Absorbent vs Adsorbent

A material said to be absorbent indicates its ability to carry a certain amount of liquid in its interstice, or little chamber like in a sponge. In the case of a textile fabric, the yarn itself can act like a mini sponge. Also the space between the yarns can be sufficiently tight to trap water and immobilize it due to the surface tension of the water. Yarns of fabric can be made to create additional "traps" for water by brushing.

The table below shows the absorption level for various textile fabrics. The test consists of weighing a fabric after it has been soaked in water.

As you can see natural fibers, (cotton, rayon and wool) provide the highest level of absorption. This is the exact reason for their comfort. Materials that we wear next to the skin feels more comfortable with yarn made of absorbent fibers. Rayon however shows such a high affinity with water that it tends to not let it evaporate thus eliminating the comfort gain.

There are, however, artificial means to alter the behavior of a yarn to mimic the comfort feel of cotton. While it is not possible for nylon, polyester or acetate to reach the absorption level of cotton, treatments exist to make them very hydrophilic to maximize the wicking effect. The wicking effect is obtained by raising the affinity between yarn and water creating a movement of water throughout the fabric in all three dimensions. Thus, a drop of perspiration is dissipated quickly on a larger area on the fabric and is diffused away from the skin more rapidly. The resulting dryness after fast perspiration evaporation gives a sensation of comfort a bit like a cotton shirt. Evaporation not only brings dryness to the skin surface but also induces a cooling effect thanks to the evaporating water taking a few calories (heat energy) while passing to the gas state. The sensation of comfort is very difficult to define scientifically but it is generally recognized that dryness, smoothness (low abrasive), and softness are the three pillars of comfort.

Despite the results that you saw on the above table, once a yarn is woven or knitted into a fabric it can absorb moisture levels beyond the theoretical value. Interstice between yarns provides its share of water traps.This is especially true with wool which is a very fluffy yarn that, when made into a fabric generate millions of sites with water holding capacity.

Now, what about adsorption, with a “d”? The word adsorption was introduced to illustrate the concept of ‘’chemical absorption’’ which occurs when a substance is caught either in nanopores or to the surface of a substrate by low energy (Van der Waal Forces). This is the principle used for activated carbon filter where billions of tiny pores will clean contaminants from a passing fluid. These contaminants can be extracted fairly easily by a steam process. The more one tries to explain the difference between absorption and adsorption, the more it appears that both concepts are in fact very similar. The main difference is that adsorption refers to trapping substances on a nanometric scale (1X10-9).

So from now on you may disregard the word adsorption from your vocabulary and use only absorption to express the capacity to hold a liquid within a structure.