Why Does Cotton Absorb So Much Water?

You've heard it before. Cotton Kills! Cotton Kills! Why does it kill? Because it absorbs lots of water—up to 27 times its weight—which rapidly wicks away body heat and can lead to potentially life-threatening hypothermia in cool or cold conditions. But what is it about cotton that allows it to absorb so much water in the first place?

There are two primary reasons: structure and chemistry. First, the easier-to-explain structure. A cotton fiber is like a tiny tube formed of six different concentric layers (see diagram). As individual cotton fibers grow on the plant, the inside of the "tube" is filled with living cells. Once the fiber matures and the cotton boll opens up to reveal its puffy white contents, these cells dry up and the fiber partially collapses, leaving behind a hollow bean-shaped canal, or "lumen" (see the ultra-magnified image below). This empty space holds lots of water.

Lumens also help provide cotton with its exceptional "wicking" ability, drawing water up along the fibers through capillary action—like sucking on a straw. (Synthetic fibers like nylon are solid, with no internal spaces within the fiber to contain water. Whatever water is absorbed is contained on the fibers' surfaces.) Lumens also radically increase the surface area of the fiber for water to interact with, which leads to the chemistry part of this.

Processed cotton fibers are 99 percent cellulose. Cellulose is a polymer composed of a long chain of connected glucose molecules that each contains three hyrodoxol groups with slight negative charges. Water, as you may remember from high school chemistry, has a slightly positive charge (the oxygen atom draws in the two hydrogen atoms' electrons). The upshot is that water molecules are attracted to—and bond with (via "hydrogen bonds")—the zillions of hydroxol groups in cotton. This, coupled with the vast amount of space contained within and between the fibers, provides cotton with its tremendous water-absorbing properties.

So now you know! To learn more (way more), check out this very informative Cotton Technical Guide from Cotton, Inc., the industry's primary advocacy group.

Equipped” is an AMC Outdoors blog, written by Matt Heid.