I Didn’t Know That...

Q: Why does a cotton towel absorb water better than one made of polyester or other synthetic fabrics?

A: Cotton’s ability to absorb water has to do with a phenomenon called hydrogen bonding, according to chemists Kerry Karukstis and Gerald Van Hecke of Harvey Mudd College. It is essentially the same phenomenon that explains why oil and water don’t mix. Sugar molecules, for example, contain many hydroxyl groups that can hydrogen bond to water molecules, so it dissolves in water easily. Oil, in contrast, has none and cannot dissolve. Cotton is a naturally occurring polymer of cellulose which, like sugar, has many hydroxyl groups. Although cotton doesn’t dissolve in water, it can hydrogen bond to water molecules and thus hold on to them tightly, removing them from your skin. Polyester, which has few hydroxyl groups, cannot hydrogen bond and so absorbs far less water. This is also why synthetic fibers dry much more quickly than cotton. Fabrics are made waterproof by chemically blocking the hydroxyl groups so that they can no longer hydrogen bond. Most treatments use silicon compounds, but the Scotchgard process uses fluorine chemicals. Neither silicon nor fluorine hydrogen bonds with water molecules, and the treated fabric thus repels water.