Zeolites: The Secret Ingredient

The next time you notice that your cat's litter box doesn't smell bad, you can thank NASA astronauts. You can also thank them when you see lush green golf courses, or when you use air fresheners and laundry detergents. The common link in these products isn't immediately obvious: it's zeolites. The link still won't be obvious until you learn more about how zeolites work. Picture a sponge. When it's dry and hard, it doesn't soak up much water. As soon as it gets wet and squeezed, though, it can absorb and trap more than its weight in liquid. It also filters impurities; when you squeeze water out of sponge, the bits and pieces of other things stay in the sponge. Think of zeolites as sponges made out of rock. Zeolites don't use squeezing to release their liquids, however; they respond to heat. In fact, the name zeolite comes from the Greek words zeo (to boil) and lithos (stone) so it means 'the rock that boils.'

Zeolites aren't really rocks in the sense most people think; zeolites are rigid crystal structures. They have networks of interconnected tunnels and cages, similar to honeycomb. There are about 50 different types of zeolites that occur naturally. Another important feature: zeolites are small. Most zeolite molecules are about 2 to 8 microns in size, and that makes it difficult to study them accurately (a human hair is about 120 microns in diameter). That's where the astronauts can help. Because they're crystals, zeolites form gradually. In space, microgravity makes that crystallization happen at a slower pace, so more materials accumulate during the crystal-forming process. More material means larger, higher quality zeolites; the ones grown in space can be up to 1,000 times the size of ones on Earth. If scientists can see the zeolites better, they can study and manipulate them better.

What do zeolites do when heated? Their pores open. They act like sponges that have been squeezed, and expel water, gas or whatever is inside. Zeolites also filter substances by trapping large molecules. This helps some chemical reactions take place. Zeolites have ions loosely attached to their framework, and they can exchange their ions with ions from other materials. For instance, zeolites in laundry detergent exhange magnesium and calcium ions from hard water with their own sodium ions. That exchange improves the lathering effect of the detergent. Since zeolites absorb liquids and gases, that makes them useful in many everyday products that need this ability. Kitty litter requires substances that absorb liquids, for example, and air fresheners absorb foul-smelling gases. Because zeolites have small pores, that keeps some molecules from entering a zeolite structure. That means zeolites can be used to filter air and water to help clean up the environment.

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