When a gas or vapor contacts a clean solid, one part will remain in contact with the surface as an adsorbed layer. The solid is the adsorbent and the gas is the “adsorbee”. Its also possible that some of the gas will be adsorbed into the metal. Adsorption and absorption are not experimentally distinguishable, sorption being the generic term used to describe the phenomenon of bonds between gases and solids. The bonds involved in adsorption of gases and vapors by solids are van der Waals bonds, which are weak and nondirectional1.
Any solid is capable of adsorbing a certain quantity of gas. The quantity depends on the temperature and pressure on the adsorbent substrate and the specific characteristics of the surface. The most notable adsorbents are extremely porous materials, like charcoal and silicon gel, which in fine powder form have surface areas close to 100m2/g. With this, only 11 grams would be needed to hypothetically2, cover an area the size of a soccer field 120 meters long by 90 meters wide. The relation between the quantity of gas adsorbed and the pressure of the gas, at a given temperature, is known as adsorption isothermals. Figure 2.3.9 shows the adsorption isotermals of ammonia gas (NH3) in charcoal. Notice that as the temperature goes down the adsorption of gas goes up. For most cases, adsorption of a gas by a solid is an exothermic reaction3.
1If compared to the covalent (directional) and ionic bonds. The heat released during physical adsorption is about 10 times lower than during the chemisorption, where there are chemical bonds with the substrate.
2If we separate particle by particle of the powder.
3Exothermic reaction: that releases heat to the environment.
