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Langmuir 1999, 15, 4752-4756
Silsesquioxane-Based Amphiphiles Ralf Knischka, Frank Dietsche, Ralf Hanselmann, Holger Frey,* and Rolf Mu¨lhaupt Institut fu¨ r Makromolekulare Chemie und Freiburger Materialforschungszentrum (FMF) der Albert-Ludwigs-Universita¨ t Freiburg, Stefan-Meier-Strasse 21/31, D-79104 Freiburg, Germany
Pierre J. Lutz Institut Charles Sadron (CNRS), 8 rue Boussingault, 67083 Strasbourg, France Received November 13, 1998. In Final Form: April 7, 1999 A novel type of amphiphilic spherosilsesquioxane derivative, 1-(1,ω-propylenemethoxy)oligo(ethylene oxide)-3,5,7,9,11,13,15-heptahydridopentacyclo[9.5.13,9.15,15.17,13]octasiloxane has been prepared from (HSiO3/2)8 and allyl-functional oligo(ethyleneoxide) (Mn ) 750 g/mol) by hydrosilylation. The monosubstituted octahydridosilsesquioxane was characterized by 1H, 13C, and 29Si NMR spectroscopy, IR, and MALDITOF mass spectroscopy as well as elemental analysis. Surface tension measurements of the water-soluble amphiphile show a cmc in the range of 6 × 10-4 mol/L. Aggregation of the uncondensed amphiphile leads to micellar and vesicular structures that can be cross-linked to liposome-like silica particles at elevated pH.
Introduction Considerable research efforts are currently directed at the preparation of nanostructured silicates and silicate nanoparticles as well as mesoporous silicate structures with uniform pore channels, based on self-assembled organic template molecules.1-4 Organosubstituted spherosilsesquioxanes (RSiO3/2)n could represent interesting building units for the construction of organic-inorganic hybrid structures but have to date only been considered to a limited extent in this context. The cube-shaped spherosilsesquioxane 1,3,5,7,9,11,13,15-octahydridopentacyclo[9.5.13,9.15,15.17,13]octasiloxane (1, referred to as H8T8 in the following; Scheme 1) is well characterized and available in reasonable yields via the synthetic route developed by Agaskar.5 Various substitution methods6-9 for H8T8 have been developed. Recently, we reported on the thermal properties of 8-fold alkyl-substituted H8T8 derivatives10 as well as a monofunctional, silsesquioxane-based monomer that was incorporated in polyolefins.11 Our current interest is centered on monosubstituted H8T8 derivatives with amphiphilic properties, combining a relatively hydrophobic spherosilsesquioxane core with a hydrophilic oligo(ethylene oxide) * To whom correspondence should be addressed. Fax: (++49) 761/203-4709. E-mail:
[email protected]. (1) Ozin, G. A.; Mann, S. Nature 1996, 382, 313. (2) Ozin, G. A.; Coombs, N.; Kuperman, A.; Khushalani, D. Chem. Mater. 1996, 8, 2188. (3) Behrens, P. Angew. Chem. 1996, 108, 561. Fyfe, C. A.; Fu, G. J. Am. Chem. Soc. 1995, 117, 9709. (4) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (5) Agaskar, P. A. Inorg. Chem. 1991, 30, 2707. (6) Agaskar, P. A. J. Am. Chem. Soc. 1989, 111, 6858. (7) Day, V. W.; Klemperer, W. G.; Mainz, V. V.; Millar, D. M. J. Am. Chem. Soc. 1985, 107, 8262. (8) Dittmar, U.; Hendan, B. J.; Florke, U.; Marsmann, H. C. J. Organomet. Chem. 1995, 489, 185. (9) Weidner, R.; Zeller, N.; Deubzer, B.; Frey, V. (Wacker-Chemie GmbH). DE 3837397, 1990. (10) Bolln, C.; Tsuchida, A.; Frey, H.; Mu¨lhaupt, R. Chem. Mater. 1997, 9, 1475. (11) Tsuchida, A.; Sernetz, F.; Bolln, C.; Frey, H.; Mu¨lhaupt, R. Macromolecules 1997, 30, 2818.
chain. Hydrosilylation can be conveniently employed to prepare substituted H8T8 derivatives with various degrees of functionalization12,13 and appeared promising also for the preparation of monosubstituted spherosilsesquioxanes.14 An alternative route to monosubstituted H8T8 derivatives, based on end-capping of an incompletely condensed spherosilsesquioxane, was described by Feher and Lichtenhan.15-17 However, this route is limited to large substituents, such as cyclohexyl or cyclopentyl groups. In this paper, we present the synthesis and first data on the aggregation of silsesquioxane-based amphiphiles that may possess potential for the preparation of novel core/shelltype silicate nanoparticles.18 Experimental Section General Procedures. All synthetic procedures were carried out under an argon atmosphere, using carefully dried solvents. For the dialysis experiments, we used benzoylated dialysis tubes (Sigma, CutOff 1200 g/mol). Preparation of 3. A solution of 2 (480 mg, 0.6 mmol) in dry toluene (50 mL) was added slowly to a solution of 1 (1.012 g, 2.4 mmol) in 40 mL of toluene under an argon atmosphere. A total of 200 µL of a 1 wt % solution of H2PtCl6/diglyme was added. The reaction mixture was then stirred at 75 °C for 8 h. Subsequently, the solvent was evaporated and acetone (150 mL) added to the residue. The organic layer was filtered and dried (MgSO4) and the solvent evaporated, affording the product 3 (yield: 598 mg, 85%). 1H NMR: δ 0.65 (t, J ) 7.1 Hz, 2 H, SiCH ), 1.11 (m, 0.4 H, 2 SiCHCH3CH2), 1.61 (m, 2 H, SiCH2CH2CH2), 3.35 (s, 3 H, CH3), 3.38 (t, J ) 7.3 Hz, 2 H, SiCH2CH2CH2O), 3.61-3.82 (m, 72 H, OCH2CH2), 4.09-4.23 (m, 7 H, SiH). 13C NMR: δ 7.81 (SiCH2), (12) Calzaferri, G.; Bu¨rgy, H. Helv. Chim. Acta 1990, 73, 698. (13) Calzaferri, G.; Bu¨rgy, H.; Herren, D. Helv. Chim. Acta 1991, 74, 24. (14) Calzaferri, G.; Herren, D.; Imhof, R. Helv. Chim. Acta 1991, 74, 1278. (15) Feher, F. J.; Budzichowski, T. A. J. Organomet. Chem. 1989, 379, 33. (16) Lichtenhan, J. D. Comments Inorg. Chem. 1995, 17, 115. (17) Lichtenhan, J. D.; Noel, C. J.; Bolf, A. G.; Ruth, P. N. Mater. Res. Soc. Symp. Proc. 1996, 435, 3. (18) Antonelli, D. M.; Ying, J. Y. Curr. Opin. Colloid Interface Sci. 1996, 1 (4), 523.
10.1021/la981594a CCC: $18.00 © 1999 American Chemical Society Published on Web 06/05/1999
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Scheme 1. Synthesis of Amphiphiles 3 and 4 (n ) 18)
22.72 (SiCH2CH2CH2O), 59.10 (OCH3), 69.95, 70.51, 70.60, 69.86, 70.72, 71.94, 73.49 (OCH2CH2). 29Si NMR: δ -65.83 (1.0 Si, SiCH2), -65.65 (0.1 Si, SiCHCH3), -83.80 to -84.10 (6.9 Si, SiH). IR (KBr): 3040, 2725 (CH2, s), 2240 (SiH, s), 1470 (m), 965, 957, 745 (SiO, m) cm-1. Anal. Calcd for C40H88Si8O31: C, 37.2; H, 6.8. Found: C, 36.3; H, 6.9. Preparation of 4. To a solution of 3 (253 mg, 0.22 mmol) in dry toluene (30 mL) was added 1 drop of a 1 wt % solution of H2PtCl6/diglyme, and the resulting mixture was stirred under an ethylene atmosphere (1 atm) at 85 °C for 15 h. The solution was concentrated via partial evaporation of the solvent and then purified by chromatography (toluene/THF, 3/1).The solvent was evaporated, affording the product 4 (yield: 297 mg, 98%). 1H NMR: δ 0.55-0.70 (m, 16 H, SiCH ), 0.98 (t, J ) 7.5 Hz, 2 21 H, SiCHCH3), 1.61 (m, 2 H, SiCH2CH2CH2), 3.35 (s, 3 H, CH3), 3.37 (t, J ) 7.3 Hz, 2 H, SiCH2CH2CH2O), 3.6-3.8 (m, 72 H, OCH2CH2). 13C NMR: δ 4.01 (CH3), 6.50 (CH2), 8.09 (SiCH2), 22.93 (SiCH2CH2CH2O), 59.02 (OCH3), 69.86, 69.93, 70.51, 70.63, 70.72, 71.94, 73.45 (OCH2CH2). 29Si NMR: δ -65.26 (3.0 Si), -65.42 (4.0 Si), -68.56 (1.0 Si). Anal. Calcd for C54H116Si8O31: C, 43.6; H, 7.8. Found: C, 43.1; H, 7.9. MALDI-TOF MS(K salt): m/z1084-1788. Methods. NMR. Solution 1H, 13C, and 29Si NMR spectra were recorded at 300, 75, and 59 MHz, respectively, in CDCl3. 1H and 13C NMR chemical shifts are referenced to the signal of the solvents used and 29Si NMR chemical shifts to the signal of TMS (tetramethylsilane). GPC. A combination of PL columns (Polymer Laboratories), 105, 104, 103, and 100 nm, was employed; CHCl3 was used as the solvent. All data are referenced to narrow polystyrene standards. Surface Tension. Solutions of 3 and 4 in distilled water using a series of different concentrations were measured using a Kru¨ss digital tensiometer K10 T at 25 °C (30 min of waiting time before measurements). DLS/SLS. Dynamic light scattering measurements were performed using the ALV 5000 correlation spectrometer system (ALV, Langen, Germany). DLS data were evaluated by CONTIN inversion. The wavelength of the argon ion laser (Spectra Physics) was 488 nm. A modified and fully computerized SOFICA (SLS, Freiburg, Germany) was used for static light scattering measurements. All measurements were performed at 20 °C. TEM. Compound 3 was dissolved (5 wt %) in water. After adjustment of the pH to 9.3 by slow addition of a 0.5 M NaOH solution, evolution of hydrogen occurred. Subsequently, after 30 min more of reaction time and filtration through a hydrophilic
microfilter (0.8 µm) and dialysation for 72 h (CutOff 1200 g/mol), the samples were deposited on a carbon-coated copper grid (EM FN 200K, SCI Science Services) and dried with filter paper. The grids were investigated by TEM. AFM. Atomic force microscopy was performed under ambient conditions with a Nanoscope III (Digital Instruments, Santa Barbara, CA) scanning probe microscope. The AFM images were obtained by operating the instrument in tapping mode. Commercial Si cantilevers with nominal force constants C ) 28-74 N/m and nominal eigenfrequencies F0 ) 320-430 kHz were used for the tapping mode AFM measurements. According to the manufacturer, these tips have a nominal radius of curvature of 5-10 nm. The topographic images were recorded with typical scan speeds of 1-4 lines s-1 and are presented as raw, unfiltered data. For particle size determination, the average size was calculated from at least 15 randomly chosen features visible in an image. MALDI-TOF MS. All MALDI spectra were obtained using a REFLEX II time-of-flight mass spectrometer (Bruker-Franzen, Bremen, Germany) operated at 20 kV accelerating voltage in reflector mode with positive ionization. 2,5-Dihydroxybenzoic acid and 9-nitroanthracene were used as matrixes: KCl or silver trifluoroacetate, respectively, was added for cationization (solvent: THF). Data were externally calibrated by means of poly(ethylene oxide) standards. An isotopic pattern was obtained using standard statistical programs (Chempute).
Results and Discussion Synthesis of the novel silsesquioxane amphiphiles 3 and 4 was achieved via three steps as shown in Scheme 1. The spherosiloxane 1 (H8T8) was prepared according to the method of Agaskar5 by controlled hydrolysis of HSi(OEt)3 or HSiCl3. We were able to obtain pure H8T8 in good yields (18%) via a slightly modified procedure, using petroleum ether instead of the solvent mixture of toluene, methanol, and hexane isomers described by Agaskar. Careful partial solvent removal permitted isolation of 1 by fractionated crystallization. Oligo(ethylene oxide) was used as the hydrophilic segment of the amphiphiles. 1,ωAllylmethoxyoligo(ethylene oxide) was prepared from monomethyl ether-functional commercial oligo(ethylene oxide) (Mn ) 750; GPC, NMR, MALDI-TOF MS) of narrow polydispersity via metalation with naphthalene/potassium
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Figure 1. (a) 29Si NMR spectrum of 3. (b) MALDI-TOF MS spectrum of 4. (c) Isotopic pattern of MALDI-TOF signals, experimental and calculated. (d) 1H NMR spectrum of 3.
and termination with allyl bromide in a batch process.19 The hydrophilic segment was coupled with H8T8 by hydrosilation,20 using Speier’s catalyst (H2PtCl6), as shown in Scheme 1, obtaining the amphiphiles 3 and 4. The synthesis of 3 and 4 was carried out in a toluene solution, using a 4-fold excess of H8T8. This permits convenient separation of 1 from 3 or 4, respectively. (Removal of disubstituted products from 3 after the reaction was expected to be tedious.) Unreacted H8T8 recovered from the reaction mixture was reused in subsequent batches. Highest yields of 3 were obtained when the hydrosilation was carried out at 80 °C. Figure 1a shows the 29Si NMR spectrum of 3. A distribution of R- and β-addition products was observed. Both products 3 and 4 showed two singlet resonances near -65.7 ppm for R- and β-addition products, respectively. At low concentrations of the allyl-terminated educt 2 and lower reaction temperatures, an increasing fraction of β addition was detected. Integration of the 29Si NMR signals yielded a ratio of 1.0/6.9 for SiR/SiH, supporting monosubstitution of H8T8 under the given reaction conditions. Furthermore, the signal pattern observed in the 29Si NMR spectrum showed no additional signals from side products that would be expected in the case of disubstitution.21 For MALDI-TOF MS (Figure 1b,c) and generally for comparison, the amphiphile 3 was hydrosilylated with (19) Knischka, R.; Lutz, P.; Frey, H., unpublished work. (20) Bassindale, A. R.; Gentle, T. E.; Taylor, P. G.; Watt, A. In Tailormade Silicon-Oxygen Compounds; Corriu, R., Jutzi, P., Eds.; Vieweg: Braunschweig, Germany, 1995; p 171. (21) Hendan, B. J.; Marsmann, H. C. J. Organomet. Chem. 1994, 483, 33.
ethylene, obtaining 4. The MALDI-TOF shows no peaks that are due to the disubstituted product. The signals in the area between m/z 1040 and 1750 with a characteristic shift of 44 g/mol are clearly due to the monosubstituted silsesquioxane and mirror the molecular weight distribution of the oligo(ethylene oxide) employed. The subdistribution shifted by 16 g/mol with respect to the main distribution is due to cationization with sodium. The isotopic pattern of the peaks correlates in the range of the margin of error with the pattern obtained from statistical calculations. Using 9-nitroanthracene, some portion of disubstituted product in the range of m/z 1800-2300 and methoxyoligo(ethylene oxide) is visible. In contrast to H8T8, 3 and 4, with HLB values of 12.5 and 10.5, respectively, are excellently soluble in various organic solvents (THF, diethyl ether, and toluene) and water. The 1H NMR spectrum (Figure 1d) as well as elemental analysis confirmed monosubstitution. Integration of the signal at 1.15 ppm, furthermore, shows the ratio of R and β adducts. Commonly, we found 12% of the β-addition product. As an additional side reaction, a rearrangement of the terminal allylic double bond to an internal one and dissociation of the enol ether bond under reformation of poly(ethylene oxide)-monomethyl ether could be observed to a low extent. Both silsesquioxane-based compounds 3 and 4 exhibit a concentration dependence of the surface tension of aqueous solutions (Figure 2). Traces of poly(ethylene glycol)-monomethyl ether that could not be removed from the nonethylated compound 3 show no surfactant properties and therefore do not alter the characteristic behavior
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Figure 2. Surface tension in water of 3 and 4 vs concentration.
of these compounds.22 Amphiphile 4 with the 7-fold ethylated hydrophobic segment showed a strong variation of the surface tension at concentrations above 4 × 10-5 mol/L, which can be interpreted in terms of micelle formation. An interpretation of this behavior as a cmc solely based on these experiments is not possible because of the noncharacteristic shape of the curve. The obtained value is, nonetheless, in the range expected for high molecular weight nonionic surfactants, which typically show cmc values between 0.2 and 20 × 10-5 mol/L.23-26 For the nonethylated compound 3, the characteristic variation of the surface tension that could be referred to as cmc is shifted to 6 × 10-4 mol/L. This is due to the smaller size of the T8 headgroup as well as decreasing hydrophobicity. To further investigate the self-organization of 3 and 4 in solution, static as well as dynamic light scattering has been carried out at different concentrations in an aqueous solution. Both methods showed the existence of aggregated species. By means of static light scattering, averages of apparent molecular weights and radii of gyration were determined. The detected radii values were in the range of 105-115 nm for 3, showing the presence of rather large aggregates. Dynamic light scattering at various concentrations above the cmc revealed the presence of aggregates with 10 and 60 nm hydrodynamic radii for 3, respectively. We tentatively explain these species as micellar- and vesicular-type precondensed aggregates of 3. The size of the micellar-type aggregates corresponds reasonably well to an estimate of the molecular dimensions of amphiphile 3, although a precise assessment of the dimensions of 3 is not feasible because of coiling of the oligo(ethylene oxide) chains. The nonflexible headgroup almost certainly requires folding of the flexible hydrophilic segment. According to Israelachvili’s theory,27 we expect aggregation (22) Gentle, T. E.; Snow, S. A. Langmuir 1995, 11, 11. (23) Kratzat, K.; Stubenrauch, C.; Finkelmann, H. Colloid Polym. Sci. 1995, 273, 257. (24) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (25) Brandrup, J.; Immergut, E. H.; Ed. Polymer Handbook; Wiley & Sons: New York, 1975; p ΙΙ-491. (26) Schmaucks, G.; Wagner, R.; Wersig, R. J. Organomet. Chem. 1993, 446, 9. (27) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
a
b Figure 3. (a) TEM image of 3, showing micellar- and vesiculartype aggregates (scale bar, 100 nm). (b) AFM image of 3, demonstrating the vesicular-type structure.
to spherical or ellipsoidal micelles. Dynamic light scattering of 4 shows aggregates in the range of 30 nm that
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showed no concentration dependence of the gyration radius and point therefore also to a critical aggregation of the amphiphilic compound. We were particularly interested in vesicular structures with a cross-linked silica shell. It is well-known from solgel chemistry that discrete spherosilsesquioxanes can be condensed to larger silicate structures at pH above 8. The condensation mechanism as well as the structure of the resulting condensed structures has been investigated in detail.28 Addition of 0.5 M NaOH to an aqueous solution of 3 was employed to raise the pH to 9.3. Evolution of hydrogen indicated condensation of the silsesquioxane cages. According to dynamic light scattering, elevation of the pH resulted in no variation of the dimensions of the aggregates. A bimodal distribution including small micellar-type and large vesicular-type aggregates was observed. To verify the formation of permanently cross-linked silica hybrid particles after elevation of the pH, dialysis experiments were carried out with dilute solutions of the amphiphiles before and after the cross-linking procedure, using a small pore size cellulose membrane with a molecular weight exclusion limit of 1200 g/mol. After several days, in the case of the non-cross-linked silsesquioxane amphiphiles 3, dialysis experiments evidenced almost complete diffusion of the amphiphiles through the membrane (retention < 9%), whereas after cross-linking above the cmc, retention of the material in a dilute solution was observed (retention > 80%). The particles formed after cross-linking of 3 were investigated by TEM (Figure 3a). Both modes from the DLS measurements could be observed. Aggregates in the range of 10 nm in diameter (micellar structures) that are (28) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, 1990; p 145.
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clustered together and vesicular-type aggregates (indicated with arrows) with a diameter of about 60 nm with internal cavities can be distinguished. Both structures show a certain polydispersity. Investigation of the cross-linked aggregates deposited on mica by AFM was expected to yield additional information about the structure of these silsesquioxanebased amphiphiles in solution after cross-linking. All AFM measurements were carried out in the tapping mode. Figure 3b shows a typical surface image of the particles deposited on mica from a dilute solution, obtained by tapping-mode AFM in different areas. The image shows a relatively flat film formed by the collapsed particles; single collapsed vesicle structures are visible. Typically, aggregates with a diameter in the range of 60 nm were observed. In summary, a novel type of amphiphile based on a spherosilsesquioxane was prepared. In this type of amphiphile the silsesquioxane cage can be considered as a molecular silicate building unit. Self-organization of the amphiphiles in solution leads to micellar and vesicular structures. An increase of the pH was employed to crosslink the self-assembled structures to spherical vesicles with a silica shell. Further studies concerning variation of the poly(ethylene oxide) chain length as well as the encapsulation of dyes in the cross-linked liposome-like silica structures are in progress. Acknowledgment. We thank Prof. Akira Tsuchida (University Gifu) for his valuable help with the synthesis and Deutsche Forschungsgemeinschaft (SFB 428) for support of this research. Dr. Ralf Thomann is acknowledged for help with the TEM measurements. LA981594A
