Staying Dry

    Kids, did you ever wonder how the new disposable diapers that are so thin can really work for your baby brothers or sisters? There are tiny beads in the filling that are able to absorb more than 300 times their own weight of water. Our purpose is to collect these beads and watch how they behave when exposed to water. All you need is an "ultra-absorbent" disposable diaper (Huggies Ultrathin, Ultra-Pampers), water, and table salt.  

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Cut open the diaper and carefully peel away the cotton-like filling. You will notice that it feels gritty. Separate the small gritty beads from the cotton fibers (tweezers and a small kitchen strainer may help, but are not really necessary). You should be able to easily get about 1/2 tsp. of beads, then pour them into a clear glass. Add about 1/2 cup of water and gently swirl, or pour the mixture back and forth between two glasses until it is too thick. (If you are able to get distilled, deionized water, it works better than hard tap water.) To "unlock" your gel, sprinkle a little salt on top and stir it into the gel. When the water is released the now syrupy liquid can be washed down the drain.

    The superabsorbent beads are a co-polymer of poly(acrylamide) and sodium polyacrylate that can undergo physical changes quickly and reversibly with water. Other uses for these polymers are for hydro-mulching plants (places like Frank's Nursery now sell small bags of colored gel for this) and removal of water from jet fuels. Try your experiment again if you like with a drop of food coloring in the water (yellow fits the diaper theme nicely).


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    February 1993


    Reference: B. Z. Shakhashiri "Chemical Demonstrations", Vol. 3, chapter 9, p. 368.