Chem. Mater. 1999, 11, 1275-1284
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A Convenient Synthesis of Silylated Silica Xerogels Gary T. Burns,*,† Qin Deng,† Rex Field,‡ James R. Hahn,† and Charles W. Lentz† Dow Corning Corporation, Midland Michigan 48686-0994, and Cabot Corporation, 700 E. U.S. Highway 36, Tuscola, Illinois 61053-9643 Received October 29, 1998. Revised Manuscript Received February 25, 1999
Treated silica xerogels with controlled porosity and surface area were prepared by the in situ treatment of hydrogels with hexamethyldisiloxane or alkylchlorosilanes in the presence of isopropyl alcohol. The resulting hydrogels were hydrophobic and readily transferred to nonpolar organic solvents upon contact. The surface area and porosity of the xerogel were controlled by varying the pH, time, and temperature used to polymerize the hydrogel prior to treatment. In general, the surface area decreased with increases in aging time or temperature, whereas the total pore volume and pore size increased. Silylation of the hydrogel in the aqueous phase retains the structure of the hydrogel and permits isolation with minimum structure collapse. By using this technique, silylated xerogels with surface areas ranging from 200 to >700 m2/g and pore volumes of 1.5 to 3.8 cm3/g were obtained.
Introduction Sodium silicate can be polymerized under both acidic and basic conditions to produce silicas with a wide range of properties. Under basic conditions polymerization occurs through the formation and growth of discrete particles. In the absence of salts, polymerization leads to the formation of colloidal silicas but in the presence of salts or other coagulants, the colloidal particles aggregate and precipitate from the aqueous phase. Precipitated silicas made in this way constitute one of the largest classes of reinforcing silica fillers. Under acidic conditions, particle growth and aggregation occur simultaneously, resulting in the formation of gels. By varying the pH, temperature, and time during the polymerization process, hydrogels with a wide range of structures can be produced. However, when dried directly from the aqueous phase much of the structure collapses, and the volume of the dried gel or xerogel is often reduced by a factor of 5 to 10 compared to the wet gel.1 Structure collapse occurs when either the capillary forces generated within the pore exceeds the gel strength or when sufficient shrinkage occurs that neighboring hydroxyl groups within the pore are brought into proximity and condense. Early attempts to make aerogels focused on the capillary forces within the pores and led to the use of supercritical fluids2,3 in the drying step and changes in the gelation chemistry designed to strengthen the hydrogel.4 During this time, several groups also investigated the conversion of hydrogels to alcogels and organogels via sequential solvent ex* Author to whom inquiries about the paper should be addressed. † Dow Corning Corporation. ‡ Cabot Corporation. (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic: Boston, 1990. (2) Kistler, S. S. U. S. Patent 2,093,454, 1937. (3) Tewari, P. H.; Hunt, A. J. U. S. Patent 4,610,863, 1986. (4) Zelinski, J. J.; Uhlmann, D. R. J. Phys. Chem. Solids 1984, 45, 1069.
changes and the subsequent surface modification of the alcogels5 and organogels6-10 with silane treating agents. As shown by Smith,7 shrinkage still occurs in the silylated gels during the drying process but the absence of condensation reactions between neighboring hydroxyl groups allows the gel to spring back or relax to its original shape and structure when dried at ambient pressure. Although this procedure eliminates the need for supercritical drying, at least one solvent exchange is required in the conversion of the hydrogel to the alcogel or organogel which limits their utility as a general synthetic method. Preliminary reports of the direct silylation of a hydrogel under acidic conditions have been made by both Lentz11 and Schwertfeger et al.12 Although similar, the Lentz procedure uses a water miscible cosolvent to facilitate the silylation step. As a result, lower acid conditions are used in the treatment step but at the expense of volume efficiency. In another preliminary report, the silylation of hydrogels aged under various neutral conditions using the Lentz procedure and their use as reinforcing fillers was described.13,14 In this paper, we describe the synthesis and conversion of hydrogels to silylated xerogels under a wide range of reaction and processing conditions. (5) Tyler, L. J. U. S. Patent 3,015,645, 1962. (6) Deshpande, R.; Smith, D. M.; Brinker, C. J. U. S. Patent 5,565,142, 1996. (7) Smith, D. M.; Stein, D.; Anderson, J. M.; Ackerman, W. J. NonCryst. Solids 1995, 186, 104. (8) Hua, D. W.; Smith, D. M. Mater. Res. Soc. Symp. Proc. 1992, 271, 547. (9) Jansen, R. M.; Zimmermann, A. European Patent Application 0690 023 A2. (10) Jansen, R. M.; Zimmermann, A.; Jacquinot, E.; Smith, D. M. European Patent Application P 0658 513A1. (11) Lentz, C. U. S. Patent 3,122,520, 1964. (12) Schwertfeger, F.; Frank, D.; Schmidt, M. J. Non-Cryst. Solids 1998, 225, 24. (13) Deng, Q.; Burns, G. T.; Hahn, J. R.; Reese, C. C.; Preston, J. D.; Winchell, S. B. Mater. Res. Soc. Symp. Proc. 1998, 520, in press. (14) Burns, G. T.; Deng, Q.; Hahn, J. R.; Reese, C. U. S. Patent 5,708,069, 1998.
10.1021/cm981037+ CCC: $18.00 © 1999 American Chemical Society Published on Web 04/24/1999
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Table 1. Synthesis and Characterization of Treated Xerogels entry no.
exp no.
colloidal silica
pH during aging
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
5 5 5 5 5 5 5 3 3 3 3 2 2 1 1 1 1 1 1 1 1 1 4 4,5 4,5
7.5 nm 7.5 nm 7.5 nm 12 nm 12 nm 12 nm 12 nm none none none none none none none none none none none none none none none none 12 nm 7.5 nm
6.9 7.4 9.8% HCl 2.5 7.0 7.1 11% HCl ∼0 1.5 1.5 1.5 6.5 6.5 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 11% HCl 10% HCl 10% HCl
aging time
aging temp (°C)
yield (g)
density (g/cm3)
M/Q ratio
calcd primary particle size (nm)
surface area (m2/g)
pore volume (cm3/g)
pore diameter (Å)
void volume (cm3/g)
10 days 2h 2h 16 h 9 days 2h 2h 5h 4h 5h 20 h 12 days 5h 3.5 h 8h 24 h 96 h 0.01 h 0.5 h 2h 3h 4h 3h 3h 3h
25 100 100 25 25 100 100 100 100 100 25 25 100 25 25 25 25 100 100 100 100 100 100 100 100
263.4 291.8 NA 354.1 279.8 360.4 299.4 155.4 139 159.3 220.9 151.2 146.8 233 214.3 215.3 210.4 204.1 196.9 172 200.6 203.9 204.7 202.7 209.8
1.79 1.90 1.79 1.62 1.80 1.91 1.83 1.71 1.53 1.53 1.43 1.67 1.90 1.49 1.50 1.57 1.59 1.65 1.66 1.74 1.77 1.71 1.66 1.83 1.78
0.121 0.078 0.114 0.217 0.113 0.064 0.122 0.180 0.287 0.269 0.394 0.201 0.088 0.298 0.265 0.239 0.218 0.201 0.175 0.148 0.117 0.133 0.157 0.122 0.115
NA NA NA NA NA NA NA 4.11 3.10 3.10 2.74 3.83 6.40 2.95 2.99 3.28 3.38 3.70 3.76 4.35 4.62 4.11 3.76 NA NA
404 267 384 595 369 235 336 477 745 689 823 538 274 839 860 835 734 699 618 525 499 510 484 333 355
2.86 2.32 2.56 1.54 2.41 2.20 2.35 3.12 3.24 3.68 2.03 3.81 2.44 2.29 3.10 3.06 3.08 2.79 2.99 2.97 3.34 3.27 3.54 2.37 2.49
195 230 181 71 179 250 185 172 126 146 72 189 246 81 105 110 122 124 145 164 175 171 197 196 196
4.92 4.13 4.59 2.38 4.40 4.01 4.32 5.96 5.81 6.15 3.34 6.73 4.58 4.25 4.59 5.42 5.35 5.21 4.82 5.08 4.94 5.50 6.87 4.67 4.90
Experimental Section Sodium silicate (40.75 baume, 40 g of SiO2 per 100 mL of solution) was purchased from PQ Corporation and used as received. All silanes were obtained from Dow Corning Corporation. All other reagents were purchased from either Aldrich Chemical Co. or Fisher Scientific. All aging studies were conducted in a 5-L round-bottomed flask fitted with a bottom drain valve and equipped with an overhead mechanical stirrer, a reflux condenser, and a gas inlet. Rapid stirring was applied during the aging step to break up the hydrogel. As a result, a free-flowing powder was obtained after treatment and solvent removal. In the cases where the hydrosol was allowed to gel outside of the roundbottomed flask, the subsequent gel was cut into 0.5-in. squares and transferred to the 5-L round-bottomed flask where subsequent aging and hydrophobing steps were conducted. In the following examples hexamethyldisiloxane is used to illustrate the in situ treatment of hydrogels. When trimethylchlorosilane was used, the amount of HCl added during the treatment step was reduced to account for the HCl generated from the hydrolysis of the chlorosilane. Procedure 1. Synthesis of Me3Si-Xerogels at a pH of 4.0. Silica sols containing 0.08 g of SiO2/mL were prepared by adding 400 mL of sodium silicate (160 g of SiO2) diluted with 960 mL of deionized water to a rapidly stirring solution of 136 mL of concentrated HCl in 504 mL of deionized water (pH ) 1.7). The pH of the resulting hydrogel was adjusted to 4.0 by back-titration with a dilute solution of sodium silicate (0.08 g of SiO2/mL). After aging at the specified time and temperature, the reaction was cooled to room temperature and 500 mL of concentrated HCl was added to the hydrogel followed by 833 mL of isopropyl alcohol and 227 mL (173.4 g) of hexamethyldisiloxane. After the solution was stirred for 1 h at room temperature, 1500 mL of toluene was added. The aqueous phase was drained, and 250 mL of isopropyl alcohol and 50 mL of deionized water were added to the toluene phase. The reaction mixture was rapidly stirred for 2 min. Deionized water (700 mL) was then added and the aqueous phase drained. The flask was fitted with a Dean-Stark trap and the residual water removed by azeotropic distillation. After all of the water had been removed, approximately one-third to one-half of the toluene was removed by distillation. The remaining toluene/ xerogel slurry was poured into 10 in. × 20 in. Pyrex pans and the solvent allowed to evaporate overnight. The treated silica was further dried at 150 °C for 14 h in an air circulating oven.
With the use of the above procedure, two series of xerogels were made by aging the hydrogel at 25 °C for 3.5 to 96 h (Table 1, entries 14-17) or at reflux for 0 to 4 h (Table 1, entries 18-22). Procedure 2. Synthesis of Me3Si-Xerogels using Gels Aged at a pH of 6.5. A sodium silicate solution made by diluting 400 mL of sodium silicate with 600 mL of deionized water was added to a rapidly stirring solution of 440 mL of concentrated HCl diluted with 560 mL of deionized water. The silica sol was filtered to remove any gels and then poured into two 16 in. × 24 in. Teflon-coated pans. After sitting for 95 min, the gel was cut into ∼0.5-in squares and transferred to a 2-gal plastic bucket equipped with an inlet near the bottom of the bucket and an outlet near the top. Deionized water was pumped through the hydrogel until the pH of the effluent was 2 ( 0.5 (∼90% of the sodium ion was removed from the hydrogel). The pH was then adjusted to 6.5 ( 0.5 with either ammonium hydroxide or potassium hydroxide. The hydrogel was then transferred to a 5-L, three-necked flask equipped with a bottom drain valve and aged at either room temperature or reflux for the designated time. After the hydrogel was aged, 727 mL of concentrated HCl was added followed by 909 mL of isopropyl alcohol and 471 mL of hexamethyldisiloxane. After the mixture was stirred for 1 h at room temperature, 2 L of toluene was added. The reaction was mildly stirred for an additional 5 min and the aqueous phase drained from the bottom of the flask. The toluene phase was washed with 500 mL of deionized water, the flask fitted with a Dean-Stark trap, and the toluene refluxed to remove the residual water. After all the water was removed, the toluene was removed by distillation under reduced pressure. The dried silica was collected and further dried overnight at 150 °C. Entries 11-13 in Table 1 were made using the above procedure. Procedure 3. Synthesis of Me3Si-Xerogels by Aging Gels at Reflux under Acidic Conditions. A 0.08 g SiO2/ mL sol made by diluting 400 mL of sodium silicate with 600 mL of deionized water was added to a rapidly stirring solution of 440 mL of concentrated HCl diluted with 560 mL of deionized water. The silica sol was filtered to remove any gels and then poured into two 16 in. × 24 in. Teflon-coated pans. After sitting for 95 min, the gel was cut into ∼0.5-in. squares and transferred to a 2-gal plastic bucket equipped with an inlet opening near the bottom of the bucket and an outlet near the top. Deionized water was pumped through the hydrogel at a
Silylated Silica Xerogels rate of ∼2.5 gal/h until the pH of the effluent was 2 ( 0.5 (∼90% of the sodium ion removed from the hydrogel). The hydrogel was transferred to a 5-L three-necked flask equipped with bottom drain valve and the pH adjusted with concentrated HCl to the indicated pH (727 mL of concentrated HCl was added to the sample aged a pH of ∼0). The reaction was then refluxed for the designated time. After the hydrogel was aged, the pH was lowered for the hydrophobing step. For the samples aged at a pH of 1.5, 727 mL of concentrated HCl was added. No additional acid was required for the hydrogels aged at a pH of ∼0. Isopropyl alcohol (909 mL) and hexamethyldisiloxane (471 mL) were then added. After the mixture stirred for 1 h at room temperature, 2 L of toluene was added. The reaction was stirred mildly for 5 min and then the aqueous phase drained from the bottom of the flask. The toluene phase was washed with 500 mL of deionized water. The flask was then fitted with a Dean-Stark trap and the toluene refluxed to remove the residual water. After all of the water had been collected, the toluene was removed by distillation under reduced pressure. The dried xerogel was collected and further dried at 150 °C for 16 h. Entries 8-10 in Table 1 were made using the above procedure. Procedure 4. Preparation of Treated Xerogels from Deionized Sols. A sol containing 0.1 g SiO2/mL was passed through a 5 in. × 2 in. polycarbonate column filled with 1500 mL of Dowex 50WX8-100 ion-exchange resin (in the acid form) at a rate of 60 mL/min. The effluent was discarded until the pH dropped from 2-3 to ∼1, indicating that the sol had begun to elute. The first 750-800 mL of the sol was collected and discarded. At this point, the pH was below 0.5 and the next 2000-2400 mL of effluent was collected. A total of 2000 mL of the deionized sol was transferred to a 5 L, three-necked round-bottom flask with a bottom drain valve. The solution was rapidly stirred, and 626 mL of concentrated HCl was added. A gel formed within a few minutes which was broken up by the constant stirring. The reaction was heated at 100 °C for 3 h and then cooled to 40 °C. To this suspension were added 872 mL of isopropyl alcohol and 112 mL of hexamethyldisiloxane. After this mixture was stirred for 45 min at room temperature, 2.4 L of toluene was added. After an additional 5 min mild stirring, the stirring was stopped and the aqueous phase drained from the flask. The toluene phase was washed with 1 L of deionized water. The flask was then fitted with a Dean-Stark trap and the toluene refluxed to remove the residual water. After all the water was removed, the solvent was evaporated. The treated silica was dried overnight at 150 °C. This procedure was used to make entry 23. Procedure 5. Synthesis of Treated Xerogels from Colloidal Silica-Modified Hydrogels. A sodium silicate solution made by diluting 400 mL of sodium silicate with 600 mL of deionized water was added to a rapidly stirring solution of 440 mL of concentrated HCl diluted with 560 mL of deionized water. The silica sol was filtered and 309 mL of Ludox HS (av. )12 nm) or 384 mL of Ludox SM (av. ) 7.5 nm) added with rapid agitation. The modified hydrosol was poured into two 16 in. × 24in. Teflon-coated pans and allowed to gel. After sitting for 95 min, the gel was cut into ∼0.5-in. squares and transferred to a bucket equipped with an inlet near the bottom of the bucket and an outlet near the top. Deionized water was pumped through the hydrogel until the pH of the effluent was 2 ( 0.5 (∼90% of the sodium ion removed from the hydrogel). The hydrogel was then transferred to a 5 L, three-necked flask equipped with a bottom drain valve and the pH adjusted with either ammonium hydroxide or concentrated HCl until the desired pH was obtained. The hydrogel was then aged at either room temperature or reflux for the designated time. After the hydrogel was aged, sufficient concentrated HCl was added to bring the HCl concentration to between 9 and 11%. Isopropyl alcohol (909 mL) and hexamethyldisiloxane (471 mL) were then added. After the mixture was stirred for 1 h at room temperature, 2 L of toluene was added. The reaction was mildly stirred for an additional 5 min and then the aqueous phase drained from
Chem. Mater., Vol. 11, No. 5, 1999 1277 the bottom of the flask. The toluene phase was washed with 500 mL of deionized water, the flask was fitted with a DeanStark trap and the toluene refluxed to remove the residual water. After all the water was removed, the toluene was removed by distillation under reduced pressure. The treated silica was collected and dried overnight at 150 °C. Entries 1-7 in Table 1 were prepared by this procedure. In some cases, the colloidal silica-modified hydrosol was deionized prior to polymerization by passing the modified hydrosol through a 5 in. × 2 in. polycarbonate column filled with 1500 mL of Dowex 50WX8-100 ion-exchange resin (in the acid form) at a rate of 60 mL/mim. The effluent was discarded until the pH dropped from 2-3 to ∼1, indicating that the sol had begun to elute. The first 750-800 mL of the sol was collected and discarded. At this point, the pH was below 0.5 and the next 2000-2400 mL of effluent was collected. The deionized sol was then polymerized using the procedure outline in procedure 4. Entries 24 and 25 in Table 1 were prepared by this process. Characterization. Surface area and porosity measurements were performed on a Micromeritics ASAP 2010 analyzer using nitrogen adsorption technique at 77 K. Samples were outgassed at room temperature until a constant pressure was achieved. Surface areas were calculated from the adsorption branch of the isotherm in the range of 0.05 e P/P0 e 0.3. Pore volume, pore size, and pore size distributions were calculated from the desorption curves using a Barrett-Joyner-Halenda15 analysis. Bulk densities were determined by compressing a weighed sample of the xerogel in a 1 in. × 4in. graduated cylinder at 10 psi of pressure and measuring the sample volume. Void volumes, or the volume of free space per gram of silica, was calculated by subtracting the skeletal density of the xerogel determined by helium pycnometry from the bulk density (void volume ) 1/Fbulk - 1/Fskeletal). Transmission electron micrographs (TEM) were obtained on a JEOL 2000FX side-entry analytical electron microscope using a lanthanum hexaboride source at an accelerating voltage of 200 kV. The images were recorded photographically on plate film and enlarged to 8 × 10 prints at magnifications of 27 000×, 67 500×, and 135 000×. Solid-state 29Si CP/MAS NMR spectra were obtained on a Varian Unity Plus 400 spectrometer equipped a wide body CP/MAS probe. A spinning speed of either 5 or 6.5 kHz and RAMP cross-polarization was used for quantitation. CHN analysis were obtained on a Perkin-Elmer 2400 Analyzer. SAXS data were collected using a Siemens Anton Paar highresolution camera equipped with a Siemens Hi-Star area detector. The data were radially averaged. The source of the Cu KR radiation was a Siemens M18XHF22-SRA rotating anode generator operating at 35 kV. Silver behenate was used as a low-angle diffraction standard according to the method of Huang et al.16 The xerogel powders were held in 1-mmdiameter glass X-ray capillary tubes, sealed by melting the open glass end. The scattering from similarly sealed glass capillaries containing only air was subtracted from each data set. Scattering data was collected for 1080 s for each sample.
Results and Discussion Synthesis of Treated Xerogels. The general scheme for making silylated silica xerogels is shown in Figure 1. The gel time of sodium silicate depends on the pH, temperature, and silicate concentration. In the first step of the synthesis, sodium silicate is added with rapid agitation to sufficient excess acid to give a final pH of 2. At this pH, the polymerization of silicic acid and the formation of silica gel is at a minimum. The resulting hydrosols are metastable and can be briefly manipu(15) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (16) Huang, T. C.; Toraya, H.; Blanton, T. N.; Wu, Y. J. Appl. Crystallogr. 1993, 26, 180.
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Figure 1. Synthesis of silylated silica xerogels.
lated with only minor changes in composition. After manipulation, additional acid or base is added to the hydrosol. At higher (pH ∼0-1) and lower (pH ∼4-7) acid concentrations, the gel times are much shorter and sols are rapidly converted to three-dimensional hydrogels. The polymerization rate increases with temperature and concentration or total area of silica surface available in the sol. Particle growth and particle aggregation occur simultaneously in both regimes through a series of silanol-silanol condensations in a reactionlimited cluster-cluster growth model. This results in small primary particles (∼3-10 nm) and a highly porous structure in the hydrogel. In an aqueous environment, the silanols on the surface of the hydrogel are fully hydrated by hydrogen bonding. Although this supports the hydrogel’s structure, it also causes the hydrogel structure to collapse if the hydrogel is dried directly from water. To retain the structure of the hydrogel in the dried gel, the mechanical strength or modulus of the gel must be greater than the capillary pressure generated inside the pores during the drying process. For cylindrical pores, the capillary pressure depends on the pore fluid/vapor surface tension, the contact angle that the meniscus makes with the pore wall and the characteristic capillary size which is related to the pore size. Typically, the capillary pressure is minimized by exchanging the pore fluid with another solvent having a lower surface tension and a higher contact angle through a series of solvent extractions. However, sufficient capillary forces can still exist to cause enough shrinkage to allow adjacent silanols within the pores to come into contact and condense, preventing the xerogel from returning to its original shape. Silylation of the pore walls prior to removing the pore liquid prevents this condensation and allows the xerogel to return to its original shape.7 In the second step of the synthesis, the hydrogel surface was directly silylated in the aqueous phase. This was accomplished by adding trimethylchlorosilane or hexamethyldisiloxane (HMDS) directly to the hydrogel
in the presence of a strong acid and a alcohol cosolvent. In the absence of the cosolvent, neither reagent sufficiently treated the silica to allow it to transfer to an organic solvent in a reasonable period of time.17 With monofunctional treating agents, acid concentrations of 10-11 wt % HCl were used. At high acid concentrations (>25 wt %) an appreciable concentration (>0.1%) of trimethylchlorosilane exists in the HMDS/ Me3SiCl equilibrium, and under these conditions, Me3SiCl may be the silylating agent.12 However, under the reaction conditions used here the concentration of Me3SiCl is very low and the actual silylating agent is probably trimethylsilanol on the basis of its solubility in water (3.5 × 104 mg/L at 25 °C),18 and the polar cosolvent acts as a phase-transfer agent by forming micelles capable of stabilizing and carrying the silylating agent to the hydrogel’s surface. To study the effect of the cosolvent, silylations were attempted using a range of alcohols at various concentrations. When methanol or ethanol was used as the solvent for the xerogel, insufficient silylation occurred for the gel to transfer to an organic phase. With higher molecular weight alcohols (n-propyl, n-butyl, tert-butyl, n-amyl, n-hexyl), silylation occurred, and the xerogel readily transferred to an organic solvent. After drying, the carbon content of the xerogel was determined by elemental analysis, and the presence or absence of alkoxy groups on the xerogel surface was determined by digesting the xerogel with aqueous KOH and analyzing the organic phase for the corresponding alcohol by gas chromatography. Both n-propyl and tert-butyl alcohol gave xerogels with a treatment equal to isopropyl alcohol. However, when n-butyl, n-amyl, or n-hexyl alcohol was used as the solvent, 1-7% of the surface treatment on the dried xerogel was due to the alcohol. The presence of alkoxy functionality on the filler surface (17) Tyler, L. J. U. S. Patent 3,015,645, 1962. (18) Mazzoni, S. M.; Roy, S.; Grigoras, S. The Handbook of Environmental Chemistry. In Organosilicon Materials; Chandra, G., Ed.; Springer-Verlag: Berlin, 1997; Vol. 3H, Chapter 3.
Silylated Silica Xerogels
with the higher alcohols is consistent with studies which show that the hydrolysis of alkoxysilanes in acidic media is reversible19 and by the effective hydrophobing of fumed silica with alcohols.20,21 In the last step of the synthesis, the silylated xerogel was transferred to an organic phase by adding an immiscible organic solvent to the aqueous suspension of the silylated hydrogel. This separates the majority of the acid and water-soluble salts from the hydrophobic xerogel. Additional impurities can be removed by carefully washing the organic phase with water or a mixture of water and isopropyl alcohol. In most cases, this is sufficient. However, higher purity materials can be obtained by either of two methods. In the first method, the initially formed hydrosol is deionized by passing the hydrosol through an ion-exchange column in the acid form (procedure 4 in the Experimental Section). This effectively removes >99.9% of the sodium ions from the hydrosol, and the final products have sodium concentrations below 0.01 wt %. Alternatively, the hydrosol can be allowed to gel without agitation. This results in a monolithic body with a certain degree of mechanical integrity, and the gel can be cut into small pieces and washed with deionized water to remove water-soluble salts and acids. However, unless controlled, the pH changes during the washing step and the structure of the hydrogel will be modified accordingly. Examples of this process are given in procedures 2 and 3 in the Experimental Section. During the transfer step, a small amount of water and acid are trapped inside the pores of the silylated gel and transfer into the organic phase. In addition, the unreacted hexamethyldisiloxane is transferred to the organic phase, and some additional treatment may occur in the organic phase. When toluene or excess hexamethyldisiloxane is used as the transfer solvent, the water and acid are removed by azeotropic distillation and completes the conversion of the silylated hydrogel to an organogel. The solvent and excess hexamethyldisiloxane are then removed by evaporation. When shear or mechanical agitation is applied during the polymerization step (either continuously as in procedure 1 or after the gel has formed as in procedures 2 and 3) the hydrogel is continually attrited. As a result, removal of the solvent at the end of the process gives a free-flowing powder. The hydrosols can also be modified by the addition of colloidal silicas. Under the reaction conditions used to polymerize the sol, the particle sizes of the colloidal silicas are maintained. As a result, treated xerogels with lower surface areas and a bimodal particle size distributions are obtained. This can be seen by comparing entries 23, 3, and 7 in Table 1. All three samples were prepared by polymerizing a hydrolysate at an acid concentration of 9.8-11 wt %. The surface area and pore volume of the unmodified xerogel (entry 23) were 484 m2/g and 3.54 cm3/g, respectively. Modification of the hydrogel with 7.5 nm colloidal particles (entry 3) reduced both the surface area and pore volume to 384 (19) Sanchez, J.; McCormick, A. J. Phys. Chem. 1992, 96, 8973. (20) Ou, Y.-C.; Yu, Z.-Z.; Vidal, A.; Donnet, J. B. J. Appl. Polym. Sci. 1996, 59, 1321. (21) Iler, R. The Chemistry of Silica; John Wiley & Sons: New York, 1979.
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Figure 2. The effect of reaction time and pH on the surface area of silylated xerogels (reaction temperature was 100 °C): pH ) 4.0 ((); pH ) 5.0 (+); pH ) 6.0 (2), pH ) 7.0 (O), and pH ) 8.0 (9).
m2/g and 2.56 cm3/g, respectively, even though a shorter aggregation time was used. Even further reductions were observed when a 12-nm colloidal silica particle was used (entry 7). Effects of Hydrogel Aging Conditions on the Structure of the Isolated Xerogel. To determine the effect of pH and time on the surface area of a treated xerogel, a series of hydrogels were prepared from a hydrosol (0.08 g of SiO2/mL) aged at reflux for 24 h at different pH values (4.0, 5.0, 6.0, 7.0, and 8.0). Aliquots of each hydrogel were removed and silylated after 2, 4, 7, and 24 h. The surface areas of the treated silica xerogels were measured by BET analysis. As shown in Figure 2, during the early stages of polymerization, the surface area decreased rapidly as the silica particles precipitated and the gel formed. The effect of pH on this process was consistent with the gel times of silica sols reported by Iler.21 Between a pH of 4.0 and 7.0, the surface area decreased as the pH increased. However, between a pH of 7 and 8, the trend began to reverse and the polymerization was actually a little slower at a pH of 8.0. The pH values shown in Figure 2 are the initial values at the beginning of the reaction. During the first few hours of the polymerization, the pH typically increased by 0.5-1 as the number of silanol species diminishes. Although the reaction time varied, it was possible to prepare xerogels with surface areas between 300 and 700 m2/g at each pH. Under strongly acidic conditions, the surface area also decreased with time and temperature although the rates were substantially slower. To compare the evolution of structure under the different sets of reaction conditions, several sets of samples (entries 8/13, 2/3, 21/23, and 6/7) were aged at 100 °C under both acid (pH ) 1.5 to ∼0) and neutral conditions (pH ) 4.0-7.4). In general, the surface area and total pore volume decreased much more rapidly in hydrogels aged at lower acid concentrations. This was accompanied by the formation of larger pores. It is important to note, however, that materials with similar surface areas and pore volumes could be obtained from both routes by adjusting the reaction temperature and time. Capture of Hydrogel Structure by Silylation. To determine how much structure was lost during the treatment step, a series of hydrogels were prepared and
1280 Chem. Mater., Vol. 11, No. 5, 1999
Burns et al. Table 2. Physical Property Comparison of Untreated and Treated Xerogels pore pore BET surface volume diameter density 2 3 area (m /g) (cm /g) (Å) (g/cm3)
sample untreated xerogela posttreated xerogelb in-situ treated xerogelc calcd values for posttreated sample
650 485 482 493
2.25 2.11 3.24 NA
91 99 166 NA
1.98 1.70 1.66 1.68
a Untreated xerogel made by converting a hydrogel to an alcogel and drying the alcogel from toluene. b Posttreatment of the dried, untreated xerogel. c In situ treatment of a hydrogel using the treating procedures outlined in the Experimental Section
Figure 3. The affect of treatment on the surface area of untreated hydrogels and the corresponding silylated xerogels: surface area of the hydrogel measured by a Sears titration (b); BET surface area of the corresponding xerogel after silylation (O); calculated surface area of the hydrogel available to nitrogen probe (9); and the calculated surface area for a hydrogel of a hydrogel with the corresponding Sears surface area after treatment assuming a particle coordination number of 4 (+).
their surface areas determined by a Sears titration.22 The hydrogels were then silylated and isolated using the procedure outlined in the Experimental Section. The surface areas of the dried powders were determined by nitrogen adsorption using the method of BrunauerEmmett-Teller (BET).23 The two sets of surface areas are summarized in Figure 3. Although there was a good correlation at low surface areas, the two sets of data diverge as the surface area increases. Part of this divergence is a consequence of the two methods used to determine the surface area and the size of the probe molecule. For discrete, spherical particles, the surface area varies inversely with the particle radius according to the following equation:24
surface area )
3 × 103 rd
(1)
where r is the particle radius (in nm), and d is the particle density (in g/cm3). However, any contact or coalescence between the particles causes a decrease in the observed surface area in proportion to the particle radius and the number of contacts of each particle with other particles. When nitrogen is used as the adsorbate, the fraction of the surface area unavailable to the nitrogen probe is given by
Lf ) 0.0885n
r (r + 0.177)2
(2)
where n is the coordination number of the primary particle and 0.177 is the radius of a nitrogen molecule.21 By assuming no loss of surface area in a Sears titration, an estimate of the primary particle size, r, in the hydrogel can be obtained from eq 1. This can then be used to calculate a “nitrogen equivalent” surface area for the hydrogel at different values of n using eq 2. As (22) Sears, G. W. Anal. Chem. 1956, 28, 1981. (23) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (24) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Studies in Surface Science and Catalysis 93; Elsevier: Amsterdam, 1995; Chapter 1.
shown in Figure 3 when a coordination number of 4 is used the values for the two sets of data are nearly parallel. However, silylation of the particle surface also increases the effective radius of the primary particle by 0.4 nm, the thickness of a trimethylsilyl layer. With large particles, r . ∆r, so the contribution of the treatment layer to the particle radius is small and can be ignored. However, as the particle size decreases, ∆r becomes increasingly significant and a larger difference between the surface area of the treated and untreated particle occurs. On the other hand, silylation of the surface can also increase the theoretical surface area if the particle density is sufficiently lowered. These two effects were approximated by calculating the density of a treated hydrogel using the particle radius obtained from eq 1 for the untreated xerogel and a coordination number of 4.25 The surface area of each hydrogel after treatment was then calculated.26 A plot of this surface area is also shown in Figure 3, and the trend is consistent with the measured surface areas. This suggests that very little surface area is lost during the treatment step. To further test this hypothesis, a hydrogel was made by polymerizing a deionized sol under acidic conditions (11% HCl) for 3 h. A portion of the hydrogel was converted to an organogel by washing the gel with isopropyl alcohol until the gravity of the effluent was the same as isopropyl alcohol (0.785 g/cm3) and then exchanging the isopropyl alcohol with toluene until the specific gravity of the effluent was the same as toluene (0.861 g/cm3). Removal of the toluene gave an untreated xerogel, a portion of which was posttreated using the same reaction conditions used to treat the hydrogels. Another portion of the hydrogel was treated in situ. All three samples were analyzed for surface area, pore volume, pore size, and density. The results are summarized in Table 2. The surface area of the untreated (25) The density was calculated using the equation
d ) 0.763 +
1.437(r - c)3 3
r - (n/4)c2(3r - c)
where r is the particle radius of the untreated hydrogel, n is the coordination number, and c is the thickness of the trimethylsilyl layer (0.4 nm). A core-shell model was assumed with a density 0.763 g/cm3 for the trimethylsilyl layer and a core density of 2.2 g/cm3. (26) The surface area was calculated from the following equation with n ) 4:
12r1[2r1 - nc] × 103 d[8r13 - 2nc2(3r1 - c)] where r1 is the radius of the silylated particle (r + 0.4). surface area )
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Table 3. Analysis of Xerogels by Small-Angle X-ray Scattering entry no. 14 15 16 18 19 20 21 21
aging conditions T (°C) t (h) pH 25 25 25 100 100 100 100 100
3.5 8.0 24.0 0.01 0.5 2.0 3.0 3.0
4 4 4 4 4 4 4 ∼0
slope in region: Porod low q -3.3 -3.8 -4.1 -4.2 -4.3 -4.4 -4.7 -4.6
-2.1 -2.1 -2.1 -1.8 -2.0 -1.8 -1.7 -2.0
crossover (nm) 0.77 0.66 0.74 0.78 0.86 1.1 1.2 0.9
Figure 4. Relationship among density, bet surface area, and M/Q ratio.
xerogel was 650 m2/g which corresponds to an equivalent primary particle size of 2.3 nm. By using the prior analysis, the calculated surface area after treatment is 493 m2/g, which is in excellent agreement with the measured value of 485 m2/g. Likewise, an excellent agreement was obtained between the calculated density (1.68 g/cm3) based on a core shell model and the measured density of 1.70 g/cm3. Significantly, the total pore volume, pore size distribution, and pore diameter in the untreated and posttreated samples were essentially identical. This suggests that the treatment process did not cause extensive collapse or rearrangement of the xerogel structure. However, when the initial hydrogel was treated in situ the resulting xerogel had a substantially higher pore volume and pore diameter than the untreated hydrogel. Since no structure was lost when the untreated xerogel was posttreated, structure must have been lost when the untreated hydrogel was converted to an organogel or when the untreated organogel was dried. By these comparisons, in situ treatment coupled with a solvent transfer and drying process, retains both the surface area and the structure of the hydrogel. Characterization of Treated Xerogels. Analysis by DRIFTS and 29Si CP/MAS NMR. The distribution and type of silanols on the xerogel surface was investigated by a combination of solid-state 29Si CP/MAS NMR and DRIFTS-FTIR. For this study, a series of treated xerogels prepared at both room temperature (entries 14-17) and at reflux (entries 18-22) at a pH of 4.0 were used. The untreated xerogel described earlier (see Table 3) was used as the control. In the CP/MAS spectra of the untreated xerogel, peaks corresponding to Q2 (-90.6 ppm), Q3 (-99.8 ppm), and Q4 (-112 ppm) species were observed in a qualitative ratio of 12:60: 29, respectively. This is consistent with the Q2, Q3, Q4 distribution found on the surface of an untreated silica gel.27 After treatment, the Q2 peak disappeared and the ratio of Q3:Q4 decreased. The Q3 peak was assigned to hydrogen-bonded silanols on the basis of the DRIFTSFTIR spectra which showed only the presence of hydrogen bonded silanols (no free or isolated silanols). Within the series of xerogels made at both room temperature and reflux, the relative Q3 content remains constant and only the ratio of M/Q4 (Me3SiO0.5/SiO2) ratio decreases as the surface area decreases. This is expected since as the surface area decreases, the total number of silanols per unit volume decreases. In addition, silylation occurs in the aqueous phase where all silanols are hydrogen-bonded and equally reactive. As
a result, the surface coverage is uniform and a linear relationship between surface area, density, and M/Q ratio is found as shown in Figure 4. Analysis of Microstructure by Nitrogen Adsorption and TEM Analysis. The surface area, pore volume, pore size, and pore size distribution were determined from nitrogen adsorption using the techniques of Brunauer, Emmet, and Teller23 (BET). The results are summarized in Table 1. All of the silylated xerogels exhibit a lower adsorption at low pressure and a monotonic increase in adsorption with increasing relative pressure characteristic of type IV isotherms.28 The isotherms also exhibit the characteristic hysteresis during desorption attributed to the existence of pore cavities larger in diameter than the openings leading into them. The nitrogen adsorption values were very high, ranging from 1500 to 2200 cm3/g. This was true even when colloidal silicas were used to reduce the surface area and structure. The adsorption values are comparable to those found for aerogels.1,21 The total pore volume was determined from the nitrogen desorption curve. However, previous studies have suggested that pore volumes and pore size distributions measured by nitrogen adsorption techniques may be inaccurate for mesoporous materials since most of the adsorption occurs near a P/P0 value of one. As such, the pore volume and pore size distribution data listed in Table 1 should be considered qualitative. For this reason, the porosity was also determined by the difference between the reciprocal of the true density measured by helium pycnometry and the reciprocal of the bulk density measured under compression (porosity ) Fbulk-1 - Ftrue-1) and in select cases, by oil absorption measurements. The porosity or void volume measured by these techniques is a combination of the internal pore volume and the interstitial volume between particles. For the samples aged at reflux, a linear relationship (r2 ) 0.96) between the latter two techniques was observed. The relationship between the total pore volume and the void volume is shown in Figure 5. Within the series of materials studied, the relationship was fairly linear, although the two values were approximately offset by a factor of 1.6.28 The evolution of porosity and structure as a function of time and temperature was studied by aging a series of hydrogels at a pH of 4.0 at both room temperature (entries 14-17) and reflux (entries 18-22). Between a pH of ∼3-6, silicic acid initially polymerizes to discrete
(27) Liu, C. C.; Maciel, G. E. J. Am. Chem. Soc. 1996, 118, 51035119.
(28) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity; Acdemic: London, 1967.
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Figure 5. Relationship between pore volume and void volume.
particles less than 4 nm in diameter which then aggregate into gels.29 Although the gel structure begins to develop at this point, oligomers and polymers of silicic acid and nonaggregated and partially aggregated particles are still present. At this point, the theoretical surface area of the various species is quite high (>2000 m2/g). However the measured surface area and porosity are much lower because of the precipitation of the smaller particles into the pores of the hydrogel during the isolation process. As the gel continues to age and the concentration of the lower molecular weight species decreases, the surface area decreases, and the measured values are more representative of the hydrogel’s structure. This effect was observed in the hydrogels aged at room temperature. Under these conditions, the surface area remained essentially unchanged for the first 24 h of reaction and then slowly decreased. However, during this time, the pore volume and average pore diameter noticeably increased (entries 14 and 15) as fewer discrete species were present. When hydrogels were aged at reflux the surface area rapidly decreased during the first 3 h (entries 18-21). This was accompanied by an increase in the total pore volume and pore diameter. After the initial 3 h, further changes occurred but at a much slower rate. Because of the large rate differences at the two temperature extremes, the effect of time and temperature on the microstructure is best illustrated by comparing the adsorption and desorption curves for treated xerogels made from hydrogels aged at room temperature (entry 14) and reflux (entry 21) for the same length of time. As shown in Figure 6, during aging a clear coarsening of the pore structure is seen. This accompanied by an increase in the total pore volume and a slight narrowing in the pore size distribution due to the presence of fewer macropores. The adsorption-desorption isotherm for a treated xerogel made from a deionized hydrogel aged under strong acid conditions (∼11 wt % HCl, entry 23) is shown in Figure 7. Compared to a hydrogel aged at reflux at a pH of 4.0, the hydrogel aged under acid conditions gave a xerogel with a slightly higher total pore volume and a wider distribution of pores in the size range of 150-450 Å. This is the same trend observed with longer aging times. A representative TEM micrograph of a treated xerogel is shown in Figure 8. As in the case with nitrogen adsorption studies, a coarsening of the pore structure (29) Coudurier, M.; Baudru, R.; Donnet, J. B. Bull. Soc. Chim. Fr. 1971, 9, 3161.
Burns et al.
was observed with either increases in aging time or temperature. The average size of the primary particles ranged from 4 to 8 nm, even under highly acidic conditions. Previous studies have shown that the primary particle size depends almost solely on the pH used to polymerize the sodium silicate solution, with larger primary particles formed at higher pH.21 In this study, nearly equivalent primary particle sizes were found over a wide pH range by varying the time and temperature used to polymerize the hydrosol. In general, increases in primary particle sizes were observed with increases in reaction time or temperature at a constant pH. The measured values were in good agreement with the primary particle sizes calculated from the experimentally determined densities using ref 17 and solving for the particle radius (see Table 1). Surface areas were also calculated from the carbon analysis, assuming a surface coverage of 0.38 nm2 for a trimethylsilyl group (or 2.63 trimethylsilyl groups per nm2).30 In general, the surface area calculated from the carbon analysis was within 10% of the measured value although the deviation increased with increasing surface area. In principle, the measured surface area should be lower since aggregation or coalescence of spherical particles reduces the available surface area. However, the experimentally determined surface areas were often higher than the calculated model suggesting other factors such as the presence of some pores only accessible to the nitrogen probe. Analysis of Porosity and Structure Evolution by Small-Angle X-ray Scattering (SAXS). The evolution of porosity and microstructure during polymerization was also studied by analyzing a series of xerogels made at a pH of 4.0 at both ambient and reflux temperatures by small-angle X-ray scattering. In addition, a xerogel made from a deionized sol gelled and aged for 3 h at reflux at a pH of ∼0 was also analyzed to determine the effect of pH. Log plots of the scatter intensity versus the scattering vector, q [) 4 π/λ(sin θ/2), where λ is the X-ray wavelength, and θ is the scattering angle] were used to calculate of the slopes in the more-or-less linear regions of the plots and the crossover point, q-1 nm, obtained from the extrapolated intersections of the two linear regions. The results are summarized in Table 3. The slopes are ascribed to either the “low q” region (down to q-1 of about 7 nm), or to the Porod law region (up to q-1 of about 0.3 nm). For the series of samples aged at 25 °C, at high q there is little indication of the slope leveling off, which shows that the features comprising the aggregates remain smaller than a nanometer or so in size. At low q there is no indication of a leveling off in the intensity, or a change to a different scaling character. As such, there is no indication of an upper limit to the cluster size, at least up to a scattering vector of 7 nm which is the upper length scale accessible in these experiments. Tangents at low q have a slope of about -2.1, a value typical for relatively unaged silica gels, and consistent with structures formed via a cluster aggregation mechanism.31,32 The slopes in this region change very little with aging times up to 24 h. However, in the high q region the slopes change from about -3.3 to about -4.1, (30) Bondi, A. J. J. Phys. Chem. 1964, 68, 441.
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Chem. Mater., Vol. 11, No. 5, 1999 1283
Figure 6. Representative adsorption/desorption curves for treated xerogels prepared by aging hydrogels at room temperature (4) and at reflux (O) at a pH of 4.0. Open markers (4, O) are adsorption isotherms and filled markers (2, b) are desorption isotherms. Aging times were 3.5 h for the sample aged at room temperature (entry 14) and 3 h for the sample aged at reflux (entry 21).
Figure 7. Representative adsorption/desorption curves for treated xerogels prepared by aging hydrogels at reflux under strong (O) and weak (4) acid conditions. Open markers (O, 4) are for adsorption isotherms and filled markers (2, b) are the desorption isotherms. Both hydrogels were refluxed for 3 h.
the latter value being close to that expected (-4.0) for particles or features with smooth surfaces (but see the results below for the xerogels aged at reflux). As the aging increases, a crossover region becomes resolved. Together these observations are consistent with the slight decrease in surface area observed in the gas adsorption experiments. The changes in the scattering pattern during aging at reflux are more pronounced both in the high and low q regions. At the shortest length scales accessible, a leveling off of the intensity becomes apparent. At q values just below this, the slopes become more negative (changing from about -4.2 to about -4.7). Such slopes are well above what would be expected from Porod law behavior (slope of -4) and are attributed to the effect of a gradation in electron density associated with the trimethyl groups attached to the silica framework. This effect has been observed in related systems and has (31) Wijnen, P. W. J. G. A Spectroscopic Study of Silica Gel Formation from Aqueous Silicate Solution. Ph.D. Thesis, Eindhoven University of Technology, 1990.
been discussed in terms of a “fuzzy boundary” by Hua et al.33 In the lower q region, slope changed for short aging time (0.5 h) from about -1.8 to -2, but thereafter decreased to -1.7 after 3 h. As aging proceeded, a crossover region became more pronounced, and shifted to lower q (from about 0.8 to about 1.2 nm). These changes are consistent both with a gradual increase in the density of the smaller scale structures, or “cores”, at the expense of the more diffuse parts of the clusters, as would occur by Ostwald ripening (the process of movement of material from regions of high local curvature to regions of lower curvature), and an increase in their size. That there appears to be a decrease in the slope (or increase in fractal dimension) at short aging times may indicate that material is moving from structures outside the q range observed (32) Dokter, W. H. Transformation in Silica Gels and Zeolite Precursors. Ph.D. Thesis, Eindhoven University of Technology, 1994. (33) Hua, D. A.; D’Souza, J. V.; Schmidt, P. W.; Smith, D. M. Characterization of Porous Solids III; Elsevier Science: New York, 1994; p 255.
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be said about the correlation of these differences and those seen in the gas adsorption experiments. The processes involved in scattering and adsorption are quite different. In particular, changes in the structures of such tenuous materials from capillary condensation during adsorption would be expected, as well as there being differences in the length scales being addressed by the two techniques. Conclusions
Figure 8. Transmission electron micrograph of a treated xerogel prepared by aging a hydrogel for 3 h at 100 °C and a pH of 4.0 (entry 21).
in these experiments so as to reinforce the “intermediate” structure. Later, these intermediate structures are affected by the same process, and themselves become more tenuous, although not to the extent that their mechanical properties reduce so much that more porosity is lost during drying compared to with the initial structures. Again the densification and smoothing of the “cores”, and increase in their size, would lead to the reduction in surface area observed by gas adsorption and TEM analysis. The scattering patterns for the gels aged for 3 h at reflux at a pH of 4 and ∼0 were similar in shape to each other except for small differences in the low q region (slopes of -1.7 and of -2, respectively) and the crossover point (1.2 and 0.9 nm, respectively). Little, however, can
Hydrogels are readily silylated under acidic conditions in the presence of isopropyl alcohol. As a result, the capillary pressure generated inside the pores during the drying process is reduced and most of the structure present in the hydrogel is retained in the silylated xerogel. In addition, since the silylation is done in the aqueous phase where all silanols are hydrogen-bonded, silylation is nonsilanol specific and occurs evenly over the available silica surface. This is reflected in the linear relationship among surface area, density, and M/Q ratio. Over the years numerous methods have been used to modify or polymerize hydrogels. In this paper we have shown that several of the more common techniques, such as time, temperature, pH, deionization, and sol modification with colloidal silicas, can be used in conjunction with in situ silylation to make treated xerogels with a wide range of surface areas and structure. Although all of the examples used trimethylchlorosilane or hexamethyldisiloxane as the treating agent, a wide variety of other silanes can be used to generate functional surfaces. Acknowledgment. The authors would like to thank J. Preston, D. Barron, and S. Winchell for their help in generating the analytical data, J. Stasser for providing the transmission electron micrographs, Dr. B. Zhong for assistance on the nitrogen adsorption technique, Dr. S. Hu for providing the solid state 29Si NMR data, C. Gould for providing the surface functionality analysis, K. Polmanteer for his help in calculating surface areas and densities for aggregated particles, and Dr. S. R. Wilson from the Department of Chemistry at University of Illinois in Urbana for making the small-angle X-ray measurements. CM981037+
