Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 114

table 5.4
Some typical γ cr values for Solid
Surfaces

Surface Group γ cr
-CF 2 -

18

-CH 2 -CH 3 - 22

Phenyl 30

Alkyl chlorine 35

Alkyl hydroxyl 40

Source: Birdi, K. S., Ed., Handbook of Surface
and Colloid Chemistry-CD Rom , CRC
Press, Boca Raton; 2002 (Second
Edition).
Note: Estimated from Zisman plots.

5.5 adSorPtIon oF GaS on a SolId SurFace
The solid surface interacts with gases or liquids in various phenomena. The adsorp-
tion of gas on a solid surface has been known to be of much importance in various
systems (especially in industries involved with catalysis). The molecules in gas are
moving very fast, but on adsorption (gas molecules are more or less fixed), there will
be a large decrease in kinetic energy (thus, a decrease in entropy, ΔS). Adsorption
takes place spontaneously,

Δ G ad = Δ H ad − T ΔS ad (5.13)

which means that ΔG ad is negative. This suggests that ΔH ad is negative (exothermic).
The adsorption of gas can be of different types. The gas molecule may adsorb as a
kind of condensation process; it may under other circumstances react with the solid
surface (chemical adsorption or chemisorption). In the case of chemiadsorption, a
chemical bond formation can almost be expected. On carbon, while oxygen adsorbs
(or chemisorbs), one can desorb CO or CO 2 . Experimental data can provide informa-
tion on the type of adsorption. On porous solid surfaces, the adsorption may give
rise to capillary condensation . This indicates that porous solid surfaces will exhibit
some specific properties. Catalytic reactions (e.g., formation of NH 3 from N 2 and H 2 )
give the most adsorption process in industry.
The surface of a solid may differ in many ways from its bulk composition.
Especially, such solids as commercial carbon black may contain minor amounts of
impurities (such as aromatics, phenol, carboxylic acid). This would render surface
adsorption characteristics different from that of pure carbon. It is therefore essential
that, in industrial production, quality control of the surface from different produc-
tion batch is maintained. Otherwise, the surface properties will affect the applica-
tion. Another example arises from the behavior of glass powder and its adsorption
character for proteins. It has been found that if glass powder is left exposed to the