“Absorbent” and “adsorbent” are two words that look very much alike, and refer to actions that may superficially appear the same. But absorbents and adsorbents work in very different ways – differences that are important in selecting which kind to use for specific tasks.
An absorbent is a material that sucks in liquids and contains them internally. Absorbents typically have a large number of tiny pores. The makeup of the absorbent makes it so effective at soaking up water and other liquids. Loose fibers create a product that is more empty space than anything else, yet form chambers that can retain liquid. The holes between the fibers soak up the liquid and cause the fibrous material itself to swell, which also prevents the liquid from sloshing right back out. Instead, the liquid is trapped inside. If you were to remove all of the empty space, you would see that the actual matter inside an absorbent product would take up less than 1/3 of the absorbent’s apparent volume.
Most so-called “universal” sorbent products are absorbents, which depend on capillarity to draw liquids of all kinds into their structure. Capillary action, or capillarity, is a phenomenon where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials. This wicking effect can cause liquids to flow against the force of gravity. It occurs because of inter-molecular attractive forces between the liquid and solid surrounding surfaces. If the diameter of the tube is sufficiently small, as in the pores of a sponge, then the combination of surface tension (which is caused by cohesion within the liquid) and forces of adhesion between the liquid and pore surfaces acts to lift the liquid.
Universal absorbents are often compounded of various cellulose products such as wood pulp, corncob or paper waste, but can be made of polypropylene or other non-organics.
An adsorbent, on the other hand, is a material that collects liquids on the surface and holds them there. Adsorbents work by selective molecular attraction – only certain molecules are attracted to the surface of the material. Typically this phenomenon is used to create “oil-only” absorbents that can be used to separate out oil from water so that only the oil is removed by the sorbent material, which in contrast repels the water molecules, leaving the water behind, stripped of polluting oils. This type of absorbent is important in cleaning up oil spills in aquatic environments such as dockyards, marinas, harbors, rivers, and lakes. In sufficient volume and in a form such as a boom that does not disperse, it can even be used in the open ocean. Additionally oil absorbents are staple supplies in machine shops, garages, and refineries where oil spills are a constant problem.
Specialized adsorbents are used for cleaning up toxic spills other than oil and in various refining processes where they are used to separate out specific components of a chemical stock flow.
Remembering this simple difference in action between absorbents and adsorbents can help keep them straight in your mind: absorbents suck liquids up; adsorbents attract specific liquids to their surfaces – like the difference between a drinking straw and a magnet.