(Inside Science) -- The stiff, crunchy feel of an air-dried cotton towel is caused by a small amount of residual water “gluing” the fibers together, new research shows. Even in the driest climates, cotton naturally retains water because its main component -- cellulose -- attracts water molecules. At 77 degrees Fahrenheit (25 C) and 60% relative humidity, about 8% of cotton’s weight is water.
Scientists in Japan have now shown how this water is present on the surface of air-dried cotton fibers. While their measurements are not the first to reveal this so-called bound water, they are the first to locate it precisely on the surface. The bound water, which in this state is more gummy than ordinary water, acts somewhat like hair spray, said Takako Igarashi, one of the scientists who worked on the new research. In hair spray, a polymer substance sticks hair fibers together. In an air-dried towel, the bound water holds the cotton fibers together. It does this through a network of strong hydrogen bonds, both between the water molecules themselves and between the bound water and the hydroxyl groups (made up of an oxygen atom and a hydrogen atom) in the cellulose.
An illustration of single cotton fibers linked together by bound water.
Credit: Takako Igarashi, et al., "Observation of Bound Water on Cotton Surfaces by Atomic Force Microscopy and Atomic Force Microscopy–Infrared Spectroscopy," Journal of Phys. Chem. C., 2020
Rights: This image may be reproduced with this Inside Science article.
Igarashi works for the Kao Corporation, which makes fabric softener, among other products. Her team first proposed the new idea that bound water could make fabrics stiff in a 2016 paper. They found that mechanical force such as what happens when clothes are tossed around in a dryer could make fabric softer, likely because it breaks the hydrogen bonds in the bound water. They also found when all the water was removed from an air-dried towel by heating it in a vacuum, the fluffiness returned. For the new research, the team collaborated with Ken-ichiro Murata at Hokkaido University in Japan to directly observe the water on the surface of the fibers. They published the results in January in the Journal of Physical Chemistry C.
Igarashi said the team hopes to use their new understanding of why towels get stiff to design improved softening products. But that’s only part of the reason for the research. She always keeps in mind these words from one of her mentors, which she shared in an email to Inside Science: “As a scientist, to write the next page of the textbook, or to replace the old theory with a new, better theory is always quite exciting."