Adsorption is the phenomenon marked by an increase in density of a fluid near the
surface, for our purposes, of a solid.* In the case of gas adsorption, this happens when
molecules of the gas occasion to the vicinity of the surface and undergo an interaction
with it, temporarily departing from the gas phase. Molecules in this new condensed
phase formed at the surface remain for a period of time, and then return to the gas
phase. The duration of this stay depends on the nature of the adsorbing surface and the
adsorptive gas, the number of gas molecules that strike the surface and their kinetic
energy (or collectively, their temperature), and other factors (such as capillary forces,
surface heterogeneities, etc.). Adsorption is by nature a surface phenomenon, governed
by the unique properties of bulk materials that exist only at the surface due to bonding
deficiencies.