Similar to
surface tension, adsorption is a consequence of
surface energy. In a bulk material, all the bonding requirements (be they
ionic,
covalent or
metallic) of the constituent
atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as
physisorption (characteristic of weak
van der Waals forces) or
chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction From Wikipedia Adsorption link.