Processes of Silica Network Structure Formation in Reverse Micellar

The processes by which the silica-network structure is formed in reverse micelles ... reverse micellar system was composed of didodecyldimethylammoniu...
0 downloads 0 Views 102KB Size
Download PDF
Langmuir 2001, 17, 4189-4195

4189

Processes of Silica Network Structure Formation in Reverse Micellar Systems Makoto Harada, Shintaro Itakura, Akihisa Shioi,† and Motonari Adachi* Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan Received July 31, 2000. In Final Form: April 17, 2001 The processes by which the silica-network structure is formed in reverse micelles were elucidated. The reverse micellar system was composed of didodecyldimethylammonium bromide (DDAB), cyclohexane, and HCl- aqueous solution, and the silica source was tetraethoxysilane (TEOS). The hydrolysis reactions for TEOS were completely different from those of usual homogeneous sol-gel processes. The rate of hydrolysis was affected by the curvature of the reverse micelles. The reverse micelles also promoted the formation of network bonds of silica even under acidic conditions. Primary silica particles spherical in shape were generated first. DDAB and partially hydrolyzed TEOS cooperatively formed the primary spherical particles, which were stable against coagulation and did not grow to large particles. The primary particles formed clusters, which were converted to linear silica rods with some branches upon neutralization of HCl with NaOH, and the system became a transparent gel. The diameter of the silica rods was similar to that of rodlike micelles of DDAB in the absence of TEOS. Thus, the DDAB micelles seemed to direct the formation of the silica microstructure as if the rodlike micelles functioned as a template. However, the formation of silica rods and a network structure arises from the connection of primary particles through condensation reactions.

Introduction Amphiphilic molecules form polymorphic molecular assemblies through self-organization. Molecular tectonics using molecular assembly as a template provide a method of chemical construction of higher order structures.1 In 1992, workers at Mobil2 proposed a new method to synthesize ordered mesoporous materials composed of (alumino) silicates using surfactant aqueous solutions. In dilute solutions of surfactant, both silicate oligomers and surfactants cooperatively form rodlike aggregates, which yield condensed matter, e.g., hexagonal phase.3,4 Recently, silica-surfactant aggregates with a random network structure, which were converted to nanotubules upon calcination, were synthesized using an aqueous solution of laurylamine.5 In both cases, hydrophobic chains of surfactants are packed in the inner domain of rodlike aggregates. Thus, the surfactants direct rodlike microstructure comprised of silica. In other words, surfactant aggregates in aqueous solutions function as a kind of template for the formation of silica microstructures. What can be expected for surfactant aggregates of a water-inoil type? Reverse micelles provide a confined space of nanometers for preparing globular particles of metals, inorganic materials, and organic polymers. However, the inner domain of the reverse micelles is filled with an aqueous solution, and the micelles are easily disrupted upon the growth of the particles. Walsh et al.6 showed that materials * To whom all corresponding should be addressed. † Current address: Department of Chemistry and Chemical Engineering, Yamagata University, Yonezawa, Yamagata, Japan. (1) Mann, S. In Biomimetic Biomaterials Chemisry; Mann, S., Ed.; VCH Publisher: New York, 1996; Chapter 1. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Chen, C. Y.; Burkett, S. L.; Davis, M. E. Microporous Mater. 1993, 2, 33. (4) Huo, Q.; Margolese, D. I.; Ciela, Demuth, D. K.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stuckey G. D. Chem. Mater. 1994, 6, 1176. (5) Adachi, M.; Harada, T.; Harada, M. Langmuir 2000, 16, 2376. (6) Walsh, D.; Hopwood, J. D.; Mann, S. Science 1994, 264, 1576.

with reticulated and interconnected beadlike microstructures were synthesized with the help of bicontinuous reverse micelles composed of didodecyldimethylammonium bromide (DDAB). They replaced the water with a supersaturated calcium phosphate solution that permitted crystals to nucleate slowly, and prepared the reticulated microstructure by solidifying the organic solvent in which DDAB microstructures were present. The radius of the elementary structure is, however, much larger than that of the original DDAB-cylindrical aggregates. Thus, the role of the reverse micelles as template is not clearly understood. Globular silica particles are readily synthesized using a sol-gel process in reverse micelles under basic conditions as reviewed by Osseo-Asare.7 Under acidic conditions, hydrolyzed silicon alkoxides in the DDAB reverse micelles form transparent gels8-10 as with the homogeneous solgel method. Watzke and Dieschboug11 prepared silicaorganic composite materials with a filamentous structure using a gelatin organogel of reverse micellar type under acidic conditions. They speculated that the silica sol-gel process always proceeds through the primary formation of dense colloidal particles. In general, sol-gel processes in homogeneous media direct the formation of fibrous structures of silica under acidic conditions and a low water/ alkoxide ratio. It is, therefore, difficult to understand why the primary colloidal silica was generated in the reverse micellar system. Probably, the hydrolysis and condensation reactions for silicon alkoxide are completely different between the homogeneous and the microheterogeneous media. (7) Osseo-Asare, K. In Handbook of Microemulsion Science and Technology; Kumar, P., Mittal, K. L., Eds.; Dekker: New York, 1999; Chapter 18. (8) Friberg, S. E.; Ahmed, A. U.; Yang, C. C.; Ahuja, S.; Bodesha, S. S. J. Mater. Chem. 1992, 2, 257. (9) Friberg, S. E.; Jones, S. M.; Yang, C. C. J. Dispersion Sci. Technol. 1992, 13, 65. (10) Burban, J. H.; He, M.; Cussler, E. L. AIChE J. 1995, 41,159. (11) Watzke, H. J.; Dieschboug, C. Adv. Colloid Interface Sci. 1994, 50, 1.

10.1021/la0010875 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/14/2001

4190

Langmuir, Vol. 17, No. 14, 2001

The aims of the present study are to examine the role of DDAB reverse micelles in the hydrolysis and condensation reactions of tetraethoxysilane(TEOS) and in the formation of a silica-network structure. First we elucidated the microstructure of DDAB molecular assemblies in the absence of TEOS. Second, the characteristics of the microheterogeneous reactions were compared with those in the homogeneous reactions under acidic conditions, and the peculiar nature of the hydrolysis and condensation reactions of silicon alkoxide in the reverse micelles was exposed. Finally, we elucidated the formation of the network structure composed of silica rods. At present, the assembly of particles into a well-defined superstructure is a demanding task. For this, it is important to understand the processes involved in the superstructure formation and how the structure is tailored. The present work will contribute to the rational design of superstructures. Experimental Section Materials. Didodecyldimethylammonium bromide (DDAB) of reagent grade purchased from Tokyo Kasei was used as supplied. Tetraethoxysilane (TEOS) of reagent grade and cyclohexane of spectroscopic grade were used as received from Aldrich and Nacalai Tesque, respectively, without further purification. Microstructure of DDAB Reverse Micelles. DDAB was dissolved in cyclohexane, and the desired amount of 0.1 M HCl aqueous solution was added. The water-to-DDAB mole ratio, Wo, was changed between 2 and 11. After equilibration at 25 °C, the reverse micellar solution was added to the cell of a small-angle X-ray scattering (SAXS) instrument (Rigaku, CN2203E7) and placed in a temperature-controlled cell holder for several minutes. Then, the SAXS data were collected at 25 °C. To monitor the connectivity of the aggregates, the electrical conductivity of the micellar solution was measured with a 12 kHz conductivity instrument (Kyoto Densi). Homogeneous and Microheterogeneous Sol-Gel Processes. The sol-gel process for TEOS in an acidic homogeneous solution was compared with that of the microheterogeneous system (i.e., the reverse micellar system). In the homogeneous system, requisite amounts of 0.1 M HCl aqueous solution were mixed with the solution composed of DDAB, ethanol, and TEOS, and then the sol-gel reaction was started at 40 °C. The feed concentrations of water, DDAB, and TEOS in the alcohol solution were 1.6, 0.2, and 0.8 M, respectively. The microheterogeneous sol-gel process was carried out at 40 °C using DDAB reverse micellar solution in cyclohexane. An aqueous solution of 0.1 M HCl was dissolved in the DDAB-cyclohexane binary solution to form the reverse micelles, and TEOS was added to the reverse micellar system. The TEOS completely dissolved in cyclohexane in the reverse micellar system to yield a transparent solution. The concentrations of water, DDAB, and TEOS were set to the same values as in the homogeneous system. Reverse micellar solutions with various water contents were prepared by mixing appropriate amounts of 0.1 M HCl aqueous solution to elucidate the effect of the Wo value on the hydrolysis reaction of TEOS. The reaction was performed at 40 or 25 °C. The water content in each system was determined by the KarlFisher method.12 29Si NMR spectra for both systems (JEOL, JNMAL400) were also recorded to monitor the hydrolysis and condensation reactions for TEOS. Sol-Gel Processes in Acidic Micellar Solution. The courses of the changes in Wo, the electric conductivity, and the microstructure of silica long term were measured by the same methods used for the reverse micellar system. In this experiment, the initial Wo conditions were adjusted to 8 and 10.4. At 24-72 h after the addition of TEOS, appropriate amounts of an aqueous solution of NaOH were added to the reverse micellar solutions to neutralize 95% of the initial HCl. This addition accelarated (12) Aelion, A.; Loebel, A.; Eirich, F. J. Am. Chem. Soc. 1950, 72, 5705.

Harada et al. the condensation of silica oligomers, and the micellar system became a gel. The SAXS data for these systems were collected at 40 °C similar to in the TEOS-free case. The electrical conductivity in the microheterogeneous system was measured to evaluate the connectivity of the water path in the micellar aggregates.

Experimental Results and Discussion DDAB/Cyclohexane/HCl/Water System without TEOS. The starting point in this report is the elucidation of micellar aggregates in the DDAB/cyclohexane/aqueous 0.1 M HCl solution system in the absence of TEOS. For this purpose, the SAXS spectra of the micelles were measured by changing the water-to-DDAB mole ratio, Wo, in the range from 2 to 11. The DDAB reverse micellar system forms rodlike aggregates in oil-rich regions.13 For thin rods, the SAXS intensity, I, satisfies eq 1 in a large q-region, where q ) (4π/λ) sin(θ/2). θ and λ are the scattering angle and the wavelength (0.154 nm). Rrod is the radius of the rods.

Iq ∝ exp(-Rrod2q2/4)

(1)

In the range Wo < 10, the observed SAXS intensities satisfied this relationship, from which the rod diameter Drod ()2Rrod) was determined. The SAXS intensity also satisfies eq 2 for the system containing rodlike aggregates with length x and diameter Drod:14

I(q,x;Drod) ∝

∫0

π/2

sin2{(qx/2) cos θ}J12{(qDrod/2) sin θ} {(qx/2) cos θ}2{(qDrod/2) sin θ}2

sin θ d θ (2)

where J1 is the first-order Bessel function of the first kind. For aggregates having the length distribution f(x), the SAXS intensity is given by

I(q,L) ∝

∫D∞

rod

I(q,x;Drod)x2f(x) dx

(3)

The length distribution f(x) is assumed15 as

f(x) ∝ exp(-x/L)/x2

(4)

where L is the characteristic length of the aggregates. The observed SAXS intensity was well-reproduced by eq 3. The resultant L and Drod values are shown in Figure 1 under the various conditions of Wo. The closed circles are the Drod values obtained by SAXS measurement. Drod is an almost constant value in the lower range of Wo. This constant lower value may be maintained due to the repulsion between headgroups in the interior of the rodlike water pool.16 Eastoe and Heenan13 determined Drod for the DDAB/ water aggregates in cyclohexane by a neutron scattering (SANS) method. In the range Wo e 8, the cyclohexane with deuterium C6D12 was used to contrast the entire aggregate of DDAB/H2O. The cross symbol in Figure 1 is the rod diameter of the cylindrical water pool, which was determined by subtracting the layer thickness of the (13) Eastoe, J.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1994, 90, 487. (14) Fournet, G. Thesis Sci., Phys., Paris, 1950, A1284, No. 3256. (15) Shioi, A.; Harada, M.; Matsumoto, K. J. Phys. Chem. 1991, 95, 7495. (16) Hyde, S. T.; Ninham, B. W.; Zemb, Z. J. Phys. Chem. 1989, 93, 1464.

Silica Network Formation in Reverse Micelles

Langmuir, Vol. 17, No. 14, 2001 4191

Figure 1. Geometry of the reverse micelles in a DDAB/ cyclohexane/0.1 M HCl aqueous solution system in the absence of TEOS, and electric conductivity σ at 25 °C. The concentration of DDAB is 0.2 M. The shape transformation occurs at around a Wo of 10. Symbols: b, Drod in this work; +, Drod by Eastoe et al.; O, mean length of cylindrical aggregate in this work; 4, electric conductivity in this work; 9, Dsp in this work; ×, Dsp, by Eastoe et al.

hydrocarbon chain tails, lc ) 1.1 nm,13 from the diameter of the entire aggregates. Our results for SAXS almost agree with those of SANS. The Drod values observed in this work are linear with respect to Wo in the range Wo > 4, indicating that the inner core space is filled with water molecules. Thus, the following relationship is obtained:

Drod = (4Vw/Σs)Wo + 2lN

(5)

Vw is the molecular volume of water, and Σs is the area occupied by a surfactant molecule on the neutral plane, the area of which is fixed with the change in aggregate curvature. lN is the length between the inner margin of the polar headgroup and the neutral plane (lN should be close to the polar head size). The Σs and lN values are about 0.75 nm2 and 0.2 nm, respectively. The value of Σs is close to the reported value, 0.68 nm2. The hydrocarbon chain tails are packed in the shell space of thickness lc under the Σs. The shell volume is almost twice the volume occupied by the chain tails as noted by Hyde et al.,16 indicating that the solvent molecules penetrated the shell space. The peak for the characteristic rod length L determined in the present study is located around the value, Wo ) 6. The connectivity of the aggregates can be evaluated from the electrical conductivity, σ, of the reverse micellar system. The observed σ is also shown in Figure 1. The change in σ with Wo is similar to that for L, indicating that long rods are connected to enhance the electrical conductivity. The rod length determined for the pure water (without salt species)/DDAB/cyclohexane system by Eastoe and Heenan13 did not show a bell shape as a function of Wo. This disagrees with our result. Our system contained large amounts of HCl. The difference in the results may be due to the presence of Cl- instead of Br-. In the range Wo > 10, the observed SAXS spectra in the low-q region fitted the intensities I(q) scattered from the spherical aggregates with the unique diameter Dsp:

I(q) ∝ {3(π/2)

1/2

J3/2(qDsp/2)/(qDav/2) }

3/2 2

(6)

Figure 2. Comparison of water consumption by TEOS hydrolysis in a ethanol homogeneous system with that in the reverse micellar system at 40 °C. The initial concentrations of water, DDAB, and TEOS in both systems were the same: [water] ) 1.6 M, [DDAB] ) 0.2 M, and [TEOS] ) 0.8 M. The aqueous solution of 0.1 M HCl was fed to the systems as the source of water and acidic catalyst. The time course of σ in the reverse micellar system is also shown.

where J3/2 is the 3/2-order Bessel function. The diameter Dsp is shown in Figure 1 (closed square symbol). The shape transition from rod to sphere occurs near a Wo of 10, as pointed out by Eastoe and Heenan.13 They obtained the water core diameter, Dsp, for the D2O/DDAB/C6H12 system by a SANS method. The Dsp′ values are also shown in the figure. The area occupied by a surfactant polar head, Σs′ can be obtained from the relationship Dsp′ ) (6Vw/Σs′)Wo, resulting in 0.55-0.60 nm2. Since Σs ) Σs′(1 + 2lN/ Dsp′)2, Σs is evaluated to 0.68-0.74 nm2, which is close to the area of the neutral plane. Effect of Reverse Micelles on Hydrolysis and Condensation of TEOS. With the usual sol-gel method, TEOS is dissolved in alcohol containing appropriate amounts of water and hydrolyzed homogeneously. By contrast, in the present reverse micellar system, TEOS mainly dissolves in a cyclohexane medium and is probably hydrolyzed near the interface of the reverse micelles. In other words, the hydrolysis and condensation reactions are heterogeneous in nature. It has been reported that normal micelles composed of alkyltrimethylammonium salts showed some catalytic actions under basic conditions.17 We compared the reactions with and without the reverse micelles. The homogeneous reaction was performed in ethanol media. The 0.1 M HCl aqueous solution and DDAB were dissolved in ethanol, and then TEOS added to this homogeneous solution. Initial concentrations of DDAB, TEOS, and water were as follows; [DDAB] ) 0.2 M, [TEOS] ) 0.8 M, and [H2O] ) 1.6 M. Note that DDAB does not self-assemble in ethanol. Figure 2 shows the change in the water concentration after the addition of TEOS. The water content expressed in terms of Wo decreased rapidly to 4, near which Si(OR)3OH begins to be hydrolyzed to Si(OR)2(OH)2. In acidic conditions, the order of the hydrolysis rate is reported as follows:18 Si(OR)4 > Si(OR)3(OH) > Si(OR)2(OH)2 > Si(OR)(OH)3. This order was observed in the present study. The heterogeneous reactions using the reverse micelles were performed as follows: TEOS was added to the reverse micellar solution composed of DDAB/cyclohexane/0.1 M HCl aqueous solution. The initial concentrations of DDAB, water, and TEOS in mol/dm3 reverse micellar phase were the same as in the homogeneous reaction system. The (17) Cheng, C.-F.; Luan, Z.; Klinowski, J. Langmuir 1995, 11, 2815. (18) Brinker, C. J.; Scherer, G. W. Sol-Gel Science, Academic Press: New York, 1990.

4192

Langmuir, Vol. 17, No. 14, 2001

Harada et al.

Figure 4. Effect of initial Wo, Wo,in on water-consumption rate. [, 40 °C; O, 25 °C. The aqueous solution of 0.1 M HCl is injected as the source of water and acidic catalyst.

Figure 3. Effect of Wo,in on water consumption by TEOS hydrolysis in the reverse micellar system at 40 °C. Initial concentrations of DDAB and TEOS were 0.2 and 0.8 M, respectively. Symbols: +, Wo,in ) 10.6; ×, Wo,in ) 8; 4, Wo,in ) 5; 0, Wo,in ) 4; ], Wo,in ) 2.5 Inserted figure: Experimental temperature was 25 °C. Initial concentrations of DDAB and TEOS were 0.2 and 0.4 M, respectively. Symbols: 2, Wo,in ) 7.7; 9, Wo,in ) 4; [, Wo,in ) 2. In both experiments, 0.1 M HCl aqueous solution was injected into the DDAB reverse micellar system as the source of water and acidic catalyst.

only difference between the two systems was the solvent species used. As shown in Figure 2, TEOS was hydrolyzed more rapidly in the heterogeneous system than in the homogeneous system. Almost a linear decrease in Wo was observed until it reached 0.2. Thereafter, Wo was kept constant in the long term. It is, therefore, concluded that the reverse micelles play an important role in the hydrolysis reaction. Figure 3 shows the effect of the initial Wo, Wo,in, on the water consumption process at 40 °C in the reverse micellar system. The initial conditions were as follows; [DDAB] ) 0.2 M and [TEOS] ) 0.8 M (the TEOS-to-surfactant mole ratio, RTEOS/surf ) 4). The results obtained at 25 °C are shown in the inset, where the initial concentrations were [DDAB] ) 0.2 M and [TEOS] ) 0.4 M (RTEOS/surf ) 2). As is clear in this figure (see the result under Wo,in ∼ 7.7), the Wo decreased rather rapidly to 3.7, and thereafter remained at this value even though large amounts of water were left in the system. The change in the water concentration ∆[H2O] is equal to 2[TEOS], indicating that only the first two steps of the hydrolysis reactions proceed even if a large amount of water coexists:

Si(OR)4 + H2O f Si(OR)3(OH) + ROH, Si(OR)3(OH) + H2O f Si(OR)2(OH)2 + ROH (7) In Figure 3, the initial Wo decrease is almost linear with respect to time. In other words, -dWo/dt takes almost a constant value k irrespective of the proceeding of hydrolysis. The effect of Wo,in on k is shown in Figure 4. The k value is very small for low values of Wo,in, but markedly increases with increasing Wo,in. With further increase in Wo,in, k jumps to higher values. This jump probably arises from the rod-to-sphere transition of the surfactant aggregates. The k value becomes scattered around the border region of the shape transition; i.e., Wo,in ∼ 9-10.

The molecular volume of TEOS was estimated from the liquid-state density as 0.34 nm3. The corresponding diameter of a TEOS molecule is 0.86 nm, which is a little less than the water-pool diameter. It is, therefore, unlikely that the TEOS is dissolved and hydrolyzed in the confined space of the water pools. Since TEOS is surface-active, it can penetrate into the gap between the polar headgroups and is hydrolyzed near the polar head monolayer of an aggregate. The surface densities of the penetrating TEOS and water molecules near the polar head monolayer are constant irrespective of the concentration change of TEOS in the solvent continuum, resulting in a nearly constant rate of water consumption. We consider the k dependency on Wo,in. In the low-Wo,in region, the water molecules are confined near the headgroups, i.e.; free water does not exist. This may affect the reactivity of water molecules.19 However, it can be seen in Figure 3 that, for Wo,in ) 4-8, Wo decreased almost linearly to at least 0.2, whereas, when Wo,in ) ∼2, the decrease of Wo is very small; i.e., the k value is very small. From this observation, the dependency of k on Wo,in cannot be interpreted in terms of the decrease in the water reactivity. As shown in Figure 1, the diameter of rodlike aggregates increases gradually with increasing Wo. This diameter change is probably responsible for the increase in k. The TEOS molecules penetrate into the polar head monolayer of a micelle. When the curvature of the micelle is large, the polar heads are packed more densely in the monolayer. In contrast, the hydrocarbon tails are packed more loosely. Thus, it is difficult for TEOS to penetrate the monolayer. The small value of k in the low range of Wo,in is attributed to the difficulty of penetration. Due to the shape transition from rod to sphere, TEOS molecules easily penetrated the monolayer, resulting in the large value of k. Figure 5 shows the comparison of the 29Si NMR spectra of the liquid samples after 4 days in the homogeneous system with that after 3 days in the reverse micellar system. The value of i in Qi denotes the number of -OSit bonds attached to a Si atom, i.e., Q1, tSi(OSit); Q2, dSi(OSit)2; Q3, -Si(OSit)3. The signal assignment follows that of Kelts et al.20 The signal due to Q2 in the homogeneous system is strong compared with Q1 and Q3, indicating that the condensation reaction yields linear silica. The result is the same for a homogeneous solution in the absence of DDAB. By contrast, in the micellar (19) Arrigada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1995, 170, 8. (20) Kelts, L. W.; Effinger, N. J.; Melpolder, S. M. J. Non-Cryst. Solids 1986, 83, 353.

Silica Network Formation in Reverse Micelles

Langmuir, Vol. 17, No. 14, 2001 4193

Figure 5. 29Si NMR spectra: (a) at 96 h after the addition of TEOS, in the ethanol homogeneous system; (b) at 72 h after the addition of TEOS, in the reverse micellar system.

Figure 7. SAXS spectra under HCl acidic condition. Experimental conditions are the same as in Figure 6. The time values in the figure were measured from the addition of TEOS. Symbols, experimental; solid curves, calculated.

Typical examples of the SAXS spectra obtained 6.25270 h following the addition of TEOS are shown in Figure 7. The scattering intensities in lower q regions increased with time, whereas in the higher q regions, the intensities obtained at different times merged into a single curve. This characteristic cannot be interpreted in terms of the formation of aggregates with a single shape. The scattering intensity is generally described as Figure 6. Full course of the changes in Wo, σ and geometry of the aggregate structure at 40 °C and Woin ) 8. Initial concentrations of DDAB and TEOS were 0.2 and 0.8 M, respectively. The aqueous solution of 0.1 M HCl is injected as the source of water and acidic catalyst. The time t is measured from the addition of TEOS or from the addition of NaOH aqueous solution.

I(q) ) f(q) S(q), f(q) ) I(0) F(q)

(9)

system, Q3 is largest, and Q4 can be observed also. The Qi values (i ) 3 and 4) are, of course, a measure of the degree of the network of silica bonds formed by the condensation of silicate. We see that the formation of network bonds is promoted more than in the homogeneous system. As mentioned previously, the hydrolysis reaction of TEOS to yield Si(OR)2(OH)2 took place rapidly in the reverse micellar system. It is no wonder that the condensation reaction occurs between the partially hydrolyzed silicates to yield water in the homogeneous system. Nevertheless, the water content is unchanged over the long term in the micellar system. Thus, the reverse micelles direct the condensation reaction which releases alcohol:

where F(q) and S(q) are the form factor and the static structure factor, respectively. The time-independent scattering intensities in the higher q range are probably attributed to the term f(q). The Guinier plot of I(q), i.e., ln I(q) vs q2, yields a straight line in the higher q region. Thus, the primary structure of the silica compounds formed by the hydrolysis and partial condensation reactions is spherical. The diameter of the sphere, Dsp, is 1.6 nm (see Figure 6). As mentioned previously, the electrical conductivity strongly increased as the hydrolysis and condensation reactions proceeded, indicating the formation of long paths for the conduction. It was assumed that the cluster formation comprised primary spheres and that a path of electrical conduction was formed between the spheres. The clusters resulted in a stronger scattering in the low-q regions. The structure factor, S(q), of the modified Ornstein-Zernike (OZ) type is assumed for the sake of simplicity:21

tSiOR + HOSit f tSi-O-Sit + ROH

S(q) ) 1 + K/{1 + (qξ)2}

(8)

Summarizing the above reaction results, there exists a catalytic action of the reverse micelle interface for the TEOS hydrolysis. Also, the reverse micellar aggregates drive the polycondensation reaction and eventually facilitate the silica-network structure as observed in the base-catalytic condition. Silica-Structure Formation Processes under Acidic Conditions. Figure 6 shows the changes in Wo and the electrical conductivity σ over 3 days under the typical conditions that Wo,in ) 8, [DDAB] ) 0.2 M, and [TEOS] ) 0.8 M (40 °C). Figure 2 shows the initial changes under the same condition as in Figure 6. The water content decreased to a constant value in the long term. The electric conductivity was initially 0.9 µS/cm, increasing to 50 µS/ cm with TEOS hydrolysis.

(10)

where K is a constant and ξ is the correlation length. Assuming ideal chains of primary spheres, the correlation length ξ is related to the end-to-end distance of the chains Rcl, where Rcl ) (12)1/2ξ. Rcl is a rough measure of the cluster size. The Rcl and K values were determined by fitting the experimental SAXS patterns and are shown in Figure 8 along with the radius Rsp of the primary sphere. The SAXS patterns are well-reproduced by eqs 6, 9, and 10 using the determined Rcl and K values (see the solid curves in Figure 7). Figure 8 also includes the results for a Wo,in of 10.6 at which the DDAB aggregates in the absence of TEOS (21) Shioi, A.; Harada, M.; Obika, M.; Adachi, M. Langmuir 1998, 14, 5790.

4194

Langmuir, Vol. 17, No. 14, 2001

Figure 8. Time dependency of the parameter values determined by fitting the SAXS spectra with the OZ equation. The time t in the abscissa was measured from the addition of TEOS. b, [, 2, 9, Wo,in ) 10.6; O, ], 4, 0, Wo,in ) 8.

become spherical. As is clear from Figure 8, the Rsp and Rcl respectively take the values 0.8-0.9 nm and 4.2-5.1 nm regardless of Wo,in. The K value increases with the reaction time, indicating an increase in the cluster concentration with time. It must be reconsidered whether the cluster formation contributes to the increase in the electric conductivity. For this purpose, the following experiments were performed: The reaction mixture obtained under the same experimental conditions as in Figure 6 was sampled 1 day following the addition of TEOS. The water content and the electrical conductivity of this sample were Wo ) 0.26 and σ ) 49 µS/cm. This sample was placed in a desiccator containing CaCl2 for 4 days. The Wo and σ values decreased to 0.10 and 0.6 µS/cm upon dehydration. On the other hand, the Wo and σ values of the reaction mixture without the above procedure were 0.31 and 44 µS/cm. Rather large amounts of ethanol were released to the cyclohexane continuum during the hydrolysis and the partial condensation of TEOS. One possible reason for the large increase in σ during these reactions is the electric conduction through the solvent continuum containing ethanol and a small amount of water. However, this is not plausible because σ decreases by 2 orders of magnitude when Wo decreases a little from 0.26 to 0.1. Thus, the drastic decrease in σ upon dehydration indicates that the passes of electric current formed between the primary particles are disconnected. The concentration of ethanol increased to 0.4 M after the hydrolysis of TEOS. Such a high alcohol concentration could destabilize the DDAB aggregates. Nevertheless, stable spherical silica compounds were produced. Thus, some mechanism must play an important role in the formation of stable primary silica particles. When Wo,in ) 10.6, the original DDAB aggregates are spherical in the absence of TEOS, whereas when Wo,in ) 8, they are rodlike. In both cases, primary spheres of silica were formed. The diameter of the primary sphere is 1.61.8 nm irrespective of Wo,in. This result indicates that the surfactant and hydrolyzed TEOS cooperatively form stable spherical particles. In aqueous solutions, the primary amine salt, CnH2n+1NH3Cl, forms micellar aggregates. Addition of TEOS to the solution directs the formation of rodlike micelles comprised of partially hydrolyzed TEOS and the amine. In this system, the silica covers the micelle surface like tiles, and the silica-to-surfactant mole ratio RSi/Sur has a value of 4.5 As mentioned previously, DDAB directs the formation of the silica network. Referring to the two experimental results, the DDAB molecules can be assumed

Harada et al.

Figure 9. SAXS spectra after the addition of NaOH aqueous solution. Experimental conditions are the same as in Figure 6. The time values in the figure were measured from the addition of NaOH aqueous solution. Symbols, experimental; solid curves, calculated.

to overlap the surface of the spherical shell composed of the silica network so that RSi/Sur ) 4. Simple geometric consideration provides that

(Rsp + lN)/Rsp ) {Σs/(ΣsiRSi/Sur)}1/2

(11)

where lN is the thickness of the DDAB polar head (0.2 nm), and Σsi is the area occupied by a SiO2 unit (0.12 nm2).22 Since Σs is 0.73 nm2 as an average, the Rsp evaluated from eq 11 is 0.9 nm, i.e., close to the observed value. To summarize the above experimental results, the primary silica particles become stable due to the coverage of DDAB molecules on their surface. Change in Silica Structure after Addition of NaOH. As mentioned previously, the hydrolysis and partial condensation reactions proceed under acidic conditions. However, at least one -OR group remains per silicate molecule at this stage. Further condensation may occur on changing the pH value. To ascertain this, a 0.4 M NaOH aqueous solution was added to the reaction mixture 3 days after the addition of TEOS. This addition of NaOH neutralized 95% of the hydrochloric acid initially fed and increased Wo to 2.1. The changes in Wo and σ are also shown in Figure 6. Wo was nearly constant after the addition of NaOH solution. σ increased a little and reached a constant value. The whole system is converted to a transparent gel about 1-1.5 h after the NaOH addition. Figure 9 shows the variation of the SAXS spectra after the addition of the NaOH solution. In the small-q regions, the scattering intensity markedly increased, indicating that large size assemblies were formed. The solid curves are the spectra calculated by eq 2 using appropriate lengths L and diameters Drod. These explain well the observed results. As shown in Figure 6, the rod length L increased linearly with time under a constant diameter (Drod ) 1.6 nm). This Drod value is nearly equal to that of the spherical silica aggregates before the addition of NaOH. Thus, the clusters of primary spherical units were converted to rodlike structures. Further condensation reactions with the addition of NaOH interconnected the units to yield the rodlike structure. The SAXS spectrum after gelation is plotted on a loglog scale in Figure 10. Here, slit height correction was made for the spectrum. In the high-q regions, the scattered (22) Hyde, S.; Andersson, S.; Larsson, K.; Blum, K. Z.; Landh, T.; Lidin, S.; Ninham, B. W. In The Language of Shape, Elsevier: Amsterdam, 1997; Chapter 2.

Silica Network Formation in Reverse Micelles

Langmuir, Vol. 17, No. 14, 2001 4195

reverse micelles and the hydrolyzed TEOS. The primary spheres are resistant to the coagulation, and only clusters composed of primary particles are formed. Further condensation reactions occur between different primary particles with the addition of NaOH, connecting the particles. Conclusion

Figure 10. log-log plot of the SAXS-intensity I and q (nm-1) for the gel obtained 330 min after the addition of NaOH aqueous solution. Experimental conditions are the same as in Figure 6.

Figure 11. Illustrative diagrams of the silica-structure evolution in a two-step process (first hydrolysis under acidic conditions and then condensation by the neutralization of HCl by NaOH).

intensity I satisfies the relationship: I ∼ q-Df, Df being 1.85. The fractal dimension, Df, is 2 for ideal polymer chains. The Df value of the silica-containing aggregates is a little less than 2, indicating the formation of a more extended fern-like structure composed of silica and DDAB. Figure 11 summarizes the processes of the silica structure formation. The results obtained in this work indicate that the DDAB microstructure directs a linear silica structure with some branches. Formation of primary silica particles is essential for the silica microstructure formation under acidic conditions. The primary particles form through the interaction between the surfactant molecules and the hydrolyzed TEOS molecules; i.e., the cooperative matching of the conformation of the surfactant

The formation of a silica network structure was elucidated for a reverse micellar system composed of didodecyldimethylammonium bromide (DDAB)/cyclohexane/HCl aqueous solution. The silica source was tetraethoxysilane (TEOS). The hydrolysis reaction of TEOS in reverse micellar solution is completely different from that in the usual homogeneous sol-gel process. In acidic conditions, the hydrolysis of TEOS rapidly proceeded until Si(OR)2(OH)2 was obtained. The hydrolysis reaction rate was nearly constant irrespective of time and increased with increasing initial water-to-DDAB mole ratio, Wo,in. This result was interpreted in terms of the curvature of the reverse micelles. In contrast to the usual sol-gel processes in homogeneous media, the polycondensation reactions formed network silica bonds even under acidic and low water-to-TEOS conditions. Primary silica spheres formed first under acidic conditions. The clusters of primary spheres then formed, which were converted to linear silica aggregates with some branching upon neutralization of the acid with NaOH. Finally, the system became a transparent gel. The process of the network structure formation in the reverse micellar system is completely different from that in a homogeneous sol-gel. The formation of the primary silica particles is due to the peculiar nature of the condensation reaction in the reverse micellar system. Primary spherical particles of similar size were formed under both conditions in which DDAB forms rodlike micelles and spherical micelles in the absence of TEOS. Therefore, DDAB and silica cooperatively form the primary spheres, which are stable against coagulation. The cluster comprised of primary spheres is formed through weak bonds between the particles, because it is broken down to primary particles upon dehydration. The interparticle condensation reaction proceeded when the hydrochloric acid was neutralized with NaOH, yielding rodlike silica aggregates, which in turn formed a network structure; i.e., a gel state. The diameter of the aggregates was close to that of DDAB rodlike micelles in the absence of TEOS; i.e., the DDAB micelles behave as if they are the template of the silica rods. However, the rodlike silica aggregates form through connection of the primary spherical particles. Acknowledgment. The authors gratefully acknowledge the financial support from a Grant in Aid for Scientific Research (Fundamental Research (B)), Ministry of Education, Science, Sports, and Culture, Japan LA0010875