Physisorption, or physical sorption, is restricted to adsorption and occurs when the forces involved are weak intermolecular forces of the same kind as those responsible for the non-ideality of gases and the condensation of vapors. Weak intermolecular forces are also known as van der Waals forces. Physisorption does not involve a significant change in the electronic structure of the species involved. Physisorption is also known as physical adsorption and it is an exothermic process. Its adsorption enthalpy is low, nearly 20 to 40 kJ/mol. Normally in physisorption, the gas is accumulated on the solid surface due to weak force. Physisorption of gas adsorbed at a lower temperature may be converted into chemisorption at a higher temperature.  Physisorption lacks specificity because the adsorbent (the surface or the material on which the process of adsorption takes place) in the given surface does not show any particular gas. It has a reversible nature, that is physisorption of gas by a solid can be reversed to a solid by gas. An example of physisorption is the adsorption of gases like hydrogen, nitrogen, etc at lower temperature on the surface of an adsorbent like charcoal. Physisorption depends on the surface area of the adsorbent. As the surface area increases, the extent of adsorption also increases. For example, finely divided metals and porous substances have a large surface area. Therefore, they are considered good adsorbents. It also depends on the nature of the adsorbate (the accumulation of molecular species or substance at the surface).

Chemisorption or chemical sorption is a surface specific phenomenon, and occurs when the interaction forces between a surface and an adsorbate are of the same general strength as found in chemical bonding in bulk compounds. In chemisorption, adsorption takes place in adsorbed substance that is held by chemical bonds. Chemisorption has high specificity that is it is highly specific, and it takes place only if there is a chemical bonding between adsorbent and adsorbate. Chemisorption has an irreversible nature and it also favours high pressure. Due to chemical bonding, enthalpy of adsorption of chemisorption is high, nearly 80 to 240 kJ/mol. Chemisorption depends on the surface area. As the surface area of the adsorbent increases, chemisorption also increases. An example of chemisorption is the adsorption of hydrogen, nitrogen, etc on the surface of the adsorbent like ferrous catalyst at a high temperature.

For a molecular adsorbate where no bond dissociation occurs, it is often difficult to draw a boundary in the energy landscape between strong physisorption and weak chemisorption. It is useful to make a distinction between molecular physisorption, in which the H-H bond in the gas phase is preserved in the sorbed state, and chemisorption, in which the H-H bond is broken during the sorption process. 

As described in more detail below, the main differences between chemisorption and physisorption is: chemisorption may occur only in a monolayer on a surface, it is unilayer, directionial and strong; whereas physisorption is usually accompanied by multilayer adsorption, it is weak, non-directtional and non-specific, depending upon the temperature.

The following excerpt from D.H. Everett’s IUPAC publication; “Manual of Symbols and Terminology for Physicochemical Quantities and Units” provides a short synopsis on differentiating chemisorption from physisorption.

“Chemisorption (or chemical adsorption) is adsorption in which the forces involved are valence forces of the same kind as those operating in the formation of chemical compounds. The problem of distinguishing between chemisorption and physisorption is basically the same as that of distinguishing between chemical and physical interaction in general. No absolutely sharp distinction can be made and intermediate cases exist, for example, adsorption involving strong hydrogen bonds or weak charge transfer."

Some features which are useful in recognizing chemisorption include:

(a) the phenomenon is characterized by chemical specificity;

(b) changes in the electronic state may be detectable by suitable physical means (e.g. u.v., infrared or microwave spectroscopy, electrical conductivity, magnetic susceptibility);

(c) the chemical nature of the adsorptive(s) may be altered by surface dissociation or reaction in such a way that on desorption the original species cannot be recovered; in this sense chemisorption may not be reversible;

(d) the energy of chemisorption is of the same order of magnitude as the energy change in a chemical reaction between a solid and a fluid: thus chemisorption, like chemical reactions in general, may be exothermic or endothermic and the magnitudes of the energy changes may range from very small to very large;

(e) the elementary step in chemisorption often involves an activation energy;

(f) where the activation energy for adsorption is large (activated adsorption), true equilibrium may be achieved slowly or in practice not at all. For example in the adsorption of gases by solids the observed extent of adsorption, at a constant gas pressure after a fixed time, may in certain ranges of temperature increase with rise in temperature. In addition, where the activation energy for desorption is large, removal of the chemisorbed species from the surface may be possible only under extreme conditions of temperature or high vacuum, or by some suitable chemical treatment of the surface;

(g) since the adsorbed molecules are linked to the surface by valence bonds, they will usually occupy certain adsorption sites on the surface and only one layer of chemisorbed molecules is formed.

Physisorption (or physical adsorption) is adsorption in which the forces involved are intermolecular forces (van der Waals forces) of the same kind as those responsible for the imperfection of real gases and the condensation of vapors, and which do not involve a significant change in the electronic orbital patterns of the species involved. The term van der Waals adsorption is synonymous with physical adsorption, but its use is not recommended.

Some features which are useful in recognizing physisorption include:

(1) the phenomenon is a general one and occurs in any solid/fluid system, although certain specific molecular interactions may occur, arising from particular geometrical or electronic properties of the adsorbent and/or adsorptive;

(2) evidence for the perturbation of the electronic states of adsorbent and adsorbate is minimal;

(3) the adsorbed species are chemically identical with those in the fluid phase, so that the chemical nature of the fluid is not altered by adsorption and subsequent desorption;

(4) the energy of interaction between the molecules of adsorbate and the adsorbent is of the same order of magnitude as, but is usually greater than, the energy of condensation of the adsorptive;

(5) the elementary step in physical adsorption from a gas phase does not involve an activation energy. Slow, temperature dependent, equilibration may however result from rate-determining transport processes;

(6) in physical adsorption, equilibrium is established between the adsorbate and the fluid phase. In solid/gas systems at not too high pressures the extent of physical adsorption increases with increase in gas pressure and usually decreases with increasing temperature. In the case of systems showing hysteresis the equilibrium may be metastable;

(7) under appropriate conditions of pressure and temperature, molecules from the gas phase can be adsorbed in excess of those in direct contact with the surface.

The terminology “adsorbate” in 6’ above refers to the adsorbed molecules of gas. However, in general “adsorbate” refers generally to the gas that has been or is capable of being adsorbed, so it may include non-adsorbed species. Line 7 above points out the important issue that the gas may experience attractive adsorptive interaction while not being in direct contact with the surface. Thus, the adsorption layer may be thicker than a monolayer of gas. For more details, the reader is encouraged to review the related publications by Everett.

figure 1: illustration of the components of a physisorption system

Physisorption is a universal interaction between a gas and an adsorbent surface (Figure 1). The origins of H2 physisorption are attractive dispersion interactions (i.e. London interactions) and short-range repulsion. The dispersion forces arise from spontaneous resonant fluctuations of electron density in one atom which induce a transient dipole moment in a neighboring atom. Since H2 contains only two electrons, the dispersion interactions are weak. Therefore, the energy minimum of the total interaction pair potential is small, and is on the same order as the thermal energies of the adsorptive particles at ambient conditions. Physisorption is, therefore, only observed in significant amounts at cryogenic temperatures. Frequently, the adsorbent sample is kept at 77 K (N2 boiling point) or 87 K (Ar boiling point) in experiments. Liquid N2-based cryostats are often used to obtain a greater temperature range above 77 K. Adsorption is exothermic, and enthalpies between 4 and 10 kJmol-1 are typically reported for H2 physisorption.

Difference between Physisorption and Chemisorption