Absorbent vs Adsorbent

Most people are familiar with the term "absorbent" whereas "adsorbent" is often mistaken for a misprint of the word absorbent. The substitution of the "b" by a "d" makes the whole difference in these two words.

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. 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.

Adsorption - Is a process that occurs when a gas or liquid accumulates on the surface of a solid or, more rarely, a liquid forming a molecular or atomic film It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. The term sorption encompasses both processes, while desorption is the reverse process.

Adsorption is operative in most natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, synthetic resins and water purification. Adsorption, ion exchange and chromatography are sorption processes in which certain adsorptives are selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column.

Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent or metallic) of the constituent atoms of the material are filled. But atoms on the (clean) surface experience a bond deficiency, because they are not wholly surrounded by other atoms. Thus it is energetically favourable for them to bond with whatever happens to be available. The exact nature of the bonding depends on the details of the species involved, but the adsorbed material is generally classified as exhibiting physisorption or chemisorption.

American Society for Testing and Materials(ASTM)
Definitions: taken from "Standard Test Methods for Sorbent Performance of adsorbents – ASTM Designation F726 – 99."