Diaper Polymers
| Chemistry: | Polymers,
Hydrophilic Molecules |
| Grade Range: | Elementary School, |
| Format: | Stage |
This demonstration is one that students could easily replicate at home. Be sure the salt is mixed thoroughly before trying to dump it, otherwise it won't go down the drain.
Materials
- Diapers
- Salt
- Scissors
- 1 Gallon Zip-lock Bag
- Glass Cups or Beakers
- Water
- Spoon
Safety Precautions
Please read the Dry Chemical section of the Demonstration Safety page before performing this demonstration.
This demonstration requires: Safety Glasses
Demonstration
- Take a Diaper and start cutting it open on the inside to reveal the absorbent pad. Carefully pull out the pad and put it inside of a 1 gallon zip-lock bag.
- Close the bag, leaving air inside of it, and start shaking it. Explain to the audience that the absorbent pad in a diaper works because of the tiny acrylic acid crystals that are added to it. After shaking it for a minute, open the bag and toss the pad. there will be a collection of crystals in the bottom of the zip-lock bag!
- Dump the crystals from the bag into a cup or beaker, and add about 4 oz (~120 ml) to it. Stir it, and let it sit for a minute or two so the crystals can absorb the water. While it is sitting, explain the difference between hydrophobic and hydrophilic.
- Once the water has been absorbed, pick up the glass and hold it upside down over your head. The mixture stays inside the glass! If the mixture looks like it has air pockets, you can even add another ounce (~30 ml) of water to it.
- Now add a spoonful of salt to the solution and stir it thoroughly. As you stir, the solution will become wet again, and the crystals stop sticking to each other! This final solution can safely be poured down a drain.
Why This Works
Short Explanation
Diapers are very absorbent because of the unique polymer that they use in them. Polymers are molecules of the same type that like to connect to each other, like plastics, and they will usually stretch and bend to stay connected. Most plastic polymers don't absorb water, and instead repel it. This means that they are Hydrophobic, or "water-fearing". However, the crystals that are in the diaper pad like to absorb water. These crystals are acrylic acid, and when dry they don't connect to each other. However, the crystals are Hydrophilic, or "water-loving". This means that they not only absorb water, but they will try to absorb as much as 300 times their mass! As they absorb water, the crystals start to stick together, becoming a polymer gel, and even sticking to the inside of the glass as well! When we add the salt, it breaks the connections between the crystals, and destroys the polymer. Although the crystals will still hold some water in them, the salt forces them to lose a lot of it, which makes the solution liquid again.
Full Explanation
A diaper needs to be able to absorb liquid without leaking. This task is done by the use of Polymers, identical molecules that connect to each other end to end. The polymers that we are all most familiar with are plastics, and the majority of plastics are hydrophobic. Hydrophobic materials are ones that, when water is poured or spilled on them, repel water and don't absorb any of it. This is because, on the molecular level, the bonds between the molecules are close enough and strong enough to prevent water from passing through them. The outer layer of a diaper is made of plastic because of this, so that the diaper won't leak onto an unsuspecting parent or caregiver.
The pad inside of a diaper is highly absorbent. Part of the absorption is because of the type of pad, but a large amount of the absorption is because of what is added to the pad. Diaper pads have crystallized Sodium Polyacrylate, a Polyacrylic Acid salt, added into them. These crystals are tiny, and when dry they don't look like much. However, these crystals become quite amazing once they are wet. Polyacrylic acid becomes a polymer when wet, causing the crystals throughout the diaper to start linking together. Polyacrylic acid is also Hydrophilic, a material that readily absorbs water and will expand several times its size to try to absorb more of it. In this case, the crystals will expand to absorb up to 300 times their mass in water!
| Sodium Polyacrylate (Anhydrate) | Polyacrylic Acid (Hydrated) |
| [C3H3NaO2]n | [C3H3O2- + Na+]n + N H2O |
When the sodium polyacrylate is in water, the sodium separates from the polyacrylic acid. Polyacrylic acid doesn't dissolve in water solutions, however, and it will start connecting with the other polyacrylic acid moelcules. Even though each of them have a net negative charge, they bond together because the negative charge is isolated to one of the oxygens, which allows the carbons to bond with each other by passing a hydrogen up to the oxygen. This makes polyacrylic acid an Anionic Polyelectrolyte, a chemical that becomes a polymer when it loses one of the positive charges in solution. The water molecules become encased in the polymer, since it can only form while the sodium is dissolved in the water, and the polymer will actively absorb as much water as possible in order to better contain the sodium ion. This is why the polymer will absorb as much as 300 times its mass in water!
When we add the salt to the solution, we are flooding the solution with excess sodium. The extra sodium starts to bond with the polyacrylic acid, which breaks the polymer and causes it to fall apart. The crystals will still hold onto water, but the salt increases the molar mass of the solution, which causes the crystals to lose some of it. This results in the crystal gel falling apart and "melting" back into solution. This final solution is safe to pour down a drain because of the added salt as well; The leaking crystals have some of the salt trapped inside of them now, so their ability to absorb water is compromised.
Additional Information
- This demonstration pairs well with the Polymer Milk Jug and the Vanishing Peanuts.
- Let students know that this can be easily done at home!