92168.pdf

The solid-gas interface 121

but this technique has largely been superseded by the use of beam-

type vacuum microbalances.

An alternative gas-adsorption method, which does not require

vacuum equipment, is that developed by Nelsen and Eggersten. It

bears some similarity to gas chromatography. A gas mixture

containing the adsorbate (usually nitrogen) and a carrier gas (usually

helium) is passed over the solid under test at room temperature. The

gas flow into and out of the sample container is monitored by means

of a pair of thermal conductivity detectors. When equilibrium has

been established, the sample is cooled (e.g. by immersion in liquid

nitrogen). Owing to gas adsorption, the outlet stream is depleted for

a time in adsorbate, the thermal conductivity detectors are thrown off

balance and the amount of gas adsorbed can be measured in terms of

the area under a peak on a recording potentiometer. On warming the

sample, desorption takes place and a negative peak of equal area is

given. This technique is extremely useful for surface area determina-

tion by the BET method (see page 134).

Classification of adsorption isotherms

Three phenomena may be involved in physical adsorption:

1. Monomolecular adsorption.


2. Muitimolecular adsorption.


3. Condensation in pores or capillaries.

Frequently, there is overlapping of these phenomena, and the
interpretation of adsorption studies can be complicated. Brunauer
has classified adsorption isotherms into the five characteristic types
shown in Figure 5.5. To facilitate comparison of isotherms, it is
preferable to plot them in terms of relative pressures (p/po), where p 0
is the saturation vapour pressure, rather than pressure itself. This
also has the advantage of giving a 0 to 1 scale for all gases.
Type I isotherms (e.g. ammonia on charcoal at 273 K) show a fairly
rapid rise in the amount of adsorption with increasing pressure up to
a limiting value. They are referred to as Langmuir-type isotherms and
are obtained when adsorption is restricted to a monolayer. Chemi-
sorption isotherms, therefore, approximate to this shape. Type I
isotherms have also been found for physical adsorption on solids