Adsorption can result either from the universal van der Waals interactions
Ž . physical adsorption, physisorption or it can have the character of a chemical
process chemical adsorption or chemisorption . Contrary to physisorption, Ž .
chemisorption occurs only as a monolayer 7 . Physical adsorption can be compared
to the condensation process of the adsorptive. As a rule, it is a reversible process
that occurs at a temperature lower or close to the critical temperature of an
adsorbed substance.