C - Lublin

The adsorption isotherm is the most popular expression of adsorption data. A
complete adsorption isotherm covers the whole range of equilibrium pressures
from very low pressures to the neighbourhood of the saturation pressure. Complete
adsorption isotherms are common pointing by plotting along the abscissa the
relative vapour pressure p/p 0 . The isotherm naturally start at the origin of the
coordinates and they end is at a nearly of the saturated vapour. No simple
interpretation can be given to the main part of the curve. It is often supposed that at
the higher relative pressures (at which adsorption hysteresis occurs) the adsorbed
substance is capillary condensed, while at the lower p/p 0 the surface of adsorbent is
covered with a thin layer of gas molecules. The beginning part of the isotherm is
used to obtain the surface area, and the end part to evaluate the pore structure in a
solid body. The volume of liquid which is adsorbed at nearly saturated vapour by 1
gram of adsorbent is called the pore volume of the adsorbent.
Theory and equation of the adsorption isotherm
In many instances an algebraic expression of the adsorption isotherm is more
convenient than its graphic presentation. A few equations have been found to
reproduce a large number of experimental isotherms.
1/ n
The older one, commonly know as Freundlich isotherm: N = kp , where N
is the amount of adsorbed of gas, p is the pressure, k and 1/n are constants. Many
gases and vapours have 1/n values between 0,3 and 0,5.
The other equation is knows as Langmuir isotherm: N = ( Nm kp)
(1 + kp)
,
where k is the constant. Langmuir regarded the surface of the solid as array of
adsorption sites, each site being capable of adsorbing one molecule, and all sites
were characterised by the same adsorption energy, and the attractive interactions
between adsorbed molecules could be ignored. The Langmuir equation described
localised monolayer adsorption on the homogeneous surface of adsorbent.
Brunauer, Emmett and Teller approach the problem of adsorption kinetically.
In 1938, they explicitly extended the Langmuir evaporation-condensation
mechanism to second and higher molecular layers. The state of affairs when
equilibrium is reached at any given pressure may be represented formally as
varying numbers of molecules being condensed on any one site. The BET model
assumes that the surface is energetic uniform i.e. that all adsorption sites are
exactly equivalent. The model neglects horizontal interactions between the
molecules within the adsorption layer, and takes into account only the vertical
interactions, and postulate that the heat of adsorption in the higher layers is equal to
the latent heat of condensation. The BET equation described localised multilayer
adsorption on the homogeneous surface of adsorbent:
131