Tony O’Lenick explains the difference between absorption and adsorption for the benefit of novice formulators.
Absorption describes a process wherein molecules enter a bulk phase; for example, a dry sponge absorbs water. The molecule being absorbed is taken into the material doing the absorption.
Adsorption describes a process wherein only the surface of the material doing the adsorption is utilized. In the adsorption process, the material being adsorbed accumulates on the surface of a adsorbent.
The difference is that in absorption, the substance being absorbed diffuses into the material, while in adsorption, the substance merely interacts with the surface of the material.
Activated carbon is an example of an absorbant. Activated carbon also is a form of carbon that has been processed to make it extremely porous and thus to have a large surface area available for adsorption.
A personal care formulator needs to understand that the two phenomenon are different when they formulate with pigments and inorganic sunscreens. If a pigment coating in a formulation merely adsorbs, the coating will be more efficient. However, if the coating is absorbed into pigment, more coating is needed and the coating will be less efficient.