God, as the hymn goes, may have made all things bright and beautiful, but for sheer weirdness first prize should go to a man-made creation instead: aerogel. A solid that's up to 99 percent gas, it is rigid to a light touch, soft to a stronger one, and shatters like glass if it's put under too much pressure too quickly; it's one of the most enigmatic of materials, as well as one of the most versatile.
It can withstand the heat of a direct flame; engineers use it for insulation on oil rigs and for warmth in the insoles of hiking boots worn in the coldest temperatures on Earth. NASA uses it to trap comet dust blowing through the universe at six kilometers per second. It even works as casual, sporty jewelry—AeroGem sells a key chain with an aerogel bob on the end, and a pendant "hermetically sealed inside silver-over-titanium end caps for added strength and long-lasting, waterproof durability."
The most recent headlines about aerogels, however, don't have anything to do with oil rigs or NASA or geeky jewelry. They instead bring the unfamiliar and exotic materials into practical, and not at all weird, territory, by suggesting a big, broad-reaching new use for them: to clean up pollution. Researchers announced recently in the journal Science that they had created a new form of aerogel capable of sopping up heavy metals, particularly mercury. It could eventually be used to purify contaminated water. There are efforts to make all sorts of new products from the stuff: rocket fuels, catalytic converters for cars, cell-phone batteries. Aerogels may be weird science, but they're turning out to be more practical than they look.
Nicknamed "frozen smoke" after its ethereal appearance, aerogel is neither frozen nor smoke. It's also surprisingly low tech—it's been known since 1931, when lore has it that chemist Steven Kistler discovered it after a colleague bet him that he could not easily take all the liquid out of a gel and replace it with a gas. Kistler heated the gel, forcing out all the liquid, and then replaced it with a gas, methanol. He published the result, his oddly behaving "aerojelly," in Nature that year. Researchers played with his formula for the next seven decades, finally settling on more suitable and safe ingredients for making the stuff: oxides, such as silicon dioxide and aluminum oxide, as the base gel, and carbon dioxide gas in place of highly flammable methanol.
Together, these ingredients can form a structure that chemically resembles glass but is so full of whorls and crevices that one cubic centimeter has a total surface area equal to a football field's. The lightest-weight solid in the world, aerogel weighs 1.2 milligrams per cubic centimeter—barely more than the air molecules around it. In fact, the material itself is almost entirely made of air, like a sponge that consists mostly of holes. Don't let its lightness fool you: it's strong. NASA photos show two grams of the material easily supporting a 2.5-kilogram brick.
The aerogel revealed last month is made with new ingredients: sulfur or selenium in place of oxides. But it's still built like a sponge, and that is why it can soak up so many heavy-metal atoms from polluted water, says Mercouri Kanatzidis, a chemist at Northwestern University in Evanston, Illinois, who conducted the new research. Aerogels made with oxides don't bind to heavy metals, but the new sulfur and selenium ones cling to them like nothing else. And because the aerogels pack an enormous surface area into a tiny volume, small pieces can clear out many liters of water. Kanatzidis's aerogels sopped up so much mercury that they diluted a solution of 645 parts per million down to 0.04 parts per million. They had similar effects on lead and cadmium, two other pollutants.
The new aerogels aren't ready for widespread use: they're made with platinum, so they're extraordinarily expensive. But if other metals can be used to make them instead (Kanatzidis says they can), chunks of them could be dropped into polluted water, removing contaminants.
Scientists are also discovering new uses for aerogels made with more traditional oxides. Biologists in California are working on an aerogel that's porous and complex, like a cell membrane, and could help them better understand molecular processes in the body. The substance is "as close to a biological membrane as we can get right now," says Marjorie Longo, a chemical engineer at the University of California, Davis. Several projects are underway to desalinize brackish water in cities such as Las Vegas and Phoenix using aerogels, though it is unclear if they'll be practical on a large scale. Then there are those rocket fuels (aerogels can purify hydrogen-based fuels and trap energy from sunlight), and the cell-phone batteries and catalytic converters (aerogels are good conductors of electricity). Keeping the world bright and beautiful, it seems, is just the beginning.
