Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 124

Area per pyridine molecule = 24 Å 2 = 24 10 −16 cm 2
Surface area of 1 gm charcoal = 1000 m 2 = 1000 10 4 cm 2
Molecules pyridine adsorbed = 1000 10 4 cm 2 /24 10 −16 cm 2 /molecule
= 40 10 20 molecules
Amount of pyridine per gram charcoal adsorbed =
(40 10 20 molecules/6 10 23 )10 0
= 0.7 g

This is a useful example to illustrate the application of charcoal (or similar substances
with large surface area per gram) in the removal of contaminants by adsorption.

5.6.2.2 adsorption in binary liquid Systems
This discussion so far has been confined to systems in which the solute species are
dilute, so that adsorption was not accompanied by any significant change in the
activity of the solvent. In the case of adsorption from binary liquid mixtures, where
the complete range of concentration, from pure liquid A to pure liquid B, is avail-
able, a more elaborate analysis is needed. The terms solute and solvent are no longer
meaningful, but it is nonetheless convenient to cast the equations around one of the
components, arbitrarily designated here as component 2.

5.6.3 h e a T S of f a d S o r p T I o n (d I f f e r e n T S u b S T a n c e S ) o n S of if I d S u r f a c e S
A solid surface interacts with its surrounding molecules (in the gas or liquid phase)
in varying degrees. For example, if a solid is immersed in a liquid, the interaction
between the two bodies will be of interest. The interaction of a substance with a solid
surface can be studied by measuring the heat of adsorption (besides other methods).
The information one needs is whether the process is exothermic (heat is produced)
or endothermic (heat is absorbed). This leads to the understanding of the mechanism
of adsorption and helps in the application and design of the system. Calorimetric
measurements have provided much useful information. When a solid is immersed in
a liquid (Figure 5.10), in most cases there is a liberation of heat:

q imm = E S − E SL (5.24)

where E S and E SL are the surface energy of the solid surface and the solid surface in
liquid, respectively.
The quantity q imm is measured from calorimetry where temperature change is
measured after a solid (in a finely divided state) is immersed in a given liquid. Since
these measurements can be carried out with microcalorimeter sensitivity, there is
much systematic data in the literature on this subject. When a polar solid surface is
immersed in a polar liquid, it can be expected that there will be a larger q imm than if
the liquid was an alkane (nonpolar). The values of some typical systems is depicted
in Table 5.5.
These data further show that such studies are sensitive to the surface purity of
solids. For example, if the surface of glass powder is contaminated with nonpolar
gas, then its q imm value will be lower than that in the case of pure glass.