By definition, “an aerogel is an open-celled, mesoporous, solid foam that is composed of a network of interconnected nanostructures and that exhibits a porosity (non-solid volume) of no less than 50%.” In practice, an aerogel is the dry, low-density, porous, solid framework of a gel, isolated intact from the gel’s liquid component. The diameters of the open cells may range from <1 nanometer (nm) up to 100 nm and are usually <20 nm. Most aerogels exhibit 90% to 99.8% porosity. Aerogels have been manufactured from silica, metals, metal oxides, semiconductor nanostructures, organic polymers, biopolymers, carbon, and carbon nanotubes. Silica is used most often as the material in commercially sold aerogel products today. With a porosity of 99.98% and a density of 0.0011 g·cm-3, a specially formulated silica aerogel is the solid material of lowest density that has ever been produced.
Zhao et al. (2018) recently provided a comprehensive review of the chemistry, properties, and applications of biopolymer aerogels and foams. As stated by the Abstract of this article, “Biopolymer aerogels were among the first aerogels produced, but only in the last decade has research on biopolymer and biopolymer–composite aerogels become popular, motivated by sustainability arguments, their unique and tunable properties, and ease of functionalization. Biopolymer aerogels and open–cell foams have great potential for classical aerogel applications such as thermal insulation, as well as emerging applications in filtration, oil–water separation, CO2 capture, catalysis, and medicine. The biopolymer aerogel field today is driven forward by empirical materials discovery at the laboratory scale, but requires a firmer theoretical basis and pilot studies to close the gap to market. This Review includes a database with over 3800 biopolymer aerogel properties, evaluates the state of the biopolymer aerogel field, and critically discusses the scientific, technological, and commercial barriers to the commercialization of these exciting materials.”
As summarized by Zhao et al., “biopolymer aerogels can be synthesized by two main routes: i) through colloid formation from molecular precursors, where the biological feedstock was reduced to its molecular form, for example, alginate, pectin or chitosan; or ii) through the use of particle-based precursors, where the particle size has been reduced to the nanoscale, for example, protein nanofiber aggregates, chitin nanofibrils, or nanofibrous cellulose. For both approaches, the aerogel properties are primarily determined by the concentration and functional groups of the precursors, the pH value of the sol, the type and concentration of cross-linkers, the optional post-gelation modification of the gels, and the drying procedure (Figure 1).”
An 8-minute video demonstrates a process used to prepare biopolymer aerogels in three hours by using only three green solvents (water, ethanol, and pressurized carbon dioxide). These products are distributed by Aerogel Technologies. Different products are prepared from polysaccharides (such as alginate, cellulose, and pectin), proteins (such as caseinate and egg protein), and lignin. The attractive properties of these products, listed below, result in their holding promise for use in applications such as pharmaceuticals (as carriers of drugs such as Ibuprofen), wound treatment, personal care products (such as cosmetics), food products (as carriers of essential nutritional components such as Omega-3 fatty acids and vitamins), and insulating packaging.
- High specific surface area (up to 500 m2/g).
- Low density.
- Superabsorbent performance (up to 70 g/g saline).
- Superior loading and delivery performance.
The images reproduced below show examples of biopolymer aerogels prepared by using this process.