C - Lublin

ASORPTION AND SURFACE AREA OF SOILS
Sokołowska Z.
Two factors, surface area and porosity are recognised to play complementary
parts in adsorption phenomena for a vast range of solids. To understand the way, in
which measurements of the adsorption of gases or vapours can be used to obtain
information about surface area and porosity, it is necessary to deal briefly the
concept of the adsorption isotherm.
ADSORPTION OF GASES OR VAPOURS ON SOLIDS
The term adsorption appears to have been introduced by Kayser in 1881 to
connote the condensation of gases on free surfaces, in contradistinction to gaseous
absorption where the gas molecules penetrate into the mass of the absorbing solid.
The term sorption proposed by McBain in 1909, embraces both types of
phenomena, adsorption and absorption. When a solid is exposed in a closed space
to a gas or vapour at some definite pressure, the solid begins to adsorb the gas or
vapour. The amount of gas adsorbed can be then calculated from the fall in
pressure (by application of the gas law if the volumes of vessel and of the solid are
known), or it can be determined directly as the increase in weight of the solid. The
adsorption is a consequence of the field force at the surface of the solid (the
adsorbent), which attracts the molecules of the gas or vapour (the adsorbate). The
forces of attraction emanating from a solid may be of two main kinds, physical and
chemical, and they give rise to physical adsorption and chemisorption,
respectively. The physical adsorption is also called as van der Waals adsorption. In
the present paper we are concerned with physical adsorption.
THE ADSORPTION ISOTHERM OF A GAS OR A VAPOUR
At the boundaries between solids and gases an accumulation of the particles
occurs. The amount adsorbed per gram of a solid depends on the equilibrium
pressure p, the temperature T, and also on the nature of the gas and the solid: N=f
(p, T, gas, solid). This function at a constant T is called the adsorption isotherm if p
increases, and desorption isotherm - if p decreases. For a given gas adsorbed on a
given solid, maintained at a fixed temperature, this equation simplifies to: N=f (p) T,
gas, solid, and if the gas is below its critical temperature the alternative isotherm
equation is: N=f (p/p 0 ) T, gas, solid , where p 0 is the saturated vapour pressure of the gas.
The adsorption, N, may be measured in any suitable units i.e. grams or milligrams,
moles or millimoles, and cm 3 (N.T.P.).
130