PHYSICAL CHEMISTRY IN BRIEF

CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 454

13.2.6 Capillary condensation

Sometimes the description of gas adsorption merely as a thin layer formed on the outer surface
of a solid does not suffice. Adsorption of gases which are under critical temperature and which
wet the adsorbent surface may occur in narrow pores of the adsorbent. This phenomenon is
known ascapillary condensation. It is a process during which vapour condensation on a
liquid occurs at pressures lower than the saturated vapour pressure of gases, while satisfying
the following relation

ln

f

f^

≈ln

p

p^

=−

2 γ Vm(l)

rRT

, (13.41)

whereris the pore radius, andp^ is the saturated vapour pressure.

Note:Capillary condensation is used to explain differences in the course of the adsorption

isotherm recorded in measurements performed while gradually increasing and subsequently

decreasing the gas pressure (the so-called hysteresis of the adsorption isotherm).

13.2.7 Adsorption from solutions on solids

On the surface of solids which are in contact with a solution, adsorption of both the solvent
and the solute occurs. We speak about adsorption of a solute in the narrow sense of the word
when the solute is adsorbed much more than the solvent.
In simple cases, both the Freundlich and Langmuir isotherms are employed for the quanti-
tative description of adsorption. For this purpose they are used in the form of equations (13.37)
and (13.39)), only this time concentrations are used in place of partial pressures

a 2 =α cβ 2 , a 2 =amax

b c 2

1 +b c 2

, (13.42)

whereα,β,amaxare temperature-dependent parameters which are characteristic of the given
trio of substances, i.e. the solvent, the solute and the adsorbate,a 2 is the amount of solute
(adsorbate) adsorbed by unit mass of the adsorbent, andc 2 is the equilibrium concentration of
the solute.