4 Sol-gel Silica: Nanoarchitectures of Being and Nothingness




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Created by Margret J. Geselbracht, Reed College (mgeselbr@reed.edu) and posted on VIPEr (www.ionicviper.org) on July 29, 2009. Copyright Margret J. Geselbracht 2009. This work is licensed under the Creative Commons Attribution Non-commercial Share Alike License. To view a copy of this license visit http://creativecommons.org/about/license/.


4
Sol-gel Silica: Nanoarchitectures of Being and Nothingness

I. Background

A very common and simple method of synthesizing mixed metal oxides is the “heat and beat” method of solid state synthesis. This method generally leads to highly crystalline materials with crystallite sizes on the order of microns (10-6 m). The recent explosion of interest in nanoscience has proven that the properties of many materials may differ greatly when they are structured on the nanometer scale as compared to the bulk crystalline versions. One such class of nanostructured materials includes aerogels, ambigels, and xerogels.

Aerogels are low density, high surface area, porous materials that are unique “composites of being and nothingness.”1 The quintessential aerogel material is silica, SiO2, but over the past several decades, numerous transition metal oxides have been prepared in the aerogel form. In an aerogel, nanoscale oxide particles (“the being”) connect in three dimensions to surround a continuous void space of pores, (“the nothingness”). The predominance of interconnected mesopores (pore sizes > 2 nm) in aerogels facilitates rapid mass transport of molecules and ions throughout the nanoarchitecture, enabling dramatic improvements in real-world applications such as sensors, catalysts, and electrochemical devices.

Synthesis of these nanostructured gels requires significantly milder conditions and temperatures than we encountered in the “heat and beat” method. Wet gels in which the pores are filled with a fluid can be synthesized by a number of different so-called “sol-gel” routes involving the controlled hydrolysis and condensation of molecular precursors. For example, in this experiment, we will explore the hydrolysis and condensation of silicon alkoxides. As shown in Figure 4.1, different methods of drying the wet gel lead to xerogels, ambigels, or aerogels with increasing surface areas and porosities. Allowing the native solvent to evaporate from the wet gels under ambient pressure leads to partial collapse of the pores due to the capillary pressures at the gas/fluid interface and xerogels of moderately low surface area and porosity. Alternatively, first exchanging the pore fluid with liquid CO2 and then drying under supercritical conditions where there is no distinction between gas and liquid phases and thus no capillary pressure leads to aerogels that retain the large pores characteristic of wet gels. In the intermediate regime, exchanging the initial pore fluid with a nonpolar, low surface tension, organic solvent such as cyclohexane, followed by drying at ambient pressure results in ambigels that retain 80-90% of the porosity of aerogels without the need for supercritical drying.

In this lab experiment, you will make a series of silica gels via either acid-catlalyzed or base-catalyzed hydrolysis and condensation of tetraethylorthosilicate (TEOS), Si(OCH2CH3)4 or tetramethylorthosilicate (TMOS), Si(OCH3)4. We will attempt to make pH sensors from these silica gels by incorporating a pH sensitive dye within the gel network. Different gel microstructures are obtained from either acid-catalyzed or base-catalyzed conditions, and we will use the visual response of the dyes to explore and probe these morphological differences.




Figure 4.1. Hydrolysis and condensation of molecular precursors results in a wet gel (shown on left) composed of a three-dimensional network of nanometer-scale oxide particles surrounding an interconnected network of pores that are filled with fluid, typically the solvent. Allowing the pore fluid to evaporate under ambient pressure leads to pore collapse and a highly densified xerogel (top, right). Exchange of the pore fluid with a low polarity solvent followed by evaporation leads to minimal pore collapse and an ambigel (middle, right) that retains 80-90% of the porosity of the wet gel. Exchange of the pore fluid with liquid CO2 followed by supercritical drying leads to an ultra high porosity, low density material known as an aerogel (bottom, right).

II. Sol-gel synthesis of silica

Silicon alkoxides are quite prone to hydrolysis reactions and subsequent formation of silica, SiO2. For example, equation 4.1 shows the hydrolysis of tetraethylorthosilicate (TEOS), Si(OCH2CH3)4 to initially form a silanol (molecule containing Si–OH).

Si(OCH2CH3)4 + H2O –––––> Si(OCH2CH3)3OH + CH3CH2OH (4.1)

Hydrolysis of the silanol can continue further, or more likely, condensation reactions of two silanols (or a silanol plus an alkoxide) will occur resulting in a bridging oxygen that links two silicon atoms together. Equation 4.2 shows an example of a condensation reaction between two silanol molecules.

Si(OCH2CH3)3OH + Si(OCH2CH3)3OH –––> (CH3 CH2O)3Si–O–Si(OCH2CH3)3 + H2O (4.2)

Further hydrolysis and condensation reactions continue to add silicon atoms and grow the polymer chain. In solution, these reactions begin simultaneously at many different locations leading to the formation of numerous growing silicon oxide/alkoxide particles. The so-called sol stage consists of a dispersion of these colloidal particles in the solvent, where the particles range from 1-100 nm in size. Over time, the particles continue to grow until they finally link in three-dimensions forming a rigid gel network that encompasses pores of trapped fluid.

The mechanisms of hydrolysis and condensation reactions of silicon alkoxides involve bimolecular nucleophilic substitution reactions that can occur under either acid- or base-catalyzed conditions. Under acid-catalyzed conditions, the rate of hydrolysis is slow compared to the rate of condensation, which leads to longer polymer chains in the sol that continue to grow and entangle, occasionally cross-linking, until the gelation point is reached (Figure 4.2). In contrast, under base-catalyzed conditions, hydrolysis occurs rapidly, resulting in more highly branched clusters in the sol that do not readily interpenetrate but link up like a pearl necklace at the gelation point. One way to think about the different microstructures of the two types of gels is to imagine a plate of spaghetti (acid-catalyzed) vs. a plate of tumbleweeds (base-catalyzed).2
Removed for copyright reasons

Please see:

C. A. Morris, D. R. Rolison, K. E. Swider-Lyons, E. J. Osburn-Atkinson,
and C. I. Merzbacher, J. Non-Cryst. Solids, 2001, 285, 29-36.
Figure 4.2. Cartoons representing the microstructural differences between acid-catalyzed and base-catalyzed silica gels. Figure adapted without permission from C. A. Morris, D. R. Rolison, K. E. Swider-Lyons, E. J. Osburn-Atkinson, and C. I. Merzbacher, J. Non-Cryst. Solids, 2001, 285, 29-36.

A variety of characterization studies of acid-catalyzed vs. base-catalyzed silica gels reveal the distinct microstructural differences in these two types of materials. These differences lead to very different properties for the two types of gels.3 For example, the scattering of visible light, whether or not the gel appears transparent or opaque, is a direct consequence of the nanoscale structural features in the gel.4 When examined by transmission electron microscopy (TEM), acid-catalyzed gels derived from the highly entangled polymer chains exhibit a very fine microstructure and very small pores that appear quite uniform throughout the material. 5 In contrast, individual silica particles (~10 nm) can be resolved in TEM images of base-catalyzed gels, and the pore sizes are generally larger. The surface chemistry of the two types of gels also differs. The surfaces of acid-catalyzed gels are richer in silanols (–SiOH) evidenced by a greater loss of water upon heating to 250˚C in thermal gravimetric analysis (TGA) experiments.Error: Reference source not found



III. Painting the Walls of the Nanoarchitecture: Building a pH Sensor

To illustrate the ability of small molecules and ions to diffuse in and out of the pores of the silica gels, we are going to attempt to incorporate pH sensitive dyes within the gel nanoarchitecture, to paint the interior walls, if you like. The plan is to add small amounts of the dyes to the sol before the sol-gel transition. Ideally, the dye molecules will chemisorb (form an electrostatic or H-bonding interaction) to the surface of the sol particles, and after gelation the dyes will then be trapped within the pore network. The behavior of these dye-modified gels will be monitored while exposing the gels to acidic and basic vapors and also to acidic and basic solutions to examine their utility as pH sensors in either the gas or liquid phases.

The structures of two of these pH sensitive dyes, Congo Red and Bromothymol Blue, are shown below in Figure 4.3. Notice that both of these dyes are sodium salts and include at least one negatively charged sulfonate group (–SO3). These features enhance the solubility of the dye in aqueous solutions. Notice also the high degree of conjugation in these structures; this results in highly colored molecules. For each dye, the conjugate base is one color while the protonated form is a distinctly different color. These dyes have slightly different pKa values and so exhibit these color changes over different pH ranges. We should also keep in mind that these dyes are different sizes and shapes and so may differ in their propensity to remain trapped within the gel network particularly when exposed to solution.





Figure 4.3. The structures of the sodium salts of the pH sensitive dyes Bromothymol Blue (top) and Congo Red (bottom).

According to the literature,Error: Reference source not found base-catalyzed silica gels that were modified with Congo Red suffered from bleeding when the dye-modified gels were exposed to low pH solutions. Protonation breaks the H-bonding interaction of the dye to the gel surface and particularly in the case of base-catalyzed gels with larger pore volumes, the dye molecules then diffuse outward into solution through the pore network. This leaching could be prevented by further modification of the gel, through the formation of a gel-in-gel composite structure. Pieces of the dye-modified base-catalyzed gel were re-immersed in an acid-catalyzed sol, which was then allowed to gel around these pieces. The smaller pore volumes of the acid-catalyzed gel effectively trapped the large Congo Red molecules within the base-catalyzed portion while still allowing acids and bases to access the dye for sensing purposes.

We are going to attempt to repeat this chemistry for Congo Red as well as extend the work to Bromothymol Blue. We will be synthesizing our silica gels from tetraethylorthosilicate (TEOS) rather than the tetramethylorthosilicate (TMOS) that was used in the literature. The outcome will likely be that our gels may not be as transparent as the gels prepared from TMOS. The first step will be to prepare base-catalyzed silica gels (both pristine and dye-modified) this week. Next week, we will prepare acid-catalyzed silica gels and try some gel-in-gel composites.

IV. Experimental Procedures

Pre-lab calculation:

In your lab notebook, calculate the molar ratio of TEOS : EtOH : H2O used in this experiment both for Week 1 and for Week 2; they are slightly different for the acid-catalyzed vs. base-catalyzed procedures. The densities of ethanol and TEOS can be easily found online or in the Aldrich catalog.



A. Week 1: Base-catalyzed sol preparation

Using a graduated cylinder, measure 12.0 mL of absolute ethanol into an Erlenmeyer flask (probably about a 250 mL sized flask). Use gloves to handle the tetraethylorthosilicate (TEOS). In a fume hood, measure out and add 15.0 mL of TEOS, Si(OCH2CH3)4, to the ethanol. Add a stir bar and begin stirring the alkoxide solution. Record observations in your lab notebook.



Base-catalyzed reaction:

Use a graduated cylinder to measure 10.5 mL absolute ethanol and 21 mL deionized water into a small beaker. Using a micropipette, deliver 85 µL of conc. NH3(aq) and 360 µL 0.5 M NH4F(aq) into the beaker. While stirring, carefully pour this aqueous basic solution into the ethanol/alkoxide solution. Record observations in your lab notebook. Are the solutions immediately miscible? Does the temperature rise upon mixing? If so, record the temperature. Using pH paper, measure and record the pH of the reaction mixture.

Continue to stir the reaction mixture vigorously for 1-2 hours to thoroughly mix the layers. Record in your notebook how long you must stir to reach the point of miscibility. Meanwhile, prepare 5 small polypropylene centrifuge vials to receive the sol.

When the sol is completely mixed, divide the liquid equally between the 5 polypropylene centrifuge vials. Use ~10 mL sol in each.



  1. Use a micro spatula to weigh out two portions of a tiny amount of solid Congo Red powder and add each portion to two of the centrifuge vials. Record in your notebook how much of the dye you add. Mix vigorously with the silica sol and note whether all of the dye dissolves in the sol.

  2. Use a micro spatula to weigh out two portions of a tiny amount of solid Bromothymol Blue powder and add each portion to two of the centrifuge vials. Record in your notebook how much of the dye you add. Mix vigorously with the silica sol and note whether all of the dye dissolves in the sol.

  3. The fifth centrifuge vial will contain the pure silica sol.

For now, you will not use the centrifuge vial caps. Cover each vial tightly with Saran Wrap and rubber bands, poking 1-2 small holes in the Saran Wrap with a syringe needle. Leave the vial containing pure silica sol in a beaker in your lab drawer to gel at room temperature. Put the other four vials in a well-labeled beaker in the drying oven to gel at 60˚C.

Important: If at all possible, come back to check on your samples after 2 days (on Monday for the Friday lab folks). Hopefully, they should have gelled in this timeframe. If they have gelled, remove the 4 vials from the oven, cap them tightly after removing the Saran Wrap and store in your lab drawer to continue to age at room temperature until your next lab meeting.

B. Week 2: Washing gels, pH sensors, and gel-in-gel composites

Retrieve all of your gels and record detailed observations on their appearance in your lab notebook. Are the gels in the form of a single monolith in the shape of the vial? Did the gels shrink at all? If so, can you estimate the size of the gels? Is there any visible solvent remaining? Are there any cracks in the gels? Are they transparent or opaque? What color are they?



Acid-catalyzed sol-gel silica prep:

Using a graduated cylinder, measure 12.0 mL of absolute ethanol into an Erlenmeyer flask (probably about a 250 mL sized flask). Use gloves to handle the tetraethylorthosilicate (TEOS). In a fume hood, measure out and add 15.0 mL of TEOS, Si(OCH2CH3)4, to the ethanol. Add a stir bar and begin stirring the alkoxide solution. Record observations in your lab notebook.

Use a graduated cylinder to measure 19 mL deionized water into a small beaker and mix in 2-3 drops of concentrated HCl with a disposable pipet. While stirring, carefully pour this aqueous acidic solution into the ethanol/alkoxide solution. Record observations in your lab notebook. Are the solutions immediately miscible? Using pH paper, measure and record the pH of the reaction mixture. Adjust to pH 3 by adding more acid, dropwise, if necessary.

Continue to stir the reaction mixture vigorously for 1-2 hours to thoroughly mix the layers. Record in your notebook how long you must stir to reach the point of miscibility. Meanwhile, work on the next steps.



Gel-in-gel composite:

We are going to attempt to prepare gel-in-gel composites where we take small pieces of the dye-modified base-catalyzed silica gels that you made last week and put them into the acid-catalyzed sol so that the acid-catalyzed gel forms around the base-catalyzed gel pieces.



Washing gels:

You will want to first rinse each base-catalyzed gel twice in acetone. Use separate large beakers for each type of gel. Use acetone or a spatula to gently transfer the gel from the centrifuge vial to the beaker. Cover completely with acetone. Allow the gel to rest in the acetone for 15 minutes. Decant off the acetone and then replace with a second batch of fresh acetone for another 15 minutes. Note any observations in your lab notebook. Does any of the dye seem to leach out of the dye-modified gels into the acetone wash solution? The acetone rinses go in the organic solvent waste jug.

You will want to divide your dye-modified gels into at least 3 pieces. If the monoliths have broken, that is fine. Otherwise, gently cut them into several pieces.

Gel-in-gel composite:

Transfer pieces of each of the two dye-modified base-catalyzed gels into separate clean, dry polypropylene centrifuge vials. Pour the acid-catalyzed sol over the gel pieces in the centrifuge vial. Cover with Saran Wrap and leave to gel at 60˚C in the oven as before.

Also prepare one pure acid-catalyzed silica gel in a centrifuge vial. This needs to gel at 60˚C in the oven with the others (the acid-catalyzed reaction will not gel at room temperature).

As you did last week, come remove your gels from the oven after 2 days, remove the Saran Wrap, cap vials, and store in your lab cabinet until next week.

If you still have time…here is what you will be investigating for both the dye-modified base-catalyzed gels and the gel-in-gel composites.

Vapor phase pH sensing:

With each of the dye-modified gels, carefully transfer one piece into a small clean beaker without any solvent. Expose this piece of gel to HCl vapor and to NH3 vapor in the small dessicators that are provided. Record any observations, particularly the time over which any color changes develop. Are the color changes reversible? Be patient! These color changes may take 15-20 minutes to fully develop.



Liquid phase pH sensing:

With each of the dye-modified gels, carefully transfer a second fresh piece into another small clean beaker without any solvent. Cover the piece with a small amount of pH 8 buffer. Record any observations. Decant off the pH 8 buffer and replace with pH 3 buffer. Record any observations. Record any observations, particularly the time over which any color changes develop. Are the color changes reversible? Be patient!



C. Week 3: Washing gels and evaluating use as pH sensors

Retrieve all of your gels and record detailed observations on their appearance in your lab notebook. Are the gels in the form of a single monolith in the shape of the vial? Did the gels shrink at all? If so, can you estimate the size of the gels? Is there any visible solvent remaining? Are there any cracks in the gels? Are they transparent or opaque? What color are they? Can you visually identify the base-catalyzed gel pieces inside acid-catalyzed gels? Would you describe these as gel-in-gel composites or did they homogenize? How does the pure acid-catalyzed silica gel compare to that of the pure base-catalyzed silica gel that you prepared last week?

As you did last week, wash your gels twice in acetone noting to what degree leaching of the dyes occurs for these composites.

After washing the gels, allow them to dry slightly, and then test the pH sensing ability in both the vapor phase and liquid phase. Carefully record all observations focusing on comparisons of the behavior of these composites to the pure base-catalyzed gels in terms of speed of response, reversibility, and leaching behavior.



1REFERENCES

 Rolison, D. R. and Dunn, B. “Electrically conductive oxide aerogels: new materials in electrochemistry,” J. Mater. Chem. 2001, 11, 963-980.

2 A. M. Buckley, M. Greenblatt, P. G. Allen, and G. C. Lisensky, Experiment 15 in Teaching General Chemistry: A Materials Science Companion, A. B. Ellis, M. J. Geselbracht, B. J. Johnson, G. C. Lisensky, and W. R. Robinson, ACS Books: Washington, DC, 1993.

3 C. A. Morris, D. R. Rolison, K. E. Swider-Lyons, E. J. Osburn-Atkinson, and C. I. Merzbacher, “Modifying nanoscale silica with itself: a method to control surface properties of silica aerogels independently of bulk structure,” J. Non-Cryst. Solids, 2001, 285, 29-36.

4 R. E. Russo and A. J. Hunt, “Comparison of Ethyl Versus Methyl Sol-Gels for Silica Aerogels Using Polar Nephelometry,” J. Non-Cryst. Solids, 1986, 86, 219-230.

5 A. M. Buckley and M. Greenblatt, “The Sol-Gel Preparation of Silica Gels,” J. Chem. Educ. 1994, 71, 599-602.



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