Adsorption

Many people confuse absorption and adsorption because the words sound similar, but they are very different processes. When a material is absorbed, it is held within a porous material by capillary action. A sponge absorbs water, and releases most of it when squeezed. A piece of chalk or a brick will also absorb water, but clearly demonstrate that absorbed materials aren't always easily removable.

Adsorption, on the other hand, is a process involving intermolecular and electrostatic forces.

All atoms and molecules are characterized by the way in which their electronic charge is distributed. There are a variety of ways for charge to distribute, and these help define the chemical and physical properties of the molecule. One of these effects, polarity, helps define the solvent properties of a compound. Another, resonance, is frequently associated with colored organic molecules. Each of these effects also creates unique possibilities for electrostatic attraction between molecules, and these attractive forces can pull molecules together from distances several times the size of the molecule.

At very close range, another attractive force exists between all atoms and molecules. These are called the van der Waals forces, named after the physicist who discovered them. Van der Waals forces are quite powerful once the molecules have been brought close enough together for them to act.

Carbon atoms in charcoal can join together in a wide variety of forms with widely different electron distributions, meaning that it will have areas on its surface that are attractive to almost every kind of molecule. Once electrostatic attraction has pulled a molecule close to the surface of the carbon, van der Waals forces take over and hold it in place tightly. Charcoal also has millions of tiny pores in its structure, which create a very large surface area for adsorption to occur.

Treatments of the carbon or charcoal can alter the distribution of pore sizes and of electrical effects, "tailoring" the carbon to primarily attract and hold certain types of compounds.

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