When my son was born last year, my wife and I began the first of thousands of diaper changes. Like most parents, we chose disposable diapers. Considering the bad reputation diaper changing has—especially from the guys—I found that it was just about the easiest part of being a new parent.

Many modern diapers have Velcro-type straps—making it easy to get them on and off—and they are so absorbent that the frequency of diaper changes is lower than in the past. It’s the absorbency of these things that is so amazing. A year later, our little guy can wear a large (stage 4) diaper for more than 12 hours (overnight) with nary a leak and still maintain a dry bottom. In case you haven’t felt the modern diaper when it is full, it can be very heavy—it feels like a half-pound of Jell-o is locked in the interior. It’s kind of squishy.

The squishiness is something special: It’s a superabsorbent polymer gel.

Since the mid 1980s, diaper manufacturers have included small beads (about 1 mm in diameter or smaller) of sodium polyacrylate in superabsorbent diapers. The amount of beads that they put in is so small that you probably wouldn’t notice them in a dry diaper. But a little goes a long way—sodium polyacrylate can absorb more than 300 times its weight of tap water and 800 times its weight in distilled water.

What is sodium polyacrylate?

Sodium polyacrylate is a polymer containing many acrylate monomers connected end-to-end in a large chain. Cross-links between chains “tether” the chains together (figure). The more cross-links the polymer has, the higher the density of the polymer. Superabsorbent polymers are actually partially neutralized polyacrylate. This means that anywhere from 50–70% of the COOH acid groups (figure) have been converted to their corresponding sodium salts.

How does it work?

Water is drawn into the beads—and hence the core of the diaper—by osmotic pressure. As the water is drawn in, the polymer swells and develops the gel-like consistency. The driving force for the osmotic pressure is the higher concentration of sodium ions in the beads than in the outside water. Water is drawn into the polymer in an attempt to balance the number of ions inside and outside the polymer. This is why the polymer can absorb more distilled water than tap water. Tap water already contains some ions and the osmotic pressure is lower between the bead and the outside water.

A further layer of complexity is added because manufacturers treat the outside of the beads to increase the cross-link density of the shell around the bead. This denser shell enables the beads, and hence the diapers, to hold water under a weight-bearing load such as a squirmy baby. Without the tighter shell, you can think of the bead as a sponge. Sure, it can soak up water, but if any pressure is applied, the water will run out. The tighter shell prevents leakage.

Can I experiment with superabsorbent polymers?

If you want to explore superabsorbent polymers, WonderNet has a diaper activity  that is easy to do. It’s designed for elementary school kids, but it is also good for the home chemistry enthusiast because it doesn’t require any special equipment.

A more advanced experiment called the “Diaper Derby”  is available for high school students from Dow (they manufacture sodium polyacrylate beads).

It’s summertime. If you have a baby, try comparing a diaper that’s been in the ocean (high salt) with one that’s been in fresh water. If you do that, you earned the right to add some letters after your name (i.e., Kevin McCue, NeRD).