Farmers need to keep moisture in the soil to promote seed germination and root development in the plants they grow. Parents need to keep their infant's cradle clean and dry. Both farmers and parents get help from the same source, a white powder called a superabsorbent polymer. This polymer is a white powder that can absorb water many times its volume, forming polymer is a white powder that can absorb water many times its volume, forming a stiff gel. The powder is incorporated in soil and in diapers to retain moisture.

You can experiment with this superabsorbent powder at home. This is what you need:

3 "superabsorbent" disposable diapers (e. g. Ultra Pampers) 5-gallon plastic bag, with twist-tie sealer kitchen sieve 2 water glasses table salt spoon

First extract the superabsorbent powder from the diaper as follows:

• Cut open the superabosrbent diapers and place the cotton-like filling into the plastic bag. (The filling will feel gritty; the grittiness results from the superabosrbent powder.)
• Seal the bag with a twist-tie.
• By manipulating through the bag, shred the diaper filling until it is in small pieces.
• Rub the filling against itself.
• Shake the bag to loosen the powder from the filling. The powder will settle to the bottom of the bag.
• Open the bag and feel the filling. If it still feels gritty, close the bag and rub the filling again. When the filling no longer feels gritty, gently shake the bag to settle the powder to the bottom of the bag.
• Open the bag and remove the filling. (Take care not to inhale dust of fibers from the filling.) The powder on the bottom of the bag can be separated from most of the remaining pieces of filling by passing it through the kitchen sieve.
• Place the powder in an empty, dry drinking glass and examine it.
• What color is it?
• Is it fine or coarse?
• Fill the second drinking glass with water.
• Pour the water from this glass into the one containing the powder from the diapers.
• Pour the mixture back and forth from one glass to the other. As you pour, the mixture will thicken, and eventually become so thick that it will no longer pour.
• How long does this take?
• What does the thickened mixture look like?
• Sprinkle about a teaspoon of table salt on top of the thickened mixture. Stir the salt into the mixture with a spoon.
• What happens as you stir?
• Does the mixture become thin enough to pour again?
• Add more salt and stir some more. Does the mixture become more fluid?

Depending on the size of the particles in the diaper, it may be possible to remove the salt from the superabsorbent material.

Pour the mixture of powder, water, and salt into a sieve lined with a paper towel and slowly run water through sieve. The flowing water will gradually remove the salt. As the salt is washed away, the mixture will thicken. When it has become almost as thick as it was before the salt was added, stop rinsing it and use another paper towel to press the excess water out of the contents of the sieve. If the thickened material in the sieve is placed in an open dish for several days, the water will evaporate from the mixture, leaving the original material from the diaper. This material can be used again to absorb more water.

The powder you extracted from the diapers is a "superabosorbent polymer." A polymer is a material whose molecules (smalles particles) are a long chain of repeating units. In the superabsorbent polymer, each of these units has a portion that holds an electrical charge. The electrical charges on the polymer attract water molecules and bind them to the polymer. Each charge binds several water molecules and each molecule of polymer can bind a large volume of water. When salt is added to the mixture of water and polymer, the polymer releases the water. This occurs because salt also contains electrical charges. The charges in salt also attract water molecules, and water molecules bind to the salt instead of the polymer. Furthermore, the charges in salt are attracted to the charged parts of trhe polymer and displace of the molecules of water from the polymer.

Superabsorbent polymers have a wide variety of uses, both in industry and in consumer products. You have seen that it is used in baby diapers. Its ability to bind water to itself has led to its use in filtering fuels, such as auto and jet fuels. Here it removes small amounts of water that otherwise might freeze and cause blockages in fuel lines. When the superabsorbent polymer is distributed in sandy soil, it improves the soil's ability to retain moisture and impoves its ability to support agricultuture. Can you think of any other uses?

Acknowledgements

For additional information, see CHEMICAL DEMONSTRATIONS: A Handbook for Teachers of Chemistry, Volume 3 (1989), by Bassam Z. Shakhashiri, The University of Wisconsin Press, 114 North Murray Street, Madison, Wisconsin 53715.

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