Langmuir 2004, 20, 5965-5968
5965
Formation Processes of the Integrated Ordered Mesostructure of Silica at Liquid-Liquid Interface Using Synchrotron Radiation X-rays Motonari Adachi,* Yusuke Murata, Kensuke Sago, and Keizo Nakagawa Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611 0011, Japan Received March 8, 2004. In Final Form: May 7, 2004 The formation processes of the integrated ordered mesostructure of silica at the liquid-liquid interface in laurylamine/tetraethoxysilane system were elucidated by measuring the small-angle X-ray scattering pattern every second during the formation processes using strong synchrotron radiation X-rays. Cylinders of silica were formed in random configurations in the very early stage of reactions at the liquid-liquid interface. Since these cylinders were restricted in a two-dimensional liquid-liquid interface, they aligned easily with each other and made an ordered structure composed of aligned cylinders at the interface. These ordered structures then accumulated with each other, making an integrated ordered structure, such as a hexagonal array.
Introduction We found that the liquid-liquid interface between the organic and the aqueous phases played an important role in the formation of an integrated ordered mesostructure, such as a hexagonal array or a bicontinuous cubic phase, in the laurylamine (LA)/tetraethoxysilane (TEOS) system, but only in a narrow pH region from 10 to 11.5, as Tanev and Pinnavaia reported.1,2 However, the formation mechanism of the integrated ordered structure at the liquidliquid interface was not elucidated, because the variation in shape and size of the molecular assemblies of silica mesostructure during the formation processes at the interface was not measured since the formation of the integrated ordered mesostructure was completed within a few minutes. The same difficulties were frequently encountered in other formation processes of ceramic integrated ordered structures. Strong synchrotron radiation X-ray enables us to measure the variation in the smallangle X-ray scattering (SAXS) pattern every second during the formation processes. Thus, this method is very useful and contributes greatly to the elucidation of the formation mechanism of many other integrated ordered mesostructures, including reactions at liquid-liquid interface shown first in this paper. A variety of mesoporous materials have been synthesized using molecular assemblies3 since researchers at Mobil succeeded in formation of mesoporous silicate and alminosilicate materials (M41S) by a templating mechanism using surfactant molecules.4 Also, many formation * Corresponding author: Telephone: 81-774-38-3518. Fax: 81774-38-3405. E-mail:
[email protected]. (1) Adachi, M. Colloid Polym. Sci. 2003, 281, 370. (2) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (3) (a) Ramann, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682. (b) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (c) Huo, Q.; Feng, J.; Shuth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14. (d) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature (London) 1995, 378, 366. (e) Lu, Y.; Gabguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Hoang, M. H.; Zink, J. I. Nature (London) 1997, 389, 364. (f) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941. (g) Mann, S.; Ozin, G. A. Nature (London) 1996, 382, 313. (h) Sun, T.; Ying, J. Y. Nature (London) 1997, 389, 704. (i) Tanev, P. T.; Liang, Y.; Pinnavaia, T. J. J. Am. Chem. Soc. 1997, 119, 8616. (j) Fyfe, C. A.; Fu, G. J. Am. Chem. Soc. 1995, 117, 9709. (k) McGrath, K. M.; Dabbs, D. M.; Yao, N.; Aksay, I. A.; Gruner, S. M. Science 1997, 277, 552. (l) Firouzi, A.; Kumar, D.; Bull, L.-M.; Besier, T.; Siegel, 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.
mechanisms have been proposed for the synthesis of mesoporous structures,4b,5 including in situ measurement using synchrotron radiation.5g,h It appears that the mechanism is highly dependent on the experimental conditions of synthesis. Thus, development of a method for increasing the mechanistic understanding of the synthesis processes is very important; it is the key to the rational design of new materials. In this paper, we first present the variation in SAXS patterns of the reaction products at the liquid-liquid interface on a time scale of seconds in a LA/TEOS system and discuss the formation processes of the integrated ordered structure at the interface, especially focusing on the very early reaction stages. Experimental Section Materials. Laurylamine (LA) with more than 95% purity was purchased from Tokyo Chemical Co. Tetraethoxysilane (TEOS) with more than 99.9% purity was purchased from Kanto Chemical Co. Distilled water was used. Experimental Procedure. We used a cell for liquid samples to measure SAXS patterns. The cell size was height 60 mm, depth 3 mm, and width 5 mm. The lower half of the cell was filled with water, and the location of the X-ray beam was adjusted to the surface. The beam strength of X-ray was 1013 photon/sec and both the width and the height of the beam were less than 200 µm. A mixed solution of LA and TEOS was introduced upon the surface of water using a peristaltic pump, and the formation (4) (a) Kresge, C. T.; Leonowicz, M. E,; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710. (b) 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. (5) (a) Vartuli, J. C.; Schmit, C. T.; Kresge, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppard, E. W. Chem. Mater. 1994, 6, 2317. (b) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamuty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (c) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, G.; Feng, P.; Gier, T. E.; Siegel, P.; Firouzi, A.; Chmelka, B. F.; Schuth, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (d) Chen, C.-Y.; Burkett, S. L.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 27. (e) Steel, A.; Carr, S. W.; Anderson, M. W. J. Chem. Soc., Chem. Commun. 1994, 1571. (f) Zhang, J.; Luz, Z.; Zimmermann, H.; Goldfarb, D. J. Phys. Chem. B 2000, 104, 279. (g) Agren, P.; Linden, M.; Rosenholm, J. B.; Schwarzenbacher, R.; Kriechbaum, M.; Amenitsch, H.; Laggner, P.; Blanchard, J.; Schuth, F. J. Phys. Chem. B 1999, 103, 5943. (h) Landy, C. L.; Tolbert, S. H.; Gallis, K. W.; Monnier, A.; Stucky, G. D.; Norby, P.; Hanson, J. C. Chem. Mater. 2001, 13, 1600. (i) Frasch, J.; Lebeau, B.; Soulard, M.; Patarin, L.; Zana, R. Langmuir 2000, 16, 9049.
10.1021/la049407y CCC: $27.50 © 2004 American Chemical Society Published on Web 06/12/2004
5966
Langmuir, Vol. 20, No. 14, 2004
Figure 1. Variation in SAXS pattern with time. The scattering vector q is defined as q ) (4π/W) sinA. Here, W and 2A represent the wavelength of X-ray and scattering angle, respectively. (a) [TEOS]/[LA] ) 1. Peaks were observed after 129 s from contact, and there was no integrated ordered structure at less than 9 s. (b) [TEOS]/[LA] ) 0.5. (c) [TEOS]/[LA] ) 4. reactions started at the liquid-liquid interface. The temperature of the cell and the mixed solution were kept at 313 K. After contact of the two liquids, SAXS patterns were measured at a constant interval of a few seconds by CCD detector.
Results and Discussion An illustration of the results is shown in Figure 1a, where q ) (4(3.14)/W) sinA. W and 2A represent the wavelength of X-ray and scattering angle, respectively. The mole ratio of TEOS to LA, [TEOS]/[LA], was 1. There was no peak in the meso-size region at less than 9 s after contact. A shoulder appeared at 69 s, clear peaks were observed after 129 s from contact, and the peak height
Adachi et al.
became higher with time. These peaks in the meso-size region indicate that an integrated ordered structure with periodical distances in meso-size was formed and grew with time. On the other hand, there was no integrated ordered structure at less than 9 s because of the absence of a peak in the meso-size region. Variation in SAXS patterns with time was also measured at [TEOS]/[LA] ) 0.5 and [TEOS]/[LA] ) 4 and are shown in Figure 1, parts b and c, respectively. All results showed almost the same variation in SAXS pattern with time regardless of the ratio of [TEOS]/[LA]. Thus, the same formation processes of the integrated ordered structure proceeded regardless of the ratio of [TEOS]/[LA] under the present experimental conditions. An interesting finding was observed when the mole ratio of TEOS to LA was changed, ranging from 0.5 to 4. The formation rate of the integrated ordered structure became faster with decreasing mole ratio of TEOS to LA, as shown in Figure 1. The growth rate of a peak in the meso-size region became slow with increasing [TEOS]/[LA]. This result may sound paradoxical, because the concentration of reactant TEOS increased with increasing the mole ratio of [TEOS]/[LA]. This paradoxical phenomena are attributed to the strong interaction between hydrolyzed TEOS and LA, which destroys the arrangement of surfactant molecules more strongly with increasing mole ratio, resulting in it taking much time to make stable molecular assemblies composed of hydrolyzed TEOS and LA. A detailed analysis will be presented elsewhere, based on the results with much wider variation in the mole ratio of [TEOS]/[LA]. Hereafter, we present our analysis mainly in Figure 1a as a typical example. The value of the scattering vector at the peak of the integrated ordered structure in Figure 1a corresponds to 4.2 nm for the periodical distance of the ordered structure. The ordered structure was inferred to be a hexagonal array, because we obtained a hexagonal array structure in the same condition [TEOS]/[LA] ) 1 as shown in Figure 2a. Clear observation of higher order peaks (110) and (200) were also obtained for [TEOS]/[LA] ) 4 as reported in the previous paper.1 (see Figure 2b) We observed sometimes the clear higher order peaks as mentioned above. Moreover, Pinnavaia et al. reported formation of hexagonal mesoporous silica (HMS).2 Thus, we believe that the ordered structures were hexagonal phase under these conditions. However, usually it is not easy to get the higher order Bragg reflections of the hexagonal structure clearly because of a weak templating force of neutral primary amine micelles. Pinnavaia et al.2 described the same understanding for the neutral surfactant system too. Assuming a hexagonal array structure, the diameter of a cylinder constituting the hexagonal array is calculated as 4.8 nm. Next, we analyze the structure of silica formed before the formation of an integrated ordered structure. The logarithmic values of the observed scattering intensity multiplied by q, log(I q) at 9 s, were plotted against the square of scattering vector q2 in Guinier type for cylinders.6 (Figure 3) The linear relationships between log(Iq) and q2 indicated formation of the cylinder-shape assembly. The diameter of the cylinders was obtained as 4.9 nm from the slope of the straight line.
ln(I q) ) -const(rcylinder2 q2/4)
(1)
(6) (a) Eastoe, J.; Heenan, R. K. J. Chem. Soc., Faraday Trans. . 1994, 90, 487. (b) Guinier, A.; Fournet, G. Small-angle Scattering of X-rays; Wiley: New York, 1955.
Integrated Ordered Mesostructure of Silica
Langmuir, Vol. 20, No. 14, 2004 5967
Figure 2. SAXS intensity vs scattering angle 2A. (a) [TEOS]/[LA] ) 1. (b) [TEOS]/[LA] ) 4.
Figure 3. Guinier type plot for cylinder at 9 s from contact. The linear relationship between log(I q) and q2 indicated formation of the cylinder-shape aggregates.
Figure 4. Assumed coaxial double cylinders with an outer hydrophilic shell and inner hydrophobic core. The rod length was l, the length distribution being (1/L) exp(-l/L).
For the SAXS analysis of the data at 9 s, the logarithmic value of the obtained intensity log I was replotted against the square of scattering vector q2 in Guinier type for particles. A linear relationship between log I and q2 was
Figure 5. Comparison of the intensity observed at 9 s with the calculated one, assuming outer diameter D ) 5.0 nm, inner diameter Di ) 3.0 nm, length L ) 200 nm, and (P2 - P1)/(P2 PS) ) 1.17.
obtained in the range q2 > 0.04, indicating particle formation.6b The particle diameter was 0.3 nm. The scattering intensity due to particles was subtracted from the observed intensity, and the obtained difference intensity was analyzed. We assumed coaxial double cylinders with an outer hydrophilic shell (outer diameter D; electron density P2), and an inner hydrophobic core (diameter Di; electron density P1). The rod length is l, with the length distribution being (1/L) exp(-l/L) (see Figure 4). PS is the electron density of the aqueous solution. Since the reaction products were sufficiently dilute in the very early stage of the reactions, the interparticle structure factor was assumed to be unity. The coaxial cylinder is composed of the hydrophobic core made of hydrocarbon chains of surfactants and the hydrophilic shell composed of silicon oxide and hydrophilic heads of surfactants surrounding the hydrophobic core. The outer part is water.
5968
Langmuir, Vol. 20, No. 14, 2004
Adachi et al.
Figure 6. Formation processes of the integrated ordered mesostructure of silica at the liquid-liquid interface.
The electron density of the hydrophobic core P1, and water PS are given as 277 and 332 e/nm3, respectively.1 The electron density of the hydrophilic shell P2 is assumed to be 641 e/nm3, taking into account that dense gel-silica is formed when pH > 7 and the density of dense gel-silica is reported to be 2.2 g/cm3.7 Thus, the parameter e.d. ) (P2 - P1)/(P2 - PS) becomes 1.17. The scattering intensity of the rod due to the form factor is given as1,6b
Irod(q) )
∫0π/2 Irod(q,β) sin β dβ
(2)
Irod(q,β) ) [(P1 - PS)v1F(q, Irod, Di) + (P2 - PS){v2F(q, Irod, D) - v1F(q, Irod, Di)}]2 (3) F(q, Irod, d) )
[
][
]
sin(q/cos β) 2J1{(qd sin β)/2} (q/cos β) (qd sin β)/2
(4)
where v1 is the volume of the rod of diameter Di, v2 is the volume of the rod of diameter D, and J1 is the Bessel function of the first order. Equation 2 can be rewritten as
Irod(q,β)/Irod(0) )
[
{ [ {
F(q, Irod, D) +
1+
(P1 - P2)
(P2 - PS)
} }
(P1 - P2)
(P2 - PS)
(Di/D)
]
2
(Di/D)2 F(q, Irod, Di)
]
2
(5)
2
The fitted shape is shown in Figure 5. The outer diameter D of the cylinder was 5.0 nm, the inner diameter D i was 3.0 nm, and the length L was 20 nm. The calculated line agreed well with the observed curve. The obtained diameters of cylinders agreed quite well with each other. The obtained Di value was close to the double length of (7) Brinker, C. J.; Scherer, G. W. Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (8) Zulauf, M.; Eicke, H.-F. J. Phys. Chem. 1979, 83, 4.
the hydrocarbon chain of LA. This is very reasonable. Thus, we can conclude that cylinders of silica with diameter of 5.0 nm were formed in random configuration in the very early stage of reactions at the liquid-liquid interface. Since these cylinders were restricted in a two-dimensional liquid-liquid interface, they aligned easily with each other and made an ordered structure composed of aligned cylinders at the interface. These ordered structures then accumulated with each other, making an integrated ordered structure such as a hexagonal array. Conclusions The formation processes of the integrated ordered mesostructure of silica at the liquid-liquid interface were observed first by measuring SAXS pattern every second during the formation processes using strong synchrotron radiation X-rays. SAXS pattern in the very early stage of reactions before formation of the integrated ordered structure was analyzed using Guinier type plot for cylinders and assuming a coaxial cylinder shape. From these results, we can conclude that the formation processes of the integrated ordered structure proceeded as shown by Figure 6. Cylinders of silica were formed in random configurations in the very early stage of reactions at the liquid-liquid interface. Since these cylinders restricted in a two-dimensional liquid-liquid interface, they aligned easily with each other and made an ordered structure composed of aligned cylinders at the interface. These ordered structures then accumulated with each other, making an integrated ordered structure such as a hexagonal array. Acknowledgment. This work was supported by a Grant in Aid for Scientific Research from the Ministry of Education, Science, Sport and Culture, Japan. The synchrotron radiation experiments were performed at the BL45XU in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) 2001B0368-NDL-np, 2002A0289-NDL2-np. LA049407Y
