Page 183 - Introduction to Colloid and Surface Chemistry

i 72 The solid-liquid interface

Isotherm equations, surface areas

In adsorption from solution, physical adsorption is far more common
than chemisorption. However, chemisorption is sometimes possible;
for example, fatty acids are chemisorbed from benzene solutions on
nickel and platinum catalysts.
Solute adsorption is usually restricted to a monomolecular layer,
since the solid-solute interactions, although strong enough to
compete successfully with the solid-solvent interactions in the first
adsorbed monolayer, do not do so in subsequent monolayers.
Multilayer adsorption has, however, been observed in a number of
cases, being evident from the shape of the adsorption isotherms and
from the impossibly small areas per adsorbed molecule calculated on
the basis of monomolecular adsorption.
The adsorption from solution of polymers has been studied
extensively. The amount of polymer adsorbed usually reaches a
limiting value as the concentration of polymer in solution is
increased, but this value is usually well in excess of that which would
be expected for a monomolecular layer of polymer adsorbed flat on
the solid surface. This suggests that the adsorbed polymer is anchored
to the surface only at a few points, with the remainder of the polymer
in the form of loops and ends moving more or less freely in the liquid
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phase .
The Langmuir and Freundlich equations (see page 128) are
frequently applied to adsorption from solution data, for which they
take the form
( x / m ) ac , ,
x/m = s^— (6.14)
1 + ac
and
l/n
x/m = kc (6.15)

respectively, where x is the amount of solute adsorbed by a mass m of
solid, c is the equilibrium solution concentration, and a, k and n are
constants.
If the monolayer capacity (jc/m) max can be estimated (either
directly from the actual isotherm or indirectly by applying the
Langmuir equation) and if the effective area occupied by each