Gas
sorption (both adsorption and desorption)
at the clean surface of dry solid powders
is the most popular method for determining
the surface area of these powders as
well as the pore size distribution of
porous materials. In a gas sorption
experiment, the material is heated and
degassed by vacuum force or inert gas
purging to remove adsorbed foreign molecules.
Controlled doses of an inert gas, such
as nitrogen, krypton, or argon, are
introduced and the gas is adsorbed,
or alternatively, withdrawn and desorbed.
The sample material is placed in a vacuum
chamber at a constant and very low temperature,
usually at the temperature of liquid
nitrogen (-195.6 °C), and subjected
to a wide range of pressures, to generate
adsorption and desorption isotherms.
The amounts of gas molecules adsorbed
or desorbed are determined by the pressure
variations due to the adsorption or
desorption of the gas molecules by the
material (the adsorbent). Various amounts
of gas molecules will be adsorbed or
desorbed at different doses of the gas
(the adsorbate). Knowing the area occupied
by one adsorbate molecule, σ (for
example, σ= 16.2 Å
2
for nitrogen), and using an adsorption
model, the total surface area of the
material can be determined. The most
well known and widely used is the BET
equation for multilayer adsorption [i]:
In Eq. 1, P, P
O,
c, n, n
m are the adsorption
pressure, the saturation vapor pressure,
a constant, the amount adsorbed (moles
per gram of adsorbent) at the relative
pressure P/P
O, and the monolayer
capacity (moles of molecules needed
to make a monolayer coverage on the
surface of one gram of adsorbent), respectively.
Through the slope and intercept of a
plot of P/[n(P
O-P)] against
(P/P
O), n
m can be resolved.
The specific surface area, S, can then
be derived:
In Eq. 2, N
A is Avogadro’s
number. The specific surface area that
can be determined by gas sorption ranges
from 0.01 to over 2000 m
2/g.
Determination of pore size and pore
size distribution of porous materials
can be made from the adsorption/desorption
isotherm using an assessment model,
suitable for the shape and structure
of the pores. The range of pore sizes
that can be measured using gas sorption
is from a few Ångstroms up to
about half a micron.
