92168.pdf

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

phase^179.

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

x/m = kcl/n (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