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Langmuir 1992,8, 372-376
Articles Amphiphilic Association Structures and the Microemulsion/ Gel Method for Ceramics: Influence on Original Phase Regions by Hydrolysis and Condensation of Silicon Tetraethoxide Stig E. Friberg,*?+Ching-Chang Yang,? and Johan Sjoblomt Center for Advanced Material Processing, Clarkson University, Potsdam, New York 13699-5814 Received January 18, 1991. In Final Form: October 11, 1991 Silicon tetraethoxide was reacted with water in inverse micellar solution, lyotropic liquid crystals, and aqueous micellar solutions in the system water, sodium dodecyl sulfate, and pentanol to compare the silica gel formation in microemulsions or lyotropic liquid crystals with that in alcohol solutions. The reaction resulted in transparent gels in a limited region of the total phase diagram. The extension of the area that formed transparent gels after the reaction depended on the original silicon tetraethoxide (TEOS) concentration in such a manner that higher TEOS concentration gave a smaller region. The lyotropic liquid crystals were changed to isotropic gels in which the long-range order was lost. The reason for this structural change was the disordering influence of the ethanol formed during the hydrolysis reaction. The system changed from a pentanol solution to a water-in-oil (W/O) microemulsion with addition of surfactant, and a comparison could be made between conditions for gel formation in an alcoholic solution (pentanol+water) and in the W/O microemulsions(pentanol+ surfactant + water). One essential difference was that the minimum water content to form a transparent gel was decisively lower in the microemulsion than in the solution. In fact gel formation in the microemulsionsrequired no excess water to that necessary for the hydrolysis and condensation to take place.
Introduction The sol/gel process' to prepare ceramics has earlier been applied using different materials such as (a) sodium silicate and acidic solution, (b) metal salt solution, hexamethylene tetramine, and urea, (c) metal alkoxides in organic solvents, (d) aqueous sols of hydrous metal oxides, and (e) aqueous solution of organic polymers. The main interest has been in method c (metal alkoxides in organic solvents), and the literature in that area is by now extensive with a number of review articles available.2-8 More specialized papers covering the chemistry at different stages of the total process are abundant. So has the structure and preparation of precursors*ll as well as the hydrolysis reaction,+ll eq 1,been described. + Current address: Department of Chemistry, Clarkson University, Potsdam, NY 13699-5814. t Current address: Department of Chemistry, University of Bergen, Bergen, N.5007, Norway. (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science. The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990. (2) Klein, L. In Sol-Gel Technology For Thin Films,Fibers,Preforms, Electronics and Specialty Shapes; Klein, L. C . ,Ed.; Noyes Publications: Park Ridge, NJ, 1988. (3) Haas, P. A. Gel Processes for Preparing Cera" and Glasses. In Glasses and Glass-Ceramics; Lewis, M. H.,Ed.;Chapman & Hall London 1FIRR: nn -___, r r 44-62. - - --, -2-7.. (4) Ray, R. Science 1987, 238,1664. (5) Ulrich, D. R. J . Non-Cryst. Solids 1988, 100,174. (6) Schmidt, H. J . Non-Cryst. Solids 1988, 100, 51. (7) Livage, J.; Henry, M.; Sanchez, C. Progr. Solid State Chem. 1989, 36, 462. (8) Ulrich, D. R. Chem. Eng. News 1990, 28. (9) Mehrotra, R. C. J. Non-Cryst. Solids 1988, 100,1. (10)Guglielmi, M.; Cartman, G. J. Non-Cryst. Solids 1988, 100,16. (11) Sanchez, C.; Livage, J.; Henry, M.; Babonneau, F. J. Non-Cryst. Solids 1988, 100, 65.
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Si(OC,H,), + 4H20 Si(OH), + 4C,H,OH Other papers12-17cover the gelation process
(1)
gelation
Si(OH), Si02+2H20 (2) as sell as the properties of the gel,laJgespecially ita chemical reactions.20 The evaporation of solvent21 during the gelzerogel transition causes pronounced shrinkage and frequently causes cracking in the zerogel, but special additives may alleviate these.22-2s The final process, the fusing of the zerogel, has also been c ~ v e r e d . ~ ~ - ~ ~ We have recently introduced a modification of this method in which the organic solution is replaced by a water(12) Pouxviel, J. C.; Boilot, J. P.; Beloeil, J. C.; Lallemand, J. Y. J. Non-Cryst. Solids 1987, 89,345. (13) Brinker, C. J. J . Non-Cryst. Solids 1988, 100,31. (14) Pouxviel, J. C.; Boilot, J. C.; Dauger, A.; Wright, A. J. Non-Cryst. Solids 1988,103, 331. (15) Abe, Y.; Sugimoto, N.; Nagao, Y.; Misono, T. J. Non-Cryst. Solids 1988, 104, 164. (16) Bowstra, A. H.; Bernards, T. N. M. J. Non-Cryst. Solids 1988, 105,207. (17) Lopez, T.;Asomoza,M.; Razo, L.; Gomez, R. J.Non-Cryst.Solids 1989, 108,45. (18) Sakka, S.; Kozuka, H.; Adachi, T. J.Non-Cryst. Solids 1988,102, 263. (19) Scherer, G. W.; Pardenek, S. A.; Swiatek, R. M. J . Non-Cryst. Solids 1988. 107. 14. (20) Brinker, C. J.;Brinker, B. C.;Tallant, D. R.;Ward, K. J.J. Chem. Phys. 1986,83, 106. (21) Scherer, G. W. J. Non-Cryst. Solids 1989, 107,135. (22) Orcel, G.; Phalippou, J.; Hench, L. L. J. Non-Cryst. Solids 1988, (23) Orcel, G.; Hench, L. L.; Artaki, I.; Jonas, J.; Zerda, T. W. J.NonCryst. Solids 1988, 105,223. (24) Prassas, M.; Phalippou, J.; Zarzycki, J. J. Mater. Sci. 1984, 19, 1656. (25) Mataon, D. W.; Smith, R. D. J. Am. Ceram. SOC.1989, 72, 871. (26) Cheng, J. J.; Wang, D.-W. J . Non-Cryst. Solids 1988, 100,288.
0 1992 American Chemical Society Q743-7463/92/24Q8-Q372$Q3.QQ/Q
Hydrolysis and Condensation of Silicon Tetraethoxide C50H
"p
SDS
Figure 1. Microemulsionbase system water (HzO),sodium dodecyl sulfate (SDS), and pentanol (C50H).The aqueous micellar solution (A) is connected with the pentanol inverse micellar solution (B) via a narrow channel. In addition, two liquid crystalline phases are found, one lamellar (C) and one (D) consisting of a hexagonal array of cylinders. in-oil (W/O) microemul~ion.~~ The reaction path and the kinetics of the reaction in microemulsions were similar to those in organic solvents, which enables an easy adaptation of this method within the framework of earlier contributions. However, the W/O microemulsion is only a part of the general amphiphilic association pattern, and we found an investigation justified into the other structures: the lyotropic crystals and the aqueous micellar solution, in order to evaluate the influence on these structures by the silica polymers and the ethanol formed. In the present paper, the system water, sodium dodecyl sulfate, and pentanol is used because its phase diagram, with its phase regions of normal Figure 1, is well and inverse micellar solutions in addition to the liquid crystals of lamellar and of hexagonally arranged cylinders.
Experimental Section Materials. Sodium dodecylsulfate (SDS)from BDH Chemical was recrystallized twice from absolute ethanol; l-pentanol (CsOH)from Aldrich Chemical (99+%) was stored with molecularsieves (mesh4)and usedwithout further purification. Silicon tetraethoxide (Si(0Et)d) was obtained from Aldrich (99+%, electron grade) and used with no further purification. Aluminum nitrate nonahydrate (Al(N0&-9HzO)(99.99% ) was fromAldrich and used as was. The water was deionized and doubly distilled. Hydrochloric acid (HC1) from Baker Co. (36.538%)was used as the acid medium to adjust the water to pH 2. The ethanol (EtOH, 200 proof) was from Pharmco Co. with no further treatment before use. Determination of Phase Diagrams with Limited Amount of EtOH in SDS/CsOH/H~O Systems. A suitable amount of EtOH (200 proof, equal amounts to what was liberated by Si(0Et)r during the reactions) was added to the samples of the amphiphilic association system and vortexed vigorously by a mixer, and isotropic solution regions were determined by visual inspection. Gel Preparation. Silicon tetraethoxide (Si(OEt),)was added to a glass vial of the association systems and vortexed vigorously until the system became a homogeneous solution. The samples were left at room temperature, and gelation was estimated by visual observation of the fluidity of the sample. (27)Irwin, A.D.;Holmgren, J. S.; Jonas, J. J.Non-Cryst. Solids 1988, 101, 249.
(28)Shukla, B.S.;Johri, G. P. J. Non-Cryst. Solids 1988,101, 263. (29)Rabinovich, E.M.;Nassau, K.; Miller, A. E.; Gallagher, P. K. J. Non-Cryst. Solids 1988,104, 107. (30)Friberg, S.E.;Yang, C. C. The Minerals, Metals and Materials Society, 1988; pp 181-191. (31)Ma, Z.;Friberg, S. E.; Neogi, P. Colloid Surf. 1988,33, 249.
Langmuir, Vol. 8, No. 2, 1992 373
Results The areas for an isotropic and transparent gel for different amounts of added TEOS are shown in Figure 2A-D. The most essential feature is the reduction of minimum water content when the surfactant was introduced. Figure 2A shows 95 5% of water needed for gelation for the water/pentanol combination, while a mixture of 95% and 5% surfactant required only 11% water. Similar tendencies were found for higher amounts of TEOS added, Figure 2B-D. Within the area of the gel, the birefringence of the liquid crystals disappeared; the gel was isotropic. The phases outside the gel region were not determined; the dashed lines in Figure 2A-D show the features of Figure 1 and should not be conceived as representing any structures hfter addition of TEOS. The phase diagrams, in which the water, Figure 1, was replaced by a concentrated solution of aluminum nitrate, but without addition of silicon tetraethoxide, were characterized by a strong reduction of the inverse micellar solution region (dashed lines, Figure 3A-C). I t should be noted that the area for the inverse micellar solution was very strongly reduced at 25% by weight (of the water) of Al(N03)3*9H20,Figure 3B. The regions for gel formation after addition of TEOS, Figure 3, depended mostly upon the concentration of the silicon tetraethoxide as revealed by a comparison of parts A and B to part C of Figure 3. The region for a stable transparent gel with 15% by weight of Al(N0&9H20 in the water and 16.7% TEOS occupied a small area close to the aqueous solution corner and a limited region emanating from the maximum water content of the inverse micellar solution. An increase of the silicon tetraethoxide from 16.7 to 28.5 w t % of the total increased the region for transparent gel to become similar to that without aluminum salt, Figure 3C. The phase diagrams with added ethanol corresponding to the amount liberated under the conditions in Figure 2A-D are presented in Figure 4A-C. The solubilityregions for isotropic solution after addition of ethanol were similar to the regions for transparent gels after addition of TEOS, but the solutionswith added ethanol required consistently less minimum water for all samples. The difference of water content between the ethanol solution and the gel region was approximately equal for all pentanol/surfactant ratios except the very high ones. In excess of a pentan01 surfactant molar ratio of approximately 0.999, the minimum water content to form a gel was significantly higher. The two-phase region along the water/pentanol axis was reduced by the addition of ethanol as expected, but no such reduction was experienced for the gel formation. Discussion The results provide information about the difference between the conditions in the traditional sol/gel method in which the reactions take place in an organic solution and in a microemulsion system in which amphiphilic association structures in the form of micelles and liquid crystals are present. There is no doubt that the presence of the colloidal silica and of the ethanol should have an effect on both the solubility regions of the total phase diagram, Figure 1. The overall chemical reaction of the silicon tetraethoxide
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Si(OC2H5),+ 2H20 SiO, + 4C2H50H (3) leads to formation of ethanol and fractal silica polymers in the system as well as to a reduction of the total water
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Friberg et al.
Langmuir, Vol. 8, No. 2, 1992 C . P
CSQH
A
D
C
6
Figure 2. Area for stable gels (-) reaching from the water corner, covering part of the phases in Figure 1 (- - -). The phase diagrams and pentanol (C60H) only. The amount of silicon tetrethoxide added reflect the sum of water (HzO),sodium dodecyl sulfate (SDS), is counted as the total of all components, including itself. The Si(OCzHd4weight percent is (A) 16.7, (B)28.5, (C) 37.5, and (D)66.7. %OH
CSOH
C5OH
SDS
A
C
6
Figure 3. Inverse micellar solution in the three-component system aqueous aluminum nitrate solution (H2O + Al(NO&SH20), sodium dodecyl sulfate (SDS),and pentanol (- - -), area of isotropic solution after addition of alcohol content corresponding to eq 3 of respective amount of silicon tetraethoxide (///), and area for the transparent gel (XXX).
"p
SDS
SDS
H@
y
SDS
"p
SDS
D Figure 4. A comparison of the regions for the isotropic transparent gels ( X X X ) and for the isotropic solutions formedwhen an alcohol content corresponding to eq 3 was added (/// + XXX). Si(OCzH& weight percent is (A) 16.7, (B)28.5, (C) 37.5, and (D)66.7. A
B
content. The reaction was complete in the transparent gels investigated; NMR spectra gave no indication of SiOCzHs or SiOH groups. These changes in the composition led to a modification of the amphiphilic colloid associationstructures in Figure 1. These structures arise because of the combination of thermodynamic forces and spatial restrictions caused by
C
the dual nature of the amphiphiles,92 and it is to be expected that an addition of polymeric 'silica" and ethanol in high amounts, according to eq 1, should have a strong influence on these structures. So was also the case. One finds that the two liquid (32) Israelachvili, J. N.; Mitchell, K. J.; Ninham, J. W.J. Chem. SOC., Faraday Trans. 2 1976, 767, 1525. (33) Sjcblom, E.; Friberg, S. E. J. Colloid Interface Sci. 1978,67, 16.
Hydrolysis and Condensation of Silicon Tetraethoxide
Table I. Minimum Water Contents, Figure 4A-D, for Transparent Gels at Constant CsOH/SDS (Weight Ratio) Si(0Et)r water figure wt% calcd 7% exptl % 4A 16.7 3.45 3.5 4B 28.5 6.9 8 4c 37.5 10.4 11.5 4D 66.7 34.6 34.5
C20H
H20
C50H
IPH 2,HCI)
Figure 5. Phase diagram for water at pH 2 (HzO), pentanol (GOH), and ethanol (C20H) (-) and area of transparent gels when silicontetraethoxidewas addedto water/pentanol (C50H)/ sodium dodecyl sulfate (SDS) mixtures (- - -). The ethanol amount for the latter case is that obtained from the hydrolysis of the silicon tetraethoxide, while the water is what is left after completedhydrolysis and condensationof the silicon compound. C20H
H20
IPH 2,HCI)
Langmuir, Vol. 8, No. 2, 1992 315
C50H/SDS
Figure 6. Area of a transparent gel, when silicon tetraethoxide was added to water/pentanol mixtures. The ethanol amount is that obtained from the hydrolysis of the silicon tetraethoxide, while the water is what is left after completed hydrolysis and condensationof the silicon compound. C50H/SDS weight ratio is ( 0 )1/0,(X) 9/1,(v)1/1, and (r)0/1.
crystal regions were changed to isotropic solutions prior to final gelation. Figure 2 reveals the fact that most of the regions for the original micellar solutions and the liquid crystals were changed to isotropic solutions and subsequently to an isotropic gel. (The dashed lines in Figure 2 are from Figure 1 and do not reflect conditions after addition of TEOS.) Since both ethanol and silica were formed, the pertinent question is to what extent this change was due to which of the two species. The query is answered by the results in Figure 4. This figure shows the isotropic solution regions formed by adding ethanol to the samples in amounts corresponding to what was liberated during reaction 3 in the samples in Figure 2A-D. This figure reveals that the ethanol formed by the reaction is sufficient to disorder the liquid crystalline structures to form isotropic solutions covering the area of transparent gels plus an extension toward lower water contents. Part of the higher water content needed for the gel than for the solution with ethanol added (two water molecules per silica molecule) is directly accounted for; reaction 3 uses water. The main question is whether the remaining increase of water required by the presence of silica was significant or not. The amount of water was calculated, and Table I shows a comparison between the need for a minimum
Table 11. Ratio between Polar (P) and Nonpolar (NP) Contributions composition,g SDS C5OH CzOHb HzO' P/NPc 0 0.200 0.883 0.127 0.5 0 0.250 0.265 0.198 0.4 0 0.175 0.177 0.290 0.4 0 0.080 0.141 0.392 0.4 0 0.010 0.0883 0.473 0.5 0 0.010 0.0442 0.481 0.5 0 0 LOO0 0 0.6 0.025 0.240 0.883 0.062 0.5 0.045 0.340 0.265 0.063 0.4 0.045 0.370 0.177 0.050 0.4 0.045 0.395 0.0883 0.043 0.3 0.035 0.340 0.0883 0.108 0.3 0.045 0.405 0.0442 0.041 0.3 0.040 0.345 0.0442 0.106 0.3 0.065 0.065 0.883 0.197 0.6 0.155 0.155 0.265 0.138 0.5 0.170 0,170 0.177 0.125 0.5 0.190 0.190 0.0883 0.103 0.5 0.195 0.195 0.0442 0.101 0.5 0.075 0 0.883 0.252 0.6 0.220 0 0.265 0.228 0.6 0.245 0 0.177 0.220 0.6 0.215 0 0.0883 0.268 0.6 0.175 0 0.0442 0.316 0.9 HzO is HzO remaining after hydrolysis and condensation reactions. CzOH is ethanol formedby the reaction. P/NP is weight ratio of polar to nonpolar contributions excluding the water.
water content with the silica tetraethoxide reaction, Figure 2A-D, and calculated values based on the experimental values for ethanol, Figure 4A-D, with 2 mol of water added according to eq 3. These values are an average for pentanol/surfactant weight ratios of 9/1, 1/1, and 1/9. The agreement is excellent for all the silica concentrations, demonstrating that the silicon polymers did not per se require "extra" water. Obviously, the silica polymers did not perturb the colloidal structure of the water poor part of the system. With acceptance of this relation between systems with and without silica, the remaining puzzle is the fact that addition of ethanol per se resulted in an increase of minimum water content for the isotropic solution. Parts A-D of Figure 4 demonstrate the fact that the minimum water/surfactant ratio without ethanol added (dashed line) was less than after addition of ethanol (striped area). There is at present no definite explanation for this behavior, but a reasonable relation is with the free energy change with added water to soap molecules. The presence of ethanol causes a reduction in the water activity. Hence, higher water concentration would be needed to provide a sufficient number of water molecules to hydrate the ionic surfactant, and the minimum water/surfactant ratio for solubility would be increased. Theoretical evaluation^^^ have shown that a certain number of water molecules as hydration shell are needed to dissolve the crystalline soap into an alcohol solution. (34) Friberg, S. E.;Flaim, T. D.; Plummer, P. L. M. In Macro- and Microemuhiom: Theory and Applications; Shah, D. O., Ed.;ACS Symposium Series 272; American Chemical Society: Washington, DC, 1985; p 33.
Friberg et al.
376 Langmuir, Vol. 8, No. 2, 1992 The second feature of interest is the fact that addition of small amounts of surfactant gave a pronounced reduction of minimum water content to form a transparent gel after addition of TEOS,Figure 2. This fact does not change the total shrinkage during the gel-xerogel transition; the reduced water content is replaced by an increase of the pentanol percentage. The advantage lies in a lower surface tension liquid in the evaporation process.' The reason for the change with surfactant addition is illustrated by the diagrams in Figures 5 and 6. Figure 5 is a comparison of the phase regions in the system water, pentanol (CBOH),and ethanol (C20H) (solid line), with a two-phase region emanating from the water/pentanol axis. The dashed line represents the minimum water content to form a transparent gel from the watedpentanol system after addition of TEOS. The line represents the water, pentanol, and formed ethanol only; the silica is not counted. The results show that at low content of TEOS the transparency, Figure 5, of the gel is determined by the solubility of the three liquid components; the curves for solubility in the system without TEOS and to form transparent gels after addition of TEOS are identical for lower TEOS additions than 27 7% ,approximately 24 % ethanol. For greater additions of TEOS, the ratio between excess water and pentanol was approximately 0.67 which corresponds to a molar ratio of 6. This result indicates that the balance between nonpolar and polar contributions may be important for the gel
to remain stable. A simple test of this hypothesis is to calculate the ratio between polar and nonpolar contributions. Table I1 shows the composition of samples involved and a weight ratio of polar to nonpolar groups of the nonaqueous components. The ratio is surprisingly constant and independent of the amount of water. I t appears that the water present has no effect on the compatibility of the silica polymer with the surfactant/alcohol/waterenvironment. Presumably the water is closely associated with the polar groups.34 Summary The colloidal changes in an amphiphilic association structure system from hydrolysis of silicon tetraethoxide was analyzed for an ionic W/O microemulsionsystem. The changes could be explained by the influence of the formed ethanol and the water requirements for the hydrolysis, while the stability of the gel depended on the hydrophilic/ lipophilic balance of the solvent with the water excluded.
Acknowledgment. This research was supported by the New York State Science and Technology Foundation through its CAMP program at Clarhon University and the Marshall Fund, Oslo, Norway. Registry No. TEOS,78-10-4;SDS,151-21-3; pentanol, 7141-0.
