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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:
- (a)
- 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;
- (b)
- evidence for the perturbation of the electronic states of adsorbent and
adsorbate is minimal;
- (c)
- 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;
- (d)
- 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;
- (e)
- 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;
- (f)
- 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;
- (g)
- 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 (multilayer adsorption or filling of micropores).
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