Although the first reports on aerogels made by Kistler1 in the 1930s dealt with aerogels from both inorganic oxides (silica and others) and biopolymers (gelatin, agar, cellulose), only recently have biomasses been recognized as an abundant source of chemically diverse macromolecules for functional aerogel materials. Biopolymer aerogels (pectin, alginate, chitosan, cellulose, etc.) exhibit both specific inheritable functions of starting biopolymers and distinctive features of aerogels (80-99% porosity and specific surface up to 800 m2/g). This synergy of properties makes biopolymer aerogels promising candidates for a wide gamut of applications such as thermal insulation, tissue engineering and regenerative medicine, drug delivery systems, functional foods, catalysts, adsorbents and sensors. This work demonstrates the use of pressurized carbon dioxide (5 MPa) for the ionic cross linking of amidated pectin into hydrogels. Initially a biopolymer/salt dispersion is prepared in water. Under pressurized CO2 conditions, the pH of the biopolymer solution is lowered to 3 which releases the crosslinking cations from the salt to bind with the biopolymer yielding hydrogels. Solvent exchange to ethanol and further supercritical CO2 drying (10 - 12 MPa) yield aerogels. Obtained aerogels are ultra-porous with low density (as low as 0.02 g/cm3), high specific surface area (350 - 500 m2/g) and pore volume (3 - 7 cm3/g for pore sizes less than 150 nm).
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机译：尽管Kistler 1 在1930年代首次发表有关气凝胶的报道，涉及无机氧化物（二氧化硅和其他）和生物聚合物（明胶，琼脂，纤维素）的气凝胶，但直到最近才发现生物质是一种功能性气凝胶材料的化学多样性大分子的丰富来源。生物聚合物气凝胶（果胶，藻酸盐，壳聚糖，纤维素等）既具有起始生物聚合物的特定遗传功能，又具有气凝胶的独特特征（孔隙度为80-99％，比表面积最高为800 m 2 / g ）。这种特性的协同作用使生物聚合物气凝胶有望成为广泛应用的候选材料，例如绝热，组织工程和再生医学，药物输送系统，功能性食品，催化剂，吸附剂和传感器。这项工作证明了使用加压的二氧化碳（5 MPa）将酰胺化的果胶离子交联到水凝胶中。最初在水中制备生物聚合物/盐分散体。在加压的CO2条件下，生物聚合物溶液的pH值降至3，这会从盐中释放出交联阳离子，从而与生物聚合物结合生成水凝胶。将溶剂交换为乙醇，然后进一步超临界CO2干燥（10-12 MPa），得到气凝胶。所获得的气凝胶是超多孔的，具有低密度（低至0.02 g / cm 3 ），高比表面积（350-500 m 2 / g）和孔体积（对于小于150 nm的孔径，为3-7 cm 3 / g）。
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