Preparation of Biopolymer Aerogels Using Green Solvents

A new way for the production of biopolymer-based aerogels by carbon dioxide (CO₂) induced gelation is shown. The technique utilizes pressurized carbon dioxide (5 MPa) for the production of biopolymer hydrogels and supercritical CO₂ (12 MPa) to convert gels into aerogels. The only solvents needed besides CO₂ are water and ethanol.

Transcript

The overall goal of this work is to visually demonstrate the production techniques of biopolymer aerogels using amidated pectin as a model biopolymer, and water, ethanol, and carbon dioxide as solvents. Biopolymer aerogel production is a three step process. Which utilizes water for the gel formation, ethanol to replace the water, and supercrystal CO2 to remove the ethanol.

Carbon dioxide has a huge impact on this process as it is a quintessential component in aerogel production. Our investigations of how carbon dioxide transforms biopolymeric solution into aerojel has led us to the understanding that CO2 cannot only gel biopolymeric solution, but also can help to produce aerogel in one step, one port approach. And we are going to discuss it in this video.

This section demonstrates how amidated pectin and calcium carbonate dispersion is made in water, however, the procedure can be adapted to making many other solution mixtures. Homogenize a 2 percent pectin and water solution at high speed for two minutes to make a viscous solution. Then check the pH.

If it is lower than 6.5, titrate with 0.5 muller sodium hydroxide to neutralize the solution. Next, weigh out and mix in the calcium carbonate. It is not necessary to adhere to this degree of cross linking.

However, too much cross linker can lead to re-precipitation and negatively affect the surface area of aerogels. Finally, homogenize the mixture at high speed again until a white homogeous dispersion is obtained. Begin with transferring the prepared suspension into open polypropylene bowls or glass petri dishes.

Then place the molds into a high pressure autoclave. Pressurize the autoclave with carbon dioxide up to 5 megapascals at room temperature. The depth of the solution in the mold determines the gelation time.

A thin two millimeter gel can be made in less that 30 minutes. After gelation, slowly depressurize the autoclave at 0.2 megapascals per minute. Any quicker and the gel can break.

When fully depressurized, open the autoclave and remove the molds. Remove the hydrogels from the molds by turning them over, and if necessary, pry them out with a spatula. A perfect hydrogel looks homogeneous.

For each gram of prepared hydrogel, make 10 grams of ethanol and water at a one to nine weight ratio. Then immerse the hydrogels in the ethanol and water mixture for 12 hours. The equilibration time is mainly dependent on the sample's thickness.

Later, immerse the gels in a three to seven weight ratio solution of ethanol and water. Shrinkage of the gel will occur. Shrinkage is strongly dependent on the concentration gradient during solvent exchange, the biopolymer type and concentration, the size of the gel, and the degree of cross linking.

Always try to minimize the shrinkage, because too much shrinkage will lead to poor aerogel properties. Ultimately, the final concentration of ethanol in the gel should be greater than 98 percent by weight. Measure it using the density meter, and once confirmed, proceed with supercritical carbon dioxide drying.

Prepare the gel sample to be dried using supercritical carbon dioxide. Fill the autoclave with ethanol. Use between two and 10 percent of the autoclave's volume to prevent premature solvent evaporation.

Complete immersion of the gels is not necessary. Now, heat the autoclave to 323 kelvin and apply 12 megapascal of pressure using carbon dioxide. Periodically replace the carbon dioxide inside the autoclave while keeping the pressure constant.

Around six or seven residence volumes are typically required. The drying time depends on the gel thickness. As before, slowly depressurize the autoclave.

Then store the prepared aerogels in a sealed container or a desiccator. These aerogels are sensitive to moisture in the atmosphere. An alternative to depressurizing is to start the solvent exchange in the pressurized autoclave.

To accomplish this, pump the ethanol-water mixtures into the autoclave stepwise, ensuring the gel remains submerged in the liquid. This takes about 90 minutes for a two millimeter gel. Once the desired solvent concentration is reached, commence with supercritical drying by increasing the pressure from five to 12 megapascal using supercritical carbon dioxide.

For a two millimeter gel, the process of gelation, solvent exchange, and drying, should only take about three hours. Biopolymer concentration plays an important role in the transparency of the hydrogels. One and two percent samples were not transparent, but one half and one quarter percent samples were.

These percentages were given by weight. Bubbles were created in the hydrogels during the depressurization, when the dissolved carbon dioxide left the gel water system due to a decreased carbon dioxide solubility. A deep reduction in the biopolymer concentration to one quarter of a percent, yielded stable but fragile hydrogels.

The obtained aerogels were ultra porous, and had low densities. Their surface areas were measured by nitrogen absorption. In the four to 150 nanometer pore size range, their volumes were measured by the kelvin model of pore filling with nitrogen.

After watching this video, you should have an understanding of how to dry gels in organic solvent with supercritical carbon dioxide, and also how to dry hydrogels using organic solvent and supercritical carbon dioxide. Also you should understand how to transform biopolymeric solutions into aerogel in one port process. Once mastered, this whole procedure.

from biopolymer solution to aerogel, can be done in as little as three hours, depending on the gel thickness. While attempting this procedure, it is important to monitor the pH value of the initial solution, the depressurization rate after the gelation, the final solvent concentration prior to supercritical drying, and the depressurization rate after supercritical drying. We do believe that this technique will attract many researchers and engineers alike.

And force them to produce various materials and products in the field of formo insulation, pharmaceuticals, neutraceuticals, and so forth. Don't forget that ethanol is a flammable solvent, and it is used in a high pressure environment for this procedure. Using personal safety precautions and safe laboratory and equipment operating procedures is essential for this procedure.

Read Article

Journal (Chemistry)

Journal (Chemistry)

Preparation of Biopolymer Aerogels Using Green Solvents

Chapters

Transcript

Related Videos

Get cutting-edge science videos from JoVE sent straight to your inbox every month.

Your free access has ended.