How does a paper towel absorb water?
Paper towels are made out of finely divided fibers of cellulose, the principal structural chemical in cotton, wood, and most other plants. Cotton is actually a polymer, which like any other plastic is a giant molecule consisting of many small molecules linked together in an enormous chain or treelike structure. The small molecules or “monomers” that make up cellulose are sugar molecules. We can’t get any nutritional value out of cellulose because we don’t have the enzymes necessary to split the sugars apart. Cows, on the other hand, have microorganisms in their stomachs that produce the necessary enzymes and allow the cows to digest cellulose.
Despite the fact that cellulose isn’t as tasty as sugar, it does have one important thing in common with sugar: both chemicals cling tightly to water molecules. The presence of many hydroxyl groups (-OH) on the sugar and cellulose molecules allow them to form relatively strong bonds with water molecules (HOH). This clinginess makes normal sugar very soluble in water and makes water very soluble in cellulose fibers. When you dip your paper towel in water, the water molecules rush into the towel to bind to the cellulose fibers and the towel absorbs water.
Incidentally, this wonderful solubility of water in cellulose is also what causes shrinkage and wrinkling in cotton clothing when you launder it. The cotton draws in water so effectively that the cotton fibers swell considerably when wet and this swelling reshapes the garment. Hot drying chases the water out of the fibers quickly and the forces between water and cellulose molecules tend to compress the fibers as they dry. The clothes shrink and wrinkle in the process.