Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 117

Gas Adsorption

Gas

Mercury

Solid

FIGure 5.7 Gas adsorption apparatus.

Different gases (such as nitrogen) are normally used as the adsorbate if the sur-
face area of a solid needs to be examined. The gas is cooled by liquid nitrogen. The
tap to the sample bulb is opened, and the drop in pressure is determined. In the
surface area calculations, a value of 0.162 nm 2 is used for the area of an adsorbed
nitrogen molecule.
Because of the toxic properties of Hg, modern apparatus use a combination of
valves to measure the change in the volume of gas adsorbed. Commercially available
instruments are designed with such modern detectors.

5.5.1.2 Gravimetric Gas adsorption methods
The amount of gas adsorbed on the solid surface is very small. A modern sensitive
microbalance is used to measure the adsorption isotherm. Its sensitivity is very high
since only the difference in weight change is measured. These microbalances can
measure weight differences in the range of nanograms to milligrams. With such
extreme sensitivity, it is possible to measure the weight change caused by the adsorp-
tion of a single monolayer on a solid if the surface area is large. The normal procedure
is to expose the sample to the adsorbate gas at a certain pressure, allowing sufficient
time for equilibrium to be reached, and then determining the mass change. This is
repeated for a number of different pressures, and the number of moles adsorbed as a
function of pressure is plotted to give an adsorption isotherm.
Microbalances (stainless steel) can be made to handle pressures as high as 120
Mpa (120 atm) since gases that adsorb weakly or boil at very low pressures can still
be used.

5.5.1.3 langmuir Gas adsorption
This equation assumes that only one layer of gas molecule adsorbs. A monolayer of
gas adsorbs in the case in which there are only a given number of adsorption sites for
a monolayer. This is the most simple adsorption model. The amount adsorbed, N s , is
related to the monolayer coverage, N sm , as follows (Appendix C):