Physisorption
The physisorption is a form of adsorption in which an adsorbed molecule through Van der Waals force on a substrate is bonded - in contrast to the chemisorption occur in the adsorption enthalpies in the order of chemical binding enthalpies.
properties
The electrostatic forces acting in physisorption are weaker than the electrostatic forces in chemical bonds . As a rule, the binding energies of physisorption are in the range from 4 to 40 kJ / mol. One form of such forces are the van der Waals forces, i.e. the electrostatic interaction between induced , fluctuating dipoles . More precisely, one speaks here of London's dispersion forces . Since the electrons are not fixed in the molecule, fluctuations in their distribution create a temporary dipole moment whose mean value over time is zero. Despite its short life, this temporary dipole moment can induce an antiparallel dipole moment in a neighboring molecule, the forces acting being inversely proportional to the sixth power of the distance r (1 / r 6 ).
However, depending on the adsorbate-adsorbent combination, higher binding energies are also possible. Much stronger forces arise through the interaction with the solid dipoles on polar surfaces ( salts ) or the mirror charges , as they occur in electrically conductive surfaces (metals). However, these interactions are already so strong that some of them have to be viewed as chemisorption.
The most important property of physisorption (as opposed to chemisorption) is the slight change in the adsorbate and adsorbent . Except for relaxation of the substrate lattice, there are no changes in the adsorbent. Only the bonds are slightly changed in the adsorbate, which is noticeable in changed vibration frequencies.
Since the chemical structures of the physisorbed substance do not change significantly, this process is in principle reversible. Since there is generally no activation energy to overcome, the adsorbent is covered very quickly. The surfaces of solid bodies are therefore always covered with a thin layer of adsorbed substances in air. This usually prevents cold welding of metal surfaces.
The direct transition of an adsorbate from the weak van der Waals bond in physisorption to the formation of a chemical bond was demonstrated with the aid of atomic force microscopy.
Vroman effect
The Vroman effect was first described by Leo Vroman based on the binding of plasma proteins . The effect comprises the successive accumulation in mixtures of proteins of different molar mass or chain length. He found that the plasma proteins albumin , globulin , fibrinogen , fibronectin , factor XII and kininogens are adsorbed one after the other.
Small molecules with a low molar mass diffuse faster and bind to a surface first, they are kinetically favored. Larger molecules have a higher binding energy, which means that smaller molecules are exchanged for larger ones. In the balance especially larger molecules are adsorbed.
Applications
Technically, the rapid adsorption of gases on fresh, uncovered surfaces is used in sorption pumps and getter pumps. Electron tubes often have a getter layer made of vapor-deposited titanium in order to maintain the vacuum they need. The titanium surface remains fresh and effective as long as the tube is undamaged.
Another application of physisorption is air or water purification with the help of activated carbon .
literature
- Andrew Zangwill: Physics at surfaces , Cambridge University Press 1988, ISBN 0-521-34752-1
Individual evidence
- ↑ Ferdinand Huber, Julian Berwanger, Svitlana Polesya, Sergiy Mankovsky, Hubert Ebert: Chemical bond formation showing a transition from physisorption to chemisorption . In: Science . September 12, 2019, ISSN 0036-8075 , p. eaay3444 , doi : 10.1126 / science.aay3444 ( sciencemag.org [accessed September 18, 2019]).
- ↑ Vroman, L., Adams, AL, Fischer, GC, Munoz, PC: Interaction of high molecular weight kininogen, factor XII, and fibrinogen in plasma at interfaces . In: Blood . 55, No. 1, 1980, pp. 156-9. PMID 7350935 .