2.2  How do Molecules Bond to Surfaces ?

There are two principal modes of adsorption of molecules on surfaces :

Physical Adsorption ( Physisorption )
Chemical Adsorption ( Chemisorption )

The basis of distinction is the nature of the bonding between the molecule and the surface. With ...

Physical Adsorption : the only bonding is by weak Van der Waals - type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface.

Chemisorption : a chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic or complete covalent character.

 
Typical Characteristics of Adsorption Processes

Chemisorption Physisorption
Material Specificity
(variation between substrates of different chemical composition)
Substantial variation between materials Slight dependence upon substrate composition
Crystallographic Specificity
(variation between different surface planes of the same crystal)
Marked variation between crystal planes Virtually independent of surface atomic geometry
Temperature Range
(over which adsorption occurs)
Virtually unlimited
(but a given molecule may effectively adsorb only over a small range)
Near or below the condensation point of the gas
(e.g. Xe < 100 K, CO2 < 200 K)
Adsorption EnthalpyWide range (related to the chemical bond strength)
- typically 40 - 800 kJ mol-1
Related to factors like molecular mass and polarity
- typically 5-40 kJ mol-1
(similar to heat of liquefaction)
Nature of AdsorptionOften dissociative
May be irreversible
Non-dissociative
Reversible
Saturation Uptake
 
Limited to one monolayer Multilayer uptake possible
Kinetics of Adsorption
 
Very variable - often an activated process Fast - since it is a non-activated process

The most definitive method for establishing the formation of a chemical bond between the adsorbing molecule and the substrate ( i.e. chemisorption ) is to use an appropriate spectroscopic technique, for example

IR ( see Section 5.4 ) - to observe the vibrational frequency of the substrate/adsorbate bond

UPS ( see Section 5.3 ) - to monitor intensity & energy shifts in the valence orbitals of the adsorbate and substrate

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