internet at http://pubs. acs.org. LA801681N
