Disposable diapers soak up liquids really well. They're designed to hold more than their weight in liquid without allowing leaks, to keep the baby dry ... but you probably didn't know that one way they do this is to add a hygroscopic chemical between the layers of padding. We didn't know this either, so we decided to investigate the interior of a disposable diaper a little more closely, to look for the chemical, and to see what it does. Hygroscopic materials are chemicals which adsorb and absorb water and undergo a chemical change, often becoming polymers. They are usually used in places where moisture isn't wanted ... those little packets of crystals that come wedged inside a new piece of electronic equipment contain hygroscopic material that absorbs water, keeping the equipment dry. Diapers contain a similar material, probably  polysodium acrylate, which is used to trap liquids by turning them into a polymer. We're going to tear apart a diaper and watch what happens when water is added to this chemical, and then explore some other interesting uses for it. The first step is to dismantle a diaper. This has to be done carefully, because, as you'll discover, the hygroscopic crystals are scattered throughout the layer of padding, and will fall out onto the bottom layer of the diaper as you remove the padding. The crystals are tiny, smaller than salt crystals, and if you accidentally turn the diaper over (or sneeze), you'll lose them. On the left you can see a close-up of the crystals scattered on the bottom layer of the diaper. If you're careful, you can collect about half a teaspoon from one regular-sized diaper. On the right, we're carefully pouring the crystals we've shaken out of the padding into a container. Make sure your hands aren't damp when handling the crystals, or they'll turn into polymers before we're ready. Here you can see the crystals we collected from about five or six diapers. There will be some fluff from the padding mixed in with the crystals ... if you shake the container, this fluff will come to the top and you can easily pluck it out. How safe is this procedure? The chemical itself is probably harmless ... its in a baby diaper, after all, and babies have been known to chew on their diapers. But you probably don't want to ingest any of it ... wash your hands when you're done, before you eat or drink anything. The biggest problem we had was the fine particles of padding material, which made the air dusty. If you don't want to breathe in diaper padding dust, wear a dust mask. None of the fibres (or the hygroscopic chemical) were identified on the packaging, but we assume they are not dangerous. Unless of course the diaper was used first ... O.K., now for some fun! Hygroscopic materials like this can hold up to 50 times their weight in water. The first thing we're going to do is to see how much water we can add to the material, and what happens to it as it gets wet. We began with a small amount of crystals ... just enough to cover the bottom of a small drinking glass, as shown on the left. We assumed that we'd have to add more eventually, but this was all we needed! A second glass of the same size was used to add water; we started with a fairly small amount, as shown on the right. No measurements were made ... we're just exploring here, to see what will happen. The crystals soaked up the water, and turned into a white fluffy material, that seemed damp to the touch, but not wet ... water would not come out of it when it was squeezed. That's because the water has combined chemically with the crystals, and is gone. O.K., let's add more water. We added water to the chemical to about the half-way mark on the glass. This is the result. The material is becoming translucent ... it's no longer white, and is quite jelly-like. It's beginning to look more like a typical polymer. It's also very cohesive. When we turned the glass upside down, the material stayed inside. Mixing it thoroughly with a finger had removed all traces of liquid water ... not a single drop escaped when we tipped the glass. Next we filled the glass nearly to the top with water. A little more stirring was needed to distribute the water throughout the compound, but again all of it disappeared, leaving a gelatinous mass that would not come out of the glass even when we turned it upside down and shook it. What has just happened is that a tiny amount of the hygroscopic crystals have turned an entire glass of water into gel, almost instantly. How can we use this to have some fun? Why would you want to make water disappear ...? One answer is if you're a magician. This chemical is well-known to magicians, who can buy it from 'magic stores' in large quantities. By concealing a small amount of the crystal material in the bottom of a glass or cup, they can make water 'disappear'. They pour water into the container, and then later turn the container upside down, making it look like the water has vanished. You can't see the gel inside, which is stuck to the bottom of the cup. We didn't know that! The next step in an investigation of this material might be to measure exactly how much water a given mass of crystals will absorb. From our simple exploration, it certainly looks like more than fifty times the weight of the crystals was absorbed. You could find out. Other questions that might be answered include: - Will the water eventually come out of the gel? Is the reaction reversible? - What are the properties of the gel? Will it burn? Will it dissolve in another liquid, like alcohol? - What other liquids will it absorb? Will it absorb an oil spill? - What other uses can you think of for the material? You might also want to research what commercial products (other than diapers) use this chemical. An internet search might give interesting results. Or perhaps you'd like to develop a magic routine using a pitcher of water and various containers, some containing  polysodium acrylate. Others might contain small amounts of  food colouring (which has been allowed to dry on the bottoms of the containers), making it possible for you to magically turn water into juice, which you can then make disappear. Whatever you decide to do, have fun! Main Page