Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 120

Gas

Solid

FIGure 5.9 Brunuer-Emmett-Teller (BET) model for multilayer adsorption on solids.

either data plots that are as type III or II. If the value of E BET is low, which means the
interaction between adsorbate and solid are weak, then type III plots are observed.
This has been explained as arising from the fact that if E1# Ev, then molecules will
tend to form multilayer in patches rather than adsorb on the naked surface.
If strong interaction exists between the gas molecule and the solid, E1 ≥ Ev, then
type II plots are observed. The monolayer coverage is clearly observed at low values
of p/po.

5.6 adSorPtIon From SolutIon on SolId SurFaceS
A clean, solid surface is actually an active center for adsorption from the surround-
ings (e.g., air or liquid). A perfectly cleaned metal surface, when exposed to air, will
adsorb a single layer of oxygen or nitrogen (or water). Or, when a completely dry
glass surface is exposed to air (with some moisture), the surface will adsorb a mono-
layer of water. In other words, the solid surface is not as inert as it may seem to the
naked eye. This has many consequences in industry, such as with corrosion control.
Accordingly, solid surfaces should always be exposed to vacuum prior to any kind
of adsorption studies.

5.6.1 T h e r m o d y n a m I c S o f a d S o r p T I o n
Activated charcoal or carbon (with surface area of over 1000 m 2 /g) is widely used for
vapor adsorption and in the removal of organic solutes from water. These materials
are used in industrial processes to purify drinking water and swimming pool water,
to decolorize sugar solutions as well as other foods, and to extract organic solvents
(especially trace amounts of dangerous substances). They are also used as a first oral
treatment in hospitals for cases of poisoning. Activated charcoal can be made by
heat degradation and partial oxidation of almost any carbonaceous matter of animal,
vegetable, or mineral origin. For convenience and economic reasons, it is usually
produced from bones, wood, lignite, or coconut shells.
The complex three-dimensional structure of these materials is determined by
their carbon-based polymers (such as cellulose and lignin), and it is this backbone
that gives the final carbon structure after thermal degradation. These materials,
therefore, produce a very porous high-surface-area carbon solid. In addition, the car-
bon has to be activated so that it will interact with and physisorb (i.e., adsorb physi-
cally, without forming a chemical bond) a wide range of compounds. This activation
process involves controlled oxidation of the surface to produce polar sites.