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Modern disposable baby diapers contain a small amount (4-5 grams) of polyacrylic acid in a powder form (sodium polyacrylate), which is mixed into the fluff in the middle layer of the diaper. The inside layer of the diaper allows water to pass through it into the absorbent middle layer, and the outer layer is waterproof, so both baby and mommy stay dry. Polyacrylic acid can absorb about more than 100 times its own weight in water, which is why an average sized diaper can absorb up to 16 ounces of water. Luckily that's more than a baby usually pees.
Why is this powder so good at absorbing water? Polyacrylic acid is a polymer made from the monomer acrylic acid. Polymer molecules are kind of like long chain made up of lots of identical links (the monomers). Polyacrylic acid chains contain thousands of monomer units, as well as cross-linking between different molecule chains, which makes them kind of tangle together. Most polymers, such as polyethylene and polystyrene (used in trash bags, plastic bottles, and Styrofoam®, for example) are hydrophobic, meaning they repel water. Polyacrylic acid, however, is very hydrophilic - it attracts water- because of the carboxylic acid groups (COOH) in the polymer, which can hydrogen-bond to water molecules. The cross-linking also helps trap lots of bonded water molecules inside the tangled chains.
Variations and related activities
You can make this activity more colorful by adding a few drops of food coloring to the water. Also try adding some table salt to the water. What happens? [Table salt is sodium chloride (NaCl) and the chloride ions will bond with the SAP and prevent water molecules from doing so.]
If you place the wet polymer on a towel or napkin and allow it to dry out for a few days it will dehydrate and shrink back to almost its original powder form and can be used over an over again.
Polyacrylic acid, often just called baby diaper polymer or super absorbent polymer (SAP), comes in many forms and has many other applications. SAP is often used to clean up hazardous waste spills- even more hazardous than what ends up in a baby diaper! Just sprinkle the dry powder on the spilled liquid and then sweep or vacuum it up. You can also find SAP in moisture-control potting soils at a garden store. If you over-water your plants the SAP will absorb the excess water. As the soil dries the SAP dehydrates and the excess water returns to the soil, maintaining just the right amount or moisture for the plant to thrive. You can also buy headbands and neckbands that contain SAP inside, after you wet them the water evaporates slowly and keeps you cool for a long time (see the reference link below to make your own!)
You can buy Instant Snow powder from many stores online (see references below). Instant snow is exactly the same material, it's just ground up into a much finer powder. This causes it to form smaller molecular structures and puff up instantly when it absorbs water, giving it the look and feel of real snow. It's often used in movies when they want snow in a scene without the need for cold temperatures- and the wet mess when it melts!
Orbeez and other water beads are clear spheres and other fun shapes made from the same polyacrylic acid SAP material, but prepared a little differently to form the shapes you see when they absorb water. Typical Orbeez spheres start as 3 or 4 mm beads, but can grow to more than 1" in diameter as they absorb water. Because they are composed almost entirely of water when fully grown, they are essentially invisible in a bowl of water (as long as both the spheres and the liquid water are clean). Water is water. The index of refraction (a measure of how light bends as it passes through a material) of the liquid water is the same as the water in the sphere, so light rays are not refracted as they pass from the water in the bowl to the water in the beads, and the spheres are therefore almost completely indistinguishable from the rest of the water. They also bounce (but can break if you bounce them too hard) and act as little magnifying glasses. Just as with the baby diaper polymer, you can add food colors to make colored Orbeez, and if you let them dry they will shrink back down to little beads that you can use over and over again.
references and links to more information
The science of sodium polyacrylate and SAP's:
Here are some other ways to do this experiment:
This video from Science Beyond at the St. Louis Science Center shows you the secret behind one of our most popular demonstrations:
Fun with Instant Snow powder:
Make your own cooling headband or neck bandana with SAP:
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