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are introduced and the gas is adsorbed, or alternatively, withdrawn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.
