Physisorption

The physisorption is a form of adsorption in which - an adsorbed molecule is bonded by physical force on a substrate - in contrast to the chemisorption. Physical forces are forces which are not caused by chemical bonds, so are relatively weak. In general, the binding energy of physisorption the range of 4 to 40 kJ / mol. One form of such forces are van der Waals forces, that the electrostatic interaction between the induced fluctuating dipoles. More precisely, one speaks of London'schen dispersion forces. Since the electrons are not fixed in the molecule, caused by fluctuations in its distribution to a temporary dipole moment whose time average is zero, however. Despite its short life can induce an anti-parallel dipole moment this temporary dipole in a neighboring molecule, wherein the forces are inversely proportional to the sixth power of the distance r ( 1/r6 ).

Depending on the adsorbate -adsorbent combination but also greater binding energies are possible. Much stronger forces are generated by the interaction with the solid dipoles at polar surfaces (salts ) or the mirror charges as they occur in electrically conductive surfaces ( metals). However, these interactions are so strong that they must be partly considered as chemisorption.

Properties

The main characteristic of the physical adsorption (as opposed to chemisorption) is the small change of the adsorbate and the adsorbent. Up to relaxations of the substrate lattice, no changes of the adsorbent takes place. In the adsorbate bonds are only slightly changed, which is reflected in changes in vibrational frequencies.

Since the chemical structures of physisorbed material not changed significantly, this process is in principle reversible. As a rule, no activation energy is overcome, the coverage of the adsorbent is very rapid. Surfaces of solids are therefore always covered in air with a thin layer of adsorbed substances. This usually prevents the cold welding of metal surfaces.

Applications

Technically, is made of the rapid adsorption of gases on fresh, uncovered surfaces in sorption and getter advantage. Have electron tubes to obtain the necessary vacuum upright in them, often a getter layer of vapor-deposited titanium. The titanium surface remains fresh and effective as long as the tube is undamaged.

Another application of the physisorption is the air or water treatment by means of activated carbon.