CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 450
13.2 Adsorption equilibria
Substances adsorbing from a gas to a solid also create a usually thin surface phase on the solid
surface.
The solid phase that adsorbs the gas is called theadsorbent, and the adsorbed gas is
referred to as theadsorbate.
The amount of adsorbate is most often expressed by way of- The dimensionless mass ratioa
a=
adsorbate mass
adsorbent mass=
madsorbate
madsorbent, (13.32)
- The amount of substance of the adsorbate per unit mass of the adsorbent
an=
nadsorbate
madsorbent, (13.33)
- The gas volume adsorbed per unit mass of the adsorbent under specified conditions
(most often atTref= 273.15 K and pressurepref= 101.325 kPa)
av=Vadsorbate(Tref, pref)
madsorbent, (13.34)
- If we know the adsorbent surface area, we can also express its amount as
aA=
adsorbate mass
adsorbent surface area=
madsorbat
A. (13.35)
U Main units:ais a dimensionless quantity,an: mol kg−^1 ,av: m^3 kg−^1 ,aA: kg m−^2.
13.2.1 Qualitative description of adsorption
Theadsorption isothermexpresses the dependence of the adsorbed amount of the gas com-
ponent on its partial pressure at constant temperature. Figure13.4shows the basic types of
adsorption isotherms. It is obvious that the adsorbed amount monotonously increases with
pressure.
Theadsorption isobarexpresses the dependence of the adsorbed amount of gas on tem-
perature at constant partial pressure. Figure13.5shows the most typical course of adsorption
isobars. It makes it clear that the adsorbed amount of gas decreases with temperature.
Theadsorption isostereshows the dependence of the partial pressure of the gas on tem-
perature at a constant adsorbed amount. Three isosteres are drawn in Figure13.6. If we draw
lnpiin dependence on 1/T, the isosters become practically straight lines.