Q. Why can a paper diaper absorb so much liquid?

A. It is not the paper diaper that absorbs liquid but rather the water-absorbing polymer (with high molecule counts) contained in the paper diaper that absorbs the liquid. Sodium polyacrylate is the common water-absorbing polymer and 1g of the polymer can absorb several hundreds grams to one kilogram of water and 50 to 70g of urine.

The sodium polyacrylate has many compartments like a high-rise apartment building. It is divided by polymer chains (many carbon atoms links). Each compartment has branches that separate the liquid into negative ions and positive ions. When the polymer absorbs liquid into each compartment, positive ions separate from the branches and negative ions remain. Since negative electrical charges repel each other, the compartments expand by several hundred times and become capable of absorbing massive amount of liquid.

This water-absorbing polymer is also used for water-retaining gel for soil and toys that expand when put into the water.