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Langmuir 1997, 13, 3700-3705
Effect of the Transient Network Structure on the Growth of Microparticles through Aggregation Ken-ichi Kurumada,* Shintaro Itakura, Shinsuke Nagamine, and Masataka Tanigaki Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-01, Japan Received September 4, 1996. In Final Form: April 20, 1997X
Growth of silica particles in microemulsion systems consisting of cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), tetraethyl orthosilicate (TEOS), and water has been investigated. The diagram of composition is given to show the region wherein a transient networklike structure is formed from CTAB micellar aggregates with cylindrical geometry. As TEOS becomes concentrated, the content of CTAB necessary for the network formation steeply increases. The growth of silica particles is studied by means of static and dynamic light scattering, and the effect of the micellar network structure is discussed. The SLS measurements give smaller values of the structural correlation length when the silica particles are formed in the network medium than in a dispersed structure. The structural relaxation behavior obtained from the DLS measurements shows a slowly decaying mode for the networklike medium, suggesting the retardation effect by the networklike structure of CTAB molecules. The networklike structure effectively prevents the interminable growth of the silica particles by blocking their free motions.
Introduction A macroscopically homogeneous phase which contains at least surfactant, water, and organic solvent is called a microemulsion and has been investigated extensively.1-6 A microemulsion possesses some significant features as follows: (1) It has intrinsically ordered microstructures of 10-101 nm in order of magnitude. (2) The unit of the microstructure shows a great variation in geometrical shape which plays a quite important role in the determination of various macroscopic properties of the microemulsion.7-10 (3) The microstructure is not permanently kept but continuously renewed; thus, the microstructure or microscopic properties need to be considered, including the dynamical aspects of structure. (4) The microstructure consists of several classes, that is, properties which could not be accounted for without considering some co-operative behavior of the unit structures.11,12 X
Abstract published in Advance ACS Abstracts, June 15, 1997.
(1) For a general survey, see: Surfactant in Solution; Mittal, K., Lindman, B., Eds.; Plenum: New York, 1984. (2) For a general survey, see: Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Proceeding of the International School of Physics (Enrico Fermi) Course XC; Italian Physical Society: Bologna, 1985. (3) For a general survey, see: Physics of Complex and Supermolecular Fluids; Safran, S. A., Clark, N. A., Eds.; John Wiley & Sons: New York, 1985. (4) For a general survey, see: Micellar Solutions and Microemulsions; Chen, S.-H., Rajagopalan, R., Eds.; Springer-Verlag: New York, 1990. (5) For a general survey, see: Micelles, Membranes, and Microemulsions, and Monolayers; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer-Verlag: New York, 1994. (6) For a general survey, see: Phase Transitions and Critical Phenomena, Volume 16: Self Assembling Amphiphilic Systems; Domp, C., Lebowitz, J. L., Gommper, G., Schick, M., Eds.; Plenum: Academic Press, 1994. (7) Safran, S. A.; Turkevich, L. A.; Pincus, P. J. Phys. Lett. 1984, 45, 69. (8) Schurtenberger, P.; Scartazzini, R.; Magid, L. J. J. Phys. Chem. 1990, 94, 3695. (9) Luisi, P. L.; Scartazzini, R.; Haering, G.; Schurtenberger, P. Colloid Polym. Sci. 1990, 268, 356. (10) Kurumada, K.; Shioi, A.; Harada, M. J. Phys. Chem. 1994, 98, 12382. (11) Kurumada, K.; Shioi, A.; Harada, M. J. Phys. Chem. 1995, 99, 16982. (12) Kurumada, K.; Shioi, A.; Harada, M. J. Phys. Chem. 1996, 100, 1020.
S0743-7463(96)00860-8 CCC: $14.00
Recently, researchers have been studying the spontaneous formation of solid materials in microemulsions utilizing the microstructure as a reaction medium.13-18 From the viewpoint of materials engineering, those researchers attempted to control the nanostructure of solid materials which cannot be attained by means of mechanical processing, and how the solid materials would resemble the microemulsion structurally is the most intriguing problem.19-21 Many of them have studied the formation of particulate substances and their kinetics in the region of the phase diagram where the microemulsion possesses the form of dilute dispersion, i.e., the waterin-oil (W/O) or oil-in-water (O/W) type. In these cases, the dynamics of the particles should be dominated by the diffusion. When the microemulsion has a networklike structure, on the other hand, it is possible that the ordinary diffusive behavior would be suppressed, and as a result, the dynamics of the particles would deviate from that in a medium allowing the diffusive motion. On this point, experimental studies on the dynamical behavior of a transient network structure formed in a micellar phases or dense microemulsion phase have pointed out the presence of a nondiffusive relaxation mode by means of dynamic light-scattering measurements.11,12,22 Nevertheless, works that attempt to relate the kinetics of the growth of solid materials with the network structure affecting their motion are still limited, and attention has been mainly focused on the morphological effect brought about by the network structure. Among recent studies, for example, Gan and co-workers have investigated the (13) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (14) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (15) Robinson, B. H.; Towey, T. F.; Zourab, S.; Visser, A. J. W. G.; Hoek, A. Colloid Surfaces 1991, 61, 175. (16) Towey, T. F.; Kahn-Lodhi, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86, 3757. (17) Nosaka, Y.; Shigeno, H.; Ikeue, T. J. Phys. Chem. 1995, 99, 8317. (18) Lianos, P.; Thomas, J. K. J. Colloid Interface Sci. 1987, 117, 505. (19) Kandori, K.; Kon-no, K.; Kitahara, A. J. Dispersion Sci. Technol. 1986, 9, 61. (20) Hirai, T.; Shiojiri, S.; Komasawa, I. J. Chem. Eng. Jpn. 1994, 27, 590. (21) Hirai, T.; Sato, H.; Komasawa, I. Ind. Eng. Chem. Res. 1994, 33, 3262. (22) Nemoto, N.; Kuwahara, N.; Osaki, K. Langmuir 1995, 11, 30.
© 1997 American Chemical Society
Growth of Microparticles through Aggregation
formation of polymeric materials via polymerization and cross-linking in bicontinuous microemulsions and found that those microemulsions could fundamentally work as a “molecular mold” of an open-cell type microporous material.23-25 However, the view that the microstructure in a microemulsion should function intact as a mold is not realistic, because a microemulsion does not keep its microscopic topological structure, being different from a real cross-linked polymeric gel. We should rather say that the size and morphology of the solid material are determined by both static and dynamic factors offered by the microemulsion. More specifically, the structure of the newly formed solid matter must be influenced by the time constants which characterize the spontaneous dynamical behavior of microstructures in a microemulsion. In the present study, we surveyed the formation of silica particles in the transient network structure comprised of surfactant-coated elongated molecular aggregates. The surfactant used here is cetyltrimethylammonium bromide (CTAB), which is well-known for its capacity to form a quite polymerlike network structure with outstanding viscoelastic properties under proper conditions. Tetraethyl orthosilicate (TEOS) contained in the CTAB aggregate is chemically converted into silica particles via hydrolysis, polymerization, and aggregation. These processes must be affected by the microstructure of the medium. In particular, the aggregation is expected to be strongly influenced by the presence of the network structure which should act on the aggregation quite directly as a geometrical constraint. To make the effect due to the network structure clear, we compared the process of particle formation in the network structure with that in a dispersed structure along time. The static structure and dynamical behavior of the samples are evaluated using static and dynamic light scattering. The process of particle formation in each reaction medium shows different time-dependent behavior, which is discussed in the light of the difference in the dynamical behavior caused by the networklike structure. Experimental Section Materials. Cetyltrimethylammonium bromide (CTAB), tetraethyl orthosilicate (TEOS), and sodium salicylate (NaSal) were provided from Nacalai Tesque Co. Ltd. CTAB was purified by recrystallization in methanol (50 % vol.)/acetone (50 % vol.), and others were used as supplied. Water was purified by ionexchanging and distillation. Preparation and Samples. CTAB, NaSal, and water were mixed at a prescribed content and kept at 25.0 °C in a thermobath until the mixture became a homogeneous state. The concentration of NaSal was equalized to that of CTAB for each sample. (In the following text, brackets, [ ], are used to stand for the concentration of the chemical species in them.) When [CTAB] is over a certain threshold, a networklike structure of elongated CTAB micellar aggregates is formed, and a distinctive elasticity is observed at this stage.22,26-31 Then, TEOS was mixed by injection. Immediately after mixing, the mixture was turbid because of the presence of oily droplets of TEOS, which (23) Chew, C. H.; Gan, L. M.; Liu, J.; Lindman, B. Langmuir 1995, 11, 3312. (24) Chieng, T. H.; Gan, L. M.; Chew, C. H.; Lee, L.; Ng, S. C.; Pey, K. L.; Grant, D. Langmuir 1995, 11, 3321. (25) Gan, L. M.; Li, T. D.; Chew, C. H.; Teo, W. K.; Gan, L. H. Langmuir 1995, 11, 3316. (26) Candau, S. J.; Hirsch, E.; Zana, R. J. Colloid Interface Sci. 1985, 105, 521. (27) Candau, S. J.; Hirsch, E.; Zana, R.; Adam, M. J. Colloid Interface Sci. 1988, 122, 430. (28) Candau, S. J.; Hirsch, E.; Zana, R.; Delsanti, M. Langmuir 1989, 5, 1225. (29) Shikata, T.; Hirata, H.; Kotaka, K. Langmuir 1987, 3, 1081. (30) Shikata, T.; Hirata, H.; Kotaka, K. Langmuir 1988, 4, 354. (31) Shikata, T.; Hirata, H.; Kotaka, K. Langmuir 1989, 5, 399.
Langmuir, Vol. 13, No. 14, 1997 3701 disappeared to make the mixture transparent irrespective of whether the mixture lost the above-mentioned elasticity after that was stirred. The time at which the stir was given is determined as τm ) 0 where τm denotes the time after mixing. Measurements. Light-scattering measurements were made along the time after mixing TEOS at the incident wave length λ ) 632.8 nm (Otsuka Denshi Co. Ltd.; SLS-600 and DLS-700). The deviation of the refractive index of samples from that of pure water is negligibly small; therefore, the scattered wavenumber k in the scattering measurements was estimated as follows:
k)
4πnwater θ sin λ 2
()
(1)
Here, θ and nwater denote the scattering angle and the refractive index of pure water, 1.33, at λ ) 632.8 nm and 25.0 °C, respectively. The details for the SLS technique have been described elsewhere.32,33 The interval and the number of channels were so determined as to profile the relevant relaxation mode in the correlation function. Electrical conductivity was measured immediately after the mixture was stirred and TEOS became homogeneously distributed to infer whether the sample possesses a networklike structure in it or not in terms of the mobility of electrical charge. The electrical current was measured between two parallel platinum sheets with identical surface areas at a frequency of 1.2 kHz (Kyoto Denshi Kogyo Co. Ltd., CM-07). All of the experiments were carried out at 25.0 °C.
Results and Discussion Preliminary Remarks. Since tetraethyl orthosilicate (TEOS), which we used as the alkoxide of silicon, is a hydrophobic liquid itself, it is contained inside the micelles coated by cetyltrimethylammonium bromide (CTAB) with the hydrophobic chains directed inside. TEOS is hydrolyzed (Si-(OC2H5)4 + 4H2O f Si(OH)4 + 4C2H5OH), and then, the hydroxide is polymerized into the polymer of SiO2 through dehydration. The polymerized silica aggregates with each other, and the resultant silica particles are formed through staged aggregation. Sodium salicylate (NaSal) added to make CTAB molecules aggregate into an elongated geometry catalyzes the hydrolysis; however, its effect is much weaker than the usually used catalyst in this chemical reaction, for example, hydrochloric acid or ammonium. The time-dependent behavior of each sample, which is measurable by experiments, changes so slowly that it is possible to trace its scattering behavior along τm regarding that as unchanged within the time scale of the measurement. In this work, the attention is mainly focused on the effect of the networklike structure formed by CTAB; therefore, we divide those samples into two regions in which the networklike structure is formed and is not formed. The difference between these two states is very clear. We call them the quasi-gel region and the nongel region, respectively. (The “quasi-gel” implies that the networklike structure is fundamentally different from a permanently cross-linked network structure; i.e., a true gel in the sense that the network structure is not renewed spontaneously due to thermal aggitations even after a long time.) The most spectacular property of the sample in the quasi-gel region is the presence of outstanding viscoelasticity22,26-31 which appears abruptly on entering the region along the [CTAB] axis on the content diagram and becomes more outstanding as [CTAB] increases. Content Region and Structure. Figure 1 shows a content diagram of the system containing water, CTAB, NaSal, and TEOS, which delineates the boundary between two regions wherein a quasi-gel structure is formed (quasi(32) Shioi, A.; Harada, M.; Tanabe, M. J. Phys. Chem. 1993, 97, 8281. (33) Shioi, A.; Harada, M.; Tanabe, M. J. Phys. Chem. 1995, 99, 4750.
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Figure 3. Photograph of samples a week after mixing water, cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and tetraorthoethyl silicate (TEOS): (A) nongel medium; (B) quasi-gel medium. Note that precipitation of silica aggregates occurs only in the nongel medium. Figure 1. Content diagram presenting the region wherein a transient networklike structure of cetylatrimethylammonium bromide (CTAB) aggregates is formed. Light-scattering measurements were made at the point indicated by × in the respective regions at [CTAB] ) 1.0 × 10-1 mol/L.
Figure 2. An example of dependence on [CTAB] of the electrical conductivity σ of the microemulsion systems containing water, cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and tetraethyl orthosilicate (TEOS) at [TEOS] ) 4.0 × 10-2 mol/L. The boundary in Figure 1 is determined as the point at which ∂σ/∂[CTAB] shows an abrupt reduction. The measurements were made just after tetraethyl orthosilicate (TEOS) was mixed and distributed homogeneously.
gel region) and wherein it is not (nongel region). The boundary is determined for each concentration of TEOS as a point at which the slope of the dependency of the electrical conductivity σ on the concentration of CTAB decreases abruptly, because the formation of a precolated networklike structure is considered to restrict strongly the motion in the medium. Figure 2 shows an example of σ against [CTAB], where the point is determined as 0.125 mol/L. The phase boundary shown in Figure 1 shifts toward the larger value of [CTAB] as [TEOS] increases. It indicates that the elongated micelle is swollen as the quantity of TEOS enclosed within the micelle increases under a fixed quantity of CTAB, and as a result, more CTAB becomes necessary for the formation of the networklike structure in the solution. Figure 3 presents a photograph of a quasi-gel and nongel sample approximately a week after mixing the constituents. In this period, the turbidity has already ceased to increase. In the nongel medium, the silica particles aggregate into a sufficiently large flock to precipitate (A).
On the other hand, they are much more homogeneously dispersed in a quasi-gel medium, and no aggregate large enough to be visible is present (B). The microstructure in the quasi-gel medium is considered to prevent silica microparticles to some extent from assembling into larger aggregates. The authors selected two compositions of the media as indicated by crosses in Figure 1, each of which corresponds to a quasi-gel and nongel one, respectively, and investigated the microstructure of the samples including the silica particles formed along the time τm after the mixing utilizing static and dynamic light scattering measurements. In particular, attention is paid to the relation between the microstructure of the quasigel or nongel medium and the growth of the silica particles. Static Light Scattering. The time-dependent behavior of the static scattered pattern of light is investigated to pursue the change of the microstructure of the sample along time τm after the mixing. Figure 4 shows the dependency of the Rayleigh ratio on the scattered wave number k for several values of τm for the quasi-gel and nongel media. For both samples, the Rayleigh ratio increases as τm increases, and this is ascribed to the silica particles which are formed accumulatively in the quasigel or nongel medium after TEOS is mixed. For the quasi-gel medium, several peaks are seen in the dependence of the Rayleigh ratio on the scattered wavenumber k. The peak position is almost the same irrespective of τm, and the corresponding distance is calculated as approximately 600 nm from the lower wave numbers around k ) 1.0 × 10-2 and 2.0 × 10-2 nm-1. This value is apparently much larger than the size of the silica particle formed in the quasi-gel medium, as shown later, and should be considered as the reflection of the spontaneous inhomogeneities in the quasi-gel medium which leads to spatial density fluctuation of the silica particles, when we take into account the fact that the peak position does not alter with τm. (Actually, a slight opacity is seen in that sample even before TEOS is mixed, indicating that there are spontaneous inhomogeneities in the spatial scale comparable to the wavelength of visible light.) Figure 5 shows the factor Re[60°]/Re[120°] against τm for both samples, and it shows a steep increase from unity indicating the growth of the structure of silica. In cases of the quasi-gel and nongel media, the increase in Re[60°]/Re[120°] is seen to start rather abruptly around the periods τm ∼ 30 h and τm ∼ 50 h, respectively. Figure 6 shows the correlation length ξ against τm, which is calculated by fitting the dependence of the Rayleigh ratio on k with the Ornstein-Zernike formula as follows:
R(k) ∝
1 1 + k2ξ2
(2)
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Figure 6. Dependence of the Ornstein-Zernike correlation length ξ on τm: (O) quasi-gel medium; (0) nongel medium. Note the respective ordinates on both side of the figure.
Figure 4. Dependence of the Rayleigh ratio R(k) on scattered wavenumber k for various values of time τm after mixing water, cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and tetraethyl orthosilicate (TEOS): (A, 0) quasi-gel medium, τm ) 46.5 h; (A, 4) quasi-gel medium, τm ) 57.5 h; (A, O) quasi-gel medium, τm ) 69 h; (B, 0) nongel medium, τm ) 26.5 h; (B, 4) nongel medium, τm ) 33.5 h; (B, O) nongel medium, τm ) 38.5 h.
Figure 5. Ratio of the Rayleigh ratio R(60°) at θ ) 60° to R(120°) at θ ) 120° against τm: (O) quasi-gel medium; (0) nongel medium. The value of R(60°)/R(120°) hardly alters from unity up to τm ) 50 and 30 h for the quasi-gel medium and the nongel medium, respectively.
Here, ξ can be regarded as the characteristic length of the microstructure in the scattering medium. For the nongel medium, ξ gives a rough estimate of the dispersed structure mainly contributing to the scattered intensity. Therefore, ξ is roughly interpreted as the size of the silica
particles when the intensity is dominated by the contribution due to the silica particles. Also, for the quasi-gel medium, ξ gives a measure of the size of the silica particles when the scattering capacity of them dominates at sufficiently large τm, whereas the mesh size of the CTAB network-like structure would be reflected in ξ for an earlier stage of τm as suggested previously.34 As shown in Figure 6, the value of ξ increases steeply when τm > 30 h and τm > 50 h for the nongel and quasi-gel samples, respectively, indicating that the silica particles grow and become increasingly dominant in scattering when τm exceeds the above period. It should be noted that the value of ξ is markedly smaller in the quasi-gel sample compared to that in the nongel sample. This result indicates that the network structure in the quasi-gel medium offers an effective geometrical constraint against the growth of the silica particles. In summary, it is the structure comprised of the molecular aggregates of CTAB up to τm ∼ 30 h and τm ∼ 50 h for the quasi-gel and nongel samples, respectively, that mainly characterize the microstructure of the sample. Around the above periods, the average size of the silica particles becomes comparable with the characteristic size of the structure formed by the CTAB aggregates and then exceeds that. The interaction between the silica aggregates and the structure of the molecular aggregates should become more significant after these periods. This effect should be particularly outstanding for the quasigel sample containing a networklike structure that would be able to entrap geometrically the silica particles more effectively. Dynamic Light Scattering. Dynamic light-scattering measurements offer information on the dynamical behavior in the media, from which we infer indirectly the characteristic structure or its size that changes along time after mixing TEOS. Figure 7 shows typical examples of the photon correlation functions which reflect the spontaneous relaxation of dynamical microstructure in the media. The photon correlation function g1(t) converges on zero when the instantaneous microstructure of the liquid sample at a certain moment is entirely relaxed and renewed, and the reciprocal of rate of relaxation in g1(t) can be considered as the measure of the time needed for the entire relaxation of the microstructure on a spatial scale 2π/k. In the quasi-gel medium, we can point out the presence of two relaxation modes (Figure 7B), one of which possesses a much longer relaxation time as compared to the other one (34) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979.
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Figure 8. Dependence of the initial relaxation rate Γin on k2 for various value of τm: (b) quasi-gel medium, τm ) 50 h; (9) quasi-gel medium, τm ) 58 h; (2) quasi-gel medium, τm ) 64 h; (O) nongel medium, τm ) 29.5 h; (0) nongel medium, τm ) 31 h; (4) nongel medium, τm ) 32.5 h. Broken lines are so drawn as to indicate Γin ∝ k2.
Figure 7. Examples of photon correlation function g1(t): (A) nongel medium (τm ) 9.5 h, k ) 1.87 × 10-2 nm-1); (B) quasi-gel medium (τm ) 50 h, k ) 1.32 × 10-2 nm-1). The autocorrelation function g1(t) decays on zero for a sufficiently long correlation time t.
( )
g1(t) ) A exp -
(
)
t t + B exp τin τlong
(3)
with τin , τlong, where A and B are constants standing for the amplitudes of the modes. The first term in the righthand side of eq 3 corresponds to the much faster initial relaxation that appears only in the initial period of the correlation time t. Similar profiles of g1(t) have been pointed out in pure micellar systems comprised of a networklike structure of CATB molecular aggregates, and the rapid relaxation has been interpreted in terms of cooperative relaxation,22 the concept of which was established in the dynamical theory of polymeric systems.34,35 Since the chemical structure of the quasi-gel medium is fundamentally the same as those of pure systems, the rapid relaxation in Figure 7B should be attributed also to the cooperative relaxation mode. However, it must be pointed out that the relaxation rate Γin (≡τin-1) in the quasigel medium is not proportional to the square of k but almost independent of k, while for the pure CTAB networklike system the rate of the rapid relaxation time is proportional to the square of k as seen in a simple diffusive process.22 Although the reason of this discrepancy has not been understood, the dynamical behavior of the CTAB aggregate must be affected by the presence of the silica particles or precursory moieties. The rate Γin of the rapid relaxation is reduced with the increase in τm as shown in Figure 8, showing that the dynamical behavior of the silica particle slows down as they become larger. The presence of a slower relaxation in g1(t) for the quasigel medium implies a strongly constrained dynamical (35) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: New York, 1986.
behavior of scattering particles by the networklike structure, and it is plausible that this constraining structure should lead to the slower growth of the silica particles contained in the quasi-gel medium. For the nongel medium, the dynamical behavior can be understood much more simply, where g1(t) is seen to be composed of a unique relaxation mode as shown in Figure 7A:
( )
g1(t) ) A exp -
t τin
(4)
The relaxation rate Γin (≡τin-1) is proportional to the square of k as shown in Figure 8, showing that a simple diffusive process takes place in the nongel medium. Here, it is obvious that the structure provided by the CTAB molecular aggregates takes the form of a dispersion. This view is quite consistent with the result that the viscosity of the nongel medium hardly deviates from that of the pure solvent, water. In such a situation, the silica particles must grow larger because the dispersed structure of the CTAB molecular aggregates would not prevent them from aggregation. Figure 9 shows the hydrodynamic radius ξd of the silica particles against τm assuming that they are immersed are and freely exhibiting Brownian motion in a medium with the same viscosity as that of water. The value of the hydrodynamically effective radius ξd is obtained based on the Stokes-Einstein formula:
ξd )
kBT 6πηwaterD
(5)
Here, ηwater denotes the viscosity of water at T ) 298 K, and D is the diffusion constant obtained from the slope in Γin against the square of k. For the same value of τm, the value of ξd roughly corresponds to that of ξ in Figure 6, i.e., the Ornstein-Zernike correlation length from the static viewpoint. In summary, the photon correlation function is distinctly indicative of the structural characteristics both of the
Growth of Microparticles through Aggregation
Figure 9. Dependence of apparent hydrodynamic radius ξd of silica particles on τm in the nongel medium calculated by the Stokes-Einstein formula as ξd ) kBTk2(6πηwater Γin)-1 at T ) 298 K.
quasi-gel medium and of the nongel one. In the former, the diffusive behavior should be restricted due to the strong geometrical constraint caused by the transient cross-linked structure of the CTAB aggregates. It is quite likely that the growth of the silica aggregates is suppressed because the CTAB network structure prevents the silica particles from moving diffusively by thermal agitations when they grow up to some extent. On the other hand, the diffusive behavior is observed predominantly in the nongel medium. This is because the nongel medium has no structure that works as a strong geometrical constraint, being different from the quasi-gel medium at this point. As a result, the silica particles could continue to grow larger through mutual aggregation. Conclusion The microemulsion systems composed of cetyltrimethylammonium bromide (CTAB), sodium salicylate (Na-
Langmuir, Vol. 13, No. 14, 1997 3705
Sal), tetraethyl orthosilicate (TEOS), and water were examined experimentally including the formation of silica particles. The region in the content diagram wherein a transient network-like structure comprised of elongated CTAB molecular aggregates is formed was determined. The minimum content of CTAB necessary to form the network structure increases steeply as the content of TEOS increases. SLS measurements give smaller values of the OrnsteinZernike correlation length for the quasi-gel medium, indicating that the network structure would restrict the growth of silica aggregates because of its geometrical obstruction. In both the quasi-gel and nongel media the scattered intensity becomes dominated by the presence of newly formed silica aggregates after a certain time lag after mixing the constituents, and this is because around those periods the silica aggregates grow up and compare in size with the characteristic spatial scale of the network structure in the media. The results of the DLS measurements indicate that the network structure hinders the diffusive behavior in the quasi-gel medium from the nonproportional dependence of Γin on the square of the scattered wavenumber. In contrast, Γin is proportional to k2 in the nongel medium; therefore, the diffusive behavior is dominant since there is no obstruction against that. The geometrical constraints due to the network structure in the quasi-gel medium should work to block the growth of silica aggregates and retard growth the silica particles. Acknowledgment. Our light scattering measurements were possible thanks to the help of Dr. A. Shioi, Dr. M. Adachi, Dr. K. Suzuki, and Prof. M. Harada in the Institute of Atomic Energy, Kyoto University. LA960860M
