Langmuir 2000, 16, 4761-4765
4761
Flow-Induced Silica Structure during in Situ Gelation of Wormy Micellar Solutions Won-Jong Kim and Seung-Man Yang* Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Kusong-dong 373-1 Yusong-ku, Taejon, 305-701, Korea Received August 31, 1999. In Final Form: March 24, 2000 In this study, we observed the flow-induced silica structure, which was formed during in situ gelation in the presence of wormlike micelles of cetyltrimethylammonium bromide and structure-forming additive, sodium salicylate. A silicon alkoxide of tetramethyl orthosilicate was used for the in situ gelation as a precursor of silica substrate. The silica formed the micrometer-sized zigzag structure in the absence of an imposed flow. In dilute regime, the morphology under steady shearing showed aligned texture and the small-scale zigzag morphology remained persistent. When the shear rate was sufficiently high that the viscosity of micellar solution shear thickened, the flow-induced interlayer coagulation was observed. Meanwhile, the degree of alignment was locally inhomogeneous at low shear rates in semidilute solution.
Introduction Recently, considerable research efforts have been centered on pursuing the studies for predicting the microstructure-macroscopic property relations for microstructured materials under equilibrium and nonequilibrium condition. The microstructured colloidal dispersions include self-assembling complex fluids such as surfactants, block copolymers, and liquid crystals, which are known as the designer soft materials. The presence of a suspended phase implies a strong interaction between external fields and macroscopic optical, mechanical, or electrical properties. Such a phase provides an opportunity for innovative manipulation of material properties through the understanding and control of the external field/ microstructure interaction. In the present work, we investigated the flow-induced silica structure, which was formed during in situ gelation through the sol-gel reaction of an alkoxide in the presence of wormlike micelles of cetyltrimethylammonium bromide (CTAB). The surfactant molecules form the wormlike micelles in the presence of the structure-forming additives such as sodium salicylate. Meanwhile, the wormlike micelles experience the complicated flow-induced coagulation or phase transition, which is strongly dependent on their concentrations and temperature. Thus, the flowinduced silica structure must be affected by the presence of the surfactant. The rheological and rheo-optical studies have been performed on various sets of surfactants and additives to investigate the flow-induced microstructure evolution.1 For example, Cates et al. suggested the socalled dynamic chain model by modifying the DoiEdwards tube model allowing chain-scission reaction and predicted the linear and nonlinear rheological behavior of the wormlike micelles.2 The flow-induced structure in dilute solution was interpreted by formation of flowinduced supramolecular structures under flow.3-5 With increasing the flow intensity, the micelles undergo coalescence or tend to be stretched toward the flow direction. * To whom correspondence should be addressed. smyang@ kaist.ac.kr. (1) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354. (2) Cates, M. E. Macromolecules 1987, 20, 2289. (3) Hu, Y. T.; Boltenhagen, P.; Pine, D. J. J. Rheol. 1998, 42, 1185. (4) Rehage, H.; Wunderlich, I.; Hoffmann, H. Prog. Colloid Polym. Sci. 1986, 72, 51. (5) Kim, W.-J.; Yang, S.-M. Submitted for publication in Langmuir.
Eventually, the shear-induced structure of wormlike micelles continuously breaks down and re-forms at high shear rates. On the other hand, the flow-induced phase transition in semidilute or concentrated regime was enhanced by the inhomogeneous flow. Several recent studies reported that the inhomogeneous flow was caused by the coexistence of isotropic and nematic phases at intermediate shear rates.6-8 However, the existence of a shear-induced nematic phase in surfactant solution has been still an issue of controversy and is far from settled. For example, the flow inhomogeneity can be also induced by the difference in experimental and relaxation time scales.9 Several direct observations on flow-induced microstructure of complex fluids were reported by use of optical microscopy,10 shear microscopy,11 and freeze fracture electron microscopy.12 The flow-induced microstructure formation of silica substrate during in situ gelation in the presence of wormy micelles is relevant to some aspects of fabrication of macroporous inorganic materials through the sol-gel process.13-15 These ordered porous materials can be applied as photonic crystals or optical waveguides.16,17 2. Experimental Section The surfactant used was cetyltrimethylammonium bromide (CTAB, Sigma) and the structure-enhancing additive was sodium salicylate (NaSal, Aldrich), both used as received. Tetramethyl orthosilicate (TMOS, Aldrich) in hydrochloric acid was selected as a gelation substrate, and ammonium hydroxide was used as (6) Berret, J. F.; Roux, D. C.; Porte, G. J. Phys. II 1994, 4, 1261. (7) Kim, W.-J.; Yang, S.-M. Submitted for publication in J. Colloid Interface Sci. (8) Decruppe, J. P.; Cressely, R.; Makhloufi, R.; Cappelaere, E. Colloid. Polym. Sci. 1995, 273, 346. (9) Grand, C.; Arrault, J.; Cates, M. E. J. Phys. II 1997, 7, 1071. (10) Toy, M. L.; Scriven, L. E.; Macosko, C. W.; Nelson, N. K.; Olmsted, R. D. J. Rheol. 1991, 35, 887. (11) Matsuzaka, K.; Koga, T.; Hashimoto, T. Phys. Rev. Lett. 1998, 80, 5441. (12) Keller, S. L.; Boltenhagen, P.; Pine, D. J.; Zasadzinski, J. A. Phys. Rev. Lett. 1998, 80, 2725. (13) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (14) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152. (15) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256. (16) Lin, S. Y.; Chow, E.; Hietala, V.; Villeneuve, P. R.; Joannopoulos, J. D. Science 1998, 282, 274. (17) Yablonovitch Phys. Rev. Lett. 1987, 58, 2059.
10.1021/la9911685 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/03/2000
4762
Langmuir, Vol. 16, No. 11, 2000
Letters
Figure 1. Scanning electron microscopy (SEM) images of the captured microstructure without shearing: (a) R ) 1.0, [CTAB] ) 0.005 M; (b) R ) 1.0, [CTAB] ) 0.05 M; (c) R ) 10.0, [CTAB] ) 0.05 M; (d) pure TMOS + HCl solution. a catalyst for the sol-gel reaction. The CTAB concentration [CTAB] was fixed at either 0.005 or 0.05 M, and NaSal concentration [NaSal] was varied with the molar ratio, R ) [NaSal]/[CTAB], fixed at R ) 1.0 or R ) 10.0. First, an aqueous wormlike micellar solution was prepared, and, at equilibrium, TMOS in hydrochloric acid was added. Then, at the same time when the shear flow was imposed, ammonium hydroxide was added to induce the gelation in basic condition. The sample was prepared in a Couette flow cell between two coaxial cylinders. The thickness of the gelified sample between the annulus region was in the range of 1-2 mm, and the height was maintained at 20 mm in the vorticity direction. Shear rate was varied from 0.03 to 12 s-1, which was strong enough to induce the microstructure evolution. When the gelation was completed, the samples were dried under vacuum for 3 h and prepared for the scanning electron microscope (SEM). The flow-induced morphologies at several locations in the cell were examined by observing the SEM images taken for the corresponding sites. To confirm that these textures were formed by the presence of the wormlike micelles with or without shear flow, SEM analyses were performed for the samples calcined to decompose the wormlike micelles. Calcination proceeded in an electric furnace at 450 °C for 10 h.
3. Results and Discussion First, let us begin with the SEM images of the captured microstructures for the unsheared CTAB/NaSal wormlike micelles. As shown in panels a and b of Figure 1, the micrometer-sized zigzag structure of silica substrate was formed in the absence of an imposed shear flow and this zigzag pattern extended very widely. In our study, the nanosized silica particles with anionic surface charge were adsorbed on the cationic micelle surface. Thus, the zigzag structures of panels a and b of Figure 1 were formed by
the silica particles and the micelles played only as a template of the zigzag structures. Consequently, the micelles were embedded in the zigzag structures and the transversal dimension of this structure was about 60-80 nm, which was considerably larger than a wormlike micelle. This zigzag structure was observed for the samples in the dilute solution of [CTAB] ) 0.005 M for either the molar ratio R ) 1.0 or R ) 10.0. On the other hand, for semidilute solution of [CTAB] ) 0.05 M, the structure was formed only for the equimolar solution, i.e., R ) 1.0. In dilute solutions with high molar ratios, the zigzag structure was formed via progressive growth of the entangled structure. Meanwhile, with a semidilute surfactant solution of [CTAB] ) 0.05 M, the structure for R ) 10.0 displayed in Figure 1c was quite different from the case for the equimolar solution. This is because the wormlike micelles were shortened by the excess salicylate ions, although the number density of micelles was similar to that of the equimolar solution; cf. panels b and c of Figure 1. Similar morphologies of the wormlike micelles in the absence of the shear flow had been observed by others through transmission electron microscopy at cryogenic temperature (cryo-TEM).18 It is also noted from panels a and b of Figure 1 that the size of repeated units was increased with [CTAB] for a given R ) 1.0. In dilute solutions, the wormlike micelles are hardly entangled with each other and the silica structures are formed from the individual micelle structures. On the other hand, in (18) Shikata, T.; Sakaiguchi, Y.; Uragami, H.; Tamura, A.; Hirata, H. J. Colloid Interface Sci. 1987, 119, 291.
Letters
Langmuir, Vol. 16, No. 11, 2000 4763
Figure 2. Steady shear viscosity as a function of the applied shear rate for dilute and semidilute solutions at 25 °C. The molar concentration ratio, [NaSal]/[CTAB], was fixed at unity.
semidilute solutions, the micelles tend to be entangled with each other. Under these circumstances, the silica structures coagulate with neighboring ones and become inhomogeneous, see Figure 1b. This is why surfactant at 0.005 M produces very fine and homogeneous textures (Figure 1a) compared to those (Figure 1b) of surfactant at 0.05 M. In Figure 1d, the SEM image of the pure substrate formed by TMOS in the absence of the surfactant was included. The structure was distinctively different from panels a-c of Figure 1 and mostly composed of the spherical silica particles of about 20-30 nm in diameter. Before considering the flow-induced silica structure, we will briefly discuss a typical rheological behavior of CTAB solutions, which will certainly help readers to understand the underlying physics. Specifically, the shear viscosity of the equimolar micellar solution is plotted as a function of the shear rate in Figure 2 for dilute and semidilute regimes. Also included for comparison are the shear viscosities of pure CTAB solutions in the absence of NaSal. It is noteworthy that in this paper, the dilute and semidilute regimes are classified by considering the rheological responses of pure CTAB solutions in the absence of NaSal. The CTAB surfactant without NaSal forms spherical or cylindrical micelles in our concentration window. It can be readily seen that the 0.005 M CTAB solution has the Newtonian plateau viscosity of 2 × 10-3 Pa‚s and shows a very weak shear thinning behavior. Thus, 0.005 M CTAB solution behaves like a Newtonian fluid with viscosity comparable to the viscosity of water. On the other hand, the 0.05 M CTAB solution exhibits a very strong shear thinning behavior, and the plateau viscosity at low shear rates appears about 1000 times as high as the viscosity of water. In the presence of NaSal, the rheological response changes dramatically. First, the shear viscosity is increased considerably by the presence of NaSal. This is because giant wormlike micelles are formed in dilute CTAB solution by the presence of NaSal. Second, when CTAB concentration is 0.005 M in the equimolar CTAB/NaSal solutions, the viscosity initially shear thins at low shear rates, then begins to shear thicken at a certain shear rate, and shear thins again at even higher shear rates. The shear thinning viscosity in low shear rates is clearly indicative of the flow-induced alignment toward the flow direction. Meanwhile, when CTAB concentration is 0.05 M in the equimolar CTAB/NaSal solutions, the micelles are much larger and entangled each other in the
Figure 3. SEM images of the captured microstructure under a low shear rate of 0.38 s-1. The flow is aligned to the diagonal direction: (a) equimolar solution of [CTAB] ) 0.005 M; (b) pure TMOS + HCl solution.
solution of CTAB/NaSal. In this case, the shear viscosity increases much higher, and the micellar solution behaves like entangled polymer solutions exhibiting typical nonlinear viscoelastic behavior such as a stress plateau. As we shall see shortly, the rheological behavior well matches the flow-induced microstructure of the silica substrate. The flow-induced microstructure was examined as functions of flow intensity, surfactant concentration [CTAB], and molar ratio R. The micellar solutions were sheared continuously at a given shear rate until the gelation was completed. In this study, the gel-point was decided as the point at which the shear stress began to increase abruptly for the solution losing its fluidity, and shearing did not persist past the point of gelation. When a shear flow was imposed, it took 3-5 min for the complete gelation, which was relatively longer than the case in the absence of the flow, 15-60 s. This was because the flow retarded the gelation forming the silica-micelle composite structure. In Figure 3a, the SEM image of the gelified sample prepared from the dilute equimolar solution of [CTAB] ) 0.005 M was reproduced. It can be readily seen that the silica substrate was aligned to the flow direction at a shear rate below the onset of shear thickening. The small-scale zigzag morphology remained persistent at low shear rates. Thus, the shearing led to the flow-induced texture. The general feature of the shear-induced structure was preserved for the higher molar ratio R ) 10.0. The SEM image of Figure 3b for the pure silica substrate formed in the weak shear flow exhibited dimly the silica particles aligned to the flow direction in the matrix.
4764
Langmuir, Vol. 16, No. 11, 2000
Letters
Figure 4. SEM images of the captured microstructure under a low shear rate of 0.38 s-1 for the equimolar solution of [CTAB] ) 0.05 M: (a) near the rotating outer cylinder; (b) near the stationary inner cylinder.
The rheological and rheo-optical responses have been monitored in our group to probe the flow-induced structural transition or coagulation in semidilute and dilute surfactant solutions.5,7 The structural transition has been believed to accompany the inhomogeneous flow field in the gap of flow cell. To examine the effects of flow in semidilute surfactant solution, we took the SEM photographs of the structures gelified at two different sites in the flow cell, namely, in the vicinities of the rotating outer cylinder and the stationary inner cylinder. The SEM images in Figure 4 showed a very fascinating feature of the aligned structures at the shear rate of 0.38 s-1 near the outer and inner cylinders, respectively. As noted, the silica substrate near the rotating outer cylinder was aligned better toward the imposed flow than the silica near the stationary inner cylinder. This implied strongly the nonuniformity of shear rate in the gap. However, the degree of flow-induced alignment for the semidilute solution of [CTAB] ) 0.05 M was shown poor relative to that observed for the dilute solution of [CTAB] ) 0.005 M; cf. Figures 3a and 4a. This weak alignment was due to the entanglement of the wormlike micelles in the semidilute surfactant solution. In Figure 4, the entangled wormlike micelles were stretched by flow and extended over several micrometers in length, but the alignment was not complete unlike that in dilute solution. Now let us then consider the flow-induced microstructure in dilute concentrations under strong flow fields. Below [CTAB] ) 0.01 M, the equimolar solutions exhibited the shear thickening as the shear rate increased highly.
Figure 5. SEM images of the captured microstructure under high shear rates for the equimolar solution of [CTAB] ) 0.005 M. Applied shear rate is 7.5 s-1 for (a) and (c) and 11.9 s-1 for (b). Flow direction is vertical in (a), diagonal in (b), and horizontal in (c).
In a dilute equimolar solution, the onset shear rate for shear thickening was 6-7 s-1 as shown in Figure 2. In Figure 5, the SEM images of the gelified structure in the flow-cell were reproduced for the equimolar solution of [CTAB] ) [NaSal] ) 0.005 M. The SEM images were taken at two different planes in the Couette flow cell, namely, the velocity-velocity gradient plane (a)-(b) and the velocity-vorticity plane (c). As noted, the silica substrate was aligned toward the flow direction but the zigzag structure was still sustained at a high shear rate 7.5 s-1 in both planes. We could observe that the stretched wormlike micelles were partly broken down by the strong
Letters
shearing. Moreover, the shear-induced structure on the velocity-vorticity plane was different from that on the velocity-velocity gradient plane. In fact, the texture alignment appeared ambiguously. As the shear rate increases strong enough for the shear thickening, the flowinduced interlayer fluctuation was observed. This is also shown in Figure 5b. These SEM images were also consistent with the rheological and rheo-optical behavior such as the viscosity buildup and intensity fluctuation above the critical shear rate. In summary, we observed the flow-induced silica structure, which was formed during in situ gelation through sol-gel chemistry in the presence of wormlike micelles of CTAB and structure-forming additive NaSal. The silica substrate formed the micrometer-sized zigzag structure, which was isotropic at a quiescent state. When
Langmuir, Vol. 16, No. 11, 2000 4765
the flow was imposed, the silica substrate was aligned to the flow direction and exhibited the microstructures depending on the surfactant concentration, molar ratio, and flow intensity. The present observation showed that the existence of the stress plateau in semidilute solutions was associated with the inhomogeneous velocity gradient or shearing caused by the difference in experimental and relaxation time scale of the micelles. On the other hand, in dilute solutions, the zigzag structure was stretched and broken down partly into the micrometersized texture at sufficiently high shear rates. Overall, the SEM images of the flow-induced silica structures were consistent with the rheological responses of the micellar solutions. LA9911685
