The Wacky World of Superabsorbent Polymers

Introduction

Pour distilled water into one “empty” cup and the water solidifies into a gel. In another beaker, add a small amount of powder, pour in distilled water and see an overflowing mass of fresh powdered “snow” produced.

Concepts

• Polymers
• Osmosis
• Superabsorbent
• Cross-linking

Materials

Safety Precautions

Sodium polyacrylate and snow powder are both nontoxic. However, they are irritating to the eyes and to nasal membranes if inhaled. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Sodium polyacrylate, PolySnow, the gel and snow material may all be disposed of according to Flinn Suggested Disposal Method #26a. Do not put these materials down the sink!

Prelab Preparation

Before class, add 0.5 grams of sodium polyacrylate to the Styrofoam^® cup and 3.0 grams of the PolySnow™ to one 250-mL beaker.

Procedure

1. Add approximately 100-mL of distilled or deionized water to the second 250-mL beaker.
2. Tell the students that the water will “disappear” when poured into the Styrofoam cup. (You can tip the cup containing sodium polyacrylate forward somewhat to show that it is “empty”; it will be difficult to see 0.5 grams against the white Styrofoam cup.)
3. Slowly pour the water into the cup containing sodium polyacrylate. Swirl the cup a bit (give the sodium polyacrylate time to absorb the water).
4. Briefly tip the cup upside down to show the students that the water has “disappeared.”
5. Set the beaker containing 3.0 grams of snow powder on a paper towel or tray.
6. Add 150-mL of distilled or deionized water to the empty 250-mL beaker.
7. Slowly add the water to the beaker containing 3.0 grams of PolySnow™. The PolySnow will absorb the water, producing white powdery “snow” that overflows the beaker.
8. Once all the water has been absorbed, both cups can be emptied onto the paper towel or tray to show the class the gel and snow products.
9. Add a small amount (1 gram) of sodium chloride to each of the 250-mL beakers. Add the gel to one and PolySnow to the other and stir each. The gel and the powder will release the water and each will transform into a slurry.

Student Worksheet PDF

13846_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to both perform the demonstration seven times, and to experiment with the polymers: 20 g of sodium polyacrylate, 60 g of PolySnow™ and seven Styrofoam^® cups.
• The sodium chloride can also be sprinkled on top of the gels and they will “melt.” Be sure the gels are on paper towels to absorb the moisture.
• Another method to demonstrate sodium polyacrylate is to use three cups. If they are different colors or are numbered, it is even better. Carefully pour about 50 mL of water into the cup containing the sodium polyacrylate. Make a big deal about which cup you are pouring the water into. Do the “three card monty” by shuffling and moving the cups around. This gives the gel time to form. Ask the class which cup has the water—they will all be surprised when you tip it upside down and nothing comes out. Ask them again which cup contains the water and they will all guess a different cup. Since they were not observant the first time, add another 30–50 mL of water and repeat the trick. When they are thoroughly confused, remind them about observations and the scientific method. If they saw the water added to cup A, it must still be there.
• Start by explaining the elements of the water cycle: evaporation, condensation, precipitation. Pour water into the cup with the sodium polyacrylate. Explain how heat causes the water to evaporate. Wait several seconds for the water to gel before turning the cup upside down to show how the water vanished. The gel will stick inside the cup briefly as you turn the cup upside down. Pour water (no more than 150 mL or it will not work) into the second cup containing 3.0 g of PolySnow. Everyone knows that water can evaporate, but what happens if the molecules condense on a dust particle and get cold enough to freeze. It’s time for a little acting as you use the cup to pretend to collect the frozen water droplets. Slowly turn the cup upside down to reveal “snow.” Tah-dah!
• Secretly add 6 g of PolySnow to an empty Styrofoam cup (8 oz). Place 200 mL of water in the second Styrofoam cup. Quickly pour the water into the cup with the polymer and cover the mouth of the cup with the mouth of the second cup (like a cocktail shaker). Keep the cups pushed together allowing the polymer to secretly swell and fill both cups. “These COLD cups are really cold. How cold? They can turn ordianary water into SNOW!”

Further Extensions

Additional Activities with PolySnow™

• Make a batch of PolySnow and freeze it. If you live in a climate where it snows, compare the frozen PolySnow to real snow. Could PolySnow be used in place of man-made snow on ski slopes?
• Place 3 g of PolySnow dry polymer into a 1000 mL graduated cylinder. Add 150 mL of water and measure the amount of swelling that occurs. Repeat the experiment using different amounts of polymer and water to determine the ratio of water to polymer that produces the greatest amount of swelling.
• Conduct experiments to determine the following:
  • How does the rate of swelling change when PolySnow is dissolved in hot deionized water versus cold deionized water?
  • Does deionized water produce greater swelling that tap water?
• Research the many uses for superabsorbents on the Internet. Use the keyword “superabsorbents” as a starting point.
• Use food coloring to color the water before adding it to the PolySnow. Fill a graduated cylinder with layers of colored “snow.” Let the snow layers sit undisturbed to see if any color mixing occurs.

Answers to Questions

1. Describe what happened in this demonstration. Also describe the difference between the two substances that were produced.

   Sodium polyacrylate was placed in the bottom of a cup. When water was added, it was absorbed by the solid, forming a thick gel-like substance. Another cup contained PolySnow, a similar solid. When water was added to this cup, the contained overflowed, with a white, powdery substance that looked exactly like snow.

2. Why do you think sodium polyacrylate is called a “superabsorbent polymer?”

   Sodium polyacrylate is called a subserabsorbent polymer because it can absorb so much water that, if too much is not added, it will absorb all the liquid that it comes in contact with.

3. Water is absorbed into the sodium polyacrylate because there is such a high sodium ion concentration inside the polymer that water is forced into it, through osmosis, in order to balance out the sodium ion concentration. What do you think would happen if sodium chloride were added to the gel?

   If sodium chloride were added to the substance, the sodium ion concentration would no longer be so much higher inside the polymer than around it. Thus, water would flow back out of the sodium polyacrylate, and the gel would become a free-flowing solution.

4. One of sodium polyacrylate’s most common uses is in diapers. Why do you think this is? Name one other product you think it might be used in.

   Sodium polyacrylate is used in diapers because it can absorb many times its own weight in liquid. It is probably also used in feminine hygiene products.

Discussion

Sodium polyacrylate and PolySnow™ are examples of superabsorbent polymers. Superabsorbents operate on the principle of osmosis: the passage of water through a membrane permeable only to water. In both sodium polyacrylate and instant snow powder, osmotic pressure results from the difference in sodium ion concentration between the inside of the polymer and the solution in which it is immersed. This osmotic pressure forces water into the solid polymer lattice in an attempt to equilibrate sodium ion concentration inside and outside the polymer membrane. The electrolyte concentration of the water will affect the osmotic pressure, subsequently affecting the amount of water absorbed by the polymer. For example, sodium polyacrylate will absorb approximately 800 times its own weight in distilled water, but will only absorb about 300 times its own weight in tap water, due to the high ion concentration of tap water.

Sodium polyacrylate and PolySnow are manufactured by the free-radical polymerization of a mixture of sodium acrylate and acrylic acid and a cross-linking molecule, such as trimethylol propanetriacrylate (see Figure 1):

The type of cross-linking molecule and the amount or density of cross-linking play a large role in determining the properties of the specific superabsorbent polymer network (see Figure 2). The crosslinking makes the polymer insoluble in water and creates a membrane barrier on the surface of the polymers that allows the passage of water molecules into the interior sites. The water molecules diffuse into the polymer network at the sites of high ionic strength. Once inside the network, the water forms hydrogen bonds with the carbonate groups (see Figure 3). The amount of water absorbed depends on:

1. the number of carboxylate sites; ratio of COO^– to COOH,
2. the amount of cross-linking,
3. the elastic nature of the cross-linking molecule,
4. the difference in sodium ion concentration inside the polymer network and the solution.

Polymers with more carboxylate sites, less cross linking and cross-linking molecules that can be stretcher farther, absorb more water. Sodium polyacrylate and PolySnow basically differ in the degree of network cross-linking. Sodium polyacrylate has less cross-linking and therefore can absorb more water per gram than PolySnow. However, in the right concentration range, the PolySnow absorbs water to form, not the normal gel, but white particles that exactly mimic snowflakes.

Sodium polyacrylate is the main ingredient in high-absorbency diapers. (It can absorb about 30 times its own weight in urine). It is also commonly used in alkaline batteries, feminine hygiene products, nursery potting soil, water beds and as a fuel filtration material to remove moisture from automobile and jet fuels. In addition to these uses, instant snow is used to create winter effects for motion pictures.

References

Special thanks to Jeff Hepburn, Dowling High School, West Des Moines, IA, for bringing this product to our attention and Steve Spangler, Cherry Creek School District, Englewood, CO, for providing us background for PolySnow.

Buchholz, F. L., J. Chem. Ed., 1996, 73 (6), 512–515.