976
J. Phys. Chem. 1991, 95, 976-979
Micellar Formatlon of Sodlum Oodecyl Sulfate in Sol-Gel Glasses Probed byqyrene Fluorescence Kazunori Matsui,* Takumi Nakazawa, and Hitoshi Morisaki College of Engineering, Kanto Gakuin University, Mutsuura, Kanazawa- ku, Yokohama 236, Japan (Received: April 12, 1990; In Final Form: July 20, 1990)
The fluorescence of pyrene probe molecules was measured in the sol-gel reaction system of tetraethyl orthosilicateand various concentrations of sodium dodecyl sulfate (SDS). The ratio of the intensity of the third peak to the first ptak (13/1,)was markedly different in the sol, gel, and xerogel stages. At low SDS concentrations, 1,/11 decreased during desiccation, indicating an increase in the environmental polarity of pyrene. In contrast, at high SDS concentrations, 13/11increased during desiccation, indicating a decrease in environmental polarity. In the latter case, 13/11 had values similar to those for micellar solutions of SDS, which means that the environment of pyrene resembled that of micelle-solubilized pyrene. The results suggest that the micelles of SDS are trapped in the sol-gel glasses.
Introduction In recent years, the sol-gel process for making various inorganic oxide glasses has become scientifically and technologically important.Is2 In this process, suitable monomers react at low temperatures to form porous gel glasses, which are then transformed into dense glasses by sintering. Aside from their use in this process, the porous gel glasses are themselves of particular interest because of the ability to trap photoactive organic molecules in an inorganic matri~.~.~ In order to make clear the complicated sol-gel process and the trapping mechanism of organic molecules in the sol-gel silica, Kaufman and Avnir used excimer and monomer fluorescence of pyrene as a probe.5 They showed that the changes in the polarity of the pyrene environment along the sol-gel process can be probed by using the intensity ratio of the vibronic structures. In a previous paper: we also reported on the sol-gel-xerogel transition during the polymerization reaction of tetraethyl orthosilicate (TEOS, Si(OC2HS)4),as revealed by probing using the fluorescence of pyrene and pyrene-3-carboxaldehyde.These results showed that fluorescence spectra of pyrene provide a good way of determining the environmental changes in the silica cage. The technique of using the fluorescence of pyrene monomer for probing was originally developed in colloid chemistry7.*and is based on the fact that the intensities of the vibronic bands are strongly dependent on the solvent en~ironment.~More specifically, the ratio I 3 / I 1 of the intensity of the third vibronic band to that of the 0-0 band decreases as the solvent polarity increases. Even though pyrene itself is strongly hydrophobic and has a very low solubility in water (a few micromolar), in the presence of micelles it becomes solubilized in the palisade layer of the micelles, where it may be in proximity with the water molecules about the head groups and probably also the first one or two methylene groups,I0 that is, the inner surface of the hydrophilic shell.”
The effect of surface-active agents on the sol-gel process was first studied by Kaufman et a1.I2 They found an oscillatory behavior of the pyrene excimer intensity when the surface-active agents were added (3 X 10-5-l.5 X M) to the sol-gel solution. When surfactants are added above the critical micelle concentration to a sol-gel solution, it is expected that micelles can be formed in the sol-gel system. The present study involved an attempt to trap micelles of sodium dodecyl sulfate in sol-gel glasses, and observations by pyrene fluorescence probing indicate that this entrapment is possible. This material system is very useful for studying the structure of micelles. Moreover, photoactive organic molecules/micelles in an inorganic matrix, which, besides the bulk form, can also be made into film, fiber, and powder,’q2 are very promising from the standpoint of the design of artificial photosynthetic system^.^,'^
( I ) Scherer, G. W. Yogyo Kyokaishi 1987, 95, 21. (2) Sakka, S.; Kamiya, K. J . Non-Crysr. Solids 1980, 42, 403. Brinker, C. J.; Scherer, G. W. J . Non-Crysr. Solids 1985, 70,301. Science of Ceramic Chemical Processing, Hench, L. L., Ulrich, D. R., Eds.; Wiley: New York, 1986. Hench, L. L.; West, J. K. Chem. Reo. 1990, 90, 33. (3) Avnir. D.; Levy, D.; Reisfeld, R. J . Phys. Chem. 1984,88, 5956. Tani, T.; Namikawa, H.; Arai, K.; Makishima, A. J . Appl. Phys. 1985, 58, 3559. Pouxviel, J. C.;Dunn, B.; Zink, J. 1. J . Phys. Chem. 1989, 93, 2134. Matsui, K.; Matsuzuka, T.; Fujita, H. J. Phys. Chem. 1989, 93, 4991. (4) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. J. Phys. Chem. 1989, 93. 7544 and references therein. (5) Kaufman, V. R.; Avnir, D. hngmuir 1986, 2, 717. (6) Matsui, K.; Nakazawa, T. Bull. Chem. SOC.Jpn. 1990, 63, 11. (7) Nakajima, A . Bull. Chem. Soc. Jpn. 1977, 50, 2473. (8) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99,
Results
2039. (9) Nakajima, A. Bull. Chem. SOC.Jpn. 1971, 44, 3272. (10) Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 86, 1019. Lianos, P.; Lang, J.; Zana, R. J. Phys. Chem. 1982, 86, 4809.
Experimental Section Chemicals. Pyrene (Aldrich) was recrystallized several times from ethanol. Tetraethyl orthosilicate (TEOS) from Tokyo Kasei, sodium dodecyl sulfate (SDS) of a biochemical grade (Wako), and ethanol of a spectroscopic grade were used without further purification. The water used was deionized and distilled. Sol-Gel Process. The method of preparing sol-gel glasses by the acidic hydrolysis of TEOS in ethanol was the same as that described in the previous report6 with only a slight modification. Ethanol containing IO” M pyrene and a solution of SDS in water with a concentration between 0 and lo-’ M were mixed together, and then TEOS was added. The solutions were adjusted to a pH of 3.1 by the addition of HCI and stirred for 1 h. Unless otherwise stated, the molar ratio of TE0S:water:ethanol was 1:6.2:3.8. Measurement. The fluorescence spectra were taken with a JASCO FP-770 spectrofluorometer at room temperature and an excitation wavelength of 340 nm. Static light scattering was measured on an Otsuka Electronics dynamic light scattering spectrophotometer (Model DLS-700) at 90° scattering angle.
13/1,was measured over time during the sol-gel-xerogel stages for various concentrations of SDS, and the results are shown in Figures 1-3. Without SDS, 13/1,initially was almost constant at 0.76, but between 300 and 400 h it dropped sharply to a value of 0.56 and remained there (Figure 1). As shown previously$ the 13/11values decrease with an increase in the water composition in the ethanol-water system. The solvents in the sol-gel system evaporated at about 0.25%/h (normalized by the initial weight) during the (11) Turro, N. J.; Kuo, P. L. J. Phys. Chem. 1986, 90, 4205. (12) Kaufman, V. R.; Levy, D.; Avnir, D. J. Non-Cryst. Solids 1986,82, 103. (13) Thomas, J. K . Chem. Reo. 1980,80, 283.
0022-3654/9 1/2095-0976%02.50/0 0 199 1 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 977
Micellar Formation of SDS in Sol-Gel Glasses
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Figure 1. 1,/1, over time during the sol-gel process for various sodium dodecyl sulfate concentrations: 0 , O M; A, 2 X IO-’ M; 0, 3 X IO” M; 0,4 X IO-’ M. Molar ratio of TE0S:water:ethanol = 1:6.2:3.8.
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Figure 3. I 3 / l 1over time during the sol-gel process for various sodium dodecyl sulfate concentrations: 3 , 3 X M; V, 4 X M; 0 , 5 X M; 0 , 1 X IO-1 M. Molzr ratio of TE0S:water:ethanol = 1:6.2:3.8.
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Figure 2. [’/I1 over time during the sol-gel process for various sodium dodecyl sulfate concentrations: 0,6 X IO-’ M; V, 8 X IO-’ M; 0,1 X M; 0, 2 X IF2M. Molar ratio of TE0S:water:ethanol = 1:6.2:3.8.
first 350 h. The water enrichment occurred during the evaporation because ethanol evaporates more easily than water and because water is attached to the matrix silanols by more and stronger hydrogen bonds than ethan01.I~ Therefore, the drop in 13/11 reflects an increase in environmental polarity. For low concentrations of SDS, 13/1,also dropped sharply, but then it recovered to some extent, with the degree of recovery depending on the concentration (Figure 1). The behavior of 13/11 began to change somewhat at a concentration of about 6 X M (Figure 2). From this point on, the drop became less and less pronounced. A radical change occurred at a concentration of 2 X l o 2 M (Figure 2). Here, a strong peak appeared, after which I J I I slowly decreased. This behavior continued up to the highest concentration tested (Figure 3). The gelation time ranges from 180 to 240 h and does not seem to depend on the SDS concentration. To clarify the reasons for this behavior, the influence of the ethanol level on micelles was investigated. Figure 4 shows that 13/11first increased from an initial value of 0.86 to a peak of 0.92 and then leveled off at a value of 0.75. The reduced scattering intensity at 90° scattering angleI5 is also plotted in Figure 4. The scattering intensity from solvents was extracted. It increased when (14) Levy, D.; Avnir, D. J . Phys. Chem. 1988, 92, 4734. (15) Imae, T.; Ikeda, S. J . Phys. Chem. 1986, 90, 5216.
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ETOHl H20 (VOL. RATIO) Figure 4. Effect of ethanol level on l 3 / l and 1 reduced scattering intensity M). at the 90’ scattering angle (A&,) in a micellar solution (5 X
a small amount of ethanol was added and then decreased with an increase in ethanol. When a volume ratio of ethanokwater was equal, the scattering intensity became zero. However, the scattering intensity did not change when an equal volume of water was added to the micelles. The result indicates that values of the micelle molecular weight, Le., the aggregation numbers of SDS, increase initially and then decrease with an increase in ethanol. The effect of alcohols on micelles is intriguing; however, it is very complex,10~i6 so detailed discussion on it is beyond the scope of this study. The behavior of the reduced scattering intensity roughly corresponds to that of 13/Il. Therefore, the increase of 13/1,upon addition of a small amount of ethanol to the micellar solutions is attributed to the increased aggregation numbers, causing less polar environment. The displacement of water molecules in the micelle palisade layer by the ethanol that penetrates the layer and/or the movement of pyrene molecules deeper into the interior At of the micelles are also responsible for the increase in 13/11.10 higher ethanol levels, the environmental polarity increiises because (16)Malliaris, A.; Lang, J.; Sturm, J.; Zana, R. J . Phys. Chem. 1987, 91, 1475. Luo, H.; Boens, N.; Van der Auweraer, M.; De Schryver, F. C.; Malliaris. A. J . Phys. Chem. 1989, 93, 3244. Muto. Y.; Ycda, K.; Yoshida, N.; Fsumi, K.;Meguro, K.; Binana-Limbele, W.; Zana, R. J . Colloid Inrerjuce Sci. 1989, 130, 165.
978 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
Matsui et al.
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400 600 800 TI MUH) Figure 5. 13/11over time during the sol-gel process for different molar ratios of ethano1:TEOS at water:TEOS ( r ) = 6.2: 0 , 1.9; 0, 3.8; V, 7.6.
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lo-* 10'' SDS CONCENTRATlON(M) Figure 7. 1,/11 at some characteristic times in the sol-gel reaction vs sodium dodecyl sulfate concentration: 0, starting solution ( r = 0); A* maxima and minima at r = ca. 400 h; 0 , xerogels ( t = ca. 900 h).
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TI ME(H) Figure 6. 1JI1 over time during the sol-gel process for different molar ratios of water:TEOS (r) at ethano1:TEOS = 3.8: A, 3.1; 0 , 6 . 2 ; 0, 12.4.
the aggregation number decreases and the micelle-solubilized pyrene becomes progressively more soluble in the ethanol-water solvent. Figure 5 shows how 13/11changes over time for three different ethano1:TEOS molar ratios. The SDS concentration was IO-1 M in all cases. The shapes of the curves are roughly the same, the main difference being that the transition occurs later for higher ethanol levels. This confirms that the change is induced by the evaporation of ethanol. Figure 6 shows how the water:TEOS molar ratio, r, affects 1 3 / I l . Again, the SDS concentration was IO-1 M. For the first 350 h, 13/11for both r = 6.2and r = 12.4exhibited the same behavior, that is, an initial period of no change followed be a rapid rise. However, for r = 12.4,13/11remained in the neighborhood of the maximum value of 0.94 for around 200 h before dropping to a lower value. In both cases, the curves leveled off at around 0.82. In contrast, the behavior of 13/11for r = 3.1 was somewhat different. It dropped to a minimum of 0.69just before reaching a peak at 0.84. And the value of 13/11in the xerogel region (ca. 900 h) was only 0.72,which is much smaller than for the other two ratios. Figure 7 summarizes the results given in Figures 1-3 in a graph of 13/11at some characteristic time during the sol-gel reaction vs the SDS concentration. In the starting solutions (0),the value of 13/11 is always approximately 0.76 and is independent of the SDS concentration. The A symbols indicate the value of 1,/11 at the maxima and minima that appeared at around 400 h as a
result of desiccation. From a value of 0.56 in the plateau region below a concentration of 5 X lW3 M, 13/1,increases with the SDS concentration until it reaches a value of 0.95 in the saturation region above a concentration of 2 X M. The critical point where the curve has the same value as for the starting solutions is at a concentration of around 1 X lo-*M. The third curve in the figure (e) is for the xerogel region above about 900 h. In this case, 13/11increases with the SDS concentration but begins to saturate above a concentration of around 1 X M.
Discussion These results can be explained in terms of three factors: the gradual change in the solvent composition in the silica ~ a g e , ~ " J ~ the gradual shrinkage of the cage,s and micellar formation. First, the case for higher concentrations of SDS, like those used for Figure 3, will be discussed. In the starting solution, pyrene is probably dissolved in the bulk phase, which consists of ethanol, water, and TEOS, because 13/11is not affected at all by the SDS concentration (at least between 0 and 1 X 10-1M) and because micellar formation is improbable in these compositions of the solutions as expected from the results of Figure 4. Hydrolysis and polymerization reactions form the silica-gel network, which encapsulates ethanol, water, pyrene, and the monomer surfactants. During the evaporation of the ethanol, the pyrene molecules become progressively solubilized into the micelles, which are formed during the evaporation process. As the evaporation proceeds and the ethanol level becomes very low, the aggregation number of SDS increases and the ethanol penetrates the micelle palisade layer and/or the pyrene moved deeper into the layer, causing a reduction in polarity and an increase in 13/11,as explained above for Figure 4. The final value of 13/11for the xerogel is similar to that for a micellar solution. This suggests that the micelles that solubilize pyrene are trapped in the silica cage. In the previous study: it was found that the polarity of the xerogels, which contain a small number of water molecules, is only slightly smaller than that of water, so it is not improbable that the micelles can be stabilized in the silica cage. The average radius of the SDS micelles is around 18.4 A,'' while the diameter of the pores of the silica gels prepared by acidic (17) Cabane, 8. In Surfucfanr Solurlons; Zana, R., Ed.;Marcel Dekker: New York, 1987; pp 57-145.
Micellar Formation of SDS in Sol-Gel Glasses hydrolysis has an enough distribution between 20 and 40 A,' and the average diameter is around 36 A,'* The pores of wet gels should naturally be larger than that, so it is certainly possible for SDS micelles to be incorporated into the wet silica cage and finally embedded in the silica cage of the xerogels. Considering the curve in Figure 7 and the previous discussion of it, the critical SDS concentration is about 1 X M. This concentration roughly corresponds to the critical micelle concentration (8 X IOd3 M),8 thus supporting the previous model. When the starting solution has a low SDS concentration, like those used for Figure 1, a similar explanation applies. The silica-gel network encapsulates ethanol, water, pyrene, and SDS monomers. As the ethanol evaporates, the environmental polarity of the pyrene molecules increases because they are dissolved in the ethanol-water solvent, and IJZI decreases. The silica cage, however, contains the SDS. As the cage shrinks, the pyrene molecules are forced into close proximity with the surfactants and sense their polarity. This leads to a decrease in environmental polarity, and 13/11increases. In the case of the concentrated pyrene, the shrinkage of the silica pores induces local dispersion of the pyrene molecules due to the repulsive force^.^ As for the oscillatory behavior during the sol-gel reaction observed by Kaufman et a1.,I2 we did not find the phenomenon. This is probably due to the difference between the properties of the probes. 13/11probes the polarity of the environment; however, excimer fluorescence probes the structure of the sol and the gel, the surface irregularity, and so The value of 13/ZIfor xerogels doped with the micelles (ca. 0.80) is slightly smaller than that for an aqueous micellar solution (ca. 0.86). This indicates that the environment of the micelle-solubilized pyrene is a bit more polar in the xerogels. This is probably because the shrinkage of the silica cage, in which the pores are the same size as or slightly smaller than the micelles, causes water molecules and/or silanol groups to penetrate the micelle palisade layer. The relationship between 13/11and the water:TEOS molar ratio, r, is rather complicated, as shown in Figure 6. The distinctive behavior of 13/11for r = 3.1 suggests that micellization does not occur during the xl-gel process at this molar ratio. The reason is that water is consumed by the sol-gel reaction Si(OC2HS), + 2 H 2 0 S O 2 + 4C2H50H
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so less water is available to stabilize the micelles in the sol-gel system. It is clear, then, that a certain amount of water is required to form micelles in the sol-gel matrix. However, when the amount of water is relatively large (an r above at least 12.4), another (18) Chen, K. C.; Tsuchiya, T.; Mackenzie, J. D. J . Non-Crysr. Solids 1986, 81, 221.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 979 distinctive feature appears; namely, 13/Z,remains at the maximum value for a long time. This contrasts sharply with the behavior of 13/ZIwhen the amount of ethanol is increased; that is, the basic pattern remains the same but the reaction is delayed. There are two factors responsible for this difference. One is the different roles of water and ethanol both in the sol-gel reaction and in micellization. The other is that ethanol and water leave the sol-gel system at different rates as discussed previously. The above discussion assumes that the micelles have a spherical structure like regular micelles. It has been reported that the adsorbed phase formed by a nonionic surfactant at the silicaaqueous solution interface can be described as an assembly of aggregates very similar to regular micelle^.'^*^^ A neutron scattering study showed that molecular assemblies formed by SDS molecules adsorbed on poly(oxyethy1ene) polymers are indistinguishable from regular micelles2' Therefore, a spherical structure in the sol-gel glass is not unlikely. Other models have also been proposed for the molecular assemblies on the solid surface, viz., hemimicellesZ2and lamellar phases.23 However, there is some doubt as to whether such structures can explain the similarity between the behavior of Z3/11in the sol-gel glasses and that in micellar solutions with ethanol. As for the adsorption stage of the monomer surfactants or the micelles, apart from the micellar structure, it is improbable that adsorption on silica particles occurs before gelation because the gelation time is hardly affected at all by the SDS concentration. The adsorption of the surfactants occurs during and/or after the evaporation of the ethanol solvent, as discussed above. Further studies are required to shed light on the structure of surfactant aggregates in sol-gel glasses and the mechanism by whidh they are formed. In summary, the change in the environmental polarity of pyrene probe molecules in the sol, gei, and xerogel stages was investigated and was found to be strongly affected by the concentration of sodium dodecyl sulfate present. The results can be explained in terms of the formation of the micelles in the sol-gel glasses. These findings may prove useful in the study of micellar structures and also in applications to artificial photosynthetic systems.
Acknowledgment. We express our gratitude to Prof. Keishiro Shirahama of Saga University for his valuable suggestions. We also express our thanks to Messrs. Kenji Maruyama, Katsuji Tanaka, and Kazushi Sasa of Otsuka Electronics Co., Ltd., for their experimental help in light scattering. (19) Levitz, P.; Van Dammd, H.; Keravis, D. J . Phys. Chem. 1984, 88, 2228. (20) Levitz, P.; Van Damme, H. J . Phys. Chem. 1986, 90, 1302. (21) Cabane, B.; Duplessix, R. J . Phys. (Les U h , Fr.) 1982, 43, 1529. (22) Gaudin, A. M.; Fuerstenau, D. W. Trans. AIME 1955, 958. (23) Cases, J. M. Bull. Mineral. 1979, 102, 684.
