Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 115

air, its surface may become covered by pollutants. This leads to lower adsorption
of proteins than on a clean surface. A silica surface has been considered to exist as
O-Si-O as well as hydroxyl groups formed with water molecules. The orientation of
the different groups may also be different at the surface.
Carbon black has been reported to possess different kinds of surface chemical
groups. These are aromatics, phenol, carboxylic, etc. The different sites can be esti-
mated by comparing the adsorption characteristics of different adsorbents (such as
hexane and toluene).
When any clean solid surface is exposed to a gas, the latter may adsorb on the
solid surface to varying degree. It has been recognized for many decades that gas
adsorption on solid surfaces does not stop at a monolayer state. Of course, more than
one layer (multilayer) adsorption will take place only if the pressure is reasonably
high. Experimental data show this when the volume of gas adsorbed, v gas , is plotted
against P gas (Figure 5.6).
These analyses showed that there were five different kinds of adsorption states
(Figure 5.6). The adsorption isotherms were classified based on v gas versus P gas data
by Brunauer.

Type I: These are obtained for Langmuir adsorption.
Type II: This is the most common type where multilayer surface adsorption
is observed.
Type III: This is a somewhat special type with usually only a multilayer forma-
tion, such as nitrogen adsorption on ice.
Type IV: If the solid surface is porous, this is similar to type II.
Type V: On porous solid surfaces type III.

The pores in a porous solid surface are found to vary from 2 nm to 50 nm ( micro-
pores). Macropores are designated for surfaces larger than 50 nm. Mesopores are
used for 2 to 50 nm range.

5.5.1 g a S a d S o r p T I o n m e a S u r e m e n T m e T h o d S

5.5.1.1 volumetric change methods
The change in the volume of gas during adsorption is measured directly in principle,
and the apparatus is comparatively simple (Figure 5.7).
A mercury reservoir beneath the manometer and the burette are used to control
the levels of mercury in the apparatus in Figure 5.7. Calibration involves measuring
the volumes of the gas (v g ) lines and the void space (Figure 5.7). All pressure mea-
surements are made with the right arm of the manometer set at a fixed zero point
so that the volume of the gas lines does not change when the pressure changes. The
apparatus, including the sample, is evacuated, and the sample is heated to remove
any previously adsorbed gas. A gas such as helium is usually used for the calibration
since it exhibits very low adsorption on the solid surface. After helium is pushed into
the apparatus, a change in volume is used to calibrate the apparatus, and the corre-
sponding change in pressure is measured.